Gullies On Mars

GULLIES ON MARS Roberto Bartali ABSTRACT Mars seems to be a dry planet because of its low temperature and low atmospheric pressure, but it was wet in some time of its history and there are many evidences that, under certain conditions, some liquid water is still flowing on its surface. There is no doubt that, relatively large quantities of water ice, are present on the surface, mainly in polar caps and in the subsurface. Several spacecraft and exploration rovers are collecting data and analysing the surface and subsurface looking for liquid water using different methods to assess the amount of water present and where it is located. I will explain briefly those methods and the evidences they found of the existence of water on the red planet, as an introduction to the main topic of this work focussed on the formation of gullies. The study of gullies is very important because they are the youngest geologic features observed on Mars and may contain the best evidence that liquid water is still flowing today (probably cominig from deposits in the subsurface), even when it is in a very low quantity and for a short period of time, respect to the past. It is very difficult to make a definitive conclusion, because there are many evidences that point to the presence of sub surface aquifers supplying gullies and sometimes, forming new ones, but there are also several, and well supported, hypothesis that not include a flow of water. EVIDENCE OF WATER ON MARS One and half centuries ago, Lowell and Schiapparelli, observed some features on the surface of Mars, that they recognized, or interpreted, as channels and believed they contain liquid water flowing from polar caps to equatorial latitudes, starting a whole set of theories about martian’s life. When, in the decade of 1960 and 1970, several spacecrafts, approached the red planet, images returned to Earth, showed that there were neither channels, water nor life. The new scenario was an oxidized and dry desert. Earlier measurements also showed that polar caps were filled with solid CO2. New, measurements and technology improvements, showed (during the decade of 1980 until now) that both polar caps (figures 1 and 2) contain also large quantity of water ice [Titus 2004]. Several spacecrafts and exploration robots were sent to assess, not only, if really there is water, but if there is also (as a direct consecuence) some kind of life. High resolution images and direct measurements on the surface, show that Lowell and Schiapparelli saw large geologic fractures, not channels; but surprisingly, 1

channels exist inside craters and other geologic features, and, million of years ago, they contained large amount of flowing water and large low and flat plaind may be filled by steady water. Today we know that Mars was very wet in the past and that there are, yet, water because there are morfological, geological, mineralogical and spectrocopic evidences for that. Grady [Grady 2006] provides a good general summary of several water evidences. I will give here a concise description of most important evidences, this is because this section wants to be only an introduction showing that Mars was, and still is, not as dry as it seems to be. Morphologic evidences Comparing satellite and high altitude images of Earth with those returned from Mars by several spacecraft, it is evident that the coincidence of morphologies are very strong. These includes river beds, rivers delta and tributaries(figure 3 and 4) [Baker 2006], showing slow erosion that are found all over the southern oldest and heavily cratered regions. While water can not be stable under present martian climatic condition, they may formed by ground water flows, perhaps during a period of different inclination of the rotation axis, when climate was hotter [ESA 1999]. Gullies, channels and valleys, aprons, flat surfaces resembling lakes and ocean floors, plateaus, sedimentary strata, outcrops, glacier displacements and morraines, on northern highlands and on slopes in volcanic regions, show that, almost in the past, large quantity of water flowed. Mars morphology shows great differences, possibly due to a large impact, during or soon after, the planet accretion. Northern flat hemisphere looks very different from the higly cratered Southern one. Possibly, billions to millions years ago, an immense ocean covered the Northern hemisphere of Mars. On our planet, Arctic glaciers [Lee 2006] and Antarctic dry valleys [Head 2007; Morgan 2007; Dikson 2007], are places where gullies are found and resembles martian present conditions. A region called “Swiss Cheese terrain” in the South polar region shows nearly circular depression of 8 meters, with a remarkable uniformity and flatness [Malin 2001; Thomas 2000] though to be composed of stable water ice covered by more volatile CO2.

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Black holes This is a new discovery on the surface of the red planet, a hole with no rims over a flat surface (figure 6). It consist of a near circular hole and judging by the absence of shadows it seems to be very deep. There is no rim, so it is inconsistent with a meteoric impact crater [Univ. Arizona 2007]. It is very similar, if not identical, to many features localized in the San Luis Potosi and Oaxaca states of Mexico; the only difference is that terrestrial “Sotano”, (figure 7 and 8) as they are called, are encircled by a dense

vegetation [Lazcano Sahagun 1986]. These holes are carved by vortex of liquid water in a calcium rich rocks, but host rocks can be every kind of evaporites or carbonates. Geologic features Sedimentary depositions (figure 9), not seen from space, were observed and analysed by the two rovers Spirit and Opportunity, but the latter landed really in the right place, just in front of a large outcrop os sedimentary layers [Squyres 2004]. Sediment lamination are of different thickness, down to millimeter size, and the composition of rocks, shows that they are sandstones consisting of reworked grains cemented in a sulfate rich environment, there are also basalt mud showing some degree of alteration. The sandstone is likely formed by dissecation [McLennan 2005]. The presence of festoon cross laminations (figure 10) in outcrops seems to be formed by the reworking action of water during sedimentary deposition. Formations at Eagle crater suggest a deposition secuence, upward, during several cycles of wetter environments.

