Planetary Science Mars Research 2

PSRD: Using TES to interpret the composition of the Martian surface

http://www.psrd.hawaii.edu/May02/MarsTES.html

posted May 22, 2002

The Tricky Business of Identifying Rocks on Mars --- A new analysis of thermal emission spectra suggests a new interpretation for the composition of the Martian surface.

Written by G. Jeffrey Taylor Hawai'i Institute of Geophysics and Planetology

The Mars Global Surveyor mission carries a remote-sensing gizmo called the Thermal Emission Spectrometer (TES). TES detects heat waves flowing from the surface of the Red Planet. The TES team, led by Phil Christensen (Arizona State University), identified two large regions on Mars that have distinctive spectral properties. Using mathematical mixing calculations based on the thermal emission spectra of numerous materials, the TES team reported in papers led by Josh Bandfield and Victoria Hamilton that the two regions had mineral abundances similar to basalt (Surface Type 1) and andesite (Surface Type 2), two common volcanic rock types on Earth. Andesite has more silicon than does basalt, giving rise to a distinctive mineralogy. Scientists had mixed reactions to the possibility of andesite on Mars, greeting the news with fascination, consternation, or skepticism. One question raised is how uniquely the spectra of Surface Type 2 matches andesite. Michael Wyatt and Harry Y. McSween (University of Tennessee) have taken another look at the TES spectra by using a larger collection of aqueous alteration (weathering) products in the spectral mixing calculations. They show that weathered basalt also matches the spectral properties of Surface Type 2. Wyatt and McSween also note that Type 2 regions are generally confined to a large, low region that is the site of a purported Martian ocean that sloshed around billions of years ago. They suggest that basalts like those in Surface Type 1 were altered in the ancient Martian sea. Independent data are needed to test the andesite vs. altered-basalt hypotheses. For now, we may have to be satisfied with at least two working hypotheses and a lively debate. References: Wyatt, M. and McSween Jr., H. Y. (2002) Spectral evidence for weathered basalt as an alternative to andesite in the northern lowlands of Mars. Nature, vol.417, p. 263-266. Bandfield, J. L., Hamilton, V. E., and Christensen, P.R. (2000) A global view of martian surface compositions from MGS-TES. Science, vol. 287, p. 1626-1630. Hamilton, V. E., Wyatt, M. B., McSween Jr., H. Y., and Christensen, P. R. (2001) Analysis of terrestrial and martian volcanic compositions using thermal emission spectroscopy: II. Application to martian surface spectra from the Mars Global Surveyor Thermal Emission Spectrometer, J. Geophys. Res., vol. 106, p. 14,733-14,746.

Thermal Eyes 1 of 8

PSRD: Using TES to interpret the composition of the Martian surface

http://www.psrd.hawaii.edu/May02/MarsTES.html

There is a wealth of information in the heat waves emitted from the surface of a planet. TES measures the intensity of the heat radiated in the wavelength range from 6 to 50 micrometers, well beyond what we humans can see. The intensity at different wavelengths (called spectra) allow experts like Phil Christensen and his team to deduce some physical properties of the surface, such as the abundance of boulders versus dust. Thermal spectra also allow us to infer what minerals are present on the surface and in what proportions. This is possible because most minerals have unique spectra--a spectral fingerprint. The trouble is that a planet's surface has more than one mineral, so all the fingerprints are on top of one another. The TES team had to separate each fingerprint, a technique called spectral deconvolution. This unmixing requires correcting for the effects of the gases and dust in the Martian atmosphere, calibrating the response of the instrument at each wavelength, and making other corrections. The whole effort has been done as well as the best forensic laboratories do in identifying the culprits of crimes from smudgy fingerprints at a crime scene. The thermal spectra of numerous minerals have been measured in the laboratory. Christensen and his colleagues have assembled all the measurements into a spectral library--a database of spectral fingerprints like that maintained by the FBI for human fingerprints. This allows them to mathematically combine the spectra of several minerals into a theoretical composite spectrum for comparison with the Martian surface. If there is a good match between calculated and theoretical spectra it suggests that the minerals used in the calculation are present in the proportions that produced the good match. The trouble is that there is not necessarily a unique combination of minerals that match the measured Martian spectra. It depends on which minerals go into the theoretical mix, the chemical compositions of the minerals, and how distinctive each mineral's spectrum is. It's a tricky business.

Distinctive Surface Types

Much of the surface of Mars is covered with reddish dust. The dust gives Mars its dramatic red color, but obscures the materials beneath it. Fortunately, there seem to be areas relatively free of dust. Josh Bandfield and other TES team members interpreted the spectra from these areas as falling into two categories. One (Surface Type 1) is similar to spectra from basalt. This was not too surprising as basalt is the most common rock type on Earth and occurs on Venus, the Moon, and even some asteroids. The other category (Surface Type 2) was a surprise. Banfield and coworkers (reaffirmed by Hamilton and others) interpreted the spectra of Surface Type 2 as indicating a volcanic rock called andesite. This interpretation is supported by analyses of rocks by an instrument on the Mars Pathfinder rover. Andesite contains more SiO 2 than does basalt (52-63 wt% in andesite vs. < 52 wt% in basalt). On Earth, andesite forms in two ways. Most andesite forms where oceanic crust descends at converging margins of tectonic plates. Water released from the wet oceanic crust rises to promote melting in the wedge of mantle above it. This happens in the Andes Mountains, from which andesite gets its name. There is no evidence, such as the presence of arc-shaped mountain ranges, that large plates dove beneath other plates on Mars.

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PSRD: Using TES to interpret the composition of the Martian surface

http://www.psrd.hawaii.edu/May02/MarsTES.html

Spectral mixing calculations of Surface Type 1 (above) and Surface Type 2 (below) indicate that they have different mineral abundances. Detailed analysis and conversion of the mineralogy to chemical composition suggests that Type 1 is basalt and Type 2 is andesite.

The other way of making andesite is by removing crystals as they form in basalt magma, a process called fractional 3 of 8

PSRD: Using TES to interpret the composition of the Martian surface

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crystallization. Removal of minerals that contain less SiO2 than does the magma causes SiO2 to increase, eventually reaching the andesite range. (Some geologist call such magma icelandite rather than andesite, to distinguish the two ways it can form. For simplicity, I'll stick with andesite.) McSween and members of the Mars Pathfinder team have argued that this process could not produce the large volume of andesite observed (if it is really andesite). They point out that the amount of SiO2 observed in andesite is not reached until 90% of the original basalt magma has crystallized. In other words, there ought to be much more basalt than andesite. It is possible to produce andesite when only about half of a basalt has crystallized if the basalt magma contains dissolved water. Michelle Minitti and Mac Rutherford (Brown University) showed that fractional crystallization of a magma with the composition of shergottites (a type of Martian meteorite) will reach 58 wt% SiO2 after only 60% crystallization. Nevertheless, Wyatt and McSween still think that there ought to be lots of basalt associated with the andesite--about equal amounts of each. This is consistent with Banfield's observations: If Surface Type 2 is andesite, maps of the distribution of basalt and andesite suggest that equal amounts of basalt and andesite exist on Mars (you can estimate the abundances of Surface Types 1 and 2 in the maps below). On the other hand, if andesite formed by fractional crystallization, it ought to be associated more intimately with the basalt, rather than concentrated in different places. This drove Wyatt and McSween to look at other ways to interpret the TES data. They tested the idea that Surface Type 2 is not andesite at all. They suggest it could be basalt altered by interaction with water. One clue to this possibility is that the andesite-like surface is confined mostly to a region suspected to have been the site of an ancient Martian sea [See PSRD article: Outflow Channels May Make a Case for a Bygone Ocean on Mars].

The basalt and andesite identified by Josh Bandfield and the TES team occur in different places on Mars (shown on the lefthand maps of the northern and southern hemispheres.) The surface interpreted as andesite (red, Surface Type 2) is concentrated in the northern hemisphere in a large, low region previously interpreted as an ancient ocean on Mars, as seen in the topographic map on the right. The white line outlines the location of the purported shoreline. Click on the image for a larger version.

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PSRD: Using TES to interpret the composition of the Martian surface

http://www.psrd.hawaii.edu/May02/MarsTES.html

Back to the Spectral Library for Some Weathering Products

To test this hypothesis, Wyatt and McSween used a different set of minerals from the Arizona State spectral library. If the Type 2 area lies at the bottom of a former ocean, they reasoned, maybe all that water chemically altered basalt. Water certainly affects the mineralogy of rocks on Earth, forming an assortment of water-bearing (hydrous) minerals. Which minerals form depends on numerous factors, including the temperature, the acidity, and the amount of oxygen and other elements dissolved in the water. Nevertheless, the basic idea of altered basalt can be tested by a greater number of hydrous minerals in their mixing calculation compared to the set used by Bandfield and Hamilton. Most important, Wyatt's calculations differed from Bandfield's and Hamilton's by not including a component of glass rich in SiO2. Such glass is a logical component of andesitic rock, but is not abundant in basalt. The new calculations produce good matches to the observed spectra. For Surface Type 1, Wyatt's calculation agrees with Banfield's and Hamilton's. It looks like relatively unweathered basalt: lots of pyroxene and plagioclase and not much weathering products (clay minerals, sulfates, carbonates). For Surface Type 2, however, the modeled mineral abundances are quite different. Bandfield and Hamilton concluded that there were high abundances of plagioclase feldspar and high-silica glass, and low abundances of pyroxene and weathering products. In contrast, Wyatt finds high abundances of hydrous minerals and other weathering products. He finds no high-silica glass, of course, because he did not use it in the calculation. Quantitative assessments of the quality of the match between measured and calculated spectra indicate that all matches are very good--another example of how tricky it is to determine mineralogy from thermal emission data.

Mike Wyatt's calculation suggests that it is possible that Surface Type 2 (below) is weathered basalt rather than andesite. His calculated spectrum contains more clay minerals than do the spectra calculated by Bandfield and Hamilton.

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PSRD: Using TES to interpret the composition of the Martian surface

http://www.psrd.hawaii.edu/May02/MarsTES.html

Wyatt's calculation differs from the others in that he substituted hydrous minerals for high-silica glass. It turns out that the clays used in Wyatt's calculation have very different spectral properties between 18 and 20 microns. Unfortunately, the Martian atmosphere (mostly carbon dioxide) is opaque in this wavelength range, so a definitive test cannot be made. Some independent measurements will be needed, perhaps made by instruments directly on the surface.

Fresh and Weathered Natural Basalt

Like silverware, rocks tarnish. They rust. They rot. Take a good look at any natural rock surface on Earth. Look at its color. Then smash off some of the surface with a rock hammer. The fresh surface will look different, usually shinier and a different shade of color than the weathered surface. Wyatt and McSween decided to test their idea by obtaining spectra of weathered and fresh basalt from the Columbia River plateau in Washington and Oregon, USA. These are extensive flows of basalt and are fairly typical of terrestrial basalts. Fresh surfaces of Columbia River basalts are somewhat browner than weathered surfaces, which are darker in color.

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PSRD: Using TES to interpret the composition of the Martian surface

http://www.psrd.hawaii.edu/May02/MarsTES.html

This road-side photograph by Thor Thordarson shows lobes of pahoehoe lava from the Columbia River basalts. Unweathered (grey-brown) pahoehoe lobes overlie strongly weathered, darker basalt pahoehoe lobes.

Wyatt measured the thermal spectra of fresh and weathered basalt and compared the results to Surface Types 1 and 2. He found that the fresh basalt is somewhat similar to Surface Type 1, the one interpreted by everyone to be basalt. On the other hand, the weathered surface of the basalt was not a bad match for Surface Type 2, the one interpreted previously by Bandfield and Hamilton to be andesite. One problem with using the Columbia River basalts as terrestrial analogs is that they were not weathered under the same conditions as Wyatt and McSween hypothesize for Surface Type 2 on Mars. The Columbia River basalts were weathered by rain and the atmosphere. They were not sitting at the bottom of a Martian ocean.

Fresh and weathered terrestrial basalt spectra match Surface Type 1 and 2, respectively. This is consistent with Wyatt and McSween's suggestion that Surface Type 2 is weathered basalt, not andesite.

Wyatt also did spectral deconvolution calculations on the Columbia River basalt spectra. He found that the fresh basalt contains mostly pyroxene and plagioclase, which matches detailed study of the Columbia River basalts. In contrast, the weathered basalt should contain lots of clay minerals (about 30%), which it does. Using a smaller number of clay minerals and adding glass, as Bandfield and Hamilton did, produces a calculated mineralogy that it should contain lots of glass (25%), which it does not. Nevertheless, the Bandfield technique still yields a good fit to the data. The difference, Wyatt and McSween suggest, may be that the silica glass mimics noncrystalline weathering products that 7 of 8

PSRD: Using TES to interpret the composition of the Martian surface

http://www.psrd.hawaii.edu/May02/MarsTES.html

could be present in the weathered rock surfaces, rather than volcanic glass.

So what is Surface Type 2?

Surface Type 2 might be andesite. It might be basalt. It might be something else nobody has considered. Finding out what it is will probably require other types of data for Mars. It is an important issue because it makes a huge difference in how we picture the igneous history of Mars and even the nature of processes operating inside the plumbing systems of Martian volcanoes. The Gamma Ray Spectrometer (GRS) carried onboard the Mars Odyssey spacecraft, currently in orbit around Mars, may help settle the argument. The GRS can measure Si quite readily, so should be able to determine if Surface Types 1 and 2 differ in Si, as they would if they were composed of basalt and andesite, respectively. If the Si concentrations are similar, it would favor the hypothesis that Surface Type 2 is made of weathered basalt. We should know this answer in a year or two as the GRS onboard Odyssey methodically determines the composition of the Martian surface.

ASU Thermal Emission Spectral Library. Bandfield, J. L., Hamilton, V. E., and Christensen, P.R. (2000) A global view of martian surface compositions from MGS-TES. Science, vol. 287, p. 1626-1630. Hamilton, V. E., Wyatt, M. B., McSween Jr., H. Y., and Christensen, P. R. (2001) Analysis of terrestrial and martian volcanic compositions using thermal emission spectroscopy: II. Application to martian surface spectra from the Mars Global Surveyor Thermal Emission Spectrometer, J. Geophys. Res., vol. 106, p. 14,733-14,746. Mars Odyssey homepage. Mars Odyssey Gamma Ray Spectrometer homepage. Minitti, M.E. and Rutherford, M.J. (2000) Genesis of the Mars Pathfinder "sulfur-free" rock from SNC parental liquids. Geochim. Cosmo. Acta, vol. 64, p. 2535-2547. Wyatt, M. and McSween Jr., H. Y. (2002) Spectral evidence for weathered basalt as an alternative to andesite in the northern lowlands of Mars. Nature, vol.417, p. 263-266.

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PSRD: Gullied Slopes on Mars

http://www.psrd.hawaii.edu/Aug03/MartianGullies.html

posted August 29, 2003

Gullied Slopes on Mars --- Do the Martian gullies tell us something about the stability and distribution of near-surface water?

Written by Linda M.V. Martel Hawai'i Institute of Geophysics and Planetology

There are a lot of gullies on certain Martian slopes and just about as many ideas of how they formed. The proposed origins of gullies include seepage of groundwater or brines, outbursts of CO2, snowmelt, geothermal activity, or dry flows of windblown dust and silt. Research teams have been publishing their hypotheses since the gullies were first announced in 2000, and the discussions are still lively. For example, a quick search of the terms "Martian or Mars or gullies or seepage" on the NASA Astrophysics Data System (link opens in a new window) delivered nearly 20 references to papers or abstracts published just in the past eight months. Gullies are such a hot topic, some researchers would argue, because they could indicate sources of liquid water at shallow depths. PSRD provides a rundown of the leading hypotheses to explain how Martian gullies form and how researchers use chemical data from Martian meteorites and knowledge of the Earth to support their points of view. The reference that started it all: (See more references at the end of the article.) Malin, M. C. and Edgett, K. S. (2000) Evidence for Recent Groundwater Seepage and Surface Runoff on Mars. Science, v. 288, p. 2330-2335.

Discovery of Gullies

Michael Malin and Kenneth Edgett (Malin Space Science Systems) made the initial announcement in 2000 of the discovery of Martian gullies in Mars Global Surveyor MOC images (2 to 8 meter/pixel resolution). They found gullies located at middle and high latitudes, particularly on interior walls and central peaks of craters, on pit walls in the south polar region, and on walls in two large valleys (Nirgal Vallis and Dao Vallis). Typical middle-latitude gullies occur within a few hundred to five hundred meters of the local surface. In early 2003, Edgett and Malin updated their account after looking at an additional two-years' worth of images and studying 10,000 gullies. We begin our coverage by summarizing their observations and interpretations. LEFT: Three main elements of Martian gullies, the alcove, channel, and apron are shown on this subframe of MOC image M03-00537/PIA01031, located near 54.8 oS, 342.5oW.

Gullies have three characteristic parts: head alcoves, channels that extend downslope from the bottoms of alcoves, and triangular aprons of debris that broaden downslope. The head alcove is a depression that tapers downslope. It may be small or absent if it occurs at an overhanging rock layer. Channels, of course, are the most striking features of the gully systems; they generally begin deep and broad at a specific exposed rock layer, then taper downslope. They are banked, sinuous, and sometimes braided. Gully channels are free of debris, suggesting to Malin and Edgett that there was enough energy during their creation to flush down materials. Depositional aprons occur below the head alcove, down the slope, and sometimes continue out past the base of the slope. Channels appear to cut down into the aprons. Malin and Edgett's mapping shows that there are more gullies in the south than in the north but that they occur, with few exceptions, poleward of 30 o latitude in both hemispheres. Gullies also occur in regional clusters. Gullies in a cluster tend to begin at the same rock layer exposed in the walls of different craters and troughs in the region. A lack of impact craters on the channels and aprons suggests that the gullies are geologically young--the youngest features on the slopes where they form. Malin and Edgett observed gully aprons that partly cover windblown dunes or polygonally patterned ground (a periglacial or freeze/thaw feature). In other cases, the aprons and channels are partially mantled by dune sand. On Earth, dunes and patterned ground are known to change easily with environmental conditions or are generally short-lived on geological timescales, that is, less than hundreds of thousands of years. This would lend support to the hypothesis that the Martian gullies are relatively young. But, in fact, we don't know their ages. Malin and Edgett compared MOC images to twenty-year-old Viking images and could not see any evidence of new gully formation or gully movement at the scale of tens of 1 of 5

PSRD: Gullied Slopes on Mars

http://www.psrd.hawaii.edu/Aug03/MartianGullies.html

meters. They estimate, based on the relationships of the gullies to underlying ground and to the lack of craters, that the gullies are probably older than 20 years but younger than a million years.

Groundwater seepage

Malin and Edgett favor gully formation by liquid water, specifically by groundwater seepage from shallow aquifers (underground layers of porous rock that yield water) and subsequent surface runoff. In their analysis, groundwater flows through and along bedrock layers a few hundred meters or less below the surface, perhaps backing up behind ice barriers, until it breaks through to the surface where the rock layers are exposed (for example in the walls of craters or troughs). Then the water flows downslope, perhaps in plunging outbursts, carving gully channels and depositing debris aprons. The clustering of gullies could be explained if the rock layers and aquifers are limited in size and lateral extent.

Gullies on a north wall of a crater captured by the Mars Orbiter Camera.

An origin requiring liquid water must resolve the issue of how to stabilize it on the surface at the mid-latitudes where the gullies are found. The answer, according to planetologists, lies in the changes in Martian climate caused by changes in the planet's obliquity--the magnitude of the tilt of its spin axis. Calculations show that the obliquity of Mars slowly increases and decreases between 15 o (axis is almost straight up) and 35o over a hundred-thousand-year cycle, but may occasionally reach 60 o over million-year cycles. (Now Mars' obliquity is 25.2 o, very similar to Earth's obliquity which is 23.5 o and only varies by 4o.) Researchers say that the obliquity cycles produce climate changes on Mars that cause melting of ground ice and ongoing redistribution of water between the poles and mid-latitudes. For example, at high obliquity the icy Martian poles warm up in the summer because of the increased sunlight. Perhaps the gullies formed during one of these periods when the obliquity was higher than it is today.

Gullies in Iceland

Hillside gullies in Iceland have identical scales and surface expressions to gullies discovered on Mars--similarities that strengthen the case for a water origin. William Hartmann (Planetary Science Institute, Tucson, AZ) and colleages Thorsteinn Thorsteinsson and Freysteinn Sigurdsson (both Icelandic researchers) report that the best Icelandic analogs are on basaltic talus slopes where gullies formed by debris flows initiated by ground water saturation and/or by drainage of water from cliffs higher on the slopes. They argue that if there was uniform water saturation of the talus slopes, then gullies would begin wherever the 2 of 5

PSRD: Gullied Slopes on Mars

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material broke loose on the cliff face. However, Martian gullies and many of the Icelandic examples appear to originate from specific rock layers--observations that support the idea that the water exited the cliff from an aquifer.

These photographs from the work by Hartmann, Thorsteinsson, and Sigurdsson show hillside gullies in Iceland with similar characteristics and dimensions to Martian gullies noted by Malin and Edgett. LEFT: Tall cliff about 500 meters high, located 15 kilometers northeast of Reykjavik, Iceland is full of gullies similar to Martian gullies. Hartmann and colleagues observe that gullies begin near the base of blocky ourcrops seen in the upper part of the cliff. RIGHT: Icelandic slope composed of hyaloclastic debris (basaltic fragments formed by the flowing or intrusion of lava or magma into water, ice, or water-saturated sediment) showing gully formation from a specific layer. Levees on these Icelandic gullies are more visible than in most of the Martian examples.

Brines Rather Than Water

Donald Burt and Paul Knauth (Arizona State University) offer an alternative to fresh water to produce Martian gullies. They raise the possibility that eutectic brines (by definition liquids containing dissolved salts that are the last to freeze and the first to melt) in near-surface aquifers have oozed out of valley walls to form the gullies. They further argue that calcium chloride brines would have the low freezing points and low vapor pressures to be stable at the middle latitudes where gullies have been observed. Brines in the Martian subsurface could be sandwiched between a top layer of ice and bottom layer of salts. The brines hypothesis is strengthened by studies of Martian meteorites that show the rocks were exposed to brines on Mars. LEFT: Back-scattered electron image of salt minerals in Martian meteorite Nakhla. Mineral symbols: h=halite, a=augite, an=anhydrite, pl=plagioclase.

Secondary mineral assemblages in Martian meteorites provide information about the interaction between fluids in the Martian crust and the parent igneous rock. Meteorite experts, such as John Bridges and Monica Grady (Natural History Museum, London), say that the salt minerals in Nakhla formed through brine evaporation on Mars. This is direct evidence that there are brines on Mars at least some of the time. The likelihood of brines on Mars has lead some researchers, for example Eric Gaidos (University of Hawaii) to suggest that brines originate from deep aquifers that are pressurized by freezing. They've termed this cold-climate process "water volcanism." Gaidos thinks that the gullies are simply an expected consequence of freezing of aquifers. Brines on Mars should be highly electrically conducting, and their presence may someday be confirmed by geophysical detection. THEMIS data from the Mars Odyssey spacecraft may be able to provide chemical evidence in support of the brine origin by detecting surface residues of salt minerals at the gullies.

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PSRD: Gullied Slopes on Mars

http://www.psrd.hawaii.edu/Aug03/MartianGullies.html

Carbon dioxide

Carbon dioxide (CO2) seasonal outbursts, perhaps due to Mars' obliquity cycles, have been evoked to explain the Martian hillside gullies. According to some researchers (for example, Donald Musslewhite, Timothy Swindle, and Jonathan Lunine of University of Arizona) liquid CO 2 aquifers build up behind plugs of dry-ice and eventually break out and vaporize at the surface during periods of seasonal heating. The gullies would form because of the fluidized flow of gas and suspended rock debris. On Earth similar landforms are produced by suspended flows of volcanic ash and gas from erupting volcanoes but without the head alcoves. Still, it's a matter of debate whether or not gas-supported flows on Mars could produce the banked and sinuous gully channels observed in the orbital images.

Snow melt

Philip Christensen (Arizona State University) proposes a different model in which melting of an overlying snowpack provides the source of water to erode gullies. Gullies form on the snow-covered slopes when melt water flows over or seeps into the slope materials. If the snow layer collapses, then water could erode the underlying ground leaving the gullies as evidence of the process. Christensen says the snow fell when water was transported from the warmer poles to colder mid-latitudes during high obliquity periods. Then melting occurred during low obliquity when mid-latitude temperatures increased and the water was stable beneath an insulating cover of snow. Christensen thinks remnants of snowpacks are still present on mid-latitude, pole-facing slopes and says melting could be happening right now. Snowmelt explains many of the gully characteristics without requiring near-surface aquifers as the source of the water. Little chemical weathering or mineralization of the surface would be expected if the gullies were formed by snowmelt because of the near-freezing temperatures and short duration, thus eliminating the need to look for evaporation deposits as evidence of the process.

Subscene of THEMIS visible band image of gullies purportedly formed by snowmelt on a crater wall with remnants of snow cover. The crater is in Terra Sirenum near 39 o S, 166 o W. The scene is illuminated from the upper left, but has been contrast stretched to show more detail. Image width is 3 kilometers. Higher Resolution will open in a new window.

Geothermal Activity

Geothermally heated ground ice has also been invoked as source water forming the gullies. Several researchers, including Hartmann and colleagues citing their work in Iceland, hypothesize that Martian volcanism provided sources of sporadic subsurface heating that could melt ground ice and produce aquifers. Thermal data from Mars Odyssey could help determine the likelihood that geothermal heating is involved in gully formation.

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PSRD: Gullied Slopes on Mars

http://www.psrd.hawaii.edu/Aug03/MartianGullies.html

Dry Flows

In a completely different mechanism, Martian gullies are not related to liquids of any sort. Dry flows of windblown dust and silt can display the same features observed in the Martian gullies according to Allan Treiman (Lunar and Planetary Institute, Houston). The locations and distribution of the gullies, he says, are consistent with atmosphere circulation models that show that wind deceleration occurs in the same areas as the gullies. Deceleration would cause suspended sediments to drop out and be deposited on wind-protected slopes. If these slopes are steep, says Treiman, then gullies will form. Treiman's hypothesis evokes massive snow avalanches on Earth, where movement is by dry granular flows. RIGHT: An avalanche in progress in the Cariboo Mountains, British Columbia, Canada. (Photo by Brad White. www.avalanche.org)

Whether or not the Martian gullies tell us something about the stability and distribution of near-surface water is yet to be answered conclusively. But the quest for the answer is keeping a lot of people busy, planetary geologists curious about the history of water on Mars and astrobiologists, just to name a few. Using multiple data sets, such as MOC images, MOLA altimeter data, and TES thermal data from Mars Global Surveyor, researchers Jennifer Heldmann and Michael Mellon (Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder) reported preliminary findings in February, 2003 at the NASA Astrobiology Institute General Meeting that support the idea that water from shallow aquifers (perhaps 200 to 300 meters beneath surface) formed the gullies. Mars Odyssey neutron spectrometer data confirm that the Martian surface poleward to 60o contains from 35wt% to 100wt% water-ice buried beneath a shallow layer of ice-free material [see PSRD article Dirty Ice on Mars]. As more data are analyzed we will have a clearer understanding of how the gullies formed. And new data should be bountiful from the currently orbiting Mars Global Surveyor and Mars Odyssey spacecrafts and future international missions as well as continuing studies of meteorites and of analogous gullies on Earth.

Bridges, J. C. and Grady, M. M. (1999) A halite-siderite-anhydrite-chlorapatite assemblage in Nakhla: Mineralogical evidence for evaporites on Mars. Meteoritics and Planetary Science, v. 34(3), p. 407-415. Burt, D. M. and Knauth, L. P. (2003) Electrically conducting, Ca-rich brines, rather than water, expected in the Martian subsurface. Journal of Geophysical Research, v. 108(E4), doi: 10.1029/2002JE001862,2003. Christensen, P. R. (2003) Formation of recent martian gullies through melting of extensive water-rich snow deposits. Nature, v. 422, p. 45-48. Edgett, K. S., Malin, M. C., Williams, R. M. E., and Davis S. D. (2003) Polar- and middle-latitude Martian gullies: A view from MGS MOC after 2 Mars years in the mapping orbit. Lunar and Planetary Science Conference XXXIV, abstract 1038. Gaidos, E. and Marion, G. (2003) Geological and geochemical legacy of a cold early Mars. Journal of Geophysical Research, v. 108(E6), doi: 10.1029/2002JE002000,2003. Hartmann, W. K., Thorsteinsson, T., and Sigurdsson, F. (2003) Martian hillside gullies and Icelandic analogs. Icarus, v. 162, p. 259-277. Heldmann, J. L. and Mellon, M. T. (2003) Gullies on Mars and Constraints Imposed by Mars Global Surveyor Data. NASA Astrobiology Institute General Meeting 2003, abstract 12653. Malin, M. C. and Edgett, K. S. (2000) Evidence for Recent Groundwater Seepage and Surface Runoff on Mars. Science, v. 288, p. 2330-2335. Mars Orbiter Camera Image Gallery at Malin Space Science Systems.

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Mellon, M. T. and R. J. Phillips (2001) Recent gullies on Mars and the source of liquid water, Journal of Geophysical Research, v. 106, p. 1-15. Musselwhite, D. S., Swindle, T. D., and Lunine, J. I. (2001) Liquid CO 2 breakout and the formation of recent small gullies on Mars. Geophysical Research Letters, v. 28(7), p. 1283-1285. Taylor, G. J. (2002) Dirty Ice on Mars. Planetary Science Research Discoveries. http://www.psrd.hawaii.edu/June02/MarsGRSice.html. Treiman, A. H. (2003) Geologic settings of Martian gullies: Implications for their origins. Journal of Geophysical Research, v. 108(E4), doi: 10.1029/2002JE001900,2003.

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PSRD:: The Multifarious Martian Mantle

http://www.psrd.hawaii.edu/review/martianMantle.html

posted June 23, 2004

The Multifarious Martian Mantle -- Detailed analyses of Martian meteorites reveal that the planet's interior preserves distinctive regions that formed 4.5 billion years ago.

Written by G. Jeffrey Taylor Hawai'i Institute of Geophysics and Planetology

Pieces of relatively young lava flows from Mars (all less than 600 million years old) preserve a record of the planet's initial segregation into core, mantle, and crust. Research by Lars Borg (University of New Mexico), his colleague David Draper, and his former colleagues at the Johnson Space Center, Chris Herd (University of Alberta, Canada), and Cyrena Goodrich (formerly at the University of Hawaii and now at Kingsborough Community College in Brooklyn, New York) shows that there are distinctive regions in the interior of Mars. These regions, or reservoirs as cosmochemists like to call them, formed early, about 4.5 billion years ago, and come in two flavors. One, dubbed "enriched," contains high concentrations of trace elements, has a high ratio of lanthanum to ytterbium (La/Yb), high strontium-87 to strontium-86 (87Sr/86Sr), a low ratio of neodynmium-143 to neodynmium-144 (143Nd/144Nd), and is relatively oxidized. The other, dubbed "depleted," contains lower levels of trace elements, has lower La/Yb and 87Sr/86Sr, higher 143Nd/144Nd, and is relatively reduced (much less oxidizing than the enriched reservoir). There are mixtures in between these extremes. The reservoirs may have formed in a global magma ocean. Their preservation for 4.5 billion years indicates that Mars, in contrast to Earth, did not have active plate tectonics since the reservoirs formed. References: Borg, L. E., Nyquist, L. E., Weissman, H., Shih, C.-Y., and Reese, Y. (2003) The age of Dar al Gani 476 and the differentiation history of the martian meteorites inferred from their radiogenic isotopic systematics. Geochimica Cosmochimca Acta, v. 67, p. 3519-3536. Borg, L. E. and Draper, D. S. (2003) A petrogenetic model for the origin and compositional variation of the martian basaltic meteorites. Meteoritics & Planetary Science, v. 38, p. 1713-1731. Goodrich, C. A., Herd, C. D. K., and Taylor, L. A. (2003) Spinels and oxygen fugacity in olivine-phyric and lherzolitic shergottites. Meteoritics & Planetary Science, v. 38, p.1773-1792. Herd, C. D. K. (2003) The oxygen fugacity of olivine-phyric martian basalts and the components within the mantle and crust of Mars. Meteoritics & Planetary Science, v. 38, p. 1793-1805.

