Today’s Presentation
23 January 2008
Recent Lunar Missions •Clementine, Lunar Prospector Missions •Recommendations for future Lunar Explorations
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Lithophile elements: An element that is concentrated in the earth’s silicate crust and mantle rather than in its core, therefore they are mostly associated with silicates rather than with metals and sulfides in the core. Examples: Oxygen, Calcium, Lithium, Sodium, Potassium etc. Siderophile elements: An element that has relatively weak affinity for oxygen or sulfur and that is readily soluble in molten iron. It is concentrated in iron meteorites and presumably in earth’s inner core. Example: Gold, Cobalt, Iron, Nickel, Platinum etc.
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Element distribution among some of the major subdivisions of the Earth* Element
Continental crust
Oceanic crust
Upper mantle
Core†
Oxygen
45.3
43.6
44.2
—
Silicon
26.7
23.1
21.0
—
Aluminium
8.39
8.47
1.75
—
Iron
7.04
8.16
6.22
85.5
Calcium
5.27
8.08
1.86
—
Magnesium
3.19
4.64
Sodium
2.29
2.08
0.25
—
Potassium
0.91
0.13
0.02
—
Titanium
0.68
1.12
0.11
—
Nickel
0.011
0.014
0.20
5.5
Sulfur
NA
NA
NA
9.0
24.0
—
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Top Ten Scientific Discoveries Made During Apollo Exploration of the Moon (NASA) The Moon is not a primordial object; it is an evolved terrestrial planet with internal zoning similar to that of Earth. Before Apollo, the state of the Moon was a subject of almost unlimited speculation. We now know that the Moon is made of rocky material that has been variously melted, erupted through volcanoes, and crushed by meteorite impacts. The Moon possesses a thick crust (60 km), a fairly uniform lithosphere (60-1000 km), and a partly liquid asthenosphere (1000-1740 km); a small iron core at the bottom of the asthenosphere is possible but unconfirmed. Some rocks give hints for ancient magnetic fields although no planetary field exists today. The Moon is ancient and still preserves an early history (the first billion years) that must be common to all terrestrial planets. The extensive record of meteorite craters on the Moon, when calibrated using absolute ages of rock samples, provides a key for unravelling time scales for the geologic evolution of Mercury, Venus, and Mars based on their individual crater records. Photogeologic interpretation of other planets is based largely on lessons learned from the Moon. Before Apollo, however, the origin of lunar impact craters was not fully understood and the origin of similar craters on Earth was highly 4 debated.
The youngest Moon rocks are virtually as old as the oldest Earth rocks. The earliest processes and events that probably affected both planetary bodies can now only be found on the Moon. Moon rock ages range from about 3.2 billion years in the maria (dark, low basins) to nearly 4.6 billion years in the terrae (light, rugged highlands). Active geologic forces, including plate tectonics and erosion, continuously repave the oldest surfaces on Earth whereas old surfaces persist with little disturbance on the Moon. The Moon and Earth are genetically related and formed from different proportions of a common reservoir of materials. The distinctively similar oxygen isotopic compositions of Moon rocks and Earth rocks clearly show common ancestry. Relative to Earth, however, the Moon was highly depleted in iron and in volatile elements that are needed to form atmospheric gases and water. The Moon is lifeless; it contains no living organisms, fossils, or native organic compounds. Extensive testing revealed no evidence for life, past or present, among the lunar samples. Even non-biological organic compounds are amazingly absent; traces can be attributed to contamination by meteorites. 5
All Moon rocks originated through high-temperature processes with little or no involvement with water. They are roughly divisible into three types: basalts, anorthosites, and breccias. Basalts are dark lava rocks that fill mare basins; they generally resemble, but are much older than, lavas that comprise the oceanic crust of Earth. Anorthosites are light rocks that form the ancient highlands; they generally resemble, but are much older than, the most ancient rocks on Earth. Breccias are composite rocks formed from all other rock types through crushing, mixing, and sintering during meteorite impacts. The Moon has no sandstones, shales, or limestones such as testify to the importance of water-borne processes on Earth. Early in its history, the Moon was melted to great depths to form a "magma ocean." The lunar highlands contain the remnants of early, low density rocks that floated to the surface of the magma ocean. The lunar highlands were formed about 4.4-4.6 billion years ago by flotation of an early, feldspar-rich crust on a magma ocean that covered the Moon to a depth of many tens of kilometers or more. Innumerable meteorite impacts through geologic time reduced much of the ancient crust to arcuate mountain ranges between basins.