Large amount of concretions in the form of hematite spherules formed in a groundwater and brine saturaded environment [Christensen 2004]. These spherules are

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harder and denser than the rock matrix where they are found and contains large amount of hematite (50% to 90%). Their dispersed presence and abundance is not consistent with a formation derived by fusion during meteorite impacts or volcanic eruptions. Many rocks in Meridiani Planum, relatively far from impact craters, show poligonal textures which are fractures across bedding structures, that may not formed during sedimentary deposition, but in a later evaporation or dehydration phase of salts contained in the rocks. This rocks are flats and dispersed on the surface or partially buried near the surface. Sedimentary rock on Earth (figure 11) are very similar to those showed by Opportunity images. Spectroscopic measurements One of the best evidences of water in the form of ice is gived by the Mars Express OMEGA instrument [Mars Express 2006]. It demonstrated that the South polar cap is composed by water ice covered by a thin layer of frozen CO2 [Titus 2004], this is because the light from the Sun is reflected with different amount and at different wavelenghts depending on the composition of the surface elements, so it is possible to distinguish the presence of water, CO2 and dust. This discovery is consistent to the data send back by the Mars orbiter Laser Altimeter [MOLA 2003] showing that the South polar cap (figure 1) do not maintain its shape if it is composed mainly by carbon dioxide. Gamma ray spectrometer onboard of Mars Odyssey [Mars Odyssey], also measured the abundance of hydrogen buried below the surface spred evenly on the planet, but the concentration is higher on polar regions. The most likely source of this hydrogen is water ice. All these intruments, together, helped the discovery that water ice is buried below the surface of Mars at different depth (figure 12). Albedo measurements Near infrared spectroscopy shows two distinct types of surface (figure 13), one with high albedo consisting of highly oxidized and fine grained materials, and another with low albedo consisting of iron in the form of pyroxene and hematite. Thermal emission spectrometer revealed that high albedo surface consist of plagioclase, magnesite [Bandfield 2000; Bandfield 2003] and zeolite [Ruff 2004], the latter contains water molecule, there is a similarity with basalt on Earth mainly composed of andesite. Low albedo surface spectra is similar to terrestrial basalt containing high abundance of plagioclase, high calcium clinopyroxene and olivine in small amount [Bandfield 2000]. Mars is composed mainly of igneous basalt, but its composition is highly influenced by surface and mantle ice. There is a close relationship between the depht of the ice layer and the chemical altered basalt. Alteration in the southern hemisphere occurs at high latitudes, decreasing slowly toward the equator. Instead, for the northern

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hemisphere, there is a more uniform alteration, clearly greather to high latitudes [Wyatt 2002; Wyatt 2004].

Water altered minerals Fine grained ferric oxydes are abundant in soil and they also cover rocks almost everywhere [Bell 2000; Squyres 2004]. These grains are sulphates, chlorides and salts formed by evaporation that are unambiguous evidence of water alteration. Spectral analysis of smooth plain in Meridiani Planum by the Thermal Emission Spectrometer (TES) [TES 2006] revealed gray crystalline hematite mixed with

plagioclase and pyroxene. Comparing those spectra with those emitted by hematite in the laboratory at different precursor phases, suggest a low temperature formation (150300 ºC) from dehydroxylation of goethite. Opprtunity rover confirmed, in situ, the presence of hematite [Christensen 2000; Christensen 2004]. Layered bedrocks in Meridiani Planum are rich in Calcium and Magnesium sulphates and also contain Jarosite [Squyres 2004] this imply an extensive alteration and fluvial activity during the formation of hematite, but it seems that only localized concentrations of standing water was responsible of such alteration. The chemical alteration was limited after the formation of hematite because the mineral is mixed with basaltic sands that do not show chemical alteration. Large amount of the OH radical in the rock, where jarosite is found,

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suggest that it was formed by precipitation of sulfate rich brine [Klingelhofer et al 2004]. INTRODUCTION TO GULLIES Water on Mars was present and abundant in the past, all the above presented evidences are irrefutable. At present, almost all the water outside polar caps, is buried. The depht of the liquid water layer may be from one meter to kilometers, and, it is greatest at the Equator and near the surface at high latitudes. If some of that buried water is allowed to reach the surface, we can observe its effect during the short time before its sublimation due to the low atmospheric pressure and cold temperature. If this happen, it can carve new gullies or rework older ones. A gully is a landform created by running water. When water flows, it lubricate and it erodes the soil creating a debris flow when carving a channel. If the activity of the gully is recurrent, and, the carried material and water flux is greather than the one of the last activity, the channel can be enlarged, these new debris material are deposited over the old one and it can be recognized because its color is lighter. Walls of gullies have a steep inclination and normally are clean, with no residual material; the bed of the channel is also very clean. Channels can be straight or sinuous, depending on the kind of the slope and the obstacle that water has to skip. This imply that the flow is slow. At the end of the channel, debris are deposited forming a cone shaped apron. The upper section of a gully is called “alcove” (figure 5, 15, 16) that act as the reservoir of ice or snow, it is a depression in the form of a theatre, it is the place from where water starts to flow downward (melted surface ice, or spring from an underground reservoir) [Treiman 2005]. Lenght and wide of a gully depend on the quantity of water available, the velocity and mass of the flow, the intensity of the flow, the time duration of the flow, the composition of the soil, the type of rock in the alcove, temperature and wind speed [Sears 2005]. They forms on a hillside or walls of chasms, pits, impact craters, peaks within craters and basins, knobs, and mesas. There must be an impermeable layer (containing water) and a permeable layer of rocks, otherwise, the gully cannot form, because there is no deposit and no outflow. To understand the formation of gullies on Mars, and, if they are formed by the outflow of liquid water from the interior of the planet, several researches are conducted on Earth, but finding places with enough similarity to the conditions observed on Mars is difficult. Present day Mars is a very dry and cold desert, the best places to find conditions like those, are in the dry valleys in Antarctica [Head 2007; Morgan 2007; Dickson 2007] and in the high latitudes glaciers near the