Samples of Martian Lava Flows

Much of the surface of Mars is made of volcanoes or lava plains, or sedimentary rocks made from them. River valleys and canyons dissect volcanoes and lava flows, and long ago, before about 4 billion years ago, chunks of asteroids and comets blasted the surface to make the cratered Martian highlands. Nevertheless, remnants of the

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lavas remain. More recent impacts flung bits of lava flows and other types of igneous rocks off the planet. Some of the ejected rocks made their way to Earth; we've found about 30 of them. These free samples of Mars are invaluable in helping us understand the geological history of the Red Planet. The main evidence that the rocks actually come from Mars is from trapped gases in three of them. The mixture of gases is a striking match for the Martian atmosphere as measured by the Viking landers in the mid-1970s. All of the Martian meteorites share a unique oxygen isotopic composition, linking them to each other and to the three with trapped Mars air in them. There are several groups of Martian meteorites. The most common, shergottites, are basalts, a common type of lava flow. The shergottites might be chips of thick surface flows or solidified magma that cooled in dikes below the surface. They have a wide range in chemical and mineralogical compositions. They tend to have much less plagioclase feldspar than do typical terrestrial basalts, but like basalts on all the planets they formed by partial melting of the interior. Quite a few processes can operate as a magma oozes through a hundred kilometers of rock to the surface, but we can often see through these processes-in fact, we can learn a lot about them-to deduce the chemical composition and much about the origin of the rocks making up the interior, or mantle. Lars Borg, Chris Herd, and their colleagues have been able to determine the time of initial differentiation of Mars, hence the time the mantle formed, and to deduce the variations in the composition within the mantle.

Polished sample of a shergottite, Sayh al Uhaymir 005, which was found in Oman. The image is an x-ray map obtained in an electron microprobe. Light green is olivine, dark green is pyroxene (actually two different minerals, one higher in calcium than the other), and pink is maskelynite (plagioclase feldspar shocked impact to a glass). Meteoriticists call such olivine-rich shergottites "olivine-phyric shergottites" to distinguish them from shergottites (similar but with little or no olivine) and lherzolitic shergotittes (lots of olivine and pyroxene, little plagioclase, and probably not lava flows).

Young Meteorites, Old Mantle

The shergottites come from lavas that erupted between about 150 million years and 500 million years ago. Nakhlites (pyroxene-rich rocks that formed in different types of lava flows from Shergottites) and the meteorite

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Chassigny are older, 1300 million years (1.3 billion years). The oldest Martian meteorite we have identified is Allan Hills 84001, which formed from a magma intruded deep in the Martian crust 4.5 billion years ago. Although the shergottites are all quite young compared to the 4.55 billion years age of the planets, they nevertheless contain a clear record of the time when their mantle source regions formed. (Cosmochemists refer to the region in a planet's mantle that melted to give rise to a magma as the magma's source region.) Lars Borg and his colleagues show this in two ways. The first was to investigate the isotopes of rubidium (Rb) and strontium (Sr). This requires painstaking work in ultraclean laboratories to prevent contamination. The samples are crushed, minerals separated, and each separate dissolved in assorted acids, run through a process that separates Rb from Sr, and isotopes measured by mass spectrometry. LEFT: Lars Borg in the isotope clean lab at the Johnson Space Center.

Using their own and analyses from other cosmochemists, Borg and his colleagues plotted the 87Rb/86Sr ratio in each rock against the 87Sr/86Sr ratio (see graph below). 87Rb is radioactive and as time passes it decays to 87Sr. If all the mantle sources for the rocks had the same 87Rb/86Sr but different amounts of Rb and Sr, they would have formed a horizontal line initially. With time, 87Rb would decay to 87Sr, leading to a line with a slope. The slope corresponds to the age of the mantle sources. If the individual points all plot close to a single line, it suggests that they had a simple history: All their mantle sources (hence much, perhaps most of the Martian mantle) formed simultaneously at the time indicated by the slope of the line. Each source subsequently melted, but at different times, producing lavas 150 to 575 million years ago (the ages of the shergottites). The slope of the line drawn between the point for Que94201 and Shergotty, which most of the other points lie close to, indicates an age of 4.49 billion years. (The points falling off the line are the nakhlites.) Note that the values of 87Rb/86Sr, which reflect the elemental ratio of Rb/Sr, range quite widely. I return to this important observation below.

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Whole rock Rb-Sr isotopic data for shergottites and nakhlites. The shergottites all fall along a line suggesting an ancient age of 4.49 billion years. This age may represent the time when distinct sources formed in the Martian mantle.

Cosmochemists get age information from samarium (Sm) and neodymium (Nd), two rare earth elements. Sm has two radioactive elements. 147Sm decays to 143Nd with a long half-life of 106 billion years. On the other hand, 146Sm decays to 142Nd with a much shorter half-life, only 103 million years. 146Sm decays fast enough that it would be almost completely transformed to 142Nd in about five half lives, about 500 million years (0.5 billion years). The shergottites have clear evidence that this short-lived isotope was present in their mantle sources, so the sources formed before 4.0 billion years ago. We have two speedometers here, one recording city driving, the other highway driving. Combined they ought to give us a more complete record of shergottite driving habits, i.e., the ages of their mantle sources. In December, 2002, Lars Borg was pondering these Sm-Nd isotopes systematics as he and his dog, Meka (who should have been named Isochron), were driving from New Mexico to his wife's parents' home in California. His wife and young child had flown to the west coast, leaving him alone with his thoughts. (Meka is not particularly talkative.) While driving through high, colorful deserts and low, hot, barren ones, Borg worked out that he could use both the long-lived and short-lived Sm isotopes to define the age of the shergottite mantle sources even better than the Rb-Sr data had, or at least make an independent assessment of it. Borg realized he could use three parameters to assess the age of differentiation of shergottite mantle sources. One is the initial ratio of 143Nd/144Nd in each shergottite, expressed as epsilon-143Nd, a measure of how the ratio deviates from the ratio in chondritic meteorites, in parts per thousand. The other is the initial ratio of 142Nd/143Nd, or epsilon-142Nd, the deviation of the ratio from chondritic meteorites, again in parts per thousand. He also needed to know the ratio of 147Sm/144Nd in the source regions. Assuming that all the epsilon values were initially like those in chondrites, that formation of the sources involved formation of reservoirs with different Sm/Nd ratios, and that the sources melted only once more (when each shergottite magma formed), Borg knew he could calculate lines of equal age (isochrons) for different times. By plotting data from shergottites on the resulting complicated graph, Borg hoped to find the age of mantle differentiation.

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Lars Borg devised this plot of the variation in Nd isotopes with time and ratio of Sm/Nd after mantle formation, assuming a simple two-stage history for Martian meteorites (early mantle formation followed billions of years later by magma formation). The circles are for shergottites (except the one labeled "ALH," which is not a basalt) and the squares are for the nakhlites and chassigny. The filled circles fall along an isochron indicating an age of 4.51 billion years, which Borg and his colleagues take as the age of formation of the shergottite source regions. The points off the line, including Shergotty and the nakhlites, probably had histories more complicated than the simple two-stage history upon which the isochron lines are based.

Distinctive Mantle Reservoirs

The isotopic data shows that the mantle sources for the shergottites formed at about 4.5 billion years ago, soon after Mars formed. To retain a record of that important event, much of the mantle cannot have been disturbed until shergottite magma formation 150 to 500 million years ago. This suggests that, unlike the Earth, there was not extensive mixing of the mantle or recycling of the crust back into it, unless that took place before 4.5 billion years ago. Other data allow Borg and his coworkers to identify other chemical properties of the mantle sources, or reservoirs. A distinctive characteristic of shergottites is that they have a range of rare earth element concentrations, especially in the concentration of light rare earth elements (those with lower atomic weights). This can be expressed in a shorthand way by the ratio of lanthanum (La) to ytterbium (Yb). Specifically, La/Yb correlates with initial 87Sr/86Sr (see diagram below) and with epsilon-143Nd. The samples with high La/Yb (about the same as in chondritic meteorites) come from "enriched" mantle reservoirs. Those with low La/Yb come from "depleted" reservoirs. The depleted reservoirs have low initial 87Sr/86Sr and high epsilon-143Nd. Scott McLennan (State University of New York, Stony Brook) also identified distinctive mantle reservoirs by comparing trace element concentrations in Martian meteorites.

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Initial 87Sr/86Sr correlates with the extent to which La/Yb is depleted compared to chondritic (C1) values. Enriched sources have high 87Sr/86Sr because they had higher 87Rb/86Sr initially, hence produced more 87Sr by decay of radioactive 87Rb.

Oxidation State of Mantle Reservoirs

The shergottites reveal another fundamental characteristic of the distinctive mantle reservoirs: the enriched reservoirs are significantly more oxidizing than the depleted ones. This is revealed by determination of a parameter called the oxygen fugacity, which is related to the partial pressure of oxygen available in a rock to oxidize elements that can occur in more than one valence state. Iron is particularly affected. Under highly oxidizing conditions it can be all Fe3+ (trivalent). Under somewhat reducing conditions it might be all Fe 2+. When conditions are very reducing, much of the iron might be metallic (Fe0). Cosmochemists determine oxygen fugacity by measuring the composition of iron-bearing minerals in a rock. Specifically, they determine the amount of Fe2+ and Fe3+ in each mineral. Examples are two iron-titanium oxide minerals (see photo on the left) or an iron-bearing oxide called spinel associated with pyroxene and olivine (also both iron-bearing minerals). It requires very careful observations of the mineralogy and the way minerals mingle in rocks to choose the right minerals to analyze. It also requires extremely accurate analyses by electron microprobe because the amount of Fe3+ cannot be determined directly-only total Fe can be measured. The amount of di- and tri-valent iron is determined by calculating the precise formula for a mineral and assuming the analysis is perfect (the total of all elements is exactly 100 percent). LEFT: The distribution of Fe3+ and Fe2+ between iron-titanium oxide minerals ilmenite and ulvöspinel can be used to determine oxygen fugacity. This shows co-existing oxide and other minerals in the shergottite EET 79001. The image is a

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backscattered electron image taken with a scanning electron microscope. [Image is 150 microns across.]

It takes careful workers like Chris Herd, Cyrena Goodrich, and Larry Taylor (University of Tennessee) to do that type of work. Their hard work paid off. They found that oxygen fugacity also correlates with La/Yb (see diagram below). The enriched reservoir (high La/Yb) is more oxidizing than the depleted one, by a factor of about 1000. (The oxygen is usually reported in logarithm units of atmospheres, but often relative to the fugacity of a known equilibrium assemblage. In the plot below, from Chris Herd's paper, it is reported relative to the quartz-fayalite-magnetite (QFM) assemblage, or buffer.)

LEFT: Graph of La/Yb (normalized to the ratio in carbonaceous chondrites) versus oxygen fugacity (compared to the quartz-fayalite-magnetite QFM buffer assemblage). Enriched (high La/Yb) mantle sources are substantially more oxidizing than depleted (low La/Yb) mantle sources.

Formation of Mantle Reservoirs

Mars appears to have at least two distinct mantle reservoirs, and probably others. The table below summarizes the properties of the two distinct reservoirs tapped by magmas that created the shergottites. Enriched Reservoir

high La/Yb low Sm/Nd ( -epsilon Nd) high Rb (high 87Sr/86Sr) oxidized

Depleted Reservoir

low La/Yb high Sm/Nd ( +epsilon low Rb (low 87Sr/86Sr) reduced

Nd)

In papers published a couple of years ago, Borg, Herd, and company called the enriched reservoir "crust-like." This followed our thinking about planetary crusts being enriched in trace elements compared to the underlying mantles. It is also consistent with observations of the Mars Odyssey Gamma-Ray Spectrometer, which shows higher levels of K and Th (both elements that behave like rare earth elements) compared to Martian meteorites. However, placing the enriched component in the crust required that depleted shergottite magmas interacted with the crustal rocks to become more oxidized, enriched in trace elements and 87Sr/86Sr, and lower in Sm/Nd in very precise ways that are require disparate chemical properties and processes to precisely balance for each magma, an unlikely event. The solution was to put the reservoirs into the mantle.

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PSRD:: The Multifarious Martian Mantle

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Model for shergottite (and Martian basalts in general) formation in the Martian mantle. Two reservoirs were produced early in Martian history. One is depleted in trace elements and is highly reducing (almost at the iron metal-iron oxide buffer "IW"). The other is enriched in trace elements and oxidizing. It contains substantially more Fe 3+ than does the depleted reservoir, but might also contain H 2 O (though this is not necessary to cause the oxidation state to be higher than in the depleted reservoir).

How did the reservoirs form? The very old age of the reservoirs drive Borg and others to suggest that Mars was surrounded by an ocean of magma when it formed, as most planetary scientists think happened to the Moon when it formed. [See magma ocean illustration in PSRD article The Oldest Moon Rocks.] If so, as the magma ocean began to crystallize, the first minerals would not take up much rare earth elements (including La, Sm, Nd, and Yb) or Rb. As crystallization continued the magma would become progressively enriched in elements excluded from the crystallizing solids, perhaps reaching values a hundred times higher. This could produce a large range from depleted mantle to highly enriched. And, because Fe3+ tends to be excluded from crystallizing minerals more than Fe2+ does, the ratio of Fe3+ to Fe2+ increases, making the enriched reservoirs more oxidizing. If H2O were present, it would also concentrate in the enriched reservoirs. Lars Borg and Dave Draper, his colleague at the University of New Mexico, modeled this process quantitatively. Such calculations have been done to try to understand the lunar magma ocean. In the Martian case, the depth of the magma ocean matters a lot because the deeper the magma ocean, the higher the pressure near its base. The pressure difference results in different minerals crystallizing and those minerals differ in the extent to which they exclude or include trace elements. This research is still in its early stages, but the basic approach is leading to hypotheses testable by analyses of Martian meteorites.

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A likely picture of the interior of Mars, based on experiments by Constance Bertka and Yingwei Fei. The uppermost mantle of Mars consists of olivine and pyroxene, with a small amount of garnet (shaded green). These are fairly common minerals on Earth, the other planets, the Moon, and asteroids. However, at a depth of about 1100 km, the olivine begins to convert to a more dense form, called gamma-spinel, without changing its chemical composition. The conversion is complete by 1300 km. Along with the conversion of olivine to a spinel crystal structure, garnet and pyroxene convert to a mineral called majorite, which has a crystal structure like garnet, but is close to pyroxene in chemical composition (shaded yellow). At higher pressures, hence deeper, there is a relatively abrupt transition at 1850 km (shaded black) to a mixture of perovskite (itself a mixture chemically of MgSiO 3 and FeSiO 3 ) and magnesiowustite (a mixture of FeO and MgO). The metallic core (shaded gray) begins at about 2000 km depth and continues to the center at a depth of 3390 km.

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Cartoon showing the crystallization sequences for a lunar magma ocean (after Gregory Snyder, then at the University of Tennessee) and three Martian magma oceans with different depths. Higher pressure in the deeper oceans results in different element distributions.

More Data Needed

It is astonishing that cosmochemists can determine so much about the formation of the Martian mantle from a few rocks that formed four billion years after the mantle did! Yet the unfolding story is reasonable. One test of the ideas will come from analysis of the composition of the Martian crust as determined by the Mars Odyssey Gamma Ray Spectrometer (papers are being written during June and July, 2004). The GRS data, particularly for the important trace elements K and Th, will allow us to compare to the compositions of shergottites and to assess how consistent the entire crustal composition is with the idea of enriched and depleted mantle reservoirs. To fully assess the reservoirs, their ages, and models for their formation we need more samples, particularly basalts from the ancient highlands. We can hope to find more meteorites (the search for Antarctic meteorites now fields two teams each Antarctic summer), but we might still be stuck to whatever was blasted off Mars

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during the past few million years. Ideally, we will obtain more samples of Mars by sending robotic (and eventually piloted) spacecraft to Mars to return samples for study in labs here on Earth. Testing our ideas of mantle reservoirs and their formation requires the highly precise analyses that can be done only on earth, at least for now.

NASA is tentatively planning to launch a sample-return mission to Mars in 2013. These paintings show two concepts for how the return vehicle will blast off. Note that both images have rovers. Recent experience with the Spirit and Opportunity rovers shows the need for mobility to collect the best possible samples.

Borg, L. E., Nyquist, L. E., Weissman, H., Shih, C.-Y., and Reese, Y. (2003) The age of Dar al Gani 476 and the differentiation history of the martian meteorites inferred from their radiogenic isotopic systematics. Geochimica Cosmochimca Acta, v. 67, p. 3519-3536. Borg, L. E. and Draper, D. S. (2003) A petrogenetic model for the origin and compositional variation of the martian basaltic meteorites. Meteoritics & Planetary Science, v. 38, p. 1713-1731. Goodrich, C. A., Herd, C. D. K., and Taylor, L. A. (2003) Spinels and oxygen fugacity in olivine-phyric and lherzolitic shergottites. Meteoritics & Planetary Science, v. 38, p.1773-1792. Herd, C. D. K. (2003) The oxygen fugacity of olivine-phyric martian basalts and the components within the mantle and crust of Mars. Meteoritics & Planetary Science, v. 38, p. 1793-1805. McLennan, S. M. (2003) Large-ion lithophile element fractionation during the early differentiation of Mars and the composition of the martian primitive mantle. Meteoritics and Planetary Science, v. 38, p. 895-904.

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PSRD: Ancient Floodwaters and Seas on Mars

http://www.psrd.hawaii.edu/July03/MartianSea.html

posted July 16, 2003

Ancient Floodwaters and Seas on Mars --- Surface deposits within the northern lowlands support the oceans hypothesis.

Written by Linda M. V. Martel Hawai'i Institute of Geophysics and Planetology

The role of water in the origin and evolution of landforms on Mars has been a main topic of planetary science research for at least the past 30 years, certainly since Mariner 9 images first showed large winding channels. The ancient immense floods that presumably formed the channels would have left behind large bodies of water at the ends of the channels. Where the bodies of water might have been, their size, or even evidence of their existence have been debated ever since. PSRD continues its coverage of water-related issues on Mars with a summary of an updated review of the evidence and possible fate of Martian oceans in the northern plains by Michael Carr (U. S. Geological Survey, Menlo Park) and James Head III (Brown University). They examined the features previously mapped as shorelines by Timothy Parker (Jet Propulsion Lab) and colleagues but found that more compelling evidence for the past presence of large bodies of water are deposits within the northern plains. They cite specifically the veneer of material of Upper Hesperian age called the Vastitas Borealis Formation (VBF). This knobby-textured surface is interpreted to be the sublimation residue from ponded flood runoff. There are multiple theories about what would have happened to the water in the northern oceans (which they predict had a volume of about 2.3x10 7 km3). Carr and Head suggest that some of it, about 30%, could have been lost to space by sublimation, almost 20% could be in the present polar caps, and the rest could be trapped in other volatile-rich surface deposits or redistributed in the groundwater system. Reference: Carr, M. H. and Head III, J. W. (2003) Oceans of Mars: An assessment of the observational evidence and possible fate. Journal of Geophysical Research, v. 108(E5), 5042, doi: 10.1029/2002JE001963, 2003.

Northern Lowlands

One of the most intriguing issues about Mars is the idea that oceans once filled the low, flat plains of the northern hemisphere. Today these gently sloping plains are marked by ridges, low hills, and sparsely scattered craters. They are considered to have a volcanic origin and to be Hesperian in age based on crater counts, but they are covered by layers of younger materials (described in a later section).

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Mars Orbiter Laser Altimeter ( MOLA ) maps clearly show a distinction between lowlands and highlands. The northern lowlands have overall elevations about five kilometers lower than the cratered uplands of the southern hemisphere. (Click image for higher resolution options from NASA Planetary Photojournal. Will open in a new window.)

Two distinct basins are recognized within the northern lowlands: North Polar basin and Utopia basin. Carr and Head describe how floodwaters that formed channels around Chryse Planitia would have flowed into the North Polar basin. [See PSRD article Outflow Channels May Make a Case for a Bygone Ocean on Mars.] Water that cut the valleys northeast of Elysium would have flowed into Utopia basin. Amazonis Planitia and the smaller Isidis basin are two other smooth, flat northern regions where floodwaters could have reached.

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Prominent basins and major landmarks in the northern hemisphere of Mars are outlined and labeled in this polar stereographic projection.

Among the early researchers to discuss possible former oceans in the northern lowlands was Baerbel Lucchitta (U. S. Geological Survey, Flagstaff) and her colleagues. Their 1986 work suggested that polygonally fractured ground downstream from outflow channels could have originated in sediments deposited in standing water. But it was the work of Timothy Parker (Jet Propulsion Lab) and colleagues beginning in 1989 that put a focus on finding evidence of a vanished ocean by looking for shorelines in the northern lowlands using Viking Orbiter images.

Proposed Shorelines

The originally proposed shorelines were two discontinuous boundary contacts between landforms thought to have formed by wave or other water-related processes. Stephen Clifford (Lunar and Planetary Institute, Houston) and Parker later refined the outlines and hypothesized that Noachian-aged bodies of water and ice covered up to one third of the surface of Mars.

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Shorelines proposed by Clifford and Parker (2001) in the Martian northern lowlands.

The two most continuous contacts, called the Arabia and Deuteronilus shorelines, generally parallel the southern boundary of the northern plains. The Arabia shoreline can be traced all around the planet except through the Tharsis region. The elevation of the Arabia contact varies by several kilometers, in some places by 11 kilometers. This large range in elevations does not support a shoreline interpretation. Features of the proposed shoreline that have been interpreted as formed by wave actions or other marine processes can be equally argued as being formed by mass wasting and volcanic processes. The Deuteronilus contact is more subtle than the Arabia contact but has a smaller range in elevations. For nearly half its length the Deuteronilus marks the southern extent of the geologic unit called the Vastitas Borealis Formation. For the rest of its length it is seen only intermittently around clusters of hills or across lava flows. There is sparse direct evidence that the Deuteronilus contact is a shoreline, such as inward-facing cliffs or channels that end abruptly at the contact. According to the report by Carr and Head clear evidence of post-Noachian shorelines around the northern plains is ambiguous. They argue that some of the previously mapped contacts are clearly of volcanic origin, that all have significant variations in elevation, and that there is no strong support at this time for most of the proposed shorelines. But his does not mean shorelines never existed. Shorelines or other marine depositional or erosional features could have been obscured or destroyed by later geologic processes such as cratering impacts, erosion, volcanism, and tectonism. The difficulties in proving the existence of shorelines would appear to weaken the oceans hypothesis, but Carr and Head show that it gains support from other geologic evidence.

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Evidence for Flooding from Within the Northern Plains

Unconvinced that the previously mapped contacts are shorelines, Carr and Head assessed the other surface features within the northern volcanic plains that could be used to support the ancient oceans hypothesis. A variety of surfaces, including polygonal ground, curvilinear ridges, highly eroded impact craters, and eroded wrinkle ridges, have been cited since the 1980s as areas formerly covered by water. What Carr and Head and other researchers have found most interesting is the question of why the northern lowlands are so much smoother than comparable plains in the south. The northern plains are buried by layers of material at least 100 meters thick. Carr and Head cite as evidence the area previously mapped as the Vastitas Borealis Formation (VBF) of Upper Hesperian age.

Mapped boundary of the Vastitas Borealis Formation. The bold line emphasizes where the VBF boundary coincides with the Deuteronilus contact.

The VBF appears to be layered deposits that have buried the older volcanic plains and craters, effectively smoothing the wrinkled surface and leaving behind shallow, almost rimless craters called stealth craters. The stippled, wrinkled, and cratered appearance shows clearly in MOLA images. Herbert Frey (Goddard Space Flight Center) and colleagues have shown that the northern lowlands were at one time as cratered as the southern highlands.

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PSRD: Ancient Floodwaters and Seas on Mars

http://www.psrd.hawaii.edu/July03/MartianSea.html

Typical Vastitas Borealis Formation (VBF) at 60oN, 140oE. Closely spaced hills create the stippled appearance. Linear patterns may be due to aligned hills or ridges. White arrows point to examples of stealth craters, interpreted to be older craters buried by the VBF. Fresher looking craters with ejecta blankets are interpreted by Carr and Head to be younger than the VBF, see examples shown by black arrows.

The Vastitas Borealis Formation is interpreted as a flood deposit left behind after a frozen ocean slowly sublimated; Carr and Head give several lines of supporting evidence: The VBF has a similar age to that of the outflow channels and it is seen in low areas at the ends of the outflow channels. There is also a similarity in the volume of the VBF (estimated at 3x106 km3)and the volume of materials eroded to form the outflow channels (estimated at 4x106 km3). Using high resolution MOLAS, MOC, and THEMIS images, researchers can see effects of the mantle deposits in the depth/diameter relationships of craters like those with black arrows in the image above. Looking closely at the shapes of craters within Utopia basin and other northern areas, Joe Boyce and colleagues (University of Hawaii) find that commonly the crater floors are at nearly the same elevation as the surrounding plains (see image below for example). They see this relationship exclusively in the northern lowlands and interpret it as the result of deflation of layered, ice-rich, geologically young material that was deposited by the last major flooding event.

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PSRD: Ancient Floodwaters and Seas on Mars

http://www.psrd.hawaii.edu/July03/MartianSea.html

THEMIS visible image centered at 44.1 o N, 101.7 o E of a filled crater in Adamas Labyrinthus. Click on the image for higher resolution options from the THEMIS website (will open in a new window).

There are other surfaces within the VBF that have been interpreted as meltwater features associated with a stationary or retreating ice sheet. Some of these features are thumbprint terrain and labyrinthine valleys.

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PSRD: Ancient Floodwaters and Seas on Mars

http://www.psrd.hawaii.edu/July03/MartianSea.html

Each image is about 300 kilometers across. Thumbprint terrain is common around the edge of the VBF. It has been interpreted as deposits of ice or rock indicating successive positions of retreating ice. Labyrinths of curved valleys, some with central ridges, have a similar appearance to terrestrial glacial landscapes where ridges of sediment mark where the sediment-laden water once flowed in tunnels under the ice.

Possible Fate of Northern Oceans

The researchers have made a case for the presence of a large body, or bodies, of standing water in the northern plains over an area roughly equal to that mapped as the Vastitas Borealis Formation. Carr and Head argue that successive floods could have flowed over ice from previous floods to progressively fill the basins over a long period of time. Once flooding ceased and the ice sublimated away, layers of sublimation residues were left covering the ridged plains. To cover all the area now mapped as the Upper Hesperian VBF would take about 2.3x107 km3 of water according to Carr and Head's assessment. Spread over the surface of Mars, this volume is equal to a global layer of water (Global Equivalent Layer or GEL) about 156 meters deep. What was the duration of the ocean and where did it go? Its fate would depend on climatic conditions. Under warm conditions, researchers such as Victor Baker (University of Arizona) argue the ocean water would have evaporated into a thick warm atmosphere, precipitated out, and returned to the ground. However, the accompanying CO2, carbonates, and evaporite deposits expected in this scenario have not been found. Under present climatic conditions on Mars, researchers say an ocean would freeze in about 10,000 years, then sublimate away at rates strongly dependent on whether or not the icy surface was covered by rocky debris. Under such cold conditions, the sublimated ice could have been redeposited at or surrounding the polar caps. Carr and Head estimate about 30 meters GEL could be in the present polar caps. Approximately 50 meters GEL could have been lost to space and the rest, approximately 80 meters GEL could be trapped in other volatile-rich surface deposits or be redistributed in a groundwater system by polar basal melting.

A Big Job Ahead

Finding geologic evidence that everyone can agree with is crucial to test any ocean hypothesis. Right now there is no consensus on the existence of shorelines in the northern lowlands. Mike Carr and Jim Head found that more compelling 8 of 9

PSRD: Ancient Floodwaters and Seas on Mars

http://www.psrd.hawaii.edu/July03/MartianSea.html

evidence lies in the deposits themselves and that the Upper Hesperian Vastitas Borealis Formation supports the ancient ocean hypothesis because it appears to be the solid remains of floodwaters from the outflow channels. Data from the gamma ray and neutron spectrometers on Mars Odyssey show that there is considerable ice in the upper few tens of centimeters of the surface in the northern plains. In fact, north of about 60 o latitude it appears that the surface is >70 vol% ice. The presence of this dirty ice would appear to be consistent with the past existence of an ocean on Mars. However, over billions of years the surface would be reworked by meteorite impacts and much of the ice would be lost by sublimation into the dry Martian atmosphere. The near-surface ice discovered by Mars Odyssey may have been deposited more recently. Much more research is needed to understand the origin of the near-surface ice on Mars and to determine if it gives us any insight into the nature of a past northern ocean. Besides new remote sensing data from Mars, geochemists are investigating how to test the existence of an ancient ocean. Water reacts with rock to form new minerals--a process called weathering. Depending on conditions in the ocean (temperature, acidity, and concentrations of dissolved compounds), specific sets of minerals may form. We can seek out these minerals by careful interpretation of chemical data returned by Mars Odyssey (especially the concentrations of potassium, thorium, uranium, and chlorine) and in the future, by using surface rovers carrying sophisticated analytical tools or by returning samples from the northern plains. Rovers may also do geophysical surveys to search for deeply buried ice that might have been deposited in the ground beneath a freezing ocean. There is still a lot to do to understand the ancient floodwaters and seas on Mars. These and other tantalizing issues will be discussed at the Sixth International Conference on Mars July 20-25, 2003.

Boyce, J. M., Mouginis-Mark, P. J., and Garbeil, H. (2003) Evidence for a thick, discontinuous mantle of volatile-rich materials in the northern high-latitudes of Mars based on crater depth/diameter measurements. Sixth International Conference on Mars, abstract 3193 pdf. Carr, M. H. (1996) Water on Mars. Oxford University Press, New York, 229 p. Carr, M. H. and Head III, J. W. (2003) Oceans of Mars: An assessment of the observational evidence and possible fate. Journal of Geophysical Research, v. 108(E5), 5042, doi: 10.1029/2002JE001963, 2003. Clifford, S. M., and Parker, T. J. (2001) The evolution of the Martian hydrosphere: Implications for the fate of a primordial ocean and the current state of the northern plains. Icarus, vol. 154: 40-79. Frey, H. V., Roark, J. H., Shockey, K. M., Frey, E. L., and Sakimoto, S. E. H. (2002) Ancient lowlands on Mars. Geophysical Research Letters, v. 29(10), 1029/2001GL013832,2002. Martel, L. M. V. (2001) Outflow channels may make a case for a bygone ocean on Mars. Planetary Science Research Discoveries. http://www.psrd.hawaii.edu/June01/MarsChryse.html Martel, L. M. V. (2001) If Lava Mingled with Ground Ice on Mars. Planetary Science Research Discoveries. http://www.psrd.hawaii.edu/June01/lavaIceMars.html Parker, T. J., R. S. Saunders, and D. M. Schneeberger (1989) Transitional Morphology in West Deuteronilus Mensae, Mars: Implications for Modification of the Lowland/Upland Boundary, Icarus, vol. 82, p. 111-145. Sixth International Conference on Mars July 20-25, 2003. 2001 Mars Odyssey mission homepage.