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The lunar magma ocean was followed by a series of huge asteroid impacts that created basins which were later filled by lava flows. The large, dark basins such as Mare Imbrium are gigantic impact craters, formed early in lunar history, that were later filled by lava flows about 3.2-3.9 billion years ago. Lunar volcanism occurred mostly as lava floods that spread horizontally; volcanic fire fountains produced deposits of orange and emerald-green glass beads. The Moon is slightly asymmetrical in bulk form, possibly as a consequence of its evolution under Earth's gravitational influence. Its crust is thicker on the far side, while most volcanic basins -- and unusual mass concentrations -- occur on the near side. Mass is not distributed uniformly inside the Moon. Large mass concentrations ("Mascons") lie beneath the surface of many large lunar basins and probably represent thick accumulations of dense lava. Relative to its geometric center, the Moon's center of mass is displaced toward Earth by several kilometers. The surface of the Moon is covered by a rubble pile of rock fragments and dust, called the lunar regolith, that contains a unique radiation history of the Sun which is of importance to understanding climate changes on Earth. The regolith was produced by innumerable meteorite impacts through geologic time. Surface rocks and mineral grains are distinctively enriched in chemical elements and isotopes implanted by solar radiation. As such, the Moon has recorded four billion years of the Sun's history to a degree of completeness that we are unlikely to find elsewhere. 7
POST-APOLLO LUNAR EXPLORATION
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Lunar Exploration: Clementine • Orbited the moon for more than 2 months while mapping and surveying the entire surface • Instrumentation included multiwavelegth imaging (11 colours) capacity from 0.415-2.78 µ m, and a laser ranging system (Nozette et al., 1994. Science, 266, 1835) - laser ranging system yielded topography (relief) of lunar surface between -75o to +75o - gravity data was derived from spacecraft’s trajectory - imaging cameras with bandpasses (filters) detected variations in mineralogy and soil maturity - bistatic radar experiment
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Clementine Mission
Ultraviolet/Visible CCD Camera 0.41kg, 4.5W, 100-325m Star Tracker Camera 1.12 kg, 9.5W
LIDAR 2.37kg, 6.8W
High Resolution Camera (HIRES)
Near Infrared CCD Camera (NIR) 1.92 kg, 11 W, 150-500m
Long wavelength Infrared Camera 10
Lunar Exploration: Clementine Findings • High resolution map of moon’s shape and crustal structure; evidence of major compositional provinces including volcanic lavas of maria, young craters, SPA; topographical and geological (volcanic vs impact origins) differences between near and far sides (Zuber et al. Science, 266, 1839; Lucey et al. Science, 266, 1855) • Global map of thickness of lunar crust from combined analysis of topography and gravity data (Zuber et al. Science, 266, 1839) • Lithologic variations (basaltic vs feldspathic) detected in impact craters Copernicus, Tycho and Giordano Bruno (Pieters et al. Science, 266, 1844) • Confirmation of ancient multiring impact basins including South Pole Aitken (SPA) and Schrodinger (Spudis et al. Science, 266, 1848; Shoemaker et al. Science, 266, 1851) 11
Lunar Exploration: Clementine Findings • Composition and geological history of Aristarchus crater (McEwen et al. Science, 266, 1858) • Radio signals beamed into moon were scattered and received by NASA’s DSN antennas; echos characteristic of ice (?) in permanently shadowed polar regions, where temperatures are probably always below 40 K
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Image of the Aristarchus Crater (40 km wide), found in the southeastern part of Oceanus Procellarum; dark gray = mare basalt; reddish unit = ejecta from Aristarchus; light blue = anorthositic 16 rock exposed in crater interior
Lunar Exploration: Lunar Prospector • Global mapping of the moon surface • Existence and size of iron core (gravity and magnetic field data) • Search for H, water ice (life support, rocket fuel) • Summary of Instrumentation γ -ray spectrometer (Fe, Ti, Th, U, K, O, Al, Si, Ca, Mg) neutron spectrometer (H, Fe, Ti, Ca, Al) magnetometer (B fields on lunar surface) α spectrometer (Rn release) Doppler tracking of spacecraft (gravity map of Moon) 17
(Binder, 1998. Science, 281, 1475)
This artist's conception of Lunar Prospector shows the Spacecraft in lunar orbit with its instrument masts fully deployed. Prospector's primary mission will keep the spacecraft in a 100 km polar mapping orbit for a full year or more. This orbit will provide higher quality science data than has previously been obtained. Prospector has an Extended Mission option of a 10km, very high resolution orbit for a brief period of time.
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Lunar Prospector Mission
Neutron Spectrometer 3.9kg, 2.5W, 10ppm H
Gamma Spectrometer 8.6kg, 3W, 150km
Magneteometer 5kg, 4.5W
Electron Spectrometer 0.01nT, 3 km
Alpha Spectrometer 4kg, 7W, 150 km
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New slide from Lecture notes • Cosmic rays (CR) were shot to surface of the moon – Reason to use CR: • Naturally produced in the solar system
• Interact with surface of the moon • Generate a lot of by products made by certain nuclear reactions which machines can detect. The stuff detected by machines can then allow us to map and study the surface fo the moon – CR can: • Admit characteristic lambda waves • Easy to map • Moon doesn’t have atmosphere to interfere w/ the CR
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Lunar Exploration: Lunar Prospector Results • Global distribution of H, Fe, Th, U, K etc. • KREEP (last melt after crust forms) is concentrated near rim of Mare Imbrium and Mare Ingenii SPA basin area; low in highlands (Lawrence et al. Science, 282, 1484) • Fe high in maria, SPA basin; abundances consistent with Clementine (Feldman et al. Science, 282, 1489) • Fast neutrons, H collide ⇒ slow down (thermal/epithermal) neutrons; H concentrations high at each pole (Feldman et al. Science, 282, 1496); total ice model estimate ~2 billion tons • Discrepancies between Fe and Ti concentrations based on LP thermal neutron data and Clementine data in the rim of Mare Imbrium • Highlands south of Mare Imbrium have high Th; farside Highlands have low Th concentrations
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LP γ -ray spectra plotted as number of counts per 32 s against energy. The top figure shows an average of all spectra, whereas the bottom figure shows spectra for Imbrium (a KREEP and basalt-rich region), and Joule, located in 22 the lunar highlands which are mainly anorthositic.
Note the decrease in the intensity of epithermal neutrons, caused by the presence of H at polar regions of the Moon.
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Lunar Exploration: Future (NRC-NAS) • Did the early flux of impacts peak at ~4 Ga or did it decay exponentially with time? • Assess the nature of the Moon’s interior (core?); test models for differentiation of the Moon’s crust and mantle • Composition and mineralogy of lower crust and mantle • Confirmation of polar H deposits; determine the history of water and other volatiles • Assess mineral resources of Moon • Sample Return Mission from South Pole Aitken Basin - Dating SPA places limits on early impact history and timing and duration of basin forming events - Deepest basin in solar system ⇒ SPA basin interior should expose lower crust and upper mantle
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