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North Pole [Lee 2004; Lee 2006] (Canada and Greenland); Neverthless, gullies are observed worldwide, but they formed under very different climate conditions. Gullies in Antarctica are of different age and evolution stage, almost half of them are active at present. They formed by surface and very near surface snow and ice melting. Due to the very dry climatologic conditions (precipitation less than 10 centimeter per year), the melted water come from glaciers, whose topography, geometry and position maintain them in shadow almost all the year. Another source of water is wind blown snow accumulated in parts of the alcove or the channel not exposed to insolation. Dark (low albedo) material can increase the rate of snow or ice melting. The occurrence of Gullies on the equator facing slopes is greather than the quantity of gulllies on slopes that are oriented toward the Pole, this is because they are exposed directly to Sun light. MARTIAN GULLIES Mars is a desert planet, liquid water can not be present on the surface because temperature and pressure are below the triple point, so if some water appear on the surface, it sublimate inmediately; but, as I showed above, this scenario was not too bad in the past when large quantity of liquid water flowed and covered the Red planet. This thirty years long paradigm, was broken when Malin and Edgett [Malin 2000] discovered several gullies in the high resolution images returned by the Mars Global Surveyor camera (figure 17). If liquid water can not persits on the surface, why and when they formed? Very few impact craters are observed over the gullies, this fact imply that they formed recently. So there are two possible explanations: gullies are formed by other way than a flow of liquid water, or there are mechanisms that allow liquid water to flow and carve them. In the following sections I will show several different mechanisms that try to explain the formation of gullies on Mars in its recent or past geologic history. Orientation of gullies On Earth, gullies seems to be oriented facing the equator, this is logic because if the alcove receive more heat, the ice can melt easier. But on Mars this is not always true, even when the full planet has been covered by several spacecrafts, maybe we need much more high resolution images covering areas not yet explored at the necessary resolution and illumination angle. Mohan and Bridges [Mohan 2004] characterized gullies between 30º and 71º South, from Mars Global Surveyor images, mesuring for each one the latitude, the longitude, the width of the gully at the top, middle, and bottom, the length, the morphology, the drainage density, the distance from the summit wall, and its azimuth. They found that most gullies are on the North and South slopes of selected craters, without any preferente. But the dimension (lenght, width and drainage density) of gullies facing toward South are greather than those facing to the North, but there is no

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relation with their morphology and the latitude. They conclude that solar insolation may be a factor that affect the quantity of fluid, favouring ice melting, but also it is possible that larger gullies are youngest and better preserved, so not all gullies were formed at the same geologic time, maybe due to different axis inclination. Berman et al. [Berman 2005], also with images of THEMIS and MOC (Mars Global Surveyor), found, instead, that there is a clearly preference of gullies respect to the latitude (figure 18). At middle latitudes (-30º to 48º S.) they observed only gullies on the polar face slope; going at higher latitudes (-44º to -56º S.) there is a reversing preference, observing only gullies on the equatorial face slope. But this trend is not definitive, because they also found that craters, with gullies on both slopes, are also present from 37º South to 67º South, even more, there are also a series of craters at latitudes between 42º South and 50º South that contains gullies on the East and West sides. Balme et al. [Balme 2006] suggest that there is an obvious relationship between the orientation of gullies and the latitude for those inside craters at mid latitudes. This imply that climate and insolation play a fundamental role in the formation of gullies because they change the content of water in the atmosphere, othrwise, too large ground water quantity are needed to explain formation from a seepage of liquid water as suggested by Malin and Edgett [Malin 2000]. With respect to gullies on the northern hemisphere, Williams [Williams 2007], found that there is no special orientation because gullies appear at nearly the same quantity on both crater’s walls. This trend is different from that of southern gullies, because, as I see above, there is a tendency to pole facing gullies. The orientation may depends on different factors like the insolation, the geometry of the acquifer, the quantity of dust in snow and rock permeability, but he also suggest that gullies may also represent (with their orientation) a different climate condition due to changes in the obliquity of Mars, so they formed, at least on two different geologic times [Lanza 2006]. Lanza also suggest that gullies formed where some ideal conditions of temperature and pressure allowed subsurface water to be released. Bridges et al. [Bridges 2005] observed a clear trend to the orientation of gullies on the northern hemisphere of Mars, specially at high latitudes. At mid latitudes there is no clear orientation trend. But they found an interesting difference between poleward facing gullies with very smooth material and equatorial facing ones, filled by bright toned material. They suggest that pole facing slope gullies are frozen in time from melting during the last change of obliquity and they may be form taday, but at a slower rate.