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PSRD: Gray hematite deposit in Terra Meridiani, Mars:

http://www.psrd.hawaii.edu/Mar03/Meridiani.html

posted March 13, 2003

Gray Iron Oxide in Meridiani, Mars --- A deposit of gray hematite in Terra Meridiani may suggest that water once circulated through the rock layers in this region of Mars.

Written by Linda M.V. Martel Hawai'i Institute of Geophysics and Planetology

In our continuing coverage of the exploration of Mars and NASA's strategy of "follow the water," PSRD highlights recent research of the hematite deposit in Terra Meridiani. The iron oxide mineral called hematite (Fe 2O3) forms on Earth in several ways, most involving water. For this reason, the announcement in 2000 of the discovery of crystalline gray hematite near the Martian equator was, in a word, a watershed for planetary geologists and astrobiologists interested in unraveling the history of water and climate on Mars. Detection of the hematite in Terra Meridiani is one of the key discoveries of the Thermal Emission Spectrometer (TES) instrument on the Mars Global Surveyor (MGS) mission. Using TES spectral data in combination with image and topographic data, Brian Hynek, Raymond Arvidson, and Roger Phillips (Earth and Planetary Sciences and McDonnell Center for the Space Sciences at Washington University in St. Louis) recently made detailed regional analyses of Terra Meridiani to better understand the origin and significance of the hematite in the Martian environment. Using Earth as a guide to hematite formation, researchers think the hematite could have formed on Mars by thermal oxidation of iron-rich volcanic eruptive products during eruption or it could have formed by chemical precipitation when iron-rich water circulated through the preexisting layers of volcanic material (ash). Hynek and his coauthors as well as other researchers studying data from Terra Meridiani prefer the chemical precipitation hypothesis because it is most consistent with their observations of the geology of the region. Reference: Hynek, B. M., Arvidson, R. E., and Phillips, R. J. (2002) Geologic setting and origin of Terra Meridiani hematite deposit on Mars. Journal of Geophysical Research, v. 107, no. E10, 5088, doi: 1029/2002001891.

How hematite was found on Mars

Crystalline gray hematite (the coarse-grained form of the iron oxide we call rust when it's powdery and red) was found by scientists analyzing remote sensing data gathered by the Thermal Emission Spectrometer (TES) instrument during the early (1997-1998 aerobraking) phase of the MGS mission. The TES instrument measures the infrared energy emitted by surface materials and by CO2, water ice, dust, and water vapor in the atmosphere. Phil Christensen (Arizona State University) and a team of scientists studied over four million TES spectra. After removing the effects of the atmosphere, they were left with the distinctive spectral curves of the surface materials. These were compared with spectra measured in the laboratory for a wide variety of minerals. Shown below, the average spectrum for Terra Meridiani matched the laboratory spectrum for hematite. The depth and shape of the hematite fundamental bands reveal that this hematite is the crystalline and coarse-grained (greater than about 10 micrometers in diameter) gray variety of 1 of 8

PSRD: Gray hematite deposit in Terra Meridiani, Mars

http://www.psrd.hawaii.edu/Mar03/Meridiani.html

the iron oxide.

Christensen and the TES science team identified hematite in the TES data by the presence and shape of dips in the curve (called oxide fundamental vibrational absorption features) centered near 300, 450, and >525 cm-1 and by the absence of dips (silicate absorptions) in the 1000 to 1400 cm-1 region. The gap in the TES spectrum is the location of an atmospheric absorption band, which gives no information about the surface. The two curves have been offset vertically for easier comparison.

Hematite in Terra Meridiani

The major hematite deposit was found in a relatively low, smooth area spanning latitudes approximately 1 oN to 3 oS and longitudes 8 oW to 1 oE in the region called Terra Meridiani (variously named Meridiani Planum or Sinus Meridiani.)

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PSRD: Gray hematite deposit in Terra Meridiani, Mars

http://www.psrd.hawaii.edu/Mar03/Meridiani.html

Image on the left is a topographic map (from the Mars Orbiter Laser Altimeter, MOLA) of the relatively smooth Terra Meridiani region and its neighboring cratered terrain to the south. Image on the right is a Viking photomosaic superimposed with TES-derived hematite abundance. Detectable hematite abundances range from approximately 5% to 15%. Corresponding latitude and longitude lines are shown in both images for easier comparison.

The Meridiani region has layered deposits covering ancient cratered terrain. The hematite occurs in one layer within a 600-meter-thick stack of layers composed of materials probably the size of coarse sand based on thermal conductivity studies and TES data. The layers are visible for hundreds of kilometers, which is remarkable on this planet where so much of the surface is obscured by dust. This layered terrain was probably even more extensive before it was eroded or buried. The thinness of the layers suggests they were deposited at regular intervals, but for how long nobody knows. What the layers are made of is an ongoing issue. Debates on the plausible origins of the layered materials generally center on whether they are sedimentary deposits or volcanic ash. Evidence is inconsistent for either origin because there is a lack of a topographic basin and large volcanoes in this region. Nevertheless, Hynek and co-authors favor a volcanic origin contending that if the ash came from volcanic air fall, then the source vents for the ash could simply be buried or distant, even several thousand kilometers away. A discussion of the hypotheses for the origin of the hematite deposit will follow the next section, which describes the geologic units mapped by Hynek and his colleagues.

Mapping the hematite

The key geologic units mapped by Hynek and colleagues in Terra Meridiani are plains, etched, and interior layered crater deposits (see map below). All these units are geologically distinct from and bury preexisting cratered terrain.

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PSRD: Gray hematite deposit in Terra Meridiani, Mars

http://www.psrd.hawaii.edu/Mar03/Meridiani.html

Typical surface features are shown in the MOC images below; their locations are indicated on the map of Terra Meridiani as black squares numbered I, II, III, and IV. Each image links to a web page at Malin Space Science Systems with options for downloading full-sized versions.

Plains units (P1, P2, P3) The units are all smooth, widespread, and somewhat eroded by wind. The upper most unit (P3) is a bright cliff-forming unit about 200 meters thick. The middle plains unit (P2) is dark basaltic plains, contains hematite, and is less than a few hundred meters thick. According to the TES data this unit has about 10 to 15% hematite. The lowermost unit is P1. It is about 200 meters thick. (See images I and III above.) Etched unit (E) is characterized by many different deposits that appear to lie between and below the plains units. Extensive erosion has made it impossible to connect individual layers between outcrops of the etched unit. The material 4 of 8

PSRD: Gray hematite deposit in Terra Meridiani, Mars

http://www.psrd.hawaii.edu/Mar03/Meridiani.html

forms ridges, mesas, pits, and troughs. (See image II above.) Interior layered crater deposits (I) are mounds of thinly layered material found inside craters. They look like they may be isolated outcrops of the plains and etched units. Cratered deposits are mapped as various units: dissected (Cd), subdued (Cs), undivided (Cu). All are rough but degraded meaning that crater rims are no longer sharp, but are rounded due to erosion. (See image IV above.) Based on superposition of units and crater counts (more craters means the surface has been exposed longer to impacts), Hynek and colleagues report that the layered plains and etched unit have a probable age of Late Noachian to Early Hesperian, that is about 3.7 to about 3.5 billion years old.

Was it chemical precipitation from iron-rich fluids?

What happened in Terra Meridiani to cause the formation of the coarse-grained gray hematite? What sequence of events led to stacks of thin, parallel-bedded deposits with only one hematite-rich layer? The events must not have been so common or else we'd expect to see more occurrences of gray hematite on Mars. As it stands now, at a detection limit of several percent, TES data show very limited exposures of gray hematite. Near-global mapping of TES data by Christensen and colleagues reveals deposits of gray hematite in only three places on Mars: Terra Meridiani, Aram Chaos (2oN, 21oW) and small scattered exposures in Valles Marineris. In Terra Meridiani some layers appear loosely packed and easily eroded while other layers are more resistant to erosion and form cliffs and flat-topped hills. People continue to look at the data for evidence to explain the layering as volcanic, wind-blown, or water-laid deposits. The hematite might have formed at the same time as the layers (primary formation) or it may have formed long after the deposition of the layers (secondary formation). Hynek and coauthors considered the plausible origins of hematite that have been proposed and compared them to their own observations of Terra Meridiani. The table below shows a summary of their findings.

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PSRD: Gray hematite deposit in Terra Meridiani, Mars

http://www.psrd.hawaii.edu/Mar03/Meridiani.html

Consistency of proposed hypotheses with observations and regional mapping of Terra Meridiani hematite by Hynek, Arvidson, and Phillips

Origin

Primary

Proposed hypothesis

Observations consistent with hypothesis

precipitation from iron-rich, smooth, layered, easily eroded low-temperature waters, as in deposits of constant thickness a lake or sea

different erosional patterns within units; possible cemented joints; association with outflow channels

Primary

precipitation from iron-rich, high-temperature waters, in hydrothermal systems

Primary

materials have volcanic compositions (basaltic, thermal oxidation of volcanic andesitic); layers are deposits: iron-rich lava flows, widespread; there are possible ignimbrites, air fall flow features in some of the Note: this hypothesis does layers; layers are thin, flat, not require liquid water to smooth, and drape the cause oxidation preexisting topography; some layers are susceptible to erosion

Observations inconsistent with hypothesis no obvious closed basins or obvious sources for lake deposits; the age of the deposits may be too old to correspond with proposed "warm and wet" conditions on Mars that would be necessary to deposit 600 meters of lakebeds; only one layer of the 600 meter-thick stack is hematite-rich; lack of other minerals in TES data that would be expected in lake deposits (evaporites) or in banded iron formations (quartz, chert, carbonates) large areal extent (>105 km2); lack of associated hydrothermal-alteration products in TES data; lack of evidence of tectonic processes or other obvious heat sources

lack of nearby volcanoes; Martian lavas are generally far less susceptible to erosion than these deposits; layers have nearly constant thickness and conform to preexisting topography

Secondary ground water - leaching

different erosional patterns red not gray hematite is more probable; within units; possible cemented sharp boundaries that correlate with local joints; layers are widespread topography

ground water - hydrothermal Secondary alteration along permeable layers

existence of one hematite layer in a 600 meter stack; different erosional patterns within units; possible joint systems; association with outflow channels

coatings of iron-rich rock by Secondary weathering from surface and/or atmospheric water

red not gray hematite is more probable; hematite coatings are common lack of substantial atmospheric water on on Earth Mars

lack of associated hydrothermal alteration products in TES data; lack of evidence of tectonic processes or other obvious heat sources

Of the primary formation options, Hynek and coauthors think it's plausible that the hematite could have formed as iron-rich deposits that were oxidized during eruptions from distant volcanoes. This kind of thermal oxidation of volcanic ash during eruption does not require water. In this scenario, however, a wider distribution of hematite-rich ash would be expected but is not seen in the TES data. Perhaps some hematite-rich ash layers are still buried or perhaps they have been eroded away. Alternatively, the hematite may have formed by a later secondary mechanism in preexisting ash beds. In this case, Hynek and his colleagues favor precipitation of the hematite when iron-rich fluids circulated within the layered volcanic ash. This kind of secondary formation of the gray hematite, they say, is most consistent with their regional 6 of 8

PSRD: Gray hematite deposit in Terra Meridiani, Mars

http://www.psrd.hawaii.edu/Mar03/Meridiani.html

geologic, topographic, and spectral observations. Researchers are continuing to study a variety of remote sensing data of Terra Meridiani. Daytime infrared images from the Thermal Emission Imaging System (THEMIS) on the Mars Odyssey spacecraft allow new views of the layered terrain in better detail. Christensen and the THEMIS team see different temperatures for the different layers. They think this could indicate the layers have different physical properties (such as particle size, mineral composition, or density) perhaps due to changes in when or how fluids circulated through. Other researchers are still looking for evidence of surface water in the region (paleolake basins, for example) or trying to evaluate the potential of the hematite deposits to preserve microfossils. New studies and results will be reported at the annual Lunar and Planetary Science Conference in Houston, March 17-21, 2003.

Likely landing location

Orbital MOLA topographic data and TES compositional data have given us testable hypotheses of how the hematite formed. We may soon be able to test these ideas right on the spot. Terra Meridiani is a leading candidate for further exploration by one of the upcoming Mars Exploration Rovers (MER) because of its distinctive mineralogy and its relative safety as a landing site (in terms of low wind shear, low abundance of boulders, and low slope angles). After years of detailed evaluations of potential landing sites, debates and recommendations on scientific merit and safety issues, and a final peer review taking place this month we will finally hear the announcement of the two site selections in early April, 2003. Launch window for the first MER spacecraft opens May 30, 2003 for an early January 2004 landing. The second MER craft is scheduled for launch on June 24, 2003 followed by a late January landing. The data gathered by these robotic field geologists will help scientists here on Earth read the stories of the rocks, stories told in the language of texture, chemistry, and mineralogy about the role of water and whether or not the environment may have ever been suitable for life.

ASU Spectral Library from Arizona State University Thermal Emission Spectroscopy Laboratory. Lists thermal infrared (2000 - 380 cm-1) emission spectra of over 150 pure minerals, with an emphasis on common rock-forming minerals. Christensen, P. R., Bandfield, J. L., Clark, R. N., Edgett, K. S., Hamilton, V. E., Hoefen, T., Kieffer, H. H., Kuzmin, R. O., Lane, M. D., Malin, M. C., Morris, R. V., Pearl, J. C., Pearson, R., Roush, T. L., Ruff, S. W., and Smith, M. D. (2000) Detection of crystalline hematite mineralization on Mars by the Thermal Emission Spectrometer: Evidence for near-surface water. Journal of Geophysical Research, v. 105 (E4), p. 9623-9642. Christensen, P. R., Morris, R. V., Lane, M. D., Bandfield, J. L., and Malin, M. C. (2001) Global mapping of Martian hematite mineral deposits: Remnants of water-driven processes on early Mars. Journal of Geophysical Research, v. 106 (E10), p. 23,873-23,885. Hynek, B. M., Arvidson, R. E., and Phillips, R. J. (2002) Geologic setting and origin of Terra Meridiani hematite deposit on Mars. Journal of Geophysical Research, v. 107, no. E10, 5088, doi: 1029/2002001891. (pdf) Lunar and Planetary Science Conference March, 2003 program with abstracts, including sessions on Mars (available as pdf files.) 7 of 8

PSRD: Gray hematite deposit in Terra Meridiani, Mars

http://www.psrd.hawaii.edu/Mar03/Meridiani.html

Mars Exploration Rovers (MER) - Athena instrument payload from Cornell University. THEMIS image of Terra Meridiani from the 2001 Mars Odyssey - Thermal Emission Imaging System from NASA/JPL/Arizona State University. Thermal Emission Spectrometer (TES) on Mars Global Surveyor (MGS).

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PSRD: Meteorites Found on Mars

Citation: Martel, L. M. V. (May, 2008) Meteorites Found on Mars. Planetary Science Research Discoveries. http://www.psrd.hawaii.edu/May08/MetsOnMars.html (date accessed).

PSRD-MetsOnMars.pdf

May 30, 2008

Meteorites Found on Mars --- No surprise that there are meteorites on other planets. Now that we've seen them on Mars, what do we know about them and what does their geochemistry tell us about the environment where they landed?

Written by Linda M. V. Martel Hawai'i Institute of Geophysics and Planetology

One meteorite and four possible others, all pieces of asteroids,

Sun glint off the jettisoned aluminum heat shield from MER Opportunity couldn't hide the interesting rock behind it (arrow). Click image for more information.

have been identified since 2005 on the plains and hills of Mars by the Mars Exploration Rover (MER) science team. Christian Schröder (NASA Johnson Space Center) and an international team of cosmochemists and planetary scientists have summarized the investigations of these five rocks. The team reports on the chemistry and mineralogy of the rocks based on data obtained from the suite of instruments onboard the rovers and discusses what these chance discoveries tell us about the Martian environment. Reference: z

Schröder, C., Rodionov, D. S., McCoy, T. J., Jolliff, B. L., Gellert, R., Nittler, L. R., Farrand, W. H., Johnson, J. R., Ruff, S. W., Ashley, J. W., Mittlefehldt, D. W., Herkenhoff, K. E., Fleischer, I., Haldemann, A. F. C., Klingelhöfer, G., Ming, D. W., Morris, R. V., de Souza, P. A., Squyres, S. W., Weitz, C., Yen, A. S., Zipfel, J., and Economou, T. (2008) Meteorites on Mars observed with the Mars Exploration Rovers, Journal of Geophysical Research, v. 113(E6), E06S22, doi: 10.1029/2007JE002990.

PSRDpresents: Meteorites Found on Mars --Short Slide Summary (with accompanying notes).

Robots Exploring Mars

Cosmochemists study meteorites from Mars and now, thanks to a couple of exploring rovers, meteorites on the surface of Mars. The two Mars Exploration Rovers (MER), Opportunity and Spirit, are equipped with a payload of cameras and instruments that allow the observation and identification of rocks and soil (no organics implied in the use of this term on Mars). The landing site for Opportunity is in Meridiani Planum, where mineral deposits (hematite) suggest Mars had a wet past and for Spirit it is Gusev Crater, which may have once held a shallow lake (see map below). As scientists navigated the rovers away from their landing sites, analyzing the geology and studying environmental conditions of the regions, the panoramic camera and instruments detected, by lucky chance, a few rocks from space that add something extra to the data bank.

http://www.psrd.hawaii.edu/May08/MetsOnMars.html

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PSRD: Meteorites Found on Mars

NASA mission landing sites are shown on this base map of Mars topography created by the Mars Orbiter Laser Altimeter (MOLA). Lowlands have colors of blue and green, and highlands are in yellow, orange, red, and white. Viking 1 and Viking 2 landed in 1976. Mars Pathfinder landed in 1997. MERs Opportunity and Spirit landed in 2004 and are still active today. Phoenix landed on May 25, 2008. Instruments on both rovers Opportunity, in Meridiani Planum region, and Spirit, in Gusev Crater, have been used to identify potential meteorites.

The MERs carry sophisticated sets of instruments. Two are used to survey the scene around the rover: The panoramic camera (Pancam) has 13 filters in the visible to near-infrared region. The miniature thermal emission spectrometer (Mini-TES) covers the 5 to 29 um wavelength region. Three additional instruments are mounted on a mechanical arm and can be placed on rock or soil targets for more detailed analyses: The Microscopic Imager (MI) for beautifully detailed images and the Alpha Particle X-ray spectrometer (APXS) for elemental compositions. A Mössbauer spectrometer is used to determine mineralogy of iron-bearing phases. In addition, a set of magnets can be used to attract dust particles and the Rock Abrasion Tool (RAT) removes surface contamination and weathering rinds off the outer layers of rock surfaces.

Artist's rendition showing instruments onboard the MERs. The PanCam and Mini-TES are not shown in this view.

http://www.psrd.hawaii.edu/May08/MetsOnMars.html

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PSRD: Meteorites Found on Mars

Unexpected Discoveries

In January 2005, the Opportunity rover used its panoramic camera to image its surroundings on Meridiani Planum near the remnants of the lander's heat shield, which had been cast off after serving to protect the rover from temperatures of up to 2,000 oF during its plung through the Martian atmosphere. A close-by rock seen in the images showed a smooth surface covered by depressions somewhat like regmaglypts (small depressions on meteorite surfaces caused by ablation during descent through the atmosphere.) As summarized by Schröder and colleagues, spectra obtained by the Mini-TES of the rock showed features akin to the Martian atmosphere, which meant it was This is the first meteorite found on another planet. Its maximum highly reflective at mid-infrared wavelengths, a dimension is 31 centimeters. Click image for more information. There characteristic of metals. This led to the logical thought were two meteorites found on the Moon. that the rock was an iron meteorite. Further classification of the meteorite was allowed by the onboard instruments: APXS, Mössbauer, MI, and the RAT. z

Alpha Particle X-ray Spectrometer - The APXS-derived bulk elemental composition of Meridiani Planum meteorite is 93% iron, 7% nickel, ~300 ppm germanium, and <100 ppm gallium.

z

Mössbauer Spectrometer - Spectra from the RAT-brushed surface show 94% of the iron is in a metal phase. On the basis of the iron/nickel ratio, this phase was assigned to kamacite (an iron-nickel mineral with low nickel content). Mössbauer spectra of both the "as is" and the RAT-brushed surface show ~5% of the iron is in the ferric state (Fe3+). Schröder and colleagues suggest some of the iron may have been oxidized during the meteorite's fall through the Martian atmosphere.

This meteorite was officially approved on October 10, 2005 with the name "Meridiani Planum" and remains the only approved meteorite on Mars. It is classified as a IAB complex iron meteorite. Iron meteorites are made, almost completely, of iron-nickel metal. Cosmochemists group them according to the abundances of trace elements such as germanium and gallium, as well as nickel. Initially, irons were classified into four groups and were identified by Roman numerals I, II, III, and IV. Today twelve groups are recognized and designated further by letters A through F according to concentrations of siderophile ("iron-loving") trace elements. When the concentration of a trace element is plotted against overall nickel content on a logarithmic plot, the iron meteorites cluster into groups. Iron meteorites that do not fit into the groups are called ungrouped. For example, the figure below shows where the meteorite Meridiani Planum plots in relation to the IAB and IIICD groups on the logarithmic plot of germanium versus nickel.

http://www.psrd.hawaii.edu/May08/MetsOnMars.html

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PSRD: Meteorites Found on Mars

This is a logarithmic plot of the concentrations of germanium versus nickel in iron meteorites based on many years of analytical work by cosmochemists. Fields for the 12 groups are shown. Meridiani Planum has Ge-Ni characteristics within the ranges of Ni-poor IAB and IIICD irons.

The second rock proposed by the MER Opportunity team to be a meteorite is a 3 centimeter-sized pebble at the rim of Endurance crater and unofficially named Barberton. It was found on sulfate-rich bedrock in the midst of basaltic soil and a hematite spherule lag deposit (see image below, left).

[LEFT] This is an approximate true color Pancam image of the reddish dust and ~3 centimeter pebble, Barberton (center), found by the Opportunity rover at the rim of Endurance crater in Meridiani Planum. The smaller beads in the scene form the hematite spherule lag deposit. [RIGHT] For comparison, this is a cut face of a mesosiderite, Barea, an observed fall in Spain in 1842. Barea is a brecciated stony-iron meteorite containing nearly equal shares of silicate rock fragments in various sizes (dark areas) and metal (white areas). The silicates are dominantly igneous rock fragments.

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PSRD: Meteorites Found on Mars Schröder and colleagues report Barberton was analyzed with the Microscopic Imager, the APXS, and the Mössbauer Spectrometer, but it was too small to be brushed or abraded with the RAT. Some of the surrounding soil was also analyzed for comparison. Barberton is olivine-rich and contains metallic iron in the form of kamacite, suggesting a meteoritic origin. However, Schröder and coauthors also report that although it is unique among samples investigated at Meridiani Planum, Barbarton's high magnesium and nickel contents and low aluminum and calcium contents would also be consistent with an ultramafic rock of Martian origin. Though it cannot yet be proven that Barberton is a meteorite, if true, then cosmochemists say it is similar in Mg/Si, Ca/Si, and Al/Si ratios to howardites and diogenites (rocks formed from basaltic magmas), but enriched in S/Si, Fe/Si, and Ni. The authors suggest Barberton, then, is chemically most consistent with a mesosiderite silicate clast with some additional metal and sulfide. Mesosiderites (see example in photo, above right) are one of two main types of stony-iron meteorite (the other type is called pallasite). Mesosiderites are complex mixtures (roughly 50:50) of smashed up volcanic rock (silicates) and iron-nickel metal. These meteorites have been brecciated by impacts and metamorphosed by burial. A 14-centimeter long cobble, dubbed Santa Catarina, is the third possible meteorite found by the Opportunity rover team. Schröder and colleagues describe Santa Catarina as a fractured, brecciated rock containing some clasts with possible igneous quench textures in olivine minerals (see images below). The cobble could not be abraded or brushed because of its geometry, but it was analyzed with the instrument suite of Microscopic Imager, APXS, and Mössbauer Spectrometer. Santa Catarina has an ultramafic composition with unusually high nickel. Schröder and team say that compared to other materials analyzed in Meridiani Planum, Santa Catarina is most similar to Barberton. Element ratios of Mg/Si, Ca/Si, Al/Si, S/Si, and Fe/Si are all very close to soilcorrected values obtained for Barberton. According to the authors, the iron-bearing mineralogy is, as in Barberton, dominated by Fe2+ in the minerals olivine (52%) and pyroxene (26%). Santa Catarina is more oxidized than Barberton with 14% of the iron as nanophase ferric oxide--a weathering product. Schröder and colleagues identified 7% troilite (iron sulfide) in the Mössbauer spectrum, but no kamacite (as had been found in Barberton or Meridiani Planum meteorite).

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PSRD: Meteorites Found on Mars

The Microscopic Imager onboard MER Opportunity acquired these detailed images of possible meteorite Santa Catarina sitting atop smaller beads of the hematite spherule lag deposit. The top image (about 5 centimeters high) shows the fractured surface and several individual clasts (outlined and shown in greater detail). Box (i) shows details of a clast consisting of light-toned crystals in a darker matrix. Box (ii) reveals what Schröder and team say might be an igneous quench texture in olivine.

The fourth and fifth possible iron meteorites were identified based on MER Spirit's remote sensing instruments in the Columbia Hills inside Gusev Crater. They are 25- to 30-centimeter boulders, named Zhong Shan and Allan Hills (see image below). Schröder and colleagues show the Mini-TES thermal infrared characteristics of these possible meteorites are similar to the Meridiani Planum meteorite (see diagram below). All three rocks display spectral characteristics similar to the Martian atmosphere because metallic iron is highly reflective in thermal infrared (as well as visible) wavelengths. But because these rocks lie on steep terrain and were discovered after the failure of Spirit's right front wheel, detailed investigations with the rover's Microscopic Imager, APXS, and Mössbauer Spectrometer were not possible.

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PSRD: Meteorites Found on Mars

Spectra of rocks on Mars obtained by mini-TES on both MER rovers. The spectrum of the known iron meteorite, Meridiani Planum (black) displays the spectral characteristics of the Martian atmosphere (pink) because of the highly reflective nature of metallic iron in thermal infrared wavelengths. The rocks called Zhong Shan (purple) and Allan Hills (blue) found in Gusev Crater by the Spirit rover have similar textural and spectral characteristics to meteorite Meridiani Planum. A nearby rock called Dome Fuji has a completely different spectrum from the others (green). It is a Martian basaltic rock.

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PSRD: Meteorites Found on Mars

Five Samples - Many Questions

In the true sense of what scientific discovery is all about, answering one question usually stimulates more questions. So is finding an iron meteorite and a handful of other rocks that could be meteorites on Mars. Why did we find iron and possibly stony iron meteorites and not other types? Why isn't the iron meteorite rusted? When did it fall? Did it make a crater when it hit the surface? Based on the populations of meteorites observed to fall on Earth, stony meteorites outnumber irons. The statistics are 94% stony meteorites (mainly chondrites), 5% irons, and 1% stony irons. The same could be expected on next-door Mars. Previous work by Albert Yen (Jet Propulsion Lab) and colleagues using nickel abundances measured by the APXS indicates that the Martian soil and certain sedimentary rocks contain 1% to 3% contamination from meteorite debris. So why haven't stony (chondritic) meteorites been identified in the MER data? Schröder and coauthors suggest chondrites may be too weak to survive impact at current atmospheric densities. Or maybe MERs Opportunity and Spirit just happened to move through strewn fields of irons. Maybe more of the cobbles on the Martian plains and hills are stony meteorites that just haven't been recognized. Rocks in Meridiani Planum indicate there was water present in the past. And we know iron will rust when in contact with water and oxygen. So the apparently almost-rust-free metallic surface of iron meteorite Meridiani Planum is consistent with the current dry, cold environment of Mars where alteration rates are extremely slow. The presence of olivine (a mineral easily altered by water) in possible meteorites Barberton and Santa Catarina also points to dry conditions since these rocks landed. But maybe the rust was sandblasted away. Or maybe rust is present on the meteorite but simply obscured by dust. Of course this brings the next question, when did the meteorites fall? If iron meteorite Meridiani Planum fell long ago, perhaps millions of years ago, it was probably buried. But we don't know if it was buried by sand moved by water or wind or for how long. Perhaps the meteorite was only recently exposed on the surface by the winds that we know have also exposed bedrock and the hematite spherules that make up the lag deposits. We don't know when the meteorites fell. If Meridiani Planum meteorite, Barberton or Santa Catarina created impact craters, we don't see them. If they fell in the past, a denser atmosphere might have decelerated the pieces enough to prevent hypervelocity impacts. If they fell recently through the existing, thin Martian atmosphere, they could be fragments of larger meteoroids that never made craters. Or if craters were made, they could have been erased by wind erosion. Schröder and coauthors also suggest the possibility that Barberton and Santa Catarina, because they were found on the rims, could be pieces of the impactors that formed Endurance crater and Victoria crater, respectively. Searching for meteorites was not, and is not, a primary objective of the MER missions, though their serendipitous discoveries show the flexibility and achievement of the team to investigate unusual rocks and identify them. If more meteorites are identified and classified on Mars, then we just might find types not yet seen in our meteorite collections on Earth. And fundamentally, finding meteorites on other planetary surfaces stimulates new ideas in cosmochemistry and planetary science.

LINKS OPEN IN A NEW WINDOW.

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PSRDpresents: Meteorites Found on Mars --Short Slide Summary (with accompanying notes).

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Athena Science Payload instruments onboard the two MERs. Malin, M. C., Edgett, K. S., Posiolova, L. V., McColley, S. M., and Noe Dobrea, E. Z. (2006) PresentDay Impact Cratering Rate and Contemporary Gully Activity on Mars, Science, v. 314(5805), p. 15731577, doi: 10.1126/science.1135156.

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PSRD: Meteorites Found on Mars z z z z

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Mars Exploration Rover Mission. Meteorite found on Mars: Meridiani Planum, data from the Meteoritical Bulletin Database. Meteorites found on the Moon, data from the Meteoritical Bulletin Database. Schröder, C., Rodionov, D. S., McCoy, T. J., Jolliff, B. L., Gellert, R., Nittler, L. R., Farrand, W. H., Johnson, J. R., Ruff, S. W., Ashley, J. W., Mittlefehldt, D. W., Herkenhoff, K. E., Fleischer, I., Haldemann, A. F. C., Klingelhöfer, G., Ming, D. W., Morris, R. V., de Souza, P. A., Squyres, S. W., Weitz, C., Yen, A. S., Zipfel, J., and Economou, T. (2008) Meteorites on Mars observed with the Mars Exploration Rovers, Journal of Geophysical Research, v. 113(E6), E06S22, doi: 10.1029/2007JE002990. Scott, E., Yang, J., and Goldstein, J. (2007) When Worlds Really did Collide. Planetary Science Research Discoveries, http://www.psrd.hawaii.edu/April07/irons.html. [Cosmochemical studies and dynamical models of protoplanetary collisions suggest a new origin for iron meteorites.] Yen, A. S., et al. (2006) Nickel on Mars: Constraints on Meteoritic Material at the Surface, Journal of Geophysical Research, v. 111(E12S11, doi: 10.1029/2006JE002797.