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Really gullies on Mars, on average, show more similarity than differences [Heldmann 2004] between those on the northern hemisphere respect to those on the southern hemisphere. On both hemisphere they are not isolated, but clustered regionally (figure 27). Most prominents clusters in the northern hemisphere are Arcadia Planitia Tempe Terra, Acidalia Planitia and Utopia Planitia; on the southern hemisphere, where they are more abundant, due to a more chaotic geology, are found in Dao Vallis, chaotic terrain around Terra Sirenum and on the South Polar Pits. FORMATION OF MARTIAN GULLIES Mars Global Surveyor, during its mission, took images of several places on a time span of a few years. Surprisingly its camera detected various new gullies, dark and light streaks (figure 19). This was a great surprise because the common tough was that gullies were old formation, because their morphology suggest the presence of liquid water; but water is not stable at current temperature and pressure. So a whole set of theories and models were presented to explain the formation of such features [Heldmann 2007]. Formation by dry granular flow Due to the apparent inconsistence of the stability of liquid water on Mars and its dry and cold climate, some researchers developed models that do not need a flowing water for gullies formation. Reiss et al. [Reiss 2007] classified the morphology of gullies and avalanche scars into four classes (figure 20) and analized them respect to the distribution, slope angle, orientation and climatic condition with the aim to understand the formation

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process involved in each one. The four types are described as follow: Type I. These features originate in small alcoves at the dune rims, which coalesce into small channel tracks showing a dendritic pattern on the steeper upper part of the slope. The small channels merge into sinuous main channels, which have a constant width from the source region to their end and show lateral embankments mainly in the lower part. This kind of formation lacks the debris apron. Type II: Characteristics of this type of formation are V-shaped alcoves, sinuous main channels and depositional aprons; small dendritic channels in the alcove merge into a single small channel; occasionally the channel runs out onto the apron. Type III: These features start in lengthy alcoves which merge directly into straight, deep and short main channels. The debris aprons are extense. Type IV: These features start at the dune rims, occasionally in small alcoves forming straight linear channels, slightly cut into the dune material, which rarely reach the base of the dune. Treiman et al. [Treiman 2004], aided with the granular flow theory [Daerr 1999], show that there is no needs of liquid water to form gullies and clear and dark slope streaks (figure 19), because they are formed by the flow of granular material. These are interpreted as an avalanche of thin layers of bright dust deposited by wind, which expose a darker layer formed by coarse or different color (or composition) material. They develop a triangular shape which is consistent to the granular flow theory. The lateral spread of the triangular avalanche can be explained as the friction exerted by the rolling small grains on their lateral neighbours. This friction is sufficient to set some of them into motion. If the grains are coarse or there are small and large grains mixed together, the avalanche develop an upward propagation. This way, large grains start to roll spontaneously, because their supports (small grains) located below them are no longer there. Formation by CO2 processes It is known that CO2 can sublimate over the whole Mars surface, but at low latitudes the temperature is too hot to form large quantity of CO2 ice and at high latitudes, this sublimation is very slow due to the lower temperature. Cedillo-Flores et al. [Cedillo-Flores 2007] suggest that gullies are formed by the fluidification of CO2. In their model carbonic dioxide snow and dust tranpoted by winds, accumulate during colder seasons. During the Spring, CO2 sublimate due to the elevation of temperature. The weight of the ice layer and the process of sublimation together produce a mixture of dust, ice and soil grains. Then, this fluidified material run downslope because of gravity, carving the gully. Gullies can form each martian year or each time Mars experiment a change of obliquity. Solid or liquid CO2 can be the alternative agent for the formation and development of gullies because it is more stable at present conditions on Mars [Ishii 2007] and the atmosphere contains up to 95% of carbon dioxide. Counting and measuring gullies on mid-latitudes impact

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craters, it seems that there is some kind of relation between the seasonal cycle of frostdefrost, condensing and sublimation of carbon dioxide ice, with the formation of gullies. CO2 ice, seems to be made of small particles with high reflectivity in the visible part of the spectrum. If the CO2 ice sublime from the bottom, and form a layer of pressurized gas between the ice and the ground, can trigger an avalanche and fluidize ice particles. Mars low gravity can also play a role in the fluidization process of CO2, this fluid and/or granular flow can carve the slope and form a gully. Figure 21 is an example of a crater with gullies covered by a CO2 frost. The seasonal cycle of CO2 works as follow [Ishii 2004]: on midwinter CO2 condense on slopes, at mid to high latitudes, (this fact also imply an orientation trend of gulllies); on the following summer the frost erodes the slope and sublime into the atmosphere because of the increased insolation. There is another model involvig CO2 for the formation of gullies, which is developed by Musselwhite and co-researchers [Musselwhite 2001]. They point out that, the generalized orientation of gullies on poleward face slopes, make them as the coldest places because they are in shadow almost all the year, with the actual inclination of (about 24º) the rotation axis. Under these conditions, CO2 ice can be stable just below the surface and in diffusive contact with the surface. In other words, it act as a cold trap. They say that the key to understand the formation of gullies is the position of the seepage respect to the crater rim, which is always about 100 neters below it. Pressure at this locations are consistents to the triple point of CO2. Poleward facing slopes at high latitudes can contain large amount of CO2 ice below the surface, and, this is a cold barrier that serve as a trap against the liquid CO2 below it. The highly cratered terrain seems to be cery porousand CO2 in the atmosphere can diffuse relatively easy into 100 meters of rocks from the surface. Lithostatic pressure below the surface is higher than atmospheric pressure (proporcional to gravity, rock density and depht below the surface), but pressure inside the void spaces, due to the porosity, is the same atmospheric pressure. This is true until voids are filled. When the temperature increase, due to insolation, CO2 subsurface ice expands, and when the voids are completely filled, pressure increase toward its lithostatic value. As temperature increase, from the surface to the interior, solid CO2 ice change its phase and become stable in the gaseous phase, but there is, now liquid CO2 behind. As the inward side of the ice trap melts (due to the continuous temperature increment), the liquid CO2 become mobile and rapidly drain out from the cliff face. In other words there is a violent eruption of CO2 that rapidly vaporize because of the much lower atmospheric pressure and cold temperature. The expansion should produce a fluidized flow of gas and particles carried from the interior. This model predict formation of gullies each martian year or during cycles of different obliquity, depending on the capacity of refill the CO2 acquifer. Formation by melting snow and ice Liquid water in the sub-surface of Mars is thought to be not enough to be considered the main source of gullies, due to its instability on the surface. Snow deposition during periods of higher obliquity and its subsecuent melting is an alternative model for gullies formation [Bridges et al. 2005; Soare et al 2006]. The source of the flow is localized on the uppermost part of the mantle hosting the alcove, so they inferr that water, that fills gullies, come from the mantle; more precisely, gullies start in the boundary of mantle and non-mantled slopes, or where they are found together. It seems that gullies begin in the non-mantled slope, when both are present. Snow pack formation and its longevity depends on latitude, orientation, insolation and temperature, but all these variables depend on the inclination of the rotation axis. As they show, gullies are found in regions where the presence of water ice in the sub-surface, and possibly extending km inside the

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mantle, is inferred by the high level of water related hydrogen. Even when gullies, under certain conditions, can be active today, most of them are old and formed during a period (or periods) of high obliquity. The presence of gullies of different orientation, is also consistent with the melting of sub-surface ice at various geologic times. So by these authors, gullies are formed by mantle ice or packed snow melting. A little different model, also involving the melting of snow and ice, is proposed by Christensen [Christensen 2003]. In this model obliquity plays a fundamental role: when it is high, water is transported from poles to mid latitudes forming water rich snow layers; when obliquity is low, mid latitude temperature favoured snow melting. Liquid water produced remain stable becase it is covered by an insulating layer of snow. Gullies form on slopes covered by snow and remain hide until the snow is removed. Liquid water erodes the slope material carving the gully, when the covering layer of snow, completely melt or sublime, the gully appears. Today, patches of snow, covered and protected against sublimation, can exist below an overlay of dessicated layer of dust sediment (figure 22). If this is the case, on favourable places, melting occurs and so, formation of gullies can be possible under present conditions. Melting is possible even if the surface temperature is bleow freezing because sunlight is absorbed at depht more efficiently than at the surface. There is also the possibility that atmospheric water vapour condensed, can melt in troughs of slopes oriented poleward, which is the most common orientation for gullies at middle latitudes [Kossacki 2004]. They run simulations of the seasonal cycle of condensation and sublimation of carbon dioxide and water, showing that water ice on the gullies walls can change to the liquid phase. Their results, when the simulation is applied to a gully of 10 meter wide, on a slope inclined 30º and oriented 50º South and 50º East, assuming a surface wind speed of 5 m/s; the amount of the obtained moisture is only 0.2 kg/m2 and can appear in a given place for a very short time, just one day. But, if the amount of water on the cold surface is larger than that calculated in simulation, gullies can form even in the present climate conditions. The mechanism of formation is related to the water cycle: atmospheric water vapour condense as water ice and is deposited on small ondulations on gullies walls (probably originated by surface wind). Insulation rise the temperature and ice melt. Spring Groundwater formation After Malin and Edgett [Malin 2000] discovered gullies in impact crater walls and showed that, almost geomorphologically speaking, they were similar to terrestrial gullies formed by liquid water flow, several researchers tryed to explain the possibility that martian gullies formed the same way.

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Recently, Malin [Malin 2006] discovered that some gullies showed sign of activity (figure 23) during past years. Comparing images, taken in 1999 and 2001 with others of the same field taken in 2005, apeear a deposit of debris with light tone, interpreted as a fresh deposited material above dark old one. Only a very fluid material (like liquid water) has the hability to divert around low relief obstacle and to freely flow even if the inclination is very low or absent. Observed at high resolution, these bright deposits, show that have relatively long, extended, digitate distal and marginal branches; they divert around obstacles, so the velocity is very low. Water needs to flow to carve the gully, so temperature and pressure at the surface must be above the triple point, otherwise it can sublimate; and temperature in the subsurface (the acquifer) must be above the freezing value (figure 24). This is interesting, because if this model is correct, the base of the alcove is the depht at which liquid water is stable under the surface of Mars. Heldmann et al [Heldmann 2004; Heldmann 2005] using data from the Thermal Emission Spectrometer, calculated the depht at which the temperature is above the melting point (273K); for 90% of the 136 gully systems observed this depth is 200 meters. Alcove basements are on average below this value (350 meters), so the temperature above the surface at that depth is expected to be higher than the melting point. An interesting result is that none of the alcove examined allow the presence of CO2 in liquid form. With this data on hand, the spring water or seepage model is consistent to observed features. Some “special case” places on Mars, allow the presence of liquid water (at least for short periods of time) on the surface. An example is Centauri Montes [Fasset 2007], where summer temperature is at least 280K. Another place where temperature and