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PSRD Hot Idea: Mining the Moon, Mars, Asteriods

http://www.psrd.hawaii.edu/Nov00/mining.html

posted November 21, 2000

Mining the Moon, Mars, and Asteroids Written by G. Jeffrey Taylor Hawai'i Institute of Geophysics and Planetology

An international group of scientists, mining and aerospace engineers, policy makers, and other specialists met in Golden, Colorado to discuss the use of space resources. Space Resources Roundtable II was held at the Colorado School of Mines, and was sponsored by the School of Mines, NASA, and the Lunar and Planetary Institute. Participants discussed lunar, martian, and asteroidal resources, along with economic and legal aspects of using extraterrestrial resources. This report focuses on lunar resources. Manufacture of useful materials on the Moon, Mars, or asteroids requires extensive use of what we know about those places through studies of lunar samples and meteorites from asteroids and Mars. It is applied cosmochemistry. References: Space Resources Roundtable II, 2000, LPI Contribution 1070. Lunar and Planetary Institute, Houston, 75 pp. Abstracts also available at Space Resources Roundtable.

Lunar Solar Power

Energy specialists point out that we need alternatives to fossil fuels. They give several reasons. There are environmental problems with burning carbon. The traditional fuels will eventually run out. Perhaps most important, increasing the standard of living in developing nations requires a huge increase in the supply of energy. Solar power has often been touted as an answer to the world's energy problems. However, it is not very efficient. A given place on Earth is dark half the time. Clouds and dust reduces the amount of solar energy by another 50%. And except near the equator, the low angle of sunlight causes loss to the air, cutting the amount of energy by yet another 50%. All those reductions amount to eight times less solar energy reaching Earth's surface than arrives from the Sun. The obvious thing to do is to tap the Sun's energy in space. The idea of space power systems has been around since the late 1960s. New technology makes it more attractive than it was at first. Its biggest problem is the cost of launching lots of stuff from the ground to orbit. But suppose almost all the needed materials were already there? David Criswell (University of Houston) has been arguing for years that the materials are already there--on the Moon. It just takes some manufacturing facilities to produce the needed parts and pieces.

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Solar power could be generated in space and beamed to the ground. The amount of material needed to construct a power satellite is large, hence expensive to transport from the surface of the Earth. It may be more cost-effective to bring the ingredients from the lunar surface--or even use the Moon to collect the solar power.

Criswell believes that solar power stations should be located on the Moon. He proposes building them on the right and left sides of the Earth-facing side of the Moon. This ensures a continuous supply of power to the earth. Solar cells would collect sunlight and transmit the energy to microwave transmitters. The microwave antennas would beam the energy to Earth, where it would be received by other antennas on the ground. Criswell says that the solar cells on the Moon would not need to be highly efficient. Instead, they could cover a lot of real estate. The trick is to make the solar cells and antennas on the Moon. Alex Ignatiev, Criswell's colleague at the University of Houston, proposed a solution. An expert in materials science, Ignatiev presented the basic design for a robotic solar-cell maker. It would roll over the lunar surface, leaving a trail of solar cells behind. As the surface passed beneath the rover, concentrated sunlight would melt the surface. This would cool quickly to make a smooth, glassy surface. Another system would extract silicon from the lunar soil by a vaporization process and deposit it in thin films on the glass surface. Depositing thin films requires a strong vacuum. The Moon provides such a vacuum. The flimsy lunar atmosphere has a pressure about a trillionth that of the Earth. The result would be an extensive network of solar cells. They would probably not be very efficient, but Ignatiev suggested that covering a large area with solar cells would overcome that problem. Although Ignatiev has lots of work to do to prove that the concept will work, most participants thought it was a promising way to produce power on the Moon. Perhaps Earthlings will prosper during the coming decades from an inexhaustible supply of solar power from the Moon.

This drawing by space artist Pat Rawlings shows Alex Ignatiev's idea of a rover making solar cells directly on the surface of the Moon.

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http://www.psrd.hawaii.edu/Nov00/mining.html

Lunar Alchemy: Dirt into Products

Schemes to extract oxygen from the lunar soil have been around for a long time. Almost all of them also produce other products as well, such as iron and titanium. Many require fairly high temperatures, hence a lot of energy. A few processes use hydrofluoric acid. These do not need a high temperature, but hydrofluoric acid is extremely toxic and corrosive. Steve Gillette (University of Nevada, Reno) studies ways to separate elements on Earth. He suggested using organic chemicals to extract useful elements at a low temperature. Once lunar soil is dissolved into a mixture of organic liquids, useful materials could be separated. For example, silicon-based ceramics could be made at low temperature. These could be useful for many purposes at a lunar base, including making molecule-sized machines (so-called molecular nanotechnology). If the Moon becomes an important part of Earth's commerce, cutting-edge technologies will be essential. Several scientists talked about their experiments on extracting oxygen, using the more traditional high-temperature techniques. These included James Blacic (Los Alamos National Laboratory), Giovanni De Maria (University of Rome), and H. Yoshida and his colleagues (Tokyo Institute of Technology). All use some kind of mechanism to fluff up moon dirt to make it easier to react with hydrogen gas. (The experiments actually use simulated moon dirt. Real lunar samples are too precious to use until a technology has been tested thoroughly with fake moon dirt.) De Maria uses ultrasound to shake a column of dirt. The others use the force of flowing hydrogen gas to make the pile of dirt behave like a fluid. Blacic's apparatus ionizes the hydrogen, making it reactive. The others heat the gas and dirt. All the approaches produced water by reaction of the hydrogen with the soil. On the Moon, this water could be used for life support. Most important, it could be split into hydrogen and oxygen to use as rocket fuel. The experiments also produced metallic iron. That could be used as a building material or for electrical cables, if we could figure out an efficient way to separate it from the rest of the dirt. Larry Taylor (University of Tennessee) has been working on lunar samples since the first batch was brought back from the Moon. He has also worked on ways to remove oxygen from lunar soil. Recently, he has been working with a team on understanding the optical properties of the lunar surface. This is important to understanding many remote-sensing observations of the Moon. While doing that work, he learned that the smallest soil grains, those smaller than 20 micrometers, are coated with tiny particles of metallic iron. The particles are only 10 to 100 nanometers across.

These two images show the distribution of iron in a collection of tiny lunar soil grains. Those labeled 'plag' and marked with arrows (for plagioclase) do not contain iron in their interiors. This is expected because lunar plagioclase contains very little iron. However, the edges of the plagioclase grains are decorated with blebs of metallic iron. This makes the plagioclase magnetic. Arrows in the righthand image point to two plagioclase grains coated with metallic iron which show up as bright rings. The other mineral grains also contain iron blebs, but the coating is indistinguishable from the iron oxide in the interiors.

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What's that have to do with lunar resources? It is important for two reasons. First, it may make it possible to filter out the finest lunar dust. Rocky dust can be a health hazard to future workers on the Moon. It also can collect on door seals, allowing air to escape from pressurized houses. The minute iron particles make all tiny lunar grains magnetic. So, magnets will be able to remove the dust from the air and could be used to clean surfaces. Second, the magnetic properties of the tiny grains give us a way to concentrate the finest dirt. By heating, the tiny iron grains will combine into larger grains that can be separated. Also, hydrogen is a useful element in lunar industry. Because it is delivered to the Moon by the solar wind, it occurs in the surfaces of soil grains. A pile of small particles has a greater surface area than a pile of large ones, so hydrogen is more abundant in small grains than large ones. Thus, separating small grains also concentrates hydrogen.

Drilling Holes in Planets

Some mining engineers are making important contributions to understanding how to explore the subsurface and how to mine asteroids. Others are trying to determine how to drill for water on Mars. Those places are very different from Earth, so the engineers must modify their equipment and techniques. Dale Boucher (Northern Centre for Advanced Technology, Sudbury, Ontario) has used his vast experience in mine construction to devise a lightweight, power-stingy drill to use on Mars. Jim Blacic has also been working on how to drill on Mars. He and his colleagues have identified many components of terrestrial drilling rigs that could be easily adapted for use on Mars. He pointed out, however, that no existing drill could be used as is. Leslie Gertsch (Michigan Technological University) described how an asteroid could be mined, once a resource was identified on it. She brought up the important point that the approach depends on the make-up of the asteroid. For example, it might be composed of a mixture of ice and rock. The asteroid might be weak, easily broken rock, or very strong rock. It might even be made of metallic iron. This shows how important it will be to thoroughly characterize an asteroid before deciding how to mine it.

The Future

People are eventually going to be working and living in space. Construction and operation of lunar solar power stations may make that happen. Or perhaps it will happen to support a thriving space tourism business. Whatever drives it, there will be a need to use the resources available in space. It is too expensive to drag all the needed ingredients up from the Earth. The resources are available on the Moon, Mars, and asteroids. Participants in the Space Resources Roundtable agree that we need to explore extraterrestrial bodies for resources and to learn how to extract those resources from them. Experts in the mineralogy and chemical composition of extraterrestrial materials will play important roles in the search and mining of space resources. Like Earth explorers through the ages, we must live off the land and a new breed of scientist, the applied cosmochemist, will be there to see it happen.

Space Resources Roundtable II, 2000, LPI Contribution 1070. Lunar and Planetary Institute, Houston, 75 pp. Abstracts also available at Space Resources Roundtable.

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PSR Discoveries: Meteorites from Mars/Rocks from Canada

http://www.psrd.hawaii.edu/Dec96/Nakhlites.html

posted December 21, 1996

Meteorites from Mars, Rocks from Canada Written by Rachel C. Friedman Department of Geology and Geophysics, University of Hawai'i

Meteorites from Mars have been in the news since last summer's announcement about possible fossils in a martian meteorite; see the October, 1996 Hot Idea of PSR Discoveries. But there is more to the study of martian meteorites than the search for life. These igneous rocks contain detailed information about volcanism and other magmatic processes on the Red Planet. Mars had a very active igneous history, dotting the landscape with huge volcanoes and covering extensive areas with sheets of lava. Images of the surface of Mars taken from orbit showed us these volcanic wonders from afar. The meteorites we have in hand provide us with more detailed information of what we glimpsed in the images.

One of the groups of meteorites thought to have come from Mars, the nakhlites [nock-lites], has an unusual composition characterized by a high abundance of the mineral pyroxene. Many experts have attributed the composition to accumulation of pyroxene in a thick magma body beneath the surface. However, some features of the rock indicate more rapid crystallization, suggesting formation in a lava flow. This has led to some confusing interpretations about the origin of these rocks. Were nakhlites formed underground or on the surface? Geologists need to answer this question so we can know if we are studying the physics of martian lava flows on the surface or the physics of how magma moves inside a volcano. What we need is information about the occurrence of the nakhlites on Mars. Collecting them directly from a martian lava flow would be ideal. But, until numerous robotic rovers and humans roam the surface of Mars, we will not have such important field information. Fortunately, a very similar rock type occurs in a few places on Earth. You are invited to come along on a field trip to eastern Ontario, Canada for a look at the rocks that we think are a match for the martian meteorites called nakhlites. For my doctoral dissertation, I am working with G. Jeffrey Taylor at the University of Hawai'i to substantiate this Earth-Mars rock connection. Allan Treiman of the Lunar and Planetary Institute in Houston first alerted us to the similarity between an unusual lava flow in Canada and the nakhlites, and is working with us on the project. Ralph Harvey of Case Western Reserve University in Cleveland also helped us in our field investigations.

What Are Nakhlites?

Nakhlite meteorites are the "N" of the SNC martian meteorites, and are named for Nakhla [nock-la], a 1 of 7

PSR Discoveries: Meteorites from Mars/Rocks from Canada

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meteorite which fell in Egypt in 1911. Reports say that the original meteorite broke into 40 pieces, one of which supposedly hit and killed a dog. The photograph below shows a slice of a nakhla stone 3 cm across.

See larger version. (170 K image)

Decades later, the other two nakhlite meteorites, Lafayette and Governador Valadares, were linked to Nakhla by their similar ages (1.3 billion years old) and by their unusual composition. All three meteorites are made almost entirely of one mineral, pyroxene, a silicate containing calcium, iron, and magnesium. Despite their history of being blasted off Mars by an impact, the three nakhlites show no mineralogic effects from the cratering event that launched them.

Did Nakhlites Form in Sills or Lava Flows?

A sill forms when a magma intrudes between layers of rock underground. The magma never makes it to the surface. Instead, it cools slowly in place, thermally insulated by the rocks above and below.

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In contrast, a lava flow forms when magma erupts at the surface and flows along the ground. Lava flows can vary in thickness, from centimeters to tens of meters. The thinner the flow, the faster it cools. Most investigators think that the large proportion of pyroxene in the nakhlites is best explained by formation in an intrusive body like a sill, where the slow cooling gives the pyroxene crystals time to sink and accumulate to high concentrations (70-80% of the rock). But the nakhlites have other features, like fine-grained areas between the pyroxene crystals and high concentration of calcium in the mineral olivine, which usually suggest formation in a rapidly cooled body, like a lava flow. The idea of nakhlites forming in a lava flow can be tested by examining a similar lava flow on Earth and comparing it to the nakhlites.

Theo's flow, Ontario: A Mars-like Rock on Earth

Theo's flow in Ontario, Canada is 2.6 billion years old. It is unusually thick for a lava flow (120 meters thick!), and is geographically associated with several thick intrusive sills, but a study by Nick Arndt in the late 1970's identified Theo's flow as an erupted lava flow. The best evidence that Arndt found for an extrusive origin is the knobby top layer of the flow.

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The top of Theo's flow is brecciated (or broken up) like the clinkery top of an 'a'a lava flow. Intrusive sills don't have brecciated tops; instead, they have a glassy or finely crystallized margin at their top boundary where they cooled rapidly against the overlying rock. Arndt described this layer thoroughly, likening it to the rubbley material produced by the interaction of hot magma with water in a submarine eruption. Another unusual aspect of Theo's flow is that while it is one flow unit, there are several rock types that make it up: a peridotite (rich in olivine) at the bottom, a thick middle layer of pyroxenite, and an upper layer of gabbro (pyroxene + feldspar). The middle layer is what interests us as an analog for the martian nakhlites. Despite coming from different planets and having formed a billion years apart, when seen through a microscope, Nakhla and Theo's pyroxenite look amazingly similar.

Photograph taken with a microscope at 25x magnification of a Nakhla meteorite thin section(field of view is about 5 millimeters across).

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Photograph taken with a microscope at 25x magnification of a thin section of a Theo's flow sample (field of view is about 5 millimeters across).

Both rocks are primarily composed of calcium-rich pyroxene and the material between pyroxene crystals is rich in feldspar. They have similar average grain sizes and compositions and record similar cooling times. This has led us to suggest the Nakhlites formed in a thick lava flow like Theo's. By studying Theo's flow in detail, then, we may shed light on the formation of nakhlites.

To follow up Arndt's work, we sampled Theo's flow at certain intervals to take a closer look at the whole of the flow. We took samples to measure mineral abundances and compositions, crystal sizes, and whole rock compositions. The next two photographs illustrate the conditions in the field.

The ancient rock of Theo's flow was tilted 90o so that a walk along the ground today lets us see the flow from bottom to top. This tape-measure extends from the base to the top of Theo's flow. It enabled us to note where we took our rock samples in relation to the height of the flow.

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Jeff Taylor (my advisor, University of Hawaii) makes himself useful. He is using a rock hammer and chisel to help collect rock samples to be analyzed back in the lab in Hawai'i.

On the left is a pictorial representation, called a stratigraphic column, of the different rock types that make up Theo's flow. The samples we took along our traverse line gave us all the pertinent information about the different rock layers to help us put together a cooling history for the flow. A detailed study of the different layers shows that Theo's flow did start as one magma body and then separated into discrete layers. As the flow cooled in place, crystals grew and settled through the liquid lava, changing the composition of the melt with the growth of each new crystal and thereby changing the composition of each subsequent crystal grown from the evolved melt. Thus, the different layers in Theo's flow are intimately related to each other. If the nakhlites did form in a similar setting, we would expect other nakhlite-like flows to be associated with peridotites and gabbros.

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Hawaiian pahoehoe lava.

Aside from shedding light on how nakhlites may have formed, this study of Theo's flow will help us to better understand the processes that occur in lava flows on Earth. Regardless of the magma type, crystal formation, growth and settling are important processes that govern the character of the solidified lava flow.

R.C. Friedman, T.J. McCoy, and G.J. Taylor, 1994, "Constraints on the Physical Details of Nakhlite Formation" in Abstracts of the Lunar and Planetary Science Conference XXV, p. 391-392. R.C. Friedman, G.J. Taylor, and A. Treiman, 1995, "Processes in Thick Lava Flows: Nakhlites (Mars) and Theo's Flow (Ontario, Earth)" in Abstracts of the Lunar and Planetary Science Conference XXVI, p. 429-430.

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PSR Discoveries: New NASA- and NSF-funded meteorite studies

http://www.psrd.hawaii.edu/Aug97/NewAwards.html

posted August 22, 1997

Testing the Evidence for Life on Mars: NASA and NSF Fund New Studies of Martian Meteorite Written by G. Jeffrey Taylor Hawai'i Institute of Geophysics and Planetology NASA and the National Science Foundation (NSF) have awarded grants to test the evidence that fossil life has been discovered in martian meteorite ALH84001. PSR Discoveries has covered the debate about the evidence in a series of articles and will continue to do so. The projects, funded after close scrutiny by other scientists, are listed below. They are organized by type of investigation and arranged alphabetically in each section. Many of the studies actually address topics across subject areas, but we list them under the topic that constitutes the major portion of the research being done.

The nature of organic compounds is important in evaluating whether life existed on Mars. The presence of organic compounds alone does not prove that life existed, as such chemicals can form by nonbiological processes, too. One of the chief research problems will be to test for contamination of the samples, on both Mars and Earth. Amino Acids and Other Organic Compounds in Antarctic Meteorites and Ice (NASA grant) Investigators: Jeffrey L. Bada, Luann Becker, and Gene D. McDonald Organizations: University of California San Diego, University of Hawaii Manoa, Cornell University An Investigation of Carbon Isotope Abundances in ALH84001 (NSF grant) Investigator: Greg H. Rau Organization: University of California Santa Cruz Carbon Characterization, Element Abundances and X-ray Near Edge Structure Measurements on ALH84001 (NASA grant) Investigators: George J. Flynn and Lindsay P. Keller Organizations: State University of New York Plattsburgh, MVA Inc. The Search for Unique Biomarkers in the Martian (SNC) Meteorites ALH84001 and EETA79001 (NASA grant)

Investiator: Richard N. Zare Organization: Stanford University

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PSR Discoveries: New NASA- and NSF-funded meteorite studies

http://www.psrd.hawaii.edu/Aug97/NewAwards.html

Biomarkers are minerals or other substances whose presence indicate that organisms were living in a rock. Many of the investigations focus on the nature of the mineral magnetite in ALH84001, in comparison to magnetite produced by bacteria on Earth. Deciphering Sulfur Isotopic Systematics as a Potential Biomarker in ALH84001 (NASA grant) Investigators: Charles K. Shearer and James J. Papike Organization: University of New Mexico High Resolution Examination of the Intact "Microfossil" - Mineral Interface of ALH84001 for Evidence of Physical and Mineralogical Changes Consistent with Microbial Activity (NSF grant) Investigators: William W. Barker and Jillian F. Banfield Organization: University of Wisconsin Madison Iron-Oxide and-Sulfide Mineral Particles as Biomarkers (NSF grant) Investigators: Richard B. Frankel, Dennis A. Bazylinski, Bruce M. Moskowitz, and Peter R. Buseck Organization: Cal Poly State University San Luis Obispo, California Microstructural Studies Bearing on the Origin of Carbonates and Associated Minerals in Martian Meteorite ALH84001 (NASA grant) Investigator: Adrian J. Brearley Organization: University of New Mexico Oxide and Sulfide Mineral Indicators of Past Biological Activity (NASA grant) Investigator: Peter P. Buseck Organization: Arizona State University The Isotopic Composition of Iron: A Chemical Fingerprint for Ancient Life (NASA grant) Investigators: Brian L. Beard, Kenneth H. Nealson, and Clark M. Johnson Organizations: University of Wisconsin Madison and University of Wisconsin Milwaukee The Isotopic Composition of Iron: A Chemical Fingerprint for Biologic Activity (NSF grant) Investigators: Brian L. Beard and Clark M. Johnson Organization: University of Wisconsin Madison

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PSR Discoveries: New NASA- and NSF-funded meteorite studies

http://www.psrd.hawaii.edu/Aug97/NewAwards.html

The possible fossils in ALH84001 are associated with carbonate minerals. There is a raging debate about how the carbonates formed, with some scientists claiming that the carbonates formed at low temperatures, hence consistent with life, while others argue that the carbonates formed at high temperatures (more than 700 oC), incompatible with life. The numerous investigations in this area are designed to determine the temperature and mode for formation of the carbonate minerals in ALH84001. An Electron Microscopy Survey of Carbonate-Bearing Regions of ALH84001 (NASA grant) Investigators: Ralph P. Harvey and John P. Bradley Organization: Case Western Reserve University, MVA Inc. Collaborative Research: Ion Microprobe Analysis of O and C Isotope Ratios in ALH84001 (NSF grants) Investigators: John Eiler, Edward M. Stolper, and John W. Valley Organizations: California Institute of Technology, University of Wisconsin Madison Experimental Investigations of the Origins of Martian Carbonates (NASA grant) Investigators: Gary E. Lofgren, Gordon A. McKay, John H. Jones, Friedrich Horz, and Douglas W. Ming Organization: NASA Johnson Space Center Isotropic and Experimental Constraints on the Genesis of Carbonates in Martian Meteorite ALH84001 (NASA grant)

Investigators: Laurie A. Leshin, Kevin D. McKeegan, Cecile Engrand, Craig E. Manning, and Ralph P. Harvey Organizations: University of California Los Angeles, Case Western Reserve University Paleomagnetic and Rock Magnetic Constraints on the Thermal History of Martian Meteorite ALH84001 (NASA grant) Investigators: Joseph L. Kirschvink and Hojatollah Vali Organizations: California Institute of Technology, McGill University Pathways of Mineral Alteration and Organic Synthesis in Hydrothermal Systems on Mars (NSF grant) Investigator: Everett L. Shock Organization: Washington University, St. Louis Petrologic Studies of Martian Carbonates in ALH84001 (NSF grant) Investigators: Edward R. D. Scott, Lauren B. Browning, and Shiv K. Sharma Organization: University of Hawaii Manoa Stable Isotopic Analysis of Secondary Minerals in ALH84001 (NASA grant) Investigator: Christopher S. Romanek Organization: University of Georgia

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PSR Discoveries: New NASA- and NSF-funded meteorite studies

http://www.psrd.hawaii.edu/Aug97/NewAwards.html

One of the most dramatic lines of evidence is the existence in ALH84001 of fossil-like objects. It is also one of the most controversial lines of evidence. Studies focus on the nature of very tiny fossils and living bacteria in rocks on Earth, for comparison to those in ALH84001. Do Nanobacteria Exist? A Microbial Landscape at Nanometer Scale (NASA grant) Investigators: Todd O. Stevens, Noelle Metting, and James McKinley Organization: Battelle Pacific Northwest Laboratory

The age of the carbonate minerals is important for understanding the evolution of climate on Mars. It is also indirectly related to the issue of fossil life in ALH84001, as some age estimates suggest that the carbonates formed about 3.5 billion years ago, during the time when the climate of Mars is thought to have been wetter and warmer than it is now. However, determining the age of the carbonates may be the most difficult experiment of all those funded. Comprehensive Microprobe Studies of Stable Isotopes, Noble Gas Isotopes and Trance Elements in Primary and Secondary Minerals in Ancient Martian Meteorites (NASA grant) Investigators: Grenville Turner, R. Burgess, G. D. Gilmour, Ian C. Lyon, and R. A. Wogelius Organization: University of Manchester Radiometric Dating of ALH84001 Carbonates (NASA grant) Investigators: Laurence E. Nyquist, James Connelly, L.E. Borg, and Donald D. Bogard Organization: NASA Johnson Space Center, University of Texas Austin

One investigation involving numerous scientists from several institutions encompasses all the categories above, in an integrated, multidisciplinary project. An Evaluation of Biogenicity in ALH84001 (NASA grant) Investigators: David F. Blake, David P. Summers, Stephen J. Mojzsis, Jack D. Farmer, David Des Marais, Sherwood Chang, and Allan H. Treiman Organizations: NASA Ames Research Center, Scripps Institute of Oceanography, Lunar and Planetary Institute

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PSRD: Olivine on Mars

http://www.psrd.hawaii.edu/Nov03/olivine.html

posted November 7, 2003

Pretty Green Mineral -- Pretty Dry Mars? Written by Linda M.V. Martel Hawai'i Institute of Geophysics and Planetology --- The discovery of olivine-bearing rocks on Mars underscores the need to understand weathering rates of silicates in the Martian environment.

Spectra of the Martian surface from the Mars Global Surveyor Thermal Emission Spectrometer (TES) have been matched with laboratory spectra of olivine. Todd Hoefen and Roger Clark (U. S. Geological Survey, Denver) and colleagues at Arizona State University and NASA Goddard Space Flight Center reported a 30,000-square-kilometer area of olivine-bearing rock in the Nili Fossae region, northeast of Syrtis Major. Olivine is the common name for a suite of iron-magnesium silicate minerals known to crystallize first from a magma and to weather first in the presence of water into clays or iron oxides. The occurrence of olivine on the surface of Mars and its susceptibility to chemical weathering has geochemists busy investigating how long it has been there and what that means about climate history. Reference: Hoefen, T. M., Clark, R. N., Bandfield, J. L., Smith, M. D., Pearl, J. C., and Christensen, P. R. (2003) Discovery of olivine in the Nili Fossae region of Mars. Science, v. 302, p. 627-630.

Discovering Olivine on Mars

The TES instrument measures the infrared energy emitted by surface materials and by CO2, water ice, dust, and water vapor in the Martian atmosphere. Hoefen and his colleagues studied the surface of Mars in locations from 60 oN to 60oS and focused on TES data in the spectral range from ~300 to ~550 cm-1, corrected to eliminate the atmospheric components. Three diagnostic spectral features of olivine were matched to the TES data: absorption features centered near 400 and 510 cm -1 (due to the bending of silicon-oxygen bonds) and a peak near 450 cm -1; see the plot shown below.

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PSRD: Olivine on Mars

http://www.psrd.hawaii.edu/Nov03/olivine.html

The olivine spectrum is for a laboratory sample with Fo 66 composition and particle size <60 microns. The TES spectrum, from the olivine-bearing area in Nili Fossae, has been corrected for atmospheric gas, dust, and water vapor. 600 to 800 cm -1 in the TES spectrum is the location of an atmospheric CO 2 absorption band, which gives no information about the surface and is therefore not shown. Spectra have been offset vertically for easier comparison.

Olivine (Mg,Fe)2SiO4 is a greenish-colored silicate mineral common in many mafic igneous rocks (dark-colored with significant iron and magnesium content). A piece of typical olivine basalt from Hawaii is pictured below. Olivine is in fact a solid solution series ranging from the magnesium end-member called forsterite, Mg2SiO4 (Fo100) to the iron end-member, fayalite, Fe2SiO4 (Foo). The Fo value is a convenient shorthand for describing olivine composition. Fo = mol%Mg / (mol%Mg + mol%Fe) x 100. Spectroscopists are able to distinguish between olivine compositions because the spectral absorption bands vary in position as a function of composition (e.g. see also work by Jack Salisbury, Johns Hopkins University and Vicky Hamilton, University of Hawaii ). As the Fo value increases (that is, decreasing FeO content), olivine absorption bands shift toward higher wavenumbers. Changing the particle size of an olivine laboratory sample does not shift the absorption bands but does change the overall band depths. Hence, the shapes, positions, and depths of the olivine fundamental bands were used to map the distribution of olivine compositions on the Martian surface.

This is a photograph of a typical Hawaiian olivine basalt. The rock is 14 centimeters across and contains about 15 to 20% olivine. A weathered face oxidized to a brownish-red is just visible at the bottom.

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PSRD: Olivine on Mars

http://www.psrd.hawaii.edu/Nov03/olivine.html

Olivine in Nili Fossae

Hoefen and coauthors report the detection of olivine in small outcrops distributed nearly globally between 60 oS and 60oN, but the largest

surface exposure occurs in the Nili Fossae region. This region is a fractured and cratered terrain thought to be of Noachian age (>3.5 billion years). It is northeast of Syrtis Major--a broad, very low shield volcano with two summit calderas whose lava flows form a plateau more than 1000 kilometers across. Syrtis Major is thought to be of late Hesperian age (~3 billion years). Its shape and surface features resemble Hawaiian shield volcanoes and suggest a mafic composition consistent with a basaltic rock type. Its calderas are thought to be located on extensions of ring fractures associated with the Isidis impact crater basin (located to the northeast). The current hypothesis is that the fractures, faults, and grabens (valleys between faults) in Nili Fossae are also related to the formation of the Isidis impact basin. Hoefen and collaborators consider that either a pre-existing, subsurface unit of olivine-bearing basalt was exposed by the impact event itself or by post-impact faulting and subsequent erosion, or olivine-bearing basalts were erupted onto the surface during post-impact volcanic activity in Nili Fossae. They favor the idea that the olivine-bearing basalt was already in the target area before the Isidis impact and that post-impact faulting exposed it.

The olivine mapped by Hoefen and team show a compositional range of Fo 60 to Fo70 in the southwest region of Nili Fossae. The northeast region shows olivine ranging from Fo40 to Fo60, which corresponds to slightly higher iron contents (see map below). This range in olivine composition is consistent with compositions of olivine-rich Martian meteorites, such as Chassigny and ALH A77005 (discussed further in a section below). Earlier work headed by Jean-Pierre Bibring (l'Institut d'Astrophysique Spatiale, Orsay, France) and John Mustard (Brown University) used the ISM imaging spectrometer near-infrared data (with a spectral range from 0.7 to 3.1 microns) from the Phobos 2 probe to show different spectral signatures for the eastern and western regions of Syrtis Major, but it was not possible then, in 1990, to determine the significance in terms of mineralogical differences.

Olivine composition mapped in the Nili Fossae region. Map on the left shows the location of the enlarged area shown on the right. Hoefen and coworkers see a trend toward lower Fo values (higher FeO content) to the northeast. They counted the pixels mapped as olivine in the map and concluded that the Nili Fossae olivine exposure covers about 30,000 square kilometers.

Based on the presumed age of ~3.6 billion years for the Nili Fossae region, Hoefen and colleagues think this could be the upper limit to when the olivine was exposed at the surface. Because olivine weathers rapidly to clays and iron oxides, this implies that no water has flowed there since then. Alternatively, the olivine may have been uncovered more recently, in the past few thousand years or so, and the current cold and dry conditions have slowed or limited chemical weathering. What's needed is a better constraint on how long olivine can exist.

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PSRD: Olivine on Mars

http://www.psrd.hawaii.edu/Nov03/olivine.html

Olivine in Ganges Chasma in Valles Marineris

In addition to Nili Fossae, spectral signatures of olivine are also found in the lower walls of Ganges Chasma--a several kilometers-deep side

canyon at the east end of Valles Marineris. Spectroscopists determine that the composition of the Ganges Chasma floor is basaltic and that the walls contain a discrete layer of basaltic rock with 10 to 15% olivine of composition Fo 68. This layer of olivine is being examined with Mars Odyssey THEMIS data (see image below). It appears to be 50 to 100 meters thick and is located 4.5 kilometers down from the top of the wall. Phil Christensen and fellow THEMIS scientists have conjectured that the olivine-bearing rock either erupted onto the surface or crystallized underground, was buried by kilometers of rock units, and has since been exposed by erosion. Alternatively, they say it could be a sedimentary layer enriched with olivine. As in Nili Fossae, the presence of olivine at Ganges Chasma has geologists pondering why it hasn't weathered away. Christensen and colleagues working with the THEMIS data note that detection of olivine may mean that significant subsurface weathering did not occur, despite the potential for liquid water to be present and stable at the temperatures expected at a depth of 4.5 kilometers and it indicates that significant surface weathering has not occurred since the olivine-bearing layer was exposed.