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pressure are above the triple point is Terra Sirenum [Fasset 2007] where temperature reach 297K. Terra Sirenum is where Malin and Edgett discovered active gullies [Malin 2006]. The problem of temperature is also addressed by Edlund [Edlund 2006], calculating the temperature and pressure on the sub-surface of the alcove base, to determine if they are above the triple point. Calculations were done with data from the Gamma Ray Spectrometer (which measured the amount of subsurface ice content) for each gully site. Approximately 81% of the gullie systems allow water in liquid form. Groundwater flow system Instead of small acquifers buried behind the alcove it is possible that very large sub surface acquifer exist. From data of the Gamma Ray Spectrometer we have a complete map of the sub-surface water (figure 26) in the form of ice, if parts of that ice is maintained liquid, then water can supply a large quantity of gullies regionally clustered. A model of mechanism that can explain the formation of gullies related to large

subterranean acquifers is presented by Marquez et al. [Marquez 2006]. They observed that gullies in Gorgonum-Newton, Dao and Nirgal Vallis regions show a pattern following the regional slope consistent with a fully developed groundwater flow system. In the Gorgonum–Newton region the groundwater would flow from different systems to the low elevation lands craters (figure 25). In Newton Crater and its surroundings, gullies are oriented radially respect to the centre of the basine basin, and also radially distributed from high-elevation sites. Marquez et al. Suggest that the source of ground water could be related to the outcrops of young and partially eroded ice rich deposits. This molten water can also recharge acquifers, like the mechanism that works on our planet, but the rate of recharge on Mars is very low and may depends not only on seasonal cycles, but on global climatic changes triggered by a change in the obliquity.

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CONCLUSIONS Magician say “The trick is there, but you do not see it”; astronomer say “The water on Mars is there, but we do not see it”. Water is buried, only a small part is present as part of the thin atmosphere and in large mass on polar caps. Mars show us many tricks and many of them inply the existence of liquid water flowing freely on the surface. But where is all that water? The wet past of Mars is also undoubtable, so how Mars acquired all its water? The answer to these questions are not easy, but time after time we are soving the puzzle which pieces are observations and measurements done by different instruments, spacecrafts and rovers. How much time it will take? Maybe we will never known the full story of Mars, because each time we are closer to place the last piece of the puzzle, a new discovery open a lot of new questions. This is the case of gullies. Everybody was happy with the image of Mars as a dry desert, but gullies appeared in the scene, when and why they formed? If liquid water is their main source, where is the water? In this work I tried to show a full set of evidences that water existed and still exist on Mars today. Obviously the red planet suffered enormous climate changes due to changes in the obliquity. Minerals, sedimentary rocks, spectroscopic measurements and landforms, all count that story. Evidences all point to an affirmative answer: Mars was filled of water in the past and still contain a relative large quantity at present time, but this water can not be present in liquid form on the surface because the temperature is too cold and the atmospheric pressure is too low. Under these conditions can be in the liquid phase only under the surface or in very localized places with the help of geology and topography. In the second part of this work, I addressed the topic of gullies. These landforms are similar to many we can see on Earth, and all known terrestrial gullies involve the action of flowing liquid water, at least at some stage of their development. The problem on Mars is that there is no flowing liquid water on the surface because temperature and pressure maintain the surface condition always below the triple point for water. Even if liquid water emerge from an acquifer, it will last liquid for a very short time because it sublime rapidly. So, gullies are there and they are in large quantity, and the question is: which is the mechanism that forms gullies? Many models were presented here, I tried to explain briefly their main arguments adding comments. Unfortunately all models work well only in limited cases and with certain conditions. Up to date, there is no complete theory that can explain the formation and development of martian gullies. One of the problem is that maybe there is no just one mechanism, but several because gullies present different features. So a unified theory for gullies I think that not apply. Many models are just a variation of the theme, so I think that there are only 4 main trends and each one can include several models: 1-dry flow of particles 2-carbon dioxide in liquid or solid form 3-water from small local or large sub surface acquifers 4-water from melting ice or atmospheric condensation I can not tell which one works better, but I am inclined to the presence of liquid water, but it is possible that different mechanism act simultaneously. It is also possible that each model is correct if applied to some specific case, and, obviously if applied to the wrong set of gullies it will appear totally inconsistent with observations.