In this false-color mosaic of Ganges Chasma (~13 o S, 318 o E), orange and red tones on the plateaus are dust, blue on the canyon floor is basalt, and the purple bands trending east-west in the canyon walls are olivine-bearing basalt. The black and white strips are THEMIS temperature images that lack compositional data. No atmospheric corrections were applied to the THEMIS data to make this mosaic.

Olivine-rich Martian Meteorites

Cosmochemists have been studying olivine in meteorites for years and are joining with spectroscopists to look for possible source regions of the Martian meteorites. A particularly successful collaboration involves spectroscopists Vicky Hamilton (University of Hawaii), Phil Christensen and Josh Bandfield (Arizona State University) with meteoriticist Hap McSween (University of Tenneessee). Their work shows that olivine-bearing rocks in Nili Fossae, Ganges Chasma, and other areas resemble the mineralogies of meteorites ALH A77005 (~55% olivine) and Chassigny (~90% olivine) with ~Fo 68 composition. But the age of the ancient terrains (>3.5 billion years for Nili Fossae) is inconsistent with the ages of the meteorites (1.3 billion years for Chassigny and 0.18 billion years for ALH A77005). The search for meteorite-like spectra from the surface of Mars is an ongoing and exciting endeavor.

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PSRD: Olivine on Mars

http://www.psrd.hawaii.edu/Nov03/olivine.html

Map of the mineral distribution in the Chassigny Martian meteorite. The map was made using an electron microprobe by measuring the intensities of X-rays from iron, aluminum, and calcium. Green is olivine, blue is pyroxene, and purple is feldspar. Other Martian meteorites also contain olivine, though not as much as in Chassigny.

First to Crystallize, First to Weather

Silicate minerals weather in the same sequence as they crystallize (described by Bowen's Reaction Series). Olivine crystallizes first from a

magma (at temperatures around 1200 oC), and is the first to weather in the presence of water. Like other silicate minerals, olivine is susceptible to chemical weathering in the following ways: dissolution (minerals dissolve in water), hydrolysis (minerals react with water forming clays), and oxidation (iron-bearing minerals react with oxygen forming iron oxides or rust). The chemical reactions occur only where the surface of the mineral and water interact. So, the smaller the particle, the higher the ratio of surface area to volume, and the faster the particle will weather chemically. The reality of how susceptible olivine is to chemical weathering does not seem to jive with its appearance on the Martian surface. Some alteration minerals have been identified in TES spectra of the Martian surface, but not necessarily in the olivine-bearing regions. So, the presence of olivine in places such as Nili Fossae and Ganges Chasma apparently without a corresponding abundance of alteration products seems inconsistent with what we know about how fast olivine weathers. There is a demonstrated need to better understand and quantify how long olivine has been exposed on the surface. Using published experimental data, University of Hawaii graduate student, Julie Stopar, is taking the first step toward determining minimum olivine residence times in water, that is, how fast olivine will dissolve in water. The length of time needed to dissolve olivine grains depends on olivine composition, particle size, temperature, and pH. The graph below shows how long it takes olivine grains of different sizes to completely dissolve. The grains have a composition of Fo 65 and the water was assumed to be slightly acidic with a pH of 5 (both values are thought to be typical of Mars). The calculations show that even at low temperature, olivine should dissolve in less than 10,000 years. Stopar and colleagues are calculating minimum dissolution rates. Actual rates may be longer due to grain coatings or other processes that work to slow dissolution.

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PSRD: Olivine on Mars

http://www.psrd.hawaii.edu/Nov03/olivine.html

Graph shows the dissolution rates for olivine of composition Fo 65 , under pH=5 conditions, for three different temperatures, 5oC (blue line), 25oC (green line), and 100oC (red line).

Forthcoming PSRD articles will follow the research on Martian olivine: how long it has been exposed on the surface, how it relates to the geochemistry of Martian meteorites, and how it will teach us about the abundance or longevity of water on the surface of the Red Planet. This research brings together meteorite studies, spectroscopy of meteorites and Mars, and experimental data on the rate and nature of olivine dissolution.

Bibring, J.-P. and many others (1989) Results from the ISM experiment. Nature, doi:10.1038/341591a0, v. 341, p. 591-593. Christensen, P. R. and many others (2003) Morphology and composition of the surface of Mars: Mars Odyssey THEMIS results. Science, v. 300, p. 2056-2061. Hamilton, V. E., Christensen, P. R., McSween, Jr., H. Y., and Bandfield, J. L. (2003) Searching for the source regions of martian meteorites using MGS TES: Integrating martian meteorites into the global distribution of igneous materials on Mars. Meteoritics and Planetary Science, v. 38(6), p. 871-885. Hamilton, V. E., Christensen, P. R., and McSween Jr., H. Y. (1997) Determining the martian meteorite lithologies and mineralogies using vibrational spectroscopy. Journal of Geophysical Research, doi:10225593-25603. Hoefen, T. M., Clark, R. N., Bandfield, J. L., Smith, M. D., Pearl, J. C., and Christensen, P. R. (2003) Discovery of olivine in the Nili Fossae region of Mars. Science, v. 302, p. 627-630. Hoefen, T. M., Clark, R. N., Pearl, J. C., and Smith, M. D. (2000) Unique spectral features in Mars Global Surveyor Thermal Emission Spectra: Implications for surface mineralogy in Nili Fossae. DPS 2000 abstract 332. Mustard, J. F., Bibring, J.-P., Erard, S., Fischer, E.M., Head, J. W., Hurtrez, S., Langevin, Y., Pieters, C. M., and Sotin, C. J. (1990) Interpretation of spectral units of Isidis-Syrtis Major from ISM-Phobos-2 observations. LPSC XXI abstract, p. 835-836. Ruff, S. W. and Christensen, P. R. (2002) Bright and dark regions on Mars: Particle size and mineralogical characteristics based on Thermal Emission Spectrometer data. Journal of Geophysical Research, v. 107, doi: 10.1029/2001JE001580 (2002). Salisbury, J. W., Walter, L. S., Vergo, N., and D'aria, D. M. (1991) Infrared (2.1-25 microns) Spectra of Minerals. Baltimore: Johns Hopkins University Press, 267 p. 6 of 7

PSRD: Olivine on Mars

http://www.psrd.hawaii.edu/Nov03/olivine.html

Stopar, J. D., Taylor, G. J., Hamilton, V. E., Browning, L., and Pickett, D. (2003) Maximum rates of olivine dissolution of Mars. Sixth International Conference on Mars abstract no. 3151. Thermal Emission Spectrometer (TES) on Mars Global Surveyor (MGS). Thermal Emission Imaging System (THEMIS) on Mars Odyssey.

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PSRD: The wet, oxidizing crust on Mars

http://www.psrd.hawaii.edu/Aug02/oxidation.html

posted August 30, 2002

The Wet, Oxidizing Crust of Mars --- Analysis of isotopes and oxide minerals in Martian meteorites indicate that many magmas interacted with a wet, oxidizing crust as they oozed from the Martian mantle to its reddish surface.

Written by G. Jeffrey Taylor Hawai'i Institute of Geophysics and Planetology

Studies have inferred that the oxidation state of Martian basaltic meteorites (the shergottites) is correlated with diagnostic geochemical parameters. For example, Meenakshi Wadhwa (Field Museum, Chicago) showed that as the ratio of triply-charged europium to doubly-charged europium (Eu3+/Eu2+) increases in a group of shergotties, the ratio of strontium-87 to strontium-86 (87Sr/86Sr) also increases, and neodynmium-143 to neodynmium-144 (143Nd/144Nd) decreases. [See PSRD article: Gullies and Canyons, Rocks and Experiments: The Mystery of Water on Mars]. Eu3+/Eu2+ is a measure of the oxidation state and can be used to infer the availability of oxygen to react chemically, a property called the oxygen fugacity. Christopher Herd, Lars Borg, and Jim Papike (University of New Mexico) and John Jones (Johnson Space Center) decided to measure the oxygen fugacity more directly by making very careful and painstaking analyses of oxide minerals in Martian meteorites. Herd and his co-workers find that as oxygen fugacity increases in a group of shergottites, 87Sr/86Sr and the ratio of lanthanum (La) to ytterbium (Yb) also increase, while 143Nd/144Nd decreases. They suggest these trends indicate that, compared to the Martian mantle, the crust is more oxidizing, has higher 87Sr/86Sr and La/Yb, and lower 143Nd/144Nd. Magmas formed in the mantle would have low oxygen fugacity. As magmas rose through the crust, they reacted with the surrounding rocks to varying extents, producing the observed chemical trends. How did the crust become more oxidizing than the mantle? They suggest that circulating hot water oxidized the crust. Alternatively, water-rich magmas might have crystallized in the crust, forming deposits of hydrous minerals. Subsequent magmas could react with the hydrated minerals to become more oxidizing. Whatever the details, the work by Herd and colleagues indicates that the mantle and crust differ significantly, that the crust has significant deposits of water, and many pristine magmas are modified by interaction with the crust. Reference: Herd, C. D. K., Borg, L. E., Jones, J. H., and Papike, J. J. (2002) Oxygen fugacity and geochemical variations in the martian basalts: Implications for martian basalt petrogenesis and the oxidation state of the upper mantle. Geochimica et Cosmochimica Acta, vol. 66, p. 2025-2036.

Meteorites and the Crust and Mantle of Mars

Understanding how planets formed and how they evolved geologically requires knowing something about the composition of their interiors and surfaces. Martian meteorites give us an indirect, somewhat blurry glimpse of the Martian core, rocky mantle, and crust. Chris Herd and his colleagues are trying to pin down the oxidation conditions in the mantle and the crust by looking at chips of lava flows sent to us for free by impacts on Mars. The trick is to figure out which features of the rocks apply to the mantle and which apply to the crust.

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PSRD: The wet, oxidizing crust on Mars

http://www.psrd.hawaii.edu/Aug02/oxidation.html

Data from Martian meteorites and orbiting spacecraft give hints about the nature of the interior of Mars. Chris Herd and his colleagues are trying to use Martian meteorites to determine the oxidation state of the mantle and crust of the red planet.

Minerals Record the Oxidation State...

Iron typically occurs in two valence states, doubly-charged (Fe2+) and triply-charged (Fe3+). The amount of each depends largely on the oxidation conditions, hence on the oxygen fugacity. The concept of oxygen fugacity is completely foreign to most of us. Even the name is forbidding. But it is actually a fairly simple concept: Oxygen fugacity is just a measure of the amount of free or uncombined oxygen that is available in an environment. One confusing thing is that oxygen makes up about half the volume of virtually all magmas and rocks. That sounds pretty oxidizing, but most of that oxygen is chemically bound to silicon (which is usually the second most abundant element) and other positively-charged ions, and hence not freely available. Thus, only a little of it is available to alter the valence state (i.e., the charge) of iron in a magma or mineral. It is as if the oxygen were in an atmosphere that permeates a magma or rock. In fact, the fugacity is measured in terms of atmospheric pressure. This atmosphere is rather tenuous: in most magmas, oxygen fugacity ranges from 10-10 to 10-18 atmospheres of pressure. (10-10 means that the oxygen partial pressure is one ten-billionth of the pressure at the surface of Earth.) With higher oxygen fugacity, there is more Fe3+ (ferric iron) and less Fe2+ (ferrous iron) in the iron-bearing minerals in a rock. In some cases the oxygen fugacity is so low that iron occurs as Fe 2+ and uncharged (metallic) iron. Moon rocks are like that: they contain tiny bits of metallic iron. Experiments and quantitative thermodynamic calculations allow us to determine the oxygen fugacity by measuring the amounts of ferrous and ferric iron in minerals, especially oxides. For example, one pair of minerals can range in composition from pure ulvospinel (Fe2TiO4, with all the Fe existing in the ferrous state) to magnetite (Fe 3O4, with one-third of the iron being in the ferrous state and the rest ferric). Many experiments show that if both minerals are in equilibrium with each other, the amount of ferric iron in each is related to the oxygen fugacity.

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PSRD: The wet, oxidizing crust on Mars

http://www.psrd.hawaii.edu/Aug02/oxidation.html

Electron microprobe image of iron-bearing minerals in Martian meteorite DaG 476. Brightness is proportional to the number of electrons bouncing off the mineral surfaces. This, in turn, is proportional to the average atomic weight of the material being bombarded with the electron beam, which is usually directly related to the percentage of iron atoms in that material. Ilm is ilmenite; TMt is titanomagnetite; Sf is iron sulfide. Dark surrounding material is composed of silicate minerals.

...But Reading the Record is Tricky

In principle, all we need to do is to analyze the chemical composition, including the amounts of Fe2+ and Fe3+, in oxide minerals in a rock, plug the data into formulas that have been determined experimentally, and calculate the oxygen fugacity. Too bad things are not that simple. One complication is that the minerals occur in fine intergrowths. This means that they cannot be separated physically from the rock and analyzed by traditional chemical techniques that give the amounts of ferrous and ferric iron. Instead, we need to use an electron microprobe. This instrument is capable of analyzing tiny spots (only a micrometer or two across) on polished slices of a rock. This gets us away from the need to physically separate the minerals, but the electron probe analyzes elements only. It does not give us their valence state. There are two ways to determine the amount of ferrous and ferric Fe from an electron microprobe analysis. One is to determine the elemental abundances very carefully and assume that the minerals have perfect, ideal chemical compositions. This allows us to divide the iron into Fe2+ and Fe3+ in just the right amounts for the composition of each mineral to be exactly correct. For example, ulvospinel has the ideal composition Fe 2TiO4. The total amount of Fe and Ti are measured, and the amount of oxygen is adjusted until the ratio of Fe+Ti to O is exactly 3 to 4. The problem is that minerals are like people--they're not perfect. The ratio might be 2.9 to 4, or 3.1 to 4. This affects the calculated oxygen fugacity. The problem is also complicated by the presence of other elements, such as magnesium, chromium, aluminum, and manganese, in each mineral. Another approach is to measure oxygen directly in the electron microprobe. This would seem to be pretty easy since there is so much oxygen. However, light elements like oxygen are notoriously difficult to analyze. For example, great care must be taken in selecting the correct oxygen peak in the x-ray spectrum produced in the electron microprobe. Furthermore, microprobe analyses must be corrected for assorted affects (such as x-ray absorption), and there are 3 of 8

PSRD: The wet, oxidizing crust on Mars

http://www.psrd.hawaii.edu/Aug02/oxidation.html

several choices available for oxygen. On top of all that, the analyses must be very precise because for modest amounts of ferric iron in an iron-bearing oxide the difference in total oxygen is quite small, less than 10% of the amount present. Given all the uncertainties and experimental difficulties, Herd and his coworkers determined the Fe 2+ and Fe3+ in both ways and calculated oxygen fugacity by two different methods. This painstaking work leads to some interesting correlations with other geochemical data and some intriguing interpretations about the Martian crust and mantle.

Oxidation State Correlated with Geochemical Parameters

Meenakshi Wadhwa measured the ratio of Europium (Eu) to gadolinium (Gd) in pyroxene crystals in Martian meteorites. Both are rare earth elements, and they behave in predictable ways during the formation and solidification of magma. Europium has the added virtue of occurring in two different oxidation states, as doubly-charged (Eu 2+) and triply-charged (Eu3+). Gadolinium is less temperamental and remains as triply-charged Gd3+. The lucky thing is that Gd behaves almost exactly like Eu3+, so geochemists can figure out the amount of Eu in each valence state from the total amount of Eu and the amount of Gd. The ratio of doubly- to triply-charged Eu is proportional to the oxygen fugacity. So in principle, if you can figure out the ratio of doubly- to triply-charged europium, you can determine the oxidation conditions. Herd and coworkers compared their calculated oxygen fugacities with Wadhwa's measured Eu and Gd concentrations in pyroxene in the same rocks. (The concentrations are expressed as the ratio of the concentration of each element in pyroxene to its concentration in the entire rock.) The result is a good positive correlation between Eu/Gd and oxygen fugacity.

Eu/Gd in pyroxene (specifically the mineral augite) correlates with oxygen fugacity determined from mineral compositions. Because the values of oxygen fugacity are so tiny, they are usually expressed as a logarithm and often compared to some kind of standard conditions. For example, a useful comparison is the free oxygen associated with an assemblage of the minerals quartz (SiO 2 ), fayalite (Fe 2 SiO 4 , and magnetite (Fe 3 O 4 ), nicknamed the QFM buffer. So, on this diagram, an oxygen fugacity of -3 is 1000 times smaller than QFM.

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PSRD: The wet, oxidizing crust on Mars

http://www.psrd.hawaii.edu/Aug02/oxidation.html

One concern with oxide minerals is that they continue to exchange oxygen after they have crystallized. This is a problem because we want to know the oxygen fugacity of the magma, not the rock after it formed. So, the correlation in the diagram above is important because the Eu and Gd are incorporated into pyroxene during crystallization. It indicates that the oxygen fugacity determined by Herd also applies to conditions during crystallization, hence in the magma. Herd examined the relation between oxygen fugacity and other geochemical parameters. Like Meenakshi Wadhwa, he found that there is a good correlation between oxygen fugacity and 87Sr/86Sr, 143Nd/144Nd, and La/Yb (lanthanum/ytterbium). These parameters are all indicators of planetary crustal materials. As the crust formed, rubidium (a radioactive element that decays to 87Sr) is separated from strontium (Sr). This causes a continual increase in the 87Sr/86Sr ratio in the crust. Similarly, radioactive samarium-147 (147Sm) is separated from neodynmium (Nd). It tends to stay behind in the mantle, causing the mantle to increase in 143Nd/144Nd with time. La is also preferentially partitioned into the crust compared to Yb.

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PSRD: The wet, oxidizing crust on Mars

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These three figures show strong correlations between three geochemical parameters and oxygen fugacity. In each case, the more oxidizing conditions (higher oxygen fugacity, which increases to the right) are associated with isotopic or elemental ratios expected for the crust of Mars. Herd and coworkers suggest that this indicates reaction between reduced (low oxygen fugacity) mantle-derived magma and more oxidized crustal materials.

What do these correlations mean? It might mean that the mantle of Mars is very heterogeneous in composition and oxidation state. The differences would all have to correlate with each other. Perhaps there are two distinct regions in the mantle and magmas from them mingle to differing extents, leading to the observed correlations. Alternatively, the two regions could represent the mantle and crust. In this case, magma formed in the mantle reacts chemically with the crust raising oxygen fugacity, 87Sr/86Sr, and La/Yb, and decreasing 143Nd/144Nd. This reaction happened to varying extent, accounting for the trends seen in the diagrams above. Herd and his colleagues prefer the mantle-crust hypothesis and consider several materials that could cause the oxidation of primary, relatively reduced magmas. They focus on the type of material that was assimilated by ascending magmas. One type could be sedimentary materials rich in ferric iron, the substance that gives the red planet its color. However, assimilation of iron oxides alone cannot explain the variations in isotopic and elemental ratios, so other material must also be involved. Two likely candidates are oxidized lava flows (which could be buried to significant depths as volcanism constructed the crust), or pockets of hydrous minerals or rock-water mixtures. Both have the virtue of being able to explain the trends seen in the diagrams above. The figures below show two possible scenarios to explain the observed correlations between oxygen fugacity and geochemical parameters.

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Alternative scenarios for producing oxidized lava flows on Mars, assuming that the mantle is relatively reduced and uniform in composition. In the top figure (a), magma interacts with altered (weathered or chemically reacted with hot water) basalt to various extents. In the bottom figure (b), magma reacts with regions containing water-bearing minerals (such as amphibole, amph, or phogopite, phl). In both cases, some magmas make the trip from the mantle to the surface without reacting with the crust; those are the Martian meteorites with lowest oxygen fugacity. The crust is assumed to be enriched in elements that concentrate in magma ("incompatible elements") compared to the mantle.

Wet Crust

This work suggests that there is a large difference between the mantle and crust of Mars, just as there is on Earth. It might also indicate that there are big differences even in the mantle. Magmas built the crust of Mars over time, though there was much more igneous activity early (before about 4 billion years ago) than more recently. This crust was modified by water, both on the surface and at depth. These modifications (making hydrous minerals, increasing the oxygen fugacity) themselves appear to have modified other magmas as they traveled through the crust. The study of these interactions is only beginning. More meteorite studies are being done, and present and future missions to Mars will help us understand the nature of the crust and its formation.

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Herd, C. D. K., Borg, L. E., Jones, J. H., and Papike, J. J. (2002) Oxygen fugacity and geochemical variations in the martian basalts: Implications for martian basalt petrogenesis and the oxidation state of the upper mantle. Geochimica et Cosmochimica Acta, vol. 66, p. 2025-2036. Herd, C. D. K., Papike, J. J., and Brearley, A.J. (2001) Oxygen fugacity of martian basalts from electron microprobe oxygen and TEM-EELS analyses of Fe-Ti oxides. American Mineralogist, vol. 86, p. 1015-1024 Taylor, G. J. "Gullies and Canyons, Rocks and Experiments: The Mystery of Water on Mars." PSR Discoveries. April 2001. . Wadhwa, M. (2001) Redox state of Mars' upper mantle and crust from Eu anomalies in shergottite pyroxenes. Science, vol. 291, p. 1527-1530.

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PSR Discoveries: Hot Idea: Evidence for Shocked Carbonates in ALH84001

http://www.psrd.hawaii.edu/May97/ShockedCarb.html

posted May 22, 1997

Shocked Carbonates May Spell in Martian Meteorite ALH84001 Written by Edward R.D. Scott Hawai'i Institute of Geophysics and Planetology, SOEST, University of Hawai'i

In an electrifying paper published in August, 1996 in the journal Science, David McKay (NASA Johnson Space Center) and his colleagues suggested there were fossils of martian organisms associated with carbonate minerals in martian meteorite ALH84001. How these carbonate minerals formed (biologic origin or not) and the temperature at which they formed (low or high) are hotly debated questions. We have proposed an entirely different origin: the carbonates in ALH84001 formed in seconds at high temperatures (>1000 oC) from melts produced during a large impact on Mars 4.0 billion years ago (Scott and others, 1997). We infer that it is unlikely that the carbonates or any minerals in them contain mineralogical evidence for ancient martian life. Reference: Scott, E.R.D., A. Yamaguchi, and A.N. Krot, 1997, "Petrological Evidence for Shock Melting of Carbonates in the Martian Meteorite ALH84001", Nature, v. 387, p.377-379.

Story of a Shocked Rock

When an asteroid or comet hits a planet its kinetic energy is transformed into high-pressure shock waves that travel into the ground, compressing, vaporizing, melting, fracturing, and moving the target rock. An important feature of ALH84001 is that it was part of the target material on Mars when a powerful impact event took place about 4.0 billion years ago, as depicted in this artist's rendition. This event no doubt moved ALH84001 from its original location in the martian crust, and heated the rock enough to record the time of the event. The shock wave also converted the silicate mineral, plagioclase, into glass. We studied meteorite ALH84001 because we wanted to know how the carbonates and other minerals had been affected by shock events. We infer that the shock wave melted and drastically redistributed the carbonate minerals, already present in the rock, obscuring how they were originally formed. (Graphics by Brooks Bays, PSR Discoveries graphic artist.)

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Close Look at the Cracks

An optical microscope and a scanning electron microscope gave us a close look at the settings where carbonate and other minerals occur in martian meteorite ALH84001. We were surprised to find other grains besides carbonates filling the cracks in the meteorite. These grains are tiny (0.005-0.05 millimeter) and are made of glass with chemical compositions very close to the minerals plagioclase and silica. Grains or veins of carbonate commonly occur in the same or parallel cracks. The cracks are found in pyroxene crystals (the mineral that makes up over 95% of the rock). Some carbonate grains enclose broken pieces of pyroxene.

Scanning electron microscope images of ALH84001 (above) show plagioclase feldspar (left), silica (center), and carbonate (right). All surround broken pieces of pyroxene. We suggest that these texures formed by high-pressure shock waves that not only fractured the pyroxene but also injected molten plagioclase, silica, or carbonate into the cracks. Plagioclase, silica, and carbonate also occur as thin veins within pyroxene crystals as seen in the three images below.

Carbonate veins vary in composition from calcium rich (Cc) to more magnesium rich (Cm). We think the cracks in the pyroxene crystals were sealed quickly under pressure squeezing and trapping the melts inside.

The shapes, compositions, and locations of glasses and carbonates in ALH84001 suggest that they all formed 2 of 5

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from melts produced by a shock wave. The melts then squirted into fractures in the pyroxene where the melts cooled quickly and the fractures resealed under pressure. As the pyroxene crystals were not heated by more than a few hundred degrees centigrade, the tiny volumes of melted minerals must have solidified in seconds before they could mix together or react with the pyroxene.

Other Shocked Meteorites Add to the Story

Comparisons with strongly shocked meteorites called L chondrites are useful in understanding the abundances, shapes, sizes, and location of carbonates and glasses in martian meteorite ALH84001. L chondrites are composed of silicate minerals (mostly olivine and pyroxene, but feldspar as well), metallic nickel-iron and iron sulfide (called troilite). Most of them are severely shock-damaged, probably by a large impact on the asteroid in which they formed. Plagioclase feldspar and silica are readily melted by shock waves to form glasses. Yet, pure melts of these minerals are not generally present in veins in shocked rocks. Instead, the shock melts normally seen in veins are made from several minerals. However, when we examined some strongly shocked L chondrites, we found tiny veins and grains of glass with compositions close to that of plagioclase feldspar. These veins and grains are found in healed fractures in silicates, just like those in ALH84001. They hadn't been described before probably because they are so tiny and difficult to analyze.

Silicate crystal in an L chondrite contains shock melts of troilite (iron sulfide) and metallic iron-nickel.

Other researchers have concluded that the troilite and metallic iron-nickel melts were injected into cracks in the silicate crystals as the shock pressures decreased. Carbonate and troilite melts both have very low viscosities, unlike plagioclase and silica melts. The difference in viscosity is comparable to the difference between water (low viscosity) and something like shampoo (relatively higher viscosity). Although carbonate is readily heated by shock it is liable to decompose to form carbon dioxide unless pressures are high enough during cooling. We do not know of other meteorites and rocks with shock-melted carbonates that are quite like those in ALH84001.

Reconciling Conflicting Interpretations

A different impact mechanism for the formation of the martian carbonates was proposed in July, 1996 by 3 of 5

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Ralph Harvey (Case Western Reserve University) and Harry Y. McSween (University of Tennessee). Harvey and McSween inferred that the carbonates formed by reaction between silicates and an impact-produced fluid rich in carbon dioxide that infiltrated fractured rock at depth. Their model differs from ours in several important respects. We find no textural or mineralogical evidence to suggest that the existing carbonates formed by reaction between silicates and a fluid. We argue instead that carbonates were already present in the rock before the impact. Because the melts were only hot for seconds, there was not enough time for the melts to react significantly with the silicates or for the chemical variations in the carbonate crystals to be erased. Supporters and some critics of the paper by McKay and coworkers argue that the carbonates in ALH84001 formed at low temperatures by precipitation from a fluid. [For example, see PSR Discoveries article " Low-temperature Origin of Carbonates consistent with Life in ALH84001."] This interpretation is based largely on the heterogeneous (nonuniform) composition of the carbonates, a common feature of carbonates produced at low temperature on Earth. In addition, the carbonates in ALH84001 could not have been at high temperatures for long periods (days or weeks) or they would have uniform compositions. The possibility of high-temperature formation in a few seconds was not previously considered as the rock itself is too large to cool in seconds. However, impacts can do amazing things: tiny volumes of various mineral melts can be produced by shock, squirted into fractures, and cooled quickly under pressure. Carbonate melts form crystals rather than glasses when cooled rapidly because atoms diffuse quickly in carbonate melts. The unusual surface texture of fractured carbonates reported by McKay and colleagues may reflect rapid growth in a rapidly cooling melt. We suggest that the key evidence necessary for understanding the origin of the carbonates in ALH84001 is the similar distribution and shape of the carbonates and glass veins and grains in the fractured pyroxene crystals. Our observations lead us to infer that the carbonates in ALH84001 crystallized from impact melts, and therefore could not have formed at the low temeratures needed for life. Experiments are now needed to test if carbonates can solidify from melts to form heterogeneous crystals like those in ALH84001. We conclude that the search for fossil evidence of life on Mars should not be focused on the carbonates in ALH84001.

Ancient Impacts

Argyre basin, about 900 kilometers in diameter, is an impact structure in the southern hemisphere of Mars (50oS, 40oW). It probably formed very early in the history of Mars, about 4 billion years ago. Other smaller impact craters decorate the area. (NASA Viking Orbiter mosaic P17002.)

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No matter how our study affects the case for life on Mars, it demonstrates the importance of impacts on ancient planetary surfaces. The surfaces of Mars, the Moon, and Mercury, as well as many of the satellites of the outer planets, were bombarded severely early in their histories. This bombardment greatly affected the crust of the Moon, as we know from samples returned by the Apollo missions, but ALH84001 is the first sample from Mars to give us a first-hand look at the effects of impact early in the history of the red planet. Whether or not there is fossil life in ALH84001, this rock contains a wealth of information about the ancient martian crust.

Editor's note: The temperature at which the carbonates in ALH84001 formed is one of the most hotly debated issues about the evidence for fossils in the meteorite. For an opposing view, see PSR Discoveries article Low-temperature Origin of Carbonates Consistent with Life in ALH84001 posted on May 22, 1997.

Harvey R.P and H. Y. McSween Jr., 1996, A possible high-temperature origin for the carbonates in the martian meteorite ALH84001, Nature, v. 382, p. 49-51. Kerr R., 1997, Martian "microbes" cover their tracks, Science, v. 276, p. 30-31. McKay D.S. et al., 1996, Search for past life on Mars: possible relic biogenic activity in martian meteorite ALH84001, Science, v. 273, p. 924-930. Scott E. R. D., A. Yamaguchi, and A. N. Krot, 1997, Petrological evidence for shock melting of carbonates in the martian meteorite ALH84001, Nature, v. 387, p. 377-379. Short list of web sites related to Mars and the search for life.

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PSRD:: Cosmochemistry and Human Exploration

http://www.psrd.hawaii.edu/Dec04/spaceResources.html

posted December 23, 2004

Cosmochemistry and Human Exploration --- Cosmochemistry plays an important role in developing local resources on the Moon and Mars, essential to sustained human presence in space.

Written by G. Jeffrey Taylor

Hawai'i Institute of Geophysics and Planetology

About 125 scientists, engineers, business men and women, and other specialists attended the sixth meeting of the Space Resources Roundtable, held at the Colorado School of Mines in Golden, Colorado. The meeting was co-sponsored by the Space Resources Roundtable, Inc. (a nonprofit organization dedicated to the use of space resources for the benefit of humankind), the Lunar and Planetary Institute, and the Colorado School of Mines. Presentations and discussions during the meeting made it clear that the knowledge gained from cosmochemical studies of the Moon and Mars is central to devising ways to use in situ resources. This makes cosmochemistry central to the human exploration and development of space, which cannot happen without extensive in situ resource utilization (ISRU). Cosmochemists at the meeting reported on an array of topics: the nature of lunar surface materials and our lack of knowledge about surface materials in permanently shadowed regions at the lunar poles; how to make reasonable simulated lunar materials for resource extraction testbeds, vehicle design tests, and construction experiments on Earth; and how to explore for resources on the Moon and Mars. Reference: Space Resources Roundtable VI, LPI Contribution No. 1224, Lunar and Planetary Institute, Houston. Abstracts available online courtesy of the Lunar and Planetary Institute. Presentations available online courtesy of the Space Resources Roundtable. [Links open in new window.]