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Due to their different morphology and orientation, gullies may be formed during different geologic eras under very different climate conditions, even more, by very different mechanisms. These mechanism can be some, or even all, that listed here. It is also possible that models explained above are only a subset of a more complete one. I think, also, that there is a strong bias in the data sets used by researchers, this is reflected in papers because if we want to know, in general form, why, when and how gullies are formed, we need to use as much data as possible. This data must reflect, almost statistically, all gullies in a planet wide census. We have images and data available from many instruments carried by half a dozen spacecrafts. Each instrument is capable to observe at different wavelenght and resolution, only a full set of observation can tell us the origin and the development of a process or, in this case, a morphologic feature. Many research are focussed and based on a reduced data set, this way their results are not consistent with others. REFERENCES Water evidence Grady M., WAT SEN: SEARCHING FOR CLUES FOR WATER (AND LIFE) ON MARS, International Journal of Astrobiology 5: 211–219, 2006. Titus T. N., WATER, WATER EVERYWHERE, Nature Vol.428, 610-611, 2004. Morphology evidences Baker V.R., GEOMORPHOLOGICAL EVIDENCES FOR WATER ON MARS, Elements 2:139-146, 2006. ESA, EXOBIOLOGY IN THE SOLAR SYSTEM AND THE SEARCH FOR LIFE ON MARS, SP1231, 1999. Head J.W., Marchant D.R., Dickson J.L., Levy J.S., Morgan G.A., MARS GULLY ANALOGS IN THE ANTARCTIC DRY VALLEYS: GEOLOGICAL SETTING AND PROCESSES, Lunar and Planetary Science XXXVIII, 2007. Morgan G.A., Head J.W., Marchant D.R., Dickson J.L., Levy J.S., GULLY FORMATION ON MARS: TESTING THE SNOWPACK HYPOTHESIS FROM ANALYSIS OF ANALOGS IN THE ANTARCTIC DRY VALLEYS, Lunar and Planetary Science XXXVIII, 2007. Dickson J.L., Head J.W., Marchant D.R., Morgan G.A., Levy J.S:, RECENT GULLY ACTIVITY ON MARS: CLUES FROM LATE-STAGE WATER FLOW IN GULLY SYSTEMS AND CHANNELS IN THE ANTARCTIC DRY VALLEYS, Lunar and Planetary Science XXXVIII, 2007. Malin et al, OBSERVATIONAL EVIDENCE FOR AN ACTIVE SURFACE RESERVOIR OF SOLID CARBON DIOXIDE ON MARS, Science 294: 21462148, 2001 Thomas P.C., NORTH-SOUTH GEOLOGICAL DIFFERENCES BETWEEN THE RESIDUAL POLAR CAPS ON MARS, Nature 404: 161-164, 2000.

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Black Holes Lazcano Sahagun C., LAS CAVERNAS DE LA SIERRA GORDA, Universidad Autonoma de Queretaro, ISBN 968-845-041-3, 1986. University of Arizona, CANDIDATE CAVERN ENTRANCE NORTHEAST OF ARSIA MONS, PSP_003647_1745, 2007 Geology Squyres S.W. et al, IN SITU EVIDENCE FOR AN ANCIENT AQUEOUS ENVIRONMENT AT MERIDIANI PLANUM, MARS, Science 306: 1709-1714, 2004. Christensen P.R., et al., MINERALOGY AT MERIDIANI PLANUMFORM THE MINI-TES EXPERIMENT ON THE OPPORTUNITY ROVER, Science 306: 1733-1739. McLennan S.M. et al., PROVENANCE AND DIAGENESIS OF THE EVAPORITE BEARING BURNS FORMATION, MERIDIANI PLANUM, MARS, Earth and planetary science Letters 240:95-121, 2005. Spectroscopic measurements Titus T. N., WATER, WATER EVERYWHERE, Nature Vol.428, 610-611, 2004. Mars Express, ESA, 2006: http://sci.esa.int/science-e/www/object/index.cfm?fobjectid=31033 Mars Odyssey, NASA, 2007: http://mars.jpl.nasa.gov/odyssey/ MOLA, NASA, 2003: http://ssed.gsfc.nasa.gov/tharsis/mola.html Albedo Bandfield J.L., Glotch T.D., Christensen P.R., SPECTROSCOPIC IDENTIFICATION OF CARBONATE MINERALS IN THE MARTIAN DUST, Science 301:1084-1087, 2003. Bandfield J.L., Hamilton V.E., Christensen P.R., A GLOBAL VIEW OF MARTIAN SURFACE COMPOSITION FROM MGS-TES, Science 287:1626-1630, 2000. Ruff S.R., SPECTRAL EVIDENCE FOR ZEOLITEIN THE DUST ON MARS, Icarus168:131-143, 2004. Wyatt M.B., McSweenH.Y. Jr, SPECTRAL EVIDENCE FOR WEATHERED BASALT AS AN ALTERNATIVE TO ANDESITE IN THE NORTHERN LOWLANDS OF MARS, Nature 417:263-266, 2002. Wyatt M.B., McSweenH.Y. Jr., Tanaka K.L., Head J.W. III, GLOBAL GEOLOGIC CONTEXT FOR ROCK TYPES AND SURFACE ALTERATION ON MARS, Geology 32:645-648, 2004.