ISRU Essential for Sustained Human Presence in Space

In January 2004, President George W. Bush announced a visionary plan to explore space, calling for sustained human presence on the Moon and then Mars. The plan emphasizes the use of the abundant resources of the Moon and Mars. In fact, sustained human habitation of the Moon and Mars is impossible without in situ resource utilization (ISRU). It is simply impossible to haul everything we need up from the deep gravity well of the Earth and plop it all down on the lunar surface, let alone onto the Martian surface. Not only will the availability of resources--materials and energy--reduce the difficulty and cost of exploration programs, by "living off the land," but will allow commerce to take root outside of the Earth. Using space resources for propellant will reduce the cost of operating in space by allowing complex extraction systems built on Earth to replace the tremendous payloads of inert propellant that now must be transported into space. Using space resources to capture, transform, and transport energy in space will replace the need for heavy, expensive payloads lifted off Earth. Production of the raw materials of space for construction and manufacturing will ultimately sever the umbilical that, until now, has bound space travel to the Earth, and will allow self-sufficiency to take root beyond the boundaries of Earth.

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This NASA painting of the lunar landscape shows a concept for a pilot plant to produce liquid oxygen propellant derived from lunar raw materials. More than merely oxygen, this picture symbolizes ISRU--the use of in situ resources on the surfaces of other planetary bodies. [ High resolution version]

Cosmochemists make up one of the main pillars of this exciting venture. They will find the resources on other planets--indeed, they already have identified and determined the chemical and mineralogical properties of several key resources such as the presence of helium-3 for use in nuclear fusion reactors or concentrations of ilmenite for use as a feedstock for oxygen extraction. Cosmochemists have been major players in devising ways to extract resources from the lunar regolith (much less work has been done for Mars). A prime example is the demonstration of extracting oxygen from the lunar regolith by reduction of ilmenite and glasses rich in iron oxide, which has been done independently by Lawrence Taylor (University of Tennessee) and Carl Allen and David McKay of the Johnson Space Center. There are many other examples. Cosmochemists will also be in the forefront of developing flight-ready testbed factories by providing expertise into the nature of lunar and Martian surface materials and by making mineralogical and chemical analyses of the products produced by prototype machinery. All that, while still continuing to extract the scientific secrets held by the Moon and Mars.

Vision for an Economic Market

Who's going to buy all those resources? Brad Blair, Mike Duke, Javier Díaz, and Begoña Ruiz (all at the Colorado School of Mines) are developing complex economic models of space development centered on propellant production from the Moon. Blair and his colleagues identified the potential markets, increasingly farther into the future, arranged below from top left to bottom right.

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Fueling space science missions throughout the solar system

Supplying the International Space Station or its successor

Refueling communication satellites in earth orbit

Providing fuel and materials for satellites that manage debris in low earth orbit

(Image from: Orbital Recovery Corporation.)

Refueling and maintaining reconnaissance and Providing fuel and life support to human terrestrial remote sensing satellites in low earth orbit exploration of the Moon and, eventually, Mars and in geosynchronous orbit

Providing fuel, life support, and materials for sustained Produce, fuel, and maintain solar power human presence on the Moon, including perhaps an satellites in earth orbit to provide energy to the elaborate solar-powered colony on the Moon that could planet market its products

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Not Your Daddy's Mining Gear

Dale Boucher and Jim Richard (Northern Centre for Advanced Technology, Inc., NORCAT, located in Sudbury, Ontario, Canada) shared their expertise in mining. It begins with searching for a potential resource (prospecting), which we have already started to do for the Moon and Mars through human (to the Moon) and robotic (to both) missions. Once identified you explore the prospect in detail (implying spacecraft that land on the surface), then mine it. The mined materials are usually beneficiated (minerals separated and concentrated), though this might not be necessary at first on the Moon. We then process the mined materials to make products (e.g., metallic iron, oxygen, water). This step produces some waste, that must be managed, and useful products. This is, among other things, applied cosmochemistry at its most artful. The trick, Dale Boucher told Roundtable participants, is to transform traditional Earth-based mining equipment into efficient and light-weight gear to use on other planets. He calls it the VAMPS process: Eliminate Volatiles (we can't use drilling fluids, for example), Automate (make extensive use of robotics), Miniaturize (lower the size and mass), redefine the Paradigm (do things completely differently if need be), and Stabilize. The result is new mining equipment and approaches. An interesting example is the NORCAT design of a bucket wheel for excavating the lunar regolith. It is small, takes little power, but can excavate efficiently. Boucher and his team plan more tests with simulate regolith.

The VAMPS process starts with traditional mining or construction equipment and changes it in suitable ways for use on another planetary body.

Experiments with a bucket wheel excavator designed at NORCAT in Sudbury, Canada. Shown from left to right are the wheel with its buckets, the test platform, and the bucket wheel excavating sand during a test.

The Cold, Mysterious Lunar Poles

There are places on the Moon where the "Sun don't shine." These regions are located at the lunar poles, shaded by mountains formed by the rims of impact craters. They exist because the Moon's spin axis is tilted only a small amount from being perpendicular to the plane of its orbit, allowing mountains to cast long shadows and creating places in permanent shadow; see PSRD article: Ice on the Bone Dry Moon. These are cold places. Without direct sunlight, the only sources of heat are the background energy in the universe (only 3 Kelvin), reflected sunlight off distant hills, and heat flowing from inside the Moon. The heat from below would cause the temperature to be only about 25 Kelvin (-248 oC). There might be rare places that cold if they are not exposed to any reflected sunlight. Most receive some indirect sunlight, but they reach at most 80 Kelvin (-193 oC) even during the hottest part of the lunar day, and many places in permanent shadow get no hotter than about 60 Kelvin. At night the temperature falls to less than 50 Kelvin. Ben Bussey (Johns Hopkins University Applied Physics Lab), a leading expert on illumination conditions at the poles, enlightened the 4 of 12

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audience with an updated report on lighting conditions (see PSRD article: The Moon's Dark, Icy Poles). Distribution of temperatures inside a large flat-floored crater in a lunar polar region were calculated by Ashwin Vasavada (UCLA), an expert in thermal modeling of planetary surfaces. During the long lunar night the surface everywhere cools to less that 80 Kelvin, with some areas less than 50 Kelvin. But even during the hottest part of the lunar day, some portions of a crater like this remain very cold, reaching no more than 80 Kelvin. These regions can trap volatiles like H 2 O, CO, and CO 2 , and might be important sources for propellant to drive commerce throughout the inner solar system. Courtesy of Ash Vasavada, Univ. of California, Los Angeles.

Permanently dark areas could collect volatile materials otherwise scarce on the Moon. The most important of these is water. The neutron detector onboard the Lunar Prospector mission showed clearly that there is an enrichment of hydrogen (H) in polar regions, but could not determine whether it was in the form of H2O, molecular hydrogen (H2), elemental hydrogen, or some other form. The Clementine mission rigged an innovative radar experiment that suggested the presence of water ice, but observations of portions of the south pole using the huge radio telescope in Arecibo, Puerto Rico did not detect ice. The issue remains highly controversial, but leaves the rest of us lots of room to speculate on the nature of the hydrogen in polar regions. If the hydrogen is in the form of H2O ice, it could be an enormously important resource that could drive commercial development of the Moon, as Brad Blair and his colleagues discussed at the Roundtable. Before any commercialization can take place, however, cosmochemical prospectors have a lot of work to do. As I noted in my talk at the meeting, we know very little about the nature of the surface in the permanently shadowed areas of the Moon. The ground up rock might be finer grained than the regolith (fragmental surface materials on the Moon) we sampled at the Apollo landing sites, including that at the Apollo 16 site in the ancient highlands. More important, if water condensed onto silicate grains at the cold temperatures in the shadows it would not be in the form of the familiar crystalline ice. It would be amorphous, lacking any long-range order to the H and O atoms--sort of an icy glass. In contrast to crystalline ice, this structure is quite accommodating to gases such as those in comets (carbon dioxide, carbon monoxide, methane). When amorphous ice (also called amorphous solid water) is heated above about 120 Kelvin, it changes to the crystalline form, releasing the trapped gases. This is one of the reasons why comets spew gas and dust. Thus, if we tried to mine the ice to make hydrogen and oxygen for propellant, heating the icy regolith might cause rapid loss of the water we were trying to mine. Heat is also released as amorphous ice transforms to crystalline ice, which could cause a runaway effect-catastrophic loss of a precious resource, at least locally. But all that is speculation at present. We need to conduct experiments on regolith with ice in it, with and without other gases. This needs to include understanding what happens when the regolith is gardened by the rain of tiny meteorites that continuously strike the Moon. Most important, we need to characterize the permanently shadowed places thoroughly from orbit and with robotic landers equipped with sophisticated analytical instruments. NASA is planning such missions during the next five years. A talk by James Powderly and colleagues at Honeybee Robotics (in collaboration with scientists at the Johnson Space Center, Ames Research Center, Jet Propulsion Laboratory, Boeing, the Army Corps of Engineers, and Los Alamos National Laboratories) highlighted one well-tested concept for sampling the icy, cryogenic regolith in polar regions. Called the Subsurface Analyzer and Sample Handler (SASH) it is basically a drill equipped with instruments to

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measure physical properties, and the concentration and distribution of water ice (or hydrogen) in the regolith, using a rover as the drilling platform.

Concept of SASH (Subsurface Analyzer and Sample Handler) operating from a rover on the lunar surface. The device can acquire cores and powdered samples from beneath the surface and deploy instruments down the holes to characterize the regolith, including detecting ice and hydrogen.

Exploring for Resources

My colleague at the University of Hawai'i, Jeff Gillis, discussed the general problem of exploring for resources using orbital remote sensing. He explained that an important step in exploration is to understand what you need and to search for those substances. Quoting Dr. Phil, Gillis noted, "In order to get what you want you have to know what it is you want." In lunar exploration terms this means that we must develop strategies for resource exploration and efficient methods of extraction. Remote sensing surveys (orbiting and roving spacecraft) will provide the ability to select a site for the first lunar base. Gillis showed the value of remote sensing measurements to determine the locations of numerous resources. Although only a few elements can be measured remotely, others can be inferred from the concentrations of just one element. A prominent example is the concentration of thorium, which has been measured by the Lunar Prospector mission. High thorium concentrations are correlated in lunar samples with the concentrations of potassium, rare earth elements, zirconium, phosphates, and others. Thorium is a tracer for a host of other potentially valuable resources.

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A large number of elements can be measured directly by remote sensing techniques or inferred from the concentrations of tracer elements or other properties. These correlations were established by cosmochemists in their analyses of lunar samples.

Richard Dissly (Ball Aerospace and Technologies Corp.) and colleagues from several university and government laboratories discussed the idea of prospecting for resources using an autonomous rover equipped with an array of sensors. The sensors would be mounted in the wheels, on the front of the rover, and on a mast on the rover. All would work in consort to identify resource targets, such as concentrations of ilmenite or volcanic glasses. The virtue of autonomy is that it increases the speed of mapping, lowers operational costs, and is safer than doing it all with humans exposed to the space environment. It is also applicable for exploration on both the Moon and Mars.

Rich Dissly's concept for an autonomous prospecting rover for use on the Moon and Mars.

Making Fake Moon Dirt

As mentioned above, permanent habitation and industrial use of the Moon and Mars will require that we live off the land-ISRU is essential. Developing techniques to excavate regolith, move it, separate its components, heat it, melt it, dissolve it, or even just pile it up will require a lot of experiments. Although Apollo astronauts returned 382 kilograms (840 pounds) of rock and regolith, the material is much too valuable to use for extensive experimentation. We need simulated lunar regolith-and lots of it. Seven participants at the Roundtable meeting indicated they would need more than 7 of 12

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10 tons, and two indicated that they would need more than 100 tons. Larry Taylor (University of Tennessee) reports that he knows of one company that expects to use 600 tons. From this somewhat limited sampling, it is apparent that there will be a near-term need for a quantity that could approach 1000 tons of lunar soil simulant. That's a lot of fake moon dirt. And it is not ordinary dirt. Larry Taylor reviewed the nature of the lunar regolith. It has some unique properties. Chief among them is the presence of glassy agglutinates, twisted, bubbly, swirly aggregations of rock and mineral fragments bound together by impact-produced glass. The glassy portions are decorated by a Milky Way, as Larry Taylor put it, of microscopic grains of metallic iron. The tiny metal grains are probably produced when hydrogen, implanted into the surface by the solar wind, reacts with iron oxide when the glass forms by micrometeorite impacts. Microscopic metal grains also form when impacts cause vaporization of surface materials. The environment is reducing, so extremely small metallic iron grains condense on the surfaces of mineral grains. This makes them magnetic.

Some of the unique properties of lunar regolith are shown here. Clockwise from the upper left: Agglutinates are glass-bonded aggregates of other regolith components. They contain countless microscopic grains of metallic iron (shown in the second image) formed by reduction of iron oxide present in the glass. The surfaces of mineral grains, even minerals that contain no iron, are coated with a mixture of silicon-rich glass and nanometer-sized grains of metallic iron (bottom image). It will not be easy to duplicate these properties.

The regolith has a distinctive distribution of grain sizes, high quantities of gases implanted by the solar wind (such as hydrogen and helium), crystals damaged by impact and radiation, and variable chemical composition. Some of these properties can be readily duplicated. Others will be difficult. Some might be impossible. Nevertheless, standardized simulants are needed so experiments in different laboratories can be compared readily to one another. Nevertheless, as Larry Taylor pointed out, one stimulant does not fit all. The chart below shows the types of uses of regolith simulants, what general type of property needs to be simulated, and a list of simulants that have been used. The "Xs" refer to the extent to which the property was successfully simulated--the greater the number the more accurately the property is matched by a stimulant. 8 of 12

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James Carter (University of Texas, Dallas) discussed the manufacturing of the widely used, but now used up, lunar stimulant JSC-1 (commissioned by the Johnson Space Center, JSC). He explained the value of simulants, the need for a standardized set of simulants, and described what properties of the regolith JSC-1 matched and did not match. It simulated the chemical composition of some Apollo soils, the types of minerals and glass present in the regolith, and the grain size distribution of the typical regolith. JSC-1 did not match the spectral properties in visible and near-infrared wavelengths, magnetic properties and the presence of microscopic metallic iron, the shapes of agglutinates, and the high contents of solar wind gases.

Lunar simulant JSC-1 was produced from a volcanic deposit located near Flagstaff, Arizona under the supervision of James Carter. Twenty-five tons of it were produced and distributed by the astromaterials curatorial laboratory at the Johnson Space Center, but none is available now. Industry will need up to 1000 tons of stimulant to develop the technology for ISRU leading to permanent habitation and industrialization of the Moon.

Most important, Carter introduced the concept of a "root stimulant," a basic mixture of minerals, rock fragments, and glassy objects. At least two of these are needed for the Moon: one with a chemical composition of a typical lunar maria and a second with a highlands composition. These basic root stimulants will be useful for many purposes, and specialized simulants (branches from the root) can be made from them. These might contain solar wind implanted gases, partly

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reduced iron, soil-ice mixtures (for the dark areas at the poles), vapor-coated grains, and many other features. Marshall Space Flight Center is organizing a workshop on lunar simulants January 24 -26, 2005 to discuss in detail the problem of making simulants, how much to make, what properties should be simulated by a root simulant, and many other issues.

ISRU on Mars

Partly because less work has been done on ISRU processes for Mars and partly because President Bush's Exploration Vision calls for going to the Moon first, there was not much discussion of ISRU on Mars. However, one particularly important issue was addressed in separate presentations given by Carol Stoker (NASA Ames Research Center) and Humbolt Mandell (Center for Space Research, University of Texas, Austin): the need for deep drilling on Mars. This is essential for scientific exploration and for resource extraction. Stoker made a compelling case for being able to drill a few hundred meters in places where liquid water might reside beneath the surface because of over pressurization of ground ice. There are hundreds of places where gullies emerge from the sides of craters. The existence of liquid water only a few hundred meters below the surrounding plains is an enticing target for human bases and for the search for existent life on Mars. We only have to drill. But drilling is not as simple as you might think, particularly if we do not use traditional lubricants, essential to prevent contamination that could compromise our search for life on Mars. The presentations by Stoker, Mandell, and Powerly (discussed above) all show that deep drilling is feasible, though some development is still necessary. The designs of drilling equipment are surprisingly similar, whether from Honeybee or from a design based on drilling technology developed by Baker Hughes, one of the foremost terrestrial drilling equipment companies. There was a strong consensus that deep drilling is an essential part of resource exploration and the search for life on Mars.

Painting of a concept for a Mars lander equipped with a drill.

Two talks centered on the exploration for other resources on Mars. A presentation by Chris Woodworth-Lynas and his colleagues at Guigné Space Systems Inc. (Golden, Colorado) focused on rover-based instruments to search for resources on Mars. This included apparatus to make seismic measurements, with the geophones (seismic receivers) made from Micro-Electro-Mechanical Systems (MEMS) and mounted in the wheels of the rover. Kim Kuhlman (Jet Propulsion Laboratory) and colleagues from JPL and NASA Langley Research Center described an updated version of a fascinating concept called Tumbleweed proposed in the late 1970s by Jacques Blamont (JPL and the University of Paris). The idea is to make inflatable, light-weight balls that would travel with the winds of Mars to sample a wide area and map the distribution of potential resources on the surface. Each Tumbleweed would carry a suite of instruments that would

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measure soil properties, water vapor, mineralogy, and chemical composition. Kuhlman's JPL colleagues, Alberto Behar and Jack Jones, have tested the concept in Greenland and Antarctica. Tumbleweeds would fill a gap between orbiters and rovers. They would go wherever the wind took them, searching for answers about Martian geology and resources--the answer, my friend, is blow'in in the wind.

Model of Tumbleweed, showing components inside the light-weight spherical prospector. It has been tested in Antarctica (inset photo) and Greenland.

Other Great Ideas

PSRD concentrates on discoveries in cosmochemistry and planetary geosciences, so this report highlights mostly those areas. However, readers will find a wealth of fascinating ideas in the abstracts and the presentations given at the Roundtable meeting. You'll find out about the mysteries of granular flow, the production of metallic parts by carbonyl vapor deposition, automated mining techniques, and more.

Next Steps

The assembled cosmochemists, remote sensing specialists, engineers, construction experts, business men and women, entrepreneurs, and other technical experts and dreamers at the Roundtable meeting discussed the need to include ISRU right from the start in planning human activities on the Moon. All of the needed materials and energy to create a self-sufficient lunar outpost are available there and their use is essential to attain self-sufficiency and prepare the way for affordable travel to Mars. At some level of development, if we plan well, a lunar outpost can achieve economic "takeoff," from which it then becomes possible to cut the umbilical from Earth and provide products to Earth and to ventures in cislunar space in free-market exchanges. When that stage of development has been reached, the ingenuity of humans on

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the Earth and in space will provide new businesses and products to improve standards of living on Earth. In the meantime, it will be a role of the Exploration Vision to blaze the technology trail and of NASA to investigate the mechanisms by which new private investment in space can be encouraged. In a white paper (pdf) written on behalf of the Space Resources Roundtable and based on the clear consensus at the meeting, Michael Duke (Colorado School of Mines) and I concluded: "We encourage NASA to establish a Space Resources Council that would be responsible for evaluating the political and economic framework in which space resources are developed and encouraging the development of the needed technology. Industry should play a role in the decision-making that must go into the integration of these important new technologies into NASA's exploration programs. Fulfilling the Exploration Vision requires that we invest immediately in the tools and techniques required to use space resources."

David, L. (2004) Mining the Moon, the Gateway to Mars, space.com. http://space.com/businesstechnology/technology/moon_mining_041110.html Duke, M. and Taylor, G. J. (2004) Statement of the Space Resources Roundtable, November 4, 2004. white paper (pdf). Space Resources Roundtable VI, LPI Contribution No. 1224, Lunar and Planetary Institute, Houston. Abstracts available online courtesy of the Lunar and Planetary Institute. Presentations available online courtesy of the Space Resources Roundtable. [Links open in new window.] Taylor, G. J. and Martel, L. M. V. (2003) Lunar prospecting. Adv. Space Resources, v. 31, p. 2403-2412. Taylor, L. A. (2004) The Need for Lunar Soil Simulants for ISRU Studies. white paper (pdf). Workshop on Production and Uses of Simulated Lunar Materials. (1991) Edited by D. S. McKay and J. D. Blacic, LPI Tech Report 91-01, Houston, Texas, 83p. References on lunar oxygen production: Allen, C. C., Graf, J. C., and McKay, D. S. (1994) Experimental reduction of lunar mare soil and volcanic glass. Journal of Geophysical Research, v. 99, p. 23173-23185. Allen, C. C., Morris, R. V., and McKay, D. S. (1996) Oxygen extraction from lunar soils and pyroclastic glass. Journal of Geophysical Research, v. 101, p. 26085-26095. Taylor, L. A. and Carrier, W. D., III (1993) Oxygen production on the Moon: An overview and evaluation. Resources of Near-Earth Space. Edited by J. S. Lewis, M. S. Matthews, and M. L. Guerrieri, Space Science Series, University of Arizona Press, p. 69.

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PSRD: Understanding Water on Mars

http://www.psrd.hawaii.edu/April01/waterFromRocks.html

posted April 24, 2001

Gullies and Canyons, Rocks and Experiments: The Mystery of Water on Mars Written by G. Jeffrey Taylor Hawai'i Institute of Geophysics and Planetology

Mars is wet. Or at least it was wet. Vast canyons, numerous gullies, and even possible ocean deposits attest to the presence of abundant water on the planet, but magmas do not seem to have contained much of it. The logical way to transport water to the surface of a planet is in magma that erupts to form volcanoes and lava flows. Where did the water come from if not from magmas? Where did it go? Why are the magmas apparently so dry? Two studies of Martian meteorites may provide answers to these questions. Both use a combination of analyses of meteorites and laboratory experiments. One study, led by Harry Y. (Hap) McSween of the University of Tennessee (UT) and coworkers from UT, the Massachusetts Institute of Technology (MIT), Oakridge National Laboratory, and the University of South Florida, measured the abundances of water-soluble trace elements in crystals in a Martian meteorite. They found the centers of the minerals, which formed first and at high pressure, had much more of these elements than the rims. McSween and coworkers cite data showing that the elements are highly soluble in the presence of very hot water. Experiments at MIT show that magma must have contained about 1.8 wt% H2O to crystallize the minerals observed in the Martian meteorite studied. The scientists conclude that magmas have delivered lots of water to the Martian surface. The other study, by Meenakshi (Mini) Wadhwa (Field Museum, Chicago) focused on the concentration of the element europium in six meteorites from Mars. By using experimental data obtained by Gordon McKay and Loan Le (Johnson Space Center), Wadhwa concludes that the magmas in which the meteorites formed experienced varying amounts of interaction with the crust of Mars, which must be oxidized. She suggests that the oxidation is due to chemical reactions of rock and water--probably the same water that carved the surface features on Mars. References: McSween Jr., Harry Y., Timothy L. Grove, Rachel C. F. Lentz, Jesse C. Dann, Astrid H. Holzheld, Lee R. Riciputi, and Jeffrey G. Ryan (2001) Geochemical evidence for magmatic water within Mars from pyroxenes in the Shergotty meteorite. Nature, vol. 409, p. 487-489. Wadhwa, Meenakshi (2001) Redox state of Mars' Upper Mantle and Crust from Eu anomalies in Shergottite pyroxenes. Science, vol. 291, p. 1527-1530.

A Surface Carved by Water

Mars orbiter missions have returned a dazzling array of pictures and other information about the Martian surface. We knew from the old Viking images that vast canyons and dendritic river systems dissected the planet's surface. There were hints of dried lake beds [see PSRD article: For a Cup of Water on Mars] and even a large northern ocean.

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High-resolution images from Mars Global Surveyor bolster those old interpretations. Using topographic data obtained by a laser altimeter (MOLA) onboard the spacecraft, James Head (Brown University) and coworkers showed that a large part of the northern hemisphere is a basin with a smooth floor. Although bone dry now, this region might once have been a Martian sea.

In the late 1980s, researchers looking at Viking images speculated that large bodies of water once filled the low, flat plains of the northern hemisphere on Mars. They cited distinctive landforms indicative of water-related erosional or depositional processes. Timothy Parker and colleagues at the Jet Propulsion Lab mapped boundary contacts between landforms in the northern lowland plains. In their 1989 paper published in Icarus, they outlined two contacts that are generally parallel to the southern boundary of the northern lowlands, and interpreted them as representing two separate highstands of a now vanished ocean. The color-coded map shown above is centered on the north pole and shows MOLA data of the topography of the northern hemisphere of Mars. Low elevations are shown in shades of blue. Black lines indicate positions of possible ancient shorelines labeled contact 1 and contact 2.

Using the MOLA topographic data and computer modeling techniques, Head and his colleagues created maps to show the northern lowlands after flooding. They observed where water would pond and how oceans might evolve with changing water depths. The three maps show MOLA elevation data, with black areas representing flooded regions. The map on the far left shows that a water depth of 500 meters would flood two distinct basins in the northern lowlands. At a water depth of 1000 meters, middle map, the basins connect. If enough water were poured back on Mars (a depth of 1490 meters), it would fill up right along the proposed contact 2, as shown in the third map.

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Other Mars Global Surveyor pictures show rock layers that look like layers of water-deposited sedimentary rocks on Earth. This indicates extensive regions covered with water at some time in the deep Martian past. Other images show young gullies that probably formed when water gushed from canyon walls relatively recently.

LEFT: These layers of rock were probably deposited by water on Mars. They look very similar to layers of sedimentary rock in many parts of the world, such as the western United States. The formation of sediment layers like these requires copious amounts of water, yet present day Mars is a barren desert. [More information on this image from Malin Space Science Systems.] RIGHT: Water gushed out of the ground to form these gullies on Mars. This indicates that the crust of Mars must contain fairly hefty pockets of water. [More information on this image from Malin Space Science Systems.]

It appears that Mars was once drastically different from the desert it is today. Where is all that water now? If it is in the crust, as water or bound to minerals such as clays or salts, the crust would be oxidizing. Mini Wadhwa shows that, sure enough, the crust is much more oxidizing than the mantle.

Two Rare Elements

Most of us do not spend a lot of time thinking about the elements europium and gadolinium. Geochemists ponder them often because these rare elements (and a bunch of others) are incredibly informative about how a planet's crust forms, interacts with the atmosphere and hydrosphere, and whether it is recycled into the underlying mantle. It's a case of better understanding through chemistry. Europium (Eu) and gadolinium (Gd) are both rare earth elements. They behave in predictable ways during geological processing. Europium has the added virtue of occurring in two different oxidation states, as doubly-charged (Eu2+) and triply-charged (Eu3+). Gadolinium is less chemically moody and remains as triply-charged Gd3+. The lucky thing is that Gd behaves almost exactly like Eu3+, so geochemists can figure out the amount of Eu in each valence state from the total amount of Eu and the amount of Gd. This is a big deal because the ratio of doubly to triply charged Eu is proportional to the amount of available oxygen--the oxidation state--of the environment in which a rock forms. So in principle, if you can figure out the ratio of doubly to triply charged europium, you can determine the oxidation conditions. Gordon McKay and his colleagues at the Johnson Space Center have spent years calibrating the relation between oxidation conditions and europium charge. They actually assess the way the ratio of europium to gadolinium changes with oxygen availability, assuming gadolinium behaves exactly like triply-charged europium. The oxidation conditions are expressed in terms of the oxygen fugacity, a measure of the availability of oxygen to react. This strange-sounding property is measured in tiny fractions of Earth's atmospheric pressure. Typical values are in the range of one billionth to one trillionth of an atmosphere.

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Because the numbers are so tiny, they are usually expressed as a logarithm and often compared to some kind of standard conditions such as the free oxygen associated with an assemblage of the minerals quartz (SiO 2), fayalite (Fe2SiO4), and magnetite (Fe3O4), or iron metal and iron oxide (FeO). Besides making the comparisons easier, it gives the experimentalists the opportunity to use sentences like, "The oxygen fugacity is plotted in log units relative to the IW buffer." IW refers to iron metal and wustite, another name for FeO. Use it a few times and you begin to think you know what it means. If there is more oxygen than present with iron and FeO, there will be no metallic iron present. You would get the quartz-fayalite-magnetite combination at an oxygen pressure over a thousand times higher (three log units) than when only iron metal and FeO are present. To determine how the ratio of europium to gadolinium varies with oxygen pressure, McKay and his coworkers use a high-temperature gas-mixing furnace. They hang a glassy experimental bead from a platinum wire loop, and place it in the hot part of a tube-shaped furnace. A thermocouple monitors the temperature to ensure it remains constant. Available oxygen (the oxygen fugacity) is controlled by passing a mixture of gases through the furnace. Typical mixtures use combinations of carbon dioxide, carbon monoxide, and hydrogen.

Ms. Loan Le suspends an experimental sample in a furnace at the Johnson Space Center.

LEFT: A close-up view of two experimental charges on platinum loops hanging from a white ceramic rod. The rod contains the thermocouple wires used to monitor the temperature inside the furnace. RIGHT: This is what you see if you look down a hot tube furnace: the thermocouple wires and round charges glowing in the hot furnace. The temperature is about 1100 oC.

The experimental charges are heated for a few days at a temperature low enough for minerals to crystallize from the molten beads. After heating, the charges are dropped from the furnace, cut up and polished, and the abundance of europium and gadolinium in the minerals is measured with an electron microprobe. McKay, Le, and their coworkers have completed a large set of experiments on charges with compositions similar to the magmas they wish to study and close enough to those of Martian meteorites to be useful. A few experiments with compositions identical to Martian meteorites confirmed that the experiments are relevant. They found that the ratio of europium to gadolinium in the mineral pyroxene (which is abundant in Martian meteorites) increases with increasing oxidation. This is important because knowing this ratio in a Martian meteorite allowed Mini Wadhwa to determine the oxidation state of Martian magma.

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Variation of the europium to gadolinium ratio versus oxidation conditions (expressed as deviation from conditions when both iron metal and iron oxide are present). The element ratio in the mineral pyroxene has been divided by the same ratio in the starting composition. This procedure gets rid of the effect of different starting concentrations of the elements. The more oxidizing the conditions (to the right on the x-axis), the higher the ratio of europium to gadolinium. If you know this ratio in a pyroxene crystal in a rock, you can determine the oxidation conditions under which the rock formed.

The Oxidation State of the Martian Crust

Mini Wadhwa is an expert on Martian meteorites. She is especially skilled in measuring the concentrations of elements present in tiny amounts, using a complicated tool called the ion microprobe. This device works by focusing a high-energy beam of ions (usually oxygen, argon, or cesium) onto a polished sample. The beam digs a hole while sputtering all the elements in the sample into a cloud of neutral atoms and ions. The ions are accelerated into a mass spectrometer, where they are magnetically separated by mass and then counted. The concentrations of elements in the sample are calculated by comparison with minerals of known composition. There are some tricky corrections to be made, but analysts have figured out how to make them.

Ion microprobe at Washington University in St. Louis, which Mini Wadhwa used to measure the concentrations of europium and gadolinium in Martian meteorites.

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Wadhwa concentrated on a group of Martian meteorites known as the shergottites. (The group was named after its first member, the Shergotty meteorite, which fell in India in 1865.) These originated as lava flows on Mars, and have not been modified much since they erupted, except for some slight weathering while on Mars and the trauma of being blasted off the planet. Wadhwa's goal was to measure the europium/gadolinium ratio in the pyroxene crystals inside the Shergotties, then use McKay's data to determine the oxidation state.

Colorized image of a polished slice of the Shergotty meteorite taken with a scanning electron microscope. Blues and greens show the pyroxene grains; yellow corresponds to plagioclase feldspar. Notice that the pyroxene grains are greenish in the interiors and bluish closer to the edges. This indicates that they are chemically zoned, which Hap McSween and his coworkers used to understand changes as the magma rose to the surface of Mars. The white spot indicated with a black arrow is an ion-beam hole (about 30 micrometers in diameter) on one of the areas Mini Wadhwa analyzed as part of her study.

Wadhwa studied seven shergottites. Their Eu/Gd ratios in pyroxene divided by that in the bulk rock sample ranged from 0.75 in Shergotty to 0.44 in Antarctic meteorite QUE 94201. These correspond to a large range in oxidation conditions, from about 100 times the iron metal-iron oxide level to about 0.1 that level.

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Eu/Gd ratios in pyroxene allowed Mini Wadhwa to determine the oxidation conditions experienced by the magmas from which each meteorite formed. To use the diagram you read the measured Eu/Gd and read across the diagram until that amount intersects the experimental curve determined by Gordon McKay and his colleagues. A line drawn straight down shows the deviation of the oxygen fugacity from that prevailing when the fugacity is controlled by the presence of iron metal and iron oxide.

Wadhwa figured out the oxidation state of the shergottite magmas, but she wondered whether it reflected different oxidation states in the mantle (where the magmas originated) or in the crust (which the magmas had to pass through). To answer this question, she used published analyses of the isotopic compositions of neodymium and strontium. The isotopes of these elements are used to determine rock ages, but they are equally useful in tracking geochemical processes. In this case, Wadhwa used the isotope ratios to assess the amount of contamination by the Martian crust experienced by the magma in which the shergottites formed. This is possible because a planet's crust has a different ratio of, for example, strontium-87 to strontium-86 than do the deeper rock layers in the mantle. Wadhwa found that the more oxidized a sample is, the higher its strontium-87/strontium-86 ratio. For example, the least oxidized sample, QUE 94201 (see diagram above), has a ratio of 0.701. In contrast, the most oxidized sample, Shergotty, has a strontium-87/strontium-86 ratio of 0.723. The other samples are in between. This suggests that Martian magmas are not very oxidized when they are made by melting in the mantle, but that they become more oxidized as they pass through the crust before erupting. Some become much more oxidized than others. Wadhwa suggests that the crust is oxidized by water circulating in the crust, forming water-bearing minerals. This circulating water may have flowed on the surface in the distant past to carve the valleys and canyons that decorate the Martian surface. How is the water transported to the surface? The work of Hap McSween and his coworkers suggests that Martian magma contained plenty of water that bubbled out as it neared the surface.

Wet Magma

The logical way to transport water to the surface of a planet is in magma that erupts to form volcanoes and lava flows. Near the surface any water dissolved in the magma escapes into the atmosphere because a decrease in pressure lowers the solubility of the water. An analogy is a can of soda which makes bubbles of carbon dioxide when pressure is reduced by popping the top. The problem with the Martian volcanic meteorites is that they contain very little water. However, Hap McSween and his coworkers show that at least one Martian meteorite, Shergotty, did contain lots of dissolved water. It lost it as its magma approached the surface. [Cartoon of loss of water in Shergotty.]

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McSween has two lines of evidence. First, his colleagues measured the abundances of trace elements such as titanium, zirconium, cerium, lithium, and boron in pyroxene mineral grains. As shown in the colorful image of Shergotty above, the pyroxenes are chemically zoned. This happens because the minerals begin to crystallize at depth (where the pressure is high) and continue to crystallize as the magma ascends (at progressively lower pressure). They measured the abundances of the trace elements with an ion microprobe at Oakridge National Laboratory. The set of elements they measured normally behave in a very predictable manner. None is incorporated readily into growing pyroxene grains, so as pyroxene crystallizes, the remaining magma ought to become enriched in those elements. This is the case for some of them, such as zirconium and titanium, as shown below.

As a magma crystallizes pyroxene, both titanium and zirconium increase in the remaining magma. This causes the edges of pyroxene crystals to have higher concentrations of both elements.

This normal behavior is not shown by boron, lithium, and cerium. Instead of increasing as crystallization proceeded, they decreased. McSween and his colleagues figured out that this seemingly perverse behavior is probably caused by the loss of substantial amounts of water from the magma. Experiments show that lithium and boron (and probably cerium, too) are quite soluble in hot water, implying that as water steamed from the magma as it neared the surface, the soluble elements followed the bubbling water. This explains why those elements are much lower in abundance on the outer portions of the pyroxene grains.

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Lithium (and also boron and cerium) decrease as pyroxene grains crystallized in Shergotty, rather than increasing as expected. This seemingly deviant behavior may have been caused by the loss of water from the magma because lithium and the other elements are quite soluble in hot water.

How much water was present in the Shergotty magma? To answer that question, Tim Grove, Jesse Dann, and Astrid Holzheid (MIT) did some experiments to figure out the conditions of formation of the two types of pyroxenes in the rock. They determined the nature of the pyroxenes formed under water-free and water-bearing conditions. Using their data and previously published experiments, they infer that the Shergotty magma contained 1.8 wt% H2O before it began to lose water. They suggest that the water was still dissolved at a depth of about 5 kilometers. McSween and colleagues suggest that the water in the Shergotty magma came from either the Martian mantle or was scavenged from a wet Martian crust, as Mini Wadhwa argues. Whatever the ultimate source for the water, McSween and his coworkers conclude that a substantial amount of water was delivered to the Martian surface by volcanic eruptions.

Water on Mars: An Interdisciplinary Problem

Deciphering the history of water on Mars is essential for understanding the geologic evolution of the surface, the extent to which the crust is recycled back into the mantle, and the chances that life originated on the planet. As befitting such an important problem, many types of studies are needed, including analyses of photographs, remote sensing data, in situ analyses by rovers, detailed analyses of samples, and a battery of supporting experimental and theoretical studies. McSween's and Wadhwa's studies use high-tech analyses of meteorites combined with experimental data to understand how water is delivered to the surface of Mars and how water might be distributed throughout the crust.

Head, J. W., III, H. Hiesinger, M. A. Ivanov, M. A. Kreslavsky, S. Pratt, and B. J. Thomson (1999) Possible ancient oceans on Mars: Evidence from Mars Orbiter Laser Altimeter Data. Science, vol. 286, p. 2134-2137. Martel, L. M. V. "For a Cup of Water on Mars." PSR Discoveries. Sept. 1998. . 9 of 10

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McKay, G., L. Le, J. Wagstaff, and G. Crozaz (1994) Experimental partitioning of rare earth elements and strontium: Constraints on petrogenesis and redox conditions during crystallization of Antarctic angrite Lewis Cliff 86010. Geochim. Cosmochim. Acta, 58, p. 2911-2919. McSween Jr., H. Y., T. L. Grove, R. C. F. Lentz, J. C. Dann, A. H. Holzheld, L. R. Riciputi, and J. G. Ryan (2001) Geochemical evidence for magmatic water within Mars from pyroxenes in the Shergotty meteorite. Nature, vol. 409, p. 487-489. Missions to Mars from Solar System Exploration, NASA Office of Space Science. MOC Images Suggest Recent Sources of Liquid Water on Mars. MOLA (Mars Orbiter Laser Altimeter) Science Investigation. Parker, T. J., R. S. Saunders, and D. M. Schneeberger (1989) Transitional morphology in West Deuteronilus Mensae, Mars: Implications for modification of the lowland/upland boundary. Icarus, vol. 82, p. 111-145. Wadhwa, M. (2001) Redox state of Mars' upper mantle and crust from Eu anomalies in Shergottite pyroxenes. Science, vol. 291, p. 1527-1530.

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PSRD Hot Idea: Regarding liquid water on Mars

http://www.psrd.hawaii.edu/May00/wetMars.html

posted May 24, 2000

Liquid Water on Mars: The Story from Meteorites Written by G. Jeffrey Taylor Hawai'i Institute of Geophysics and Planetology

Two studies shed light on the nature and timing of alteration by water of rocks from Mars. One is an experimental study of the

alteration of a rock similar to Martian meteorites, conducted by Leslie Baker, Deborah Agenbroad, and Scott Wood (University of Idaho). They exposed crushed pieces of terrestrial lava flows to water at 23 oC and 75 oC and normal atmospheric pressure, and to hot water at 200 oC to 400 oC and a pressure 1000 times normal atmospheric to see what minerals would form. On the basis of a detailed comparison between the experimental products and the Martian meteorites Baker and colleagues conclude that the rocks from which Martian meteorites derived were intermittently exposed to water or water vapor; they were not exposed for a long time to large volumes of water. In an independent study, a team led by Tim Swindle (University of Arizona) tried to determine the time of formation of a reddish-brown alteration product in the Martian meteorite Lafayette. This meteorite appears to have formed from magma 1.3 billion years ago, but the rusty-looking weathering product, a mixture of clay minerals, iron oxide, and iron hydride, formed long after the original rock had crystallized. Although the precise time is not pinned down, their measurements indicate formation during the past 650 million years. Taken together, these studies suggest that water flowed intermittently on the surface of Mars during the past 650 million years. References: Baker, L. L., Agenbroad, D. J., and Wood, S. A., 2000, Experimental hydrothermal alteration of a martian analog basalt: Implications for martian meteorites. Meteoritics and Planetary Science, vol. 35, p. 31-38. Swindle, T. D., Treiman, A. H., Lindstrom, D. J., Burkland, M. K., Cohen, B. A., Grier, J. A., Li, B., and Olson, E. K., 2000, Noble gases in iddingsite from the Lafayette meteorite: Evidence for liquid water on Mars in the last few hundred million years. Meteoritics and Planetary Science, vol. 35, p. 107-115.

Wet Mars and Altered Meteorites

Volcanoes and vast lava plains form much of the bedrock on the surface of Mars. No doubt other igneous rocks reside beneath the rusty surface. However, the ground is cut by vast dendritic networks and huge channels that appear to have been carved by flowing water, leading scientists to conclude that at least in the past, liquid water was abundant on Mars. [See, for example, the image at left of Nanedi Vallis.] There is even some evidence that an ocean once existed in the Northern Hemisphere of Mars. This clement period in Martian history is thought to have occurred over three billion years ago. Martian meteorites also show evidence that liquid water flowed through them; the evidence is in the form of alteration products (minerals that formed at low temperature after creation of the original rock). However, the Martian meteorites are relatively young: all but one is 1.3 billion years old, or younger. The alteration products had to have something to alter, so they must have formed during the past 1.3 billion years, long after the suspected wet and warm climate on Mars had changed to the dry, cold conditions found today. How recently did the alteration minerals form? How much water was required to make the alteration products? How hot was the water? These are the questions Baker, Swindle, and their colleagues are trying to answer.

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Martian meteorites possess an interesting, though complicated, array of alteration products. One type of Martian meteorite, the shergottites, are much like terrestrial basalts, and formed in lava flows. They consist mostly of the minerals plagioclase feldspar and pyroxene, but contain patches and veins of calcium carbonate and calcium sulfate, the alteration products. The nakhlites are quite different in composition. They are made mostly of pyroxene and olivine, with only a small amount of feldspar. Alteration products include calcium carbonate (like chalk) and calcium sulfate, sodium chloride (table salt), and iddingsite, a rust-colored mixture of clay minerals, iron oxide, and iron hydride.

Alteration products in two meteorites from Mars, Lafayette (left) and ALH 84001 (right). Lafayette has prominent occurrences of rusty-brown veins that are a mixture of clay minerals, iron oxide, and iron hydride. ALH 84001 is famous for its complicated globules of (brownish) carbonate minerals, in which the very controversial evidence for fossil life was found.

The only very ancient Martian meteorite is ALH 84001, which formed more than 4 billion years ago. It is composed almost entirely of pyroxene, but is decorated with tiny globules of carbonate minerals. The meteorite has been at the center of a controversy concerning the evidence for fossil life in it, and the carbonate globules are at the hub of that argument. Several ideas for the origin of the globules have been proposed, and estimates for the temperature at which they formed range from about 0 oC to 800 oC. Now there's a disagreement! It is also an opportunity to explore carbonate formation experimentally, and that is what Leslie Baker and her colleagues did.

Martian or Terrestrial Weathering?

There is absolutely no doubt that some of the alteration products formed on Mars. Allan Treiman (Lunar and Planetary Institute,

Houston) and his colleagues reported the most dramatic example in 1993. While studying the nakhlite Lafayette they found that the rusty alteration product was truncated by the fusion crust, the melted zone on the outside of a meteorite that formed when the rock blazed through the Earth's atmosphere. Furthermore, the rusty veins are depleted in sulfur, chlorine, and phosphorous near the fusion crust. These elements are easily lost by heating. These observations indicate that the rusty veins must have been present when the meteorite hit the Earth's atmosphere.

On the other hand, it is not clear that all alteration products formed on Mars. Most of the Martian meteorites have been found on Earth, rather than observed to fall and picked up immediately. They could have weathered on Earth. Even the observed falls might be weathered somewhat. Thus, several researchers are trying to develop criteria to distinguish Martian from terrestrial weathering. Nevertheless, some of the products clearly formed on Mars and they may contain information about the Martian climate.

Soaking Rocks

In a nutshell, Leslie Baker and her colleagues put pieces of rock into little buckets of water. They chose a basalt from the Columbia

River plateau for their experiments. The composition was not an exact match for Martian meteorites, but had similar levels of iron oxide, an important constituent when considering how weathering takes place. They crushed pieces of basalt and, for the high temperature experiments, ground it to a powder. The samples were then cleaned in an ultrasonic cleaner in dilute hydrochloric acid and rinsed in pure water to be sure pre-existing terrestrial weathering products were removed. Before placing a sample into the experimental apparatus, Baker sterilized it with hydrogen peroxide to prevent, as she writes, "an inadvertent life-on-Mars experiment." They ran two sets of experiments, one at low temperature (23 oC and 75 oC) and another at high temperature (200 oC and 400 oC). The

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low temperature experiments were done in an apparatus in which carbon dioxide gas (the main constituent of the thin Martian atmosphere) forced water through a sample container packed with the crushed basalt. The sample container was held at 23 oC or 75 oC by wrapping it in heating tape, sort of an electric blanket. The water that left the sample container was saved for analysis. The low-temperature experiments were run for four to seven days.

The diagram above shows the experimental set up used by Leslie Baker and her colleagues, and the photograph on the right is the actual apparatus. Carbon dioxide gas flows from the grey tank into a large bottle of water, which becomes saturated with carbon dioxide (starting fluid.) The water leaves the container and flows through the sample column, which contains crushed basalt. Wastewater flows down into the second bottle.

Research assistant Becca Carpenter prepares a sample holder for a new set of experiments.

The high temperature experiments were done in special pressure containers so the water would not change to steam and rupture the apparatus. The samples were placed in gold capsules and sealed to prevent leaks. The investigators made sure that carbon dioxide was also present in the system because it is the main constituent of the Martian atmosphere and must play a role in chemical reactions on the surface. The ratio of carbon dioxide to water varied from 1:3 to 3:1. The experiments ran for seven days.

Schematic diagram for the high temperature experiments. The samples, consisting of basalt powder, were placed in a sealed container with water and carbon dioxide, and heated for seven days.

After each experiment Baker and her colleagues examined the samples by electron microscopy and x-ray diffraction to identify the compounds produced. All the experiments produced an impressive assortment of carbonate minerals, including those containing calcium, iron, magnesium, and manganese. Several forms of pure silica dioxide were produced (opal, cristobalite, and quartz). Hematite (rusty iron oxide) appeared in many of the experiments, as did an array of hideously complicated water-bearing minerals with exotic names such as sacrofanite, vesuvianite, and sepiolite. Temperature had the most pronounced effect on the nature of the products produced in the experiments. The lowest temperature experiments (23 oC) generally produced only calcium carbonate, magnesium carbonate, and opal or cristobalite. The experiments at 75 oC produced all of those minerals plus iron carbonate, iron and magnesium oxides, quartz, and sacrofanite, one of the complicated

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water-bearing minerals. The high-temperature experiments formed these minerals in various combinations, plus an assortment of water-bearing minerals. Exactly which mineral formed depended on the ratio of carbon dioxide to water.

Martian Meteorites: Intermittent Wetting by a Little Bit of Water

Baker and co-workers compared the products of the experiments to the minerals found in ALH 84001 and the nakhlites. The ALH meteorite contains calcium-iron-magnesium carbonates, iron oxides, and traces of silica, but no water-bearing minerals. In contrast, the nakhlites are more extensively altered and contain water-bearing minerals such as clays and hydrated iron oxides. On the basis of the experiments, Baker concludes that the alteration products in ALH 84001 formed at different temperatures than those in the nakhlites. The experiments suggest that ALH 84001 did not interact with water for a long period. Similarly, long-duration interaction with hot water-poor vapor is also unlikely. Baker and colleagues note that this is consistent with Paul Warren's (University of California, Los Angeles) suggestion that the ALH 84001 parent rock was altered by intermittent soaking of the Martian surface by floods. However, the rapid rate of alteration observed in the experiments also suggests that the wet periods must have been fast, lasting only days rather than weeks or months. Baker also points out that the experiments do not rule out formation of the carbonates and oxides in ALH 84001 by very short exposure to high-temperature gases, as some other scientists have suggested. Baker and her colleagues also conclude that the nakhlites were not exposed to water for long periods, although they are more altered than ALH 84001. This is consistent with conclusions reached several years ago by James Gooding (Johnson Space Center) and Allan Treiman (Lunar and Planetary Institute). They suggested that the Lafayette meteorite (one of the three nakhlites) was altered by occasional interaction with small amounts of water. So, although altered more than ALH 84001, the nakhlites also did not sit around in either cool or hot water for a long time. Baker hopes that further experiments will be able to allow a more precise estimate of the amount of time and water involved.

When?

Tim Swindle and his coworkers from the University of Arizona, the Lunar and Planetary Institute, and the Johnson Space Center wanted to find out when the alteration of one of the nakhlites took place. They analyzed samples from the Lafayette meteorite, which contains the most alteration products of any Martian meteorite. Previous measurements of the age of the meteorite concentrated on the main minerals in the rock, which formed when the rock solidified in magma. Three different isotopic techniques (potassium-argon, rubidium-strontium, and samarium-neodymium) all gave an age of 1.3 billion years. The alteration products must have formed after that time, showing that water was present on the surface in small quantities during the past billion years.

Mass spectrometer used by Tim Swindle and colleagues at the University of Arizona to measure the ages of small samples of iddingsite.

Swindle used the potassium-argon technique to determine the age of iddingsite, the rusty mixture of minerals that decorates Lafayette. Allan Treiman carefully picked 25 tiny samples of iddingsite from the meteorite, weighing from 0.5 to 34 micrograms. Then David Lindstrom (Johnson Space Center) measured the amount of potassium in each sample. Finally, Swindle and his colleagues at the University of Arizona measured the argon in the samples, and converted all these measurements into ages. The fact that they could measure the amount of potassium and argon in such minuscule samples is remarkable, but such microanalytical techniques are now commonplace. [See PSRD article Analyzing Next to Nothing.]

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Age determinations are almost always tricky, but these were exceptionally so because of the presence of argon from the terrestrial atmosphere, from the Martian atmosphere, and produced by cosmic rays in space. They had to worry about argon loss from the iddingsite--on Mars, in space, during passage through the Earth's atmosphere, and even in the experimental apparatus. Swindle and his colleagues examined all these possibilities in detail, correcting for some, showing that others were insignificant. The ages of the samples ranged from 0 to about 650 million years. The authors could not determine if the ages represent a range in the formation time of iddingsite or times of alteration of an iddingsite formed earlier. However, because a previously-reported rubidium-strontium age of iddingsite was 679 (plus or minus 66) million years, they suspect that the formation of iddingsite occurred beginning about 650 million years ago and continued for several hundred million years. The old ages also provide further proof that the alteration took place on Mars because the Lafayette meteorite is a very recent arrival on Earth.

Implications for Mars

Mars experts agree that the wettest period in Martian history occurred early in its history, probably 3 to 4 billion years ago.

Nevertheless, these two studies of weathering products in Martian meteorites suggest that small amounts of water were available near the surface of Mars during the past several hundred million years. This might indicate that some process (volcanism, impact) melted permafrost in the upper few hundred meters (or less) of the surface, liberating the water needed to alter the nakhlites. ALH 84001 is much less altered than the nakhlites despite the fact that it is much older. This indicates that it remained dry for almost 4 billion years, suggesting that not all regions on Mars had the intermittent presence of water. Swindle and co-workers suggest that ALH 84001, which probably comes from the ancient highlands of Mars, remained drier because the depth to permafrost in the highlands is much greater than on the lower volcanic plains. Study of the weathering products in Martian meteorites is just beginning. Further study of the alteration products themselves, additional experiments, and refined age-dating measurements will help us understand the water/climate cycle on Mars. Baker and Swindle are collaborating on a series of experiments to determine how much argon and other noble gases can be incorporated into the alteration products as they form. Even more understanding will come when future missions land on Mars, make careful measurements, and return samples to Earth.

Baker, L. L., Agenbroad, D. J., and Wood, S. A., 2000, Experimental hydrothermal alteration of a martian analog basalt: Implications for martian meteorites. Meteoritics and Planetary Science, vol. 35, p. 31-38. Meteorites from Mars from Astromaterials Curation, Johnson Space Center. Swindle, T. D., Treiman, A. H., Lindstrom, D. J., Burkland, M. K., Cohen, B. A., Grier, J. A., Li, B., and Olson, E. K., 2000, Noble gases in iddingsite from the Lafayette meteorite: Evidence for liquid water on Mars in the last few hundred million years. Meteoritics and Planetary Science, vol. 35, p. 107-115. Treiman, A. H., Gooding, J. L., and Barrett, R. A., 1993, Preterrestrial aqueous alteration of the Lafayette (SNC) meteorite. Meteoritics and Planetary Science, vol 28, p. 86-97.

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PSRD: Show Me the Carbonates on Mars

http://www.psrd.hawaii.edu/Oct03/carbonatesMars.html

posted October 28, 2003

Show Me the Carbonates Written by Linda M.V. Martel Hawai'i Institute of Geophysics and Planetology --- Carbonate minerals intermingle with silicates in the Martian surface dust.

The Martian surface dust is 2 to 5 weight % carbonate minerals. Joshua Bandfield, Timothy Glotch, and Philip Christensen (Arizona State University) reported the result after examining Mars Global Surveyor Thermal Emission Spectrometer (TES) data from 21 high-albedo, dusty surfaces on Mars located between 30 oS and 15oN. Trace amounts of carbonates are widely distributed in the silicate-rich dust, but no evidence has been found in the TES data for widespread deposits of exposed carbonate rock. The small amount of detected carbonate is more consistent with the idea that Mars has long been cold and mostly dry rather than a place formerly warm and wet with a thick carbon dioxide atmosphere, and especially favorable for life. Reference: Bandfield, J. L., Glotch, T. D., and Christensen, P. R. (2003) Spectroscopic identification of carbonate minerals in the Martian dust. Science, v. 301, p. 1084-1087.

Why Look for Carbonates on Mars?

The motivation to search for carbonates on Mars is the mineral's relationship to water.

Carbonates form when carbon dioxide gas dissolves in water releasing negatively charged carbonate ions (anions, CO32-) that bind to a variety of positively charged ions (cations) such as calcium or magnesium. This means that carbonate minerals precipitate out of carbon dioxide-rich solutions; they form readily in the presence of water and a carbon dioxide atmosphere. If the hypotheses for a ancient thick carbon dioxide atmosphere and water on Mars are true, including an ancient northern ocean, widespread smaller standing bodies of water, and outflow channels (as depicted in the graphic on the right), then one line of evidence would be the presence of carbonate rocks.

Carbonates in Martian Meteorites

Cosmochemical studies of Martian meteorites give us a direct look at water-precipitated minerals including carbonates, salts, sulfates, and

clays of extraterrestrial origin (e.g. James Gooding, NASA Johnson Space Center and colleagues). This cosmochemical evidence indicates that water was chemically active on Mars for at least the time span represented by the radiometric ages of the meteorites, that is the past 200-1300 million years. But the Martian meteorites are not severely weathered. The alteration products found in these rocks suggest only intermittent contact with water on Mars [see PSRD article "Liquid Water on Mars: The Story from Meteorites."] Martian carbonate minerals stirred up a commotion in 1996 when a group of scientists from Johnson Space Center, Lockheed Martin, and three universities published a paper in Science called "Search for Past Life on Mars: Possible Relic Biogenic Activity in Martian Meteorite ALH84001." ALH 84001 was a slowly-cooled igneous rock in the Martian crust before it was excavated by an impact, altered by fluids, sent to 1 of 5

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Earth 13,000 years ago by another impact, and finally collected off the Antarctic ice in 1984. The researchers suggested that the meteorite contained evidence of potential fossils within the carbonate globules that were found in crushed zones and cracks in the rock. Fluids rich in carbon dioxide presumably flowed through cracks in the Martian rock depositing globules, plates, and veins of carbonate minerals. Whether these carbonate minerals formed by biologic origin or not and the temperature at which they formed are still issues of debate. [For background, see PSRD articles "Shocked Carbonates may Spell N-o L-i-f-e in Martian Meteorite ALH84001," "Low-temperature Origin of Carbonates Consistent with Life in ALH84001," and "Life on Mars?"] Simply and plainly, the existence of carbonates in Martian meteorites confirms the presence of the minerals on Mars.

Finding Carbonates on Mars Remotely

Carbonate minerals have unique absorptions in the near-infrared and thermal infrared spectral regions because of vibrations due to stretching and bending of the carbon-oxygen bonds. Examples of thermal infrared emission spectra of carbonate minerals collected in the laboratory are shown below next to a photo of the instrument used by the researchers.

LEFT: Spectra of laboratory samples. The absorption band between 1350 and 1580 cm-1 is diagnostic of carbonate minerals. Spectra are offset vertically for easier comparison. RIGHT: Spectrometer.

Laboratory spectra serve as the standards against which TES data of Mars are compared. Since these comparisons began, no areas of carbonate rocks have been found anywhere on Mars within the resolution limits of the instrument (e.g. areas a few tens of square kilometers.) But this year's new work on the TES spectra by Bandfield and colleagues resulted in the detection of carbonate minerals in the Martian surface dust. Josh Bandfield and Michael Smith (NASA Goddard Space Flight Center) developed a way to mathematically separate effects of the atmosphere from the surface in the TES data. Their work led to the first detailed spectrum of the dusty surface isolated from the interfering effects of atmospheric dust, water ice aerosols, carbon dioxide, and water vapor. Then Bandfield, Glotch, and Christensen examined 21 TES sequences from a variety of dusty regions between 30 oS and 15oN. Though the 21 sites were in different places there were no detectable variations in the shapes of the spectra, probably because the Martian wind is so effective in mixing and moving the dust over the entire globe.

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The spectrum of the Martian surface dust is shown in the upper curve on the graph shown on the left. To determine the compositions of the materials that can produce such a spectrum, Bandfield and coauthors mixed pure labradorite (calcium sodium aluminum silicate) dust separately with several different carbonate minerals: calcite, dolomite, siderite, and magnesite. Other non-carbonates were also mixed with the labradorite for comparison, including hematite, gypsum, and high silica glass. Bandfield and colleagues chose labradorite as the silicate mineral in their model because of its general similarity to the Martian high-albedo surface spectrum, its purity, and its easy availability. Of all the mineral mixtures tested, only the addition of carbonate minerals and gypsum had any effect on the spectrum of the fine-grained labradorite. Furthermore, the spectral shape between 1350 and 1580 cm-1 is unique to materials that contain carbonate minerals, and no other mixture matched the Martian spectrum at these wavelengths. Only the addition of magnesite to the labradorite produced a match with the Martian surface dust spectrum in both magnitude and shape at wavenumbers greater than 1300 cm-1. The researchers tested the effects of particle sizes of the laboratory samples on the resulting spectrum. They separated labradorite and magnesite into particle size fractions of 0-5, 0-10, 10-20, 20-30, and 30-41 microns. The magnesite dust was added to the labradorite dust in 0.5 weight % increments until the resulting spectrum matched the Martian surface dust spectrum at wavenumbers greater than 1300 cm-1. The 0 to 10 micron particle sizes were the most sensitive to the addition of small amounts of carbonate. They found that the Martian spectrum could be reproduced with at least 2 to 3 weight % magnesite. A tentative upper limit on the abundance of carbonates in the Martian surface dust is set at 5 weight % as suggested by other remote sensing research in the near- and mid-infrared spectral regions. In the final analysis, magnesite may not be a unique answer if future work shows that other carbonate mineral mixtures also match the Martian spectrum. Significantly, these new TES results demonstrate that where there is dust on Mars, there are carbonates in low concentrations. ABOVE: The TES spectrum of the Martian dust (top) matches well with the spectrum of a laboratory sample mixture (bottom) of 98 weight % labradorite silicate and 2 weight % magnesite carbonate (MgCO3) of particle sizes between 0-10 microns. The spectra have been offset vertically for comparison.

What a Trace of Carbonates Might Mean

The work by Bandfield and colleagues found that carbonate minerals at concentrations of <5 weight % are common in the Martian surface dust (see the image below for a typical dusty surface). Previous work also recognized carbonates (1 to 3% volume) in the airborne dust using thermal emission spectra of Mars from telescopes (e.g. the Kuiper Airborne Observatory data used by James Pollack and colleagues in 1990). Bits of carbonate minerals have been studied in Martian meteorites. The task is to figure out how these findings of carbonate minerals fit into the history of water and climate on Mars.

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Martian dust appears to have ubiquitous low levels of carbonate minerals yet the planet appears to lack carbonate cliffs or outcrops. Does this mean there were no oceans, lakes, or running water on the Red Planet? No, it's not that simple. Carbonates are simply expected as a natural product of weathering in a wet Martian environment, and there are good reasons for thinking that Mars was wet. A variety of surface features are attributed to ancient surface water (e.g. channels, outflow channels) and recent groundwater seepage or snow (gullies) even though water is not stable on the surface of Mars today. Subsurface water ice was detected last year by the Mars Odyssey suite of instruments (see PSRD article Dirty Ice on Mars). It is possible that the small amounts of carbonate minerals in the dust are erosional remnants of ancient carbonate source rocks whose formation served as a buffer for atmospheric CO2. These carbonate rock layers may simply be hidden now beneath layers of the silicate-rich dust. It's also possible that the carbonates were washed underground and are now too deep to be detected by TES. On the other hand, there are reasons favoring a colder, dryer Mars where extensive carbonate rock layers never formed. It is most probable, say Bandfield and coauthors, that the small amounts of carbonate minerals detected in the surface dust were not derived from carbonate outcrops but formed through the ages by simple reactions between the dust and moisture in the thin Martian atmosphere. Additionally, spectroscopic evidence for clays, which are known indicators of aqueous weathering, remains inconclusive. The questions of how carbonate minerals formed on Mars and what that means about the environment and climate in which they formed are still being answered. Ultimately, they'll lead us to answers about the duration of surface and near-surface water in Mars' past. In August, 2003 in reference to finding traces of carbonates and what that may mean for water on Mars, Phil Christensen, principal investigator for the TES instrument said, "Maybe instead of calling them oceans, we should call them glaciers. A frozen ocean will not form carbonate. I believe Mars has a lot of water, but it is cold and frozen most of the time. That is consistent with what we have seen."

Archived news release, August 22, 2003, from Arizona State University. Bandfield, J. L., Glotch, T. D., and Christensen, P. R. (2003) Spectroscopic identification of carbonate minerals in the Martian dust. Science, v. 301, p. 1084-1087. Bandfield, J. L. and Smith, M. D. (2003) Multiple emission angle surface-atmosphere separations of thermal emission spectrometer data. Icarus, v. 161, p. 47. Gooding, J. L. (1992) Soil mineralogy and chemistry on Mars: Possible clues from salts and clays in SNC meteorites. Icarus, v. 99, p. 28-41. Lane, M. D. and Christensen, P. R. (1997) Thermal infrared emission spectroscopy of anhydrous carbonates, Journal of Geophysical Research, v. 102, p. 25581-25592. Mars Meteorite Compendium-2003 McKay, D.S., Gibson, Jr. E.K., Thomas-Keprta, K.L., Vali, H., Romanek, C.S., Clemett, S.J., Chillier, X.D.F., Maechling, C.R. and Zare, R.N. (1996) Search for Past Life on Mars: Possible Relic Biogenic Activity in Martian Meteorite ALH84001, Science, v. 273, p. 924-930. Pollack, J. B, Roush, T., Witteborn, F., Bregman, J., Wooden, D., Stoker, C., Toon, O. B., Rank, D., Dalton, B., and Freedman, R. (1990) Thermal emission spectra of Mars (5.4-10.5 microns): evidence for sulfates, carbonates, and hydrates, Journal of Geophysical Research, v. 95 (B9), p. 14595-14627. Scott, E. R. D. (1997) Shocked Carbonates may Spell N-o L-i-f-e in Martian Meteorite ALH84001. Planetary Science Research Discoveries. http://www.psrd.hawaii.edu/May97/ShockedCarb.html. Taylor, G. J., (2002) Dirty Ice on Mars. Planetary Science Research Discoveries. http://www.psrd.hawaii.edu/June02/MarsGRSice.html. 4 of 5

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Taylor, G. J. (2000) Liquid Water on Mars: The Story from Meteorites. Planetary Science Research Discoveries. http://www.psrd.hawaii.edu/May00/wetMars.html. Taylor, G. J. (1997) Low-temperature Origin of Carbonates Consistent with Life in ALH84001. Planetary Science Research Discoveries. http://www.psrd.hawaii.edu/May97/LowTempCarb.html. Taylor, G. J. (1996) Life on Mars? Planetary Science Research Discoveries. http://www.psrd.hawaii.edu/Oct96/LifeonMars.html. Thermal Emission Spectrometer (TES) on Mars Global Surveyor (MGS).

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PSRD:Squeezing Meteorites to Reveal the Martian Mantle

http://www.psrd.hawaii.edu/Dec06/Y-980459.html

posted December 19, 2006

Squeezing Meteorites to Reveal the Martian Mantle --- Experiments at high temperature and pressure give clues to the composition of the interior of Mars.

Written by G. Jeffrey Taylor Hawai'i Institute of Geophysics and Planetology

A piece of a Martian lava flow, Antarctic meteorite Yamato-980459, appears to represent the composition of a magma produced by partial melting of the Martian interior. That's the view of researchers Don Musselwhite, Walter Kiefer, and Allan Treiman (Lunar and Planetary Institute, Houston) and Heather Dalton (Arizona State University). Musselwhite and his colleagues determined that this basaltic Martian meteorite represented a primary melt from the mantle. This was an important discovery because magma produced inside a planet contains significant clues to the composition of the region of the interior in which it formed. The lava flows that decorate the surface of planets tell us about the mantle, the rocky region beneath the crust and above the metallic core. The researchers used apparatus at the Johnson Space Center to determine what minerals are present when samples with the composition of Y-980459 are heated to a range of temperatures and squeezed to a range of pressures like those that planetary scientists expect to exist in the interior of Mars. The results indicate that the magma represented by this special meteorite formed at a depth of about 100 kilometers and a temperature of about 1540 oC. From the high temperature and high ratio of magnesium to iron in the magma, Musselwhite and his colleagues infer that the amount of melting to produce the Y-980459 parent magma was high, which suggests that the temperature at the boundary between the metallic core and the rocky mantle was higher than previous estimates. This work gives us clues to the composition and dynamics of the Martian interior--all from a rock chipped off a lava flow on Mars and flung to Earth by an impact. Reference: Musselwhite, D. S., H. A. Dalton, W. S. Kiefer, and A. H. Treiman (2006) Experimental petrology of the basaltic shergottite Yamato-980459: Implications for the thermal structure of the Martian mantle. Meteoritics and Planetary Science, v. 41, p.1271-1290.

A Special Rock from Mars

All meteorites from Mars are special, of course. Some, such as the shergottites, are pieces of more-or-less familiar lava flows. Others are bits of unusual lava flows, such as the nakhlites. One is ancient, ALH 84001, and represents an accumulation of the mineral orthopyroxene deep inside the crust of Mars, but later modified by 1 of 9

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water and possibly containing evidence for past life on Mars (see the first PSRD article: Life on Mars?). But Y-980459 has the added virtue of having the composition of a magma that has not been modified much since it formed by partial melting of the Martian interior a few hundred million years ago. The age of Y-980459 is 472 ± 47 million years based on Rb-Sr and Sm-Nd isotopic measurements. Samples of a partial melt of a planetary interior are like the Holy Grail. They were at one time in equilibrium with minerals in the interior, so they contain a record of the chemical and mineralogical composition of their place of formation. They are messengers from the interior. The shapes and sizes of the minerals in Y-980459 and the way the minerals are intergrown indicate that Y-980459 is a piece of a lava flow. It consists of large grains of olivine embedded in a matrix of smaller crystals, including lath-shaped plagioclase feldspar. There is no question that the minerals crystallized in a lava flow. Two views of a thin slice of Y-980459. TOP: Photomicrograph of a thin slice of the meteorite as viewed in polarized light in a microscope. The large grains are olivine. They are surrounded by a finer-grained intergrowth of plagioclase feldspar and pyroxene. The straight edges of the olivine suggest that they formed in a magma.

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BOTTOM: Map of the magnesium (Mg) concentration in the minerals in Y-980459, showing the same area as in the photograph on the top. Many of the grains have high Mg concentrations, indicative of olivine.

What Don Musselwhite and other experimentalists want is to know the composition of the magma (also called "melt" by experimental petrologists) produced by partial melting. The problem is that many magmas begin to crystallize and the early-formed minerals accumulate in some portions of the magma (see illustration below). Olivine is usually the first major mineral to crystallize, so often a lava flow will contain large olivine crystals dragged up with the magma from a storage area (magma chamber) beneath the ground. If olivine has accumulated, the chemical composition of the lava does not represent a melt because it has been modified by addition of crystals. In many cases, one mineral forms first as a magma begins to crystallize. Crystals of this mineral can accumulate inside the magma chamber. If a batch of the magma containing the extra crystals erupted, its total composition (crystals plus liquid magma) would not represent the composition of the original crystal-free magma. Y-980459, however, appears not to have accumulated extra olivine crystals, hence its chemical composition is the same as it was when it formed by partial melting of the Martian mantle.

There is a way to determine if olivine has been accumulated. If olivine crystallized directly from the lava it is found in, it will have the composition (specifically the ratio of magnesium to iron) predicted from experiments to form from a magma of the composition of the lava rock (large olivine crystals plus the finer-grained groundmass surrounding them) in which they occur. Many lava flows contain a olivine crystals with a large range of compositions, which indicate that they accumulated in the magma. Their presence affects the bulk chemical composition and the lava does not represent a pristine liquid derived from the mantle. The composition of Y-980459 has been measured by Japanese researchers E. Koizumi and his coworkers. Musselwhite's calculations indicate that the olivine crystals in Y-980459 would have formed from a melt with the composition of the meteorite. Thus, it is likely that Y-980459 represents an unmodified melt from the Martian mantle. It is a probe of the interior of Mars. 3 of 9

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Revealing Mantle Mineralogy

Once planetary scientists know that a rock is a primary melt from the interior of a planet, they want to find out as much as they can about the composition of the mantle rock in which the magma formed and the pressure (hence depth) at which it formed. To do this, Musselwhite and Dalton made a homogeneous glass with a chemical composition equal to that measured by other investigators for Y-980459. Making starting compositions for experiments is part of the art of experimental petrology. In this case, the investigators made a mixture of oxides and carbonates and ground them together in an agate mortar and pestle, with the powdery mixture immersed in acetone to prevent contamination and loss of the finest powder. The mixed powder was then melted in a high-temperature furnace and ground up again to ensure that it was homogeneous. Because multiple experiments were going to be done on the same starting material, it was essential that it be homogeneous. Experiments were done at high temperature and pressure in what PSRD calls a Squeeze-O-Matic. Experimental petrologists call this particular piece of gear a "Quickpress non-end-loaded piston cylinder apparatus." The sample was placed inside a graphite capsule that in turn was placed inside a pressure cell made of barium carbonate and magnesium oxide. This was squeezed to a pressure between 7 and 15 kilobars (one bar is the pressure of air at sea level) and heated to temperatures ranging from 1410 to 1615 oC. After holding the sample at a given pressure and temperature for up to 24 hours, it was quenched to room temperature first, then the pressure was released. Musselwhite and his colleagues took great care to ensure that the measurements of pressure and temperature were accurately calibrated, and include in their paper an extensive Appendix explaining their procedure. Products of the experiments were made into polished pieces and analyzed with an electron microprobe at the Johnson Space Center.

The Quickpress apparatus used in the experiments on Y-980459. This device generates very high pressures at high temperatures.

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Electron microscope image of a polished sample of an experimental run product. The dark material at the top is the graphite capsule the sample was placed in. The light gray is glass (which was molten during the high temperature and pressure at which the experiment was run), and the gray area contains olivine crystals. Electron microprobe analysis of the glass and olivine gives their chemical compositions.

A central purpose of the experiments was to determine the pressure and temperature at which the Y-980459 magma formed. But how do we figure that out? The experiments show what minerals are present at different compositions, what their compositions are, and what the composition of the co-existing melt is. At some pressure the experimental product might be olivine and melt (which is preserved as glass in the experimental samples). At another pressure and temperature pyroxene might be present with glass. This means that if the Y-980459 magma formed at those conditions the left over solid rock would contain just one mineral, olivine or pyroxene. This is unlikely. For a typical mantle rock inside a planet, it takes about 50% melting to leave just one mineral behind. Hot magmas are so mobile (low viscosity) that they readily move up in the planet when the amount of melting is as low as a few percent, and quite rapidly at 20%. Thus, experimental petrologists look for the pressure and temperature at which a melt co-exists with two or more minerals. For Y-980459 that pressure is 12 kilobars (plus or minus 0.5 kilobars), equivalent to a depth of about 100 kilometers inside Mars.

A quick reminder: When most substances melt they do not go from solid to liquid at a single temperature. Pure materials like ice do melt at a single temperature, but complex ones like rocks melt over a range of temperatures and the chemical composition of the molten material and the minerals present change as the amount of melting increases.

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PSRD:Squeezing Meteorites to Reveal the Martian Mantle

http://www.psrd.hawaii.edu/Dec06/Y-980459.html

The plot, above, shows results of the experiments done by Don Musselwhite and his colleagues. Above the uppermost diagonal line, a magma with the composition of Y-980459 would be completely molten; this line is called the "liquidus," which means that everything is molten above it. In the lefthand field labeled "olivine + melt" it would consist of olivine and molten silicate; this corresponds to conditions as the magma was nearing eruption at low pressure. At high pressure the magma consists of pyroxene and melt. There is a region in between where melt is accompanied by olivine and pyroxene. The liquidus at this point is the likely formation pressure and temperature of the Y-980459 magma, 12 kilobars (equivalent to a depth of 100 kilometers on Mars).

This approach to determining the depth of origin and mineralogy of the mantle was applied to basalt lava flows from the lunar maria, too. Investigators (mostly in the 1970s and 1980s) sought primary magmas like Musselwhite and his colleagues did for Martian meteorites. Experiments at high pressure and temperature showed that the pressure at which a melt co-existed with two or more solids varied, corresponding to depths of 100 to 400 kilometers.

Amount of Melting and Composition of the Mantle

The experiments suggest that when the Y-980459 magma formed the leftover solid minerals were olivine and low-calcium pyroxene. Neither contains much aluminum or calcium, so the amount of melting must have been high enough to completely dissolve minerals that contain those elements (plagioclase feldspar, oxides, or garnet). This implies that the amount of melting was at least 15-20%. The experiments tell us what the olivine composition was when the magma separated. It contained 86 mole percent forsterite (Mg2SiO4) and 14 mole percent fayalite (Fe2SiO4). This composition can be abbreviated as the Mg#, simply the ratio of magnesium to magnesium plus iron [Mg/(Mg+Fe)], in this case 0.86. This means that the leftover olivine in the mantle after the Y-980459 magma had migrated away had an Mg# of 0.86. Knowing the composition of the residual minerals is useful, but we really want to know the composition of the mantle before it melted. That gets us a step closer to figuring out the composition of the entire Martian mantle. Unfortunately, this cannot be determined uniquely from knowing the composition of the residual minerals because different amounts of melting give different calculated Mg#. Musselwhite and his colleagues calculated the Mg# of the initial, pre-melting Y-980459 mantle source rock as a function of the amount of melting. To do this, they used well-established geochemical equations that describe the partial melting process. As shown in the 6 of 9

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diagram below, the amount of melting could range from an infinitesimal amount (corresponding to Mg# of 0.86) to over 70% (Mg# of 0.75 in the original mantle rock).

This graph shows calculations of the Mg# of the mantle for different amounts of partial melting to produce Y-980459. The common view is that the average Martian mantle has a Mg# of about 0.75. If so, then formation of Y-980459 required over 70% melting, an unreasonably large value in a single event. In fact, if the amount of melting was much over 40% all the pyroxene would have been melted, leaving only olivine in the residual solid. Musselwhite and his colleagues suggest that the source in the mantle had already experienced one or more melting events before the one that produced the magma in which Y-980459 crystallized.

Percentages of melting higher than 50% are not reasonable, except in the early stages of a planet?s life when it could have been mostly molten. This means that either the Martian mantle has a higher Mg# than we think (say 0.80 instead of 0.75), or that multiple episodes of melt extraction occurred. Either is reasonable and we have no unique way of determining which is correct. It is possible that there was an initial large melting event, say the Martian magma ocean (see PSRD article: A Primordial and Complicated Ocean of Magma on Mars), resulting in formation of a region with relatively high Mg#. Subsequent melting could have formed the Y-980459 magma, possibly after an intermediate melting event or events that produced other, unsampled magmas.

Thermal State of the Interior

Results of these experiments and calculations have implications for the temperature inside Mars as a function of time. Co-author Walter Kiefer used a complex geophysical computer model to estimate temperature inside Mars, from the core-mantle boundary to the base of the lithosphere (the upper, relatively cool, rigid part of the planet that extends from the surface to a depth of about 200 kilometers). In this case he updated the approach he took previously, taking into account the high temperature reached to melt the mantle that gave rise to Y-980459, and including some other tricks of the geophysics trade. The fact is that these calculations are both informative and uncertain. The uncertainty stems from the large number of variables that must be accounted for, including temperature, viscosity of the solid mantle, the volume of magma produced over time and over specific time periods, the percentage of melting, how many melting episodes, to name a few.

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http://www.psrd.hawaii.edu/Dec06/Y-980459.html

This shows the temperature inside Mars at about 280 million years ago (the middle of the range in ages of the shergottie Martian basalts), using Walter Kiefer's previous calculations. The central hot feature (red) is a plume of hot mantle rock that flowed upwards from the core-mantle boundary. When shallow enough, it began to melt, indicated by the white region. The updated calculations are similar, but tying them to the high melting temperature needed to make Y-980459 raises the temperature at the core-mantle boundary. ("Potential temperature" means the physical temperature minus the effects that pressure has on temperature caused by the work done to squeeze solid rock.)

The new calculations suggest that the temperature at the boundary between the mantle and the core is hotter than calculated previously. This might be caused by the core containing less sulfur than thought, a higher viscosity than used in the calculations, or by a high concentration of radioactive elements (potassium, thorium, and uranium) at the base of the mantle. These elements are important because they release heat when they decay, raising the local temperature. The last hypothesis may be most likely and is consistent with geophysical models of what could have happened in a Martian magma ocean. This is itself a very complicated process, but it seems likely that it would have overturned, depositing dense rock at its base. Some of the dense minerals, such as garnet, would have concentrated thorium and uranium. See PSRD article: A Primordial and Complicated Ocean of Magma on Mars for details.

Just the Beginning . . .

Identifying a primary melt from the mantle of Mars is a another step on the road to unraveling the detailed composition of the Martian interior. The shergottite Martian meteorites have already told a complicated story of the mantle (see PSRD article: The Multifarious Martian Mantle). Nevertheless, we need more samples that represent magma compositions, and preferably magmas that represent formation over a range of time back to more than four billion years ago. To find them we need more samples. The search for Antarctic meteorites and meteorite finds in desert regions on Earth are helping fill in gaps, but they are biased towards younger samples. Returning samples from volcanic areas and the ancient Martian surface would help enormously. There might even be mantle rocks excavated when huge basins such as Hellas (2500 kilometers in diameter) formed. The sample returns could be supplemented by installation of a geophysical network that could use Mars quakes to probe the interior. The chemical composition of Mars is an important piece of the puzzle of planet formation, and Martian samples help us put those pieces together.

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LINKS OPEN IN A NEW WINDOW.

Kiefer, W. S. (2003) Melting in the Martian mantle: Shergottite formation and implications for present-day mantle convection on Mars.Meteoritics and Planetary Science, v. 38, p. 1815-1832. Musselwhite, D. S., H. A. Dalton, W. S. Kiefer, and A. H. Treiman (2006) Experimental petrology of the basaltic shergottite Yamato-980459: Implications for the thermal structure of the Martian mantle. Meteoritics and Planetary Science, v. 41, p.1271-1290. Taylor, G.J. (1996) Life on Mars? Planetary Science Research Discoveries. http://www.psrd.hawaii.edu/Oct96/LifeonMars.html Taylor, G.J. (2004) The Multifarious Martian Mantle. Planetary Science Research Discoveries. http://www.psrd.hawaii.edu/June04/martianMantle.html Taylor, G.J. (2006) A Primordial and Complicated Ocean of Magma on Mars. Planetary Science Research Discoveries. http://www.psrd.hawaii.edu/Mar06/mars_magmaOcean.html

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PSR Discoveries:Feature: Effects of gamma rays on rocks

http://www.psrd.hawaii.edu/Dec99/gammaRays.html

posted December 17, 1999

Zapping Mars Rocks with Gamma Rays Written by G. Jeffrey Taylor Hawai'i Institute of Geophysics and Planetology

Because we do not know what deadly microorganisms might be lurking inside samples returned from Mars, the samples will either have to be sterilized before release or kept in isolation until biological studies declare them safe. One way to execute microorganisms is with radiation, such as gamma rays. Although quite effective in snuffing out bacteria and viruses, gamma rays might also affect the mineralogical, chemical, and isotopic compositions of the zapped rocks and soils. Carl Allen (Lockheed Martin Space Operations, Houston) and a team of 18 other analysts tested the effect of gamma rays on rock and mineral samples like those we expect on Mars. Except for some darkening of some minerals, high doses of gamma rays had no significant effect on the rocks, making gamma radiation a feasible option for sterilizing samples returned from Mars. Reference: Allen, Carlton C. and 18 others, 1999, Effects of sterilizing doses of gamma radiation on Mars analog rocks and minerals. Journal of Geophysical Research, v. 104, p. 27,043-27,066.

The Possibility of Tiny, Hostile Extraterrestrials

The Andromeda Strain by Michael Crichton is a scary and, some people fear, realistic story about a dangerous disease that fell to Earth from a satellite in space. If life exists on Mars, the samples we expect to return during the next decade might be teeming with microorganisms, some of which might be out to get us here on Earth. Although we have samples of Mars already (Martian meteorites) with no apparent ill effects, the fear and the possibility are still there, so the samples will be treated as if they were biohazards. The Space Studies Board, an arm of the National Academy of Sciences, studied the issue and stated in a report that, "Controlled distribution of unsterilized materials from Mars should occur only if rigorous analyses determine that the materials do not constitute a biological hazard. If any portion of the sample is removed from containment prior to completion of these analyses it should first be sterilized." The trouble with biohazards is that they have to be studied inside a containment facility that ensures that the organisms cannot escape into the surrounding environment. This means that all studies of the non-biological nature of the samples will have to wait until the samples are deemed safe by a team of biologists, take place inside the containment facility, or use sterilized samples. Waiting would be torture for scientists who have waited for years to study samples returned from Mars, and would deprive the public of the excitement of their discoveries. Doing all the work inside the special laboratory would limit the number of scientists involved to a small, elite (and possibly elitist) group. The samples should be studied by the best scientists with the best equipment all over the world. Besides, some equipment is too 1 of 6

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large to be housed inside the containment facility. Sterilization of part of each sample is an interesting alternative, but it needs to be done without changing the nature of the samples. Dry or steam heating the samples can alter the structures of some minerals and result in loss of Martian gases. Exposure to ultraviolet light or plasmas would sterilize only the surfaces of the samples, leaving the interiors potentially crawling with Andromeda-strain-like organisms. Gamma rays might do the trick with minimal damage, but no studies have been done until now.

Effectiveness of Gamma Rays as Germ Killers

Radiation causes ionization inside tissues, which damages cells, including their DNA. A common measure of the amount of radiation is the rad (radiation absorbed dose). One rad is the amount of radiation delivered by about ten chest x-rays. Studies have shown that some bacteria are more resistant to radiation than others. The record holder for radiation resistance is deinococcus radiodurans, which requires more than one million rads to decrease its concentration in a culture to 0.1% of its starting value. Increasing the dose by a factor of ten increases the effectiveness by about a factor of 8000. Common bacteria, such as Escherichia coli, require about 100,000 rads to knock off. Viruses are rendered inactive by about a million rads. Even the feared Ebola, Lassa, and Marburg viruses are deactivated with 700,000 rads. So, assuming Martian organisms have the same resistance to gamma radiation, doses of a million or more rads should sterilize a sample, though caution might call for much larger doses, perhaps 10 million rads.

The Experiments

To cover the range in which bacteria and viruses are killed, Carl Allen zapped samples with four different levels of radiation: 300,000 rads, 3,000,000 rads, 30,000,000 rads, and 100,000,000 rads. He used the High Dose Research Irradiator at the Centers for Disease Control and Prevention (located in Atlanta). This is a six-foot tall box lined with lead (see photo on left). Allen used the radioactive isotope cobalt-60 to supply gamma rays, which produced 31,500 rads per minute. (Lucky that box is lined with lead!) Samples were exposed inside the chamber for as short as 9 minutes to as long as 53 hours. The samples used in the experiments represented a variety of geological materials expected to be found on Mars: rock (basalt, carbonaceous chondrite, chert), minerals (quartz, feldspar, olivine, pyroxene, clay minerals, halite, aragonite, gypsum), and a simulated Mars soil. Both before and after irradiation, these materials were analyzed by numerous techniques. These measured chemical compositions, the atomic structures of the minerals, and optical properties. The techniques and results are briefly outlined below. (Allen's team did not study the effect of gamma rays on organic compounds or on bacteria in their samples. However, such studies would be done on non-sterilized Martian samples inside the containment laboratory as part of the biological assessment.)

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Gamma ray spectroscopy: This technique searched for radioactivity induced by the gamma rays. Radioactivity levels were the same before and after the samples were irradiated.

Isotopic analysis and age dating: Age dating is a crucial geological measurement, but it requires that processes other than radioactive decay do not affect the abundances of the isotopes. Rubidium and strontium are crucial elements for dating old rocks, so the team tested the effect of gamma rays on rubidium and strontium isotopes. Analysis of the basalt sample indicated that these isotopes were not affected by the irradiation.

Instrumental neutron activiation analysis: This is a well established technique for measuring the abundances of both major (abundant) and trace elements in rocks and minerals. In this method, samples are exposed to radiation in the form of neutrons. The neutrons cause numerous isotopic changes and the formation of short-lived radioactive isotopes. These decay and emit gamma rays, which are counted. Comparison to a rock of known composition establishes the composition of the sample. The gamma rays used for sterilization might interfere by making some elements radioactive before the neutron irradiation. Fortunately, neutron activation analysis of the basalt and simulated Martian soil showed that there was no effect from the gamma irradiation. The concentrations of twenty-seven elements were the same within analytical uncertainty before and after irradiation.

Inductively Coupled Plasma Emission Spectrometry: In this analytical method samples are dissolved in solutions and aspirated into a plasma (a cloud of hot, ionized gases) to determine their concentrations. This excites the atoms in the samples, and the atoms emit light when they return to their initial state. The light is characteristic of each element and the amount of light is measured with a special detector. The analyses were not affected by the gamma irradiation: all elements fell within experimental uncertainty of their pre-irradiation values.

X-ray diffraction: This is a classic technique to characterize the crystal structure of a mineral. It uses the fact that x-rays bounce off planes of atoms inside a crystal. By varying the angle between the sample and a x-ray detector, peaks in x-ray intensity occur. The peaks correspond to the spacing between planes of atoms in the crystal. The minerals were not affected by the gamma irradiation. No new x-ray peaks were observed and the observed peaks were in their correct positions.

Thermal emission spectroscopy: Minerals emit radiation in the infrared, with characteristic peaks and valleys on plots of emission intensity versus wavelength. This is a useful technique that can identify minerals and estimate some physical properties such as grain size, although it is not routinely used in the laboratory to characterize rocks. Its main use is in remote sensing. In fact, a thermal emission spectrometer is onboard the Mars Global Surveyor spacecraft, trying to determine the mineralogy of the Martian surface. The minerals tested showed no difference in the positions of the peaks and valleys between the irradiated and unirradiated samples, except for halite (sodium chloride). The halite sample used contains tiny inclusions of other minerals, and variations in the abundance of these might cause the small differences observed. The authors conclude that the differences are so small that they would not prevent identification of halite in irradiated samples.

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Right: The Mars Global Surveyor spacecraft is mapping the surface of Mars. The instrument payload includes a thermal emission spectrometer, built by a team headed by Phil Christensen at Arizona State University.

Raman spectroscopy: This is an analytical technique for identifying molecules in gases, liquids, and solids. It involves the scattering of laser light. When a laser is shined on a mineral, molecules making up the mineral scatter the light. Most is scattered at the same wavelength (hence the same energy), but some is scattered at a different wavelength. This is called Raman scatter. The energy difference between the incident light and the Raman scattered light is called the Raman shift. Different minerals have characteristic sets of Raman shifts and intensities of the effect, thus allowing identification of a mineral. Allen and coworkers found no differences in the Raman shifts or their intensities between unirradiated and irradiated samples.

Specific surface area: An interesting property of soils is the total surface area of the individual minerals in them, expressed as square meters per gram of soil. This property affects the way fluids and a soil or other particulate materials adsorb gases. Specific surface areas were not affected by the irradiation. For example, the specific surface area of the Mars soil simulant remained about 140 m2/g, even after irradiation with 30,000,000 rads. (The amazing thing is how large the surface area is of all the small grains in a soil. It is not easy to visualize that a thimble full of soil has a total internal surface area of over 100 square meters.)

Fluid inclusions: When a crystal form in the presence of a fluid (gas, liquid, or magma) it may trap some of the fluid as it grows. Study of the trapped fluid sheds light on the environment in which the mineral formed. The analysis involves heating samples to determine the temperature at which gas bubbles appear or dissolve, and at which the inclusion becomes homogeneous. The homogenization temperatures of fluid inclusions in quartz did not change after irradiation, so critical information was not lost.

Visible and near infrared reflectance spectroscopy: When light shines on a mineral, light at some wavelengths is absorbed by the mineral while light at other wavelengths is not. This is why minerals have color. The wavelengths at which the light is absorbed is effectively a fingerprint of each mineral, so the technique can be used to identify minerals. This is not commonly used in laboratories to identify minerals, but, like thermal emission spectroscopy, it is used in remote sensing observations of planets. Irradiation with gamma rays greatly affected the color properties of the minerals. The quartz, for example, was initially completely colorless. After exposure to 30,000,000 rads it was so dark that it was almost opaque. The darkening was not uniform throughout the quartz crystals. Instead, it concentrated in bands parallel to crystal faces. There were higher aluminum concentrations in the bands (250 parts per million) compared to the remainder of each crystal (about 100 parts per million), so the amount of darkening depends on the abundances of impurities inside the quartz. The same interaction of radiation with aluminum produces naturally-occurring smoky quartz.

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Left: This quartz crystal was originally clear, but is nearly opapue after being zapped by 30,000,000 rads. Crystal is 2.5 cm across.

Right:The darkening of the quartz crystals occurs in bands inside the crystal, as shown in this cross section. The dark specs are fluid inclusions. Crystal is 2.5 cm across.

Halite turned blue with irradiation. This is a common effect, and was observed in the purple and blue halite discovered recently in meteorites. [See PSRD article, "Purple Salt and Tiny Drops of Water in Meteorites".] On the other hand, carbonate minerals became brighter with irradiation. The positions of the main absorption bands did not change in any of the minerals studied.

Thermoluminescence: Light can be emitted from minerals when they are heated. In this case, the investigators heated samples from room temperature to 500 oC. As the sample is heated at a uniform rate, the amount of light emitted increases to a maximum value, then begins to decline. The temperature of the maximum emission provides information about the structural state of a mineral. After samples are heated, they are given a dose of radiation from strontium-90, and measured again. The amount of light emitted at the peak temperature is compared to the initial amount. This parameter is called thermoluminescence sensitivity. Quartz and feldspar showed significant changes in thermoluminescence sensitivity with increasing dose of radiation, but not in the peak temperature. Other minerals were not significantly affected by the gamma irradiation.

What It Means for Martian Samples

NASA is planning two sample-return missions to Mars during the next ten years. The returned samples will contain cores and soil collected by a rover, as well as core material collected by a drill on the lander. The goals of the missions are to search for evidence that the conditions for life existed on Mars (especially standing water), the raw ingredients for life (organic compounds), and for evidence for life (fossils, diagnostic organic compounds, minerals produced by organisms). Sample return mission will also provide rocks and soil for inorganic analysis. Landing sites will be chosen to maximize the chances for finding life and studying the history of the Martian climate.

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Rocks and soil, like those at the Pathfinder landing site shown above, will be collected by future sample-return missions to Mars. Some might harbor microscopic organisms. These will be of intense scientific interest, but there is a small chance that they could threaten life on Earth if released into the terrestrial environment. The samples will be studied in a special biological laboratory to search for Martian organisms. This will take months. However, some samples could be released soon after return to Earth for non-biological studies if the samples were sterilized. Carl Allen and his colleagues have shown that very high doses of gamma radiation, known to be quite effective at snuffing out terrestrial bacteria and viruses, will not harm most of the studies planned for the returned samples. If biologists conclude that gamma radiation is a safe way to sterilize Martian samples, then work on the chemical and mineralogical compositions and ages of the samples could begin within weeks after the samples were returned to earth. Allen notes that their studies were done with cobalt-60. Radioactive elements that emit gamma rays with much higher energy than those spewed out by cobalt-60 might cause more alteration than they observed in their experiments. Thus, their conclusions apply only to radiation like that emitted from cobalt-60. Fortunately, sterilization using cobalt-60 is a standard practice. Gamma sterilizers are available commercially and are a proven technology.

Allen, Carlton C. and 18 others, 1999, Effects of sterilizing doses of gamma radiation on Mars analog rocks and minerals. Journal of Geophysical Research, v. 104, p. 27,043-27,066. Athena (Mars Rover) homepage at Cornell University. Space Studies Board, 1997, Mars Sample Return Issues and Recommendations, National Research Council, Washington, DC. on-line from the National Academy of Sciences. Thermal Emission Spectrometer on Mars Global Surveyor: homepage at Arizona State University.

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