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Mineral alteration Squyres S.,et al., THE OPPORTUNITY ROVE’S ATHENA SCIENCE INVESTIGATION AT MERIDIANI PLANUM, MARS, Science 306, 1698-1703, 2004 Bell et al., MINERALOGIC AND COMPOSITIONAL PROPERTIES OF MARTIAN SOIL AND DUST: RESULTS FROM MARS PATHFINDER, Journal of geophysical research, 105 1721-1756, 2000. Christensen P.R. et al, FORMATION OF THE HEMATITE-BEARING UNIT IN MERIDIANI PLANUM: EVIDENCE FOR DEPOSITION IN STANDING WATER,Journal of geophysical research 109, 2004. Christensen P.R. et al, DETECTION OF CRYSTALLINE HEMATITE MINERALIZATION ON MARS BY THE THERMAL EMISSION SPECTROMETER, EVIDENCE FOR NEAR SURFACE WATER, Jounal of geophysical research 105, 2000. Klingelhofer G. Et al., JAROSITE AND HEMATITE AT MERIDIANI PLANUM FROM OPPORTUNITY MOSSBAUER SPECTROMETER, Science 306:17401745, 2004. TES, THERMAL EMISSION SPECTROMETER, Mars Global Surveyor, 2006: http://tes.asu.edu/ Gullies in general and on Earth Head J.W., Marchant D.R., Dickson J.L., Levy J.S., Morgan G.A., MARS GULLY ANALOGS IN THE ANTARCTIC DRY VALLEYS: GEOLOGICAL SETTING AND PROCESSES, Lunar and Planetary Science XXXVIII, 2007. Morgan G.A., Head J.W., Marchant D.R., Dickson J.L., Levy J.S., GULLY FORMATION ON MARS: TESTING THE SNOWPACK HYPOTHESIS FROM ANALYSIS OF ANALOGS IN THE ANTARCTIC DRY VALLEYS, Lunar and Planetary Science XXXVIII, 2007. Dickson J.L., Head J.W., Marchant D.R., Morgan G.A., Levy J.S:, RECENT GULLY ACTIVITY ON MARS: CLUES FROM LATE-STAGE WATER FLOW IN GULLY SYSTEMS AND CHANNELS IN THE ANTARCTIC DRY VALLEYS, Lunar and Planetary Science XXXVIII, 2007. Lee P, Glass B.J., Osinski G.T., Parnell J., Schutt J.W., McKay C.P.,.GULLIES ON MARS: FRESH GULLIES IN DIRTY SNOW, DEVON ISLAND, HIGH ARCTIC, AS ENDMEMBER ANALOGS, Lunar and Planetary Science XXXVII, 2006. Lee P., Cockell C.S., McKay C.P., GULLIES ON MARS: ORIGIN BY SNOW AND ICE MELTING AND POTENTIAL FOR LIFE BASED ON POSSIBLE ANALOGS FROM DEVON ISLAND, HIGH ARCTIC, Lunar and Planetary Science, 2004.

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Treiman A.H., MARTIAN GULLIES AND GROUNDWATER: A SERIES OF UNFORTUNATE EXCEPTIONS, Lunar and Planetary Science XXXVI, 2005. Martian gullies Orientation of gullies Lanza N.L., Gilmore M.S., DEPTHS, ORIENTATION AND SLOPES OF MARTIAN HILLSIDE GULLIES IN THE NORTHERN HEMISPHERE, Lunar and Planetary Science XXXVII, 2006. Mohan S., Bridges N.T., ANALYSIS OF ORIENTATION-DEPENDENCE OF MARTIAN GULLIES, Lunar and Planetary Science XXXV, 2004. Balme M., Mangold N., Baratoux D., Costard F., Gosselin M., Masson P., Pinet P., Neukum G., HRSC team, ORIENTATION AND DISTRIBUTION OF RECENT GULLIES IN THE SOUTHERN HEMISPHERE OF MARS: OBSERVATIONS FROM HRSC/MEX AND MOC/MGS DATA, Lunar and Planetary Science XXXVII, 2006. Williams R.M.E., A REASSESSMENT OF THE SPATIAL ORIENTATION OF GULLIES IN THE MARTIAN MID-LATITUDES, Lunar and Planetary Science XXXVIII, 2007. Bridges N.T., NORTHERN MANTLING MIGRATION XXXVI, 2005.

Lackner C.N., AGE-ORIENTATION RELATIONSHIPS OF HEMISPHERE MARTIAN GULLIES AND “PASTED-ON” UNIT: IMPLICATIONS FOR NEARSURFACE WATER IN MARS’ RECENT HISTORY, Lunar and Planetary Science

Berman D.C. et al, SURVEY OF CRATERS WITH ARCUATE RIDGES AND GULLIES IN THE NEWTON BASIN REGION ON MARS, Lunar and Planetary Science XXXVI, 2005. Sears D., Roe1 L., Moore S., STABILITY OF WATER AND GULLY FORMATION ON MARS, Lunar and Planetary Science XXXVI, 2005. Heldmann J.L., Carlsson E., Johansson H., Michael T. Mellon M.T., Owen B. Toon O.B., OBSERVATIONS OF MARTIAN GULLIES AND CONSTRAINTS ON POTENTIAL FORMATION MECHANISMS II. THE NORTHERN HEMISPHERE, Icarus 188: 324–344, 2007. Dry granular flow Treiman A.H., Louge M.Y., MARTIAN SLOPE STREAKS AND GULLIES: ORIGINS AS DRY GRANULAR FLOWS, Lunar and Planetary Science XXXV, 2004. Reiss D., Jaumann R. Kereszturi A., Sik A., Neukum G., GULLIES AND AVALANCHE SCARS ON MARTIAN DARK DUNES, Lunar and Planetary Science XXXVIII, 2007.

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Daerr A., Douady S., TWO TYPES OF AVALANCHE BEHAVIOUR IN GRANULAR MEDIA, Nature 399:241-243, 1999. CO2 formation Musselwhite D.S., Swindle T.D., Lunine J.I., LIQUID CO2 BREAKOUT AND THE FORMATION OF RECENT SMALL GULLIES ON MARS, Lunar and Planetary Science XXXII, 2001. Cedillo-Flores Y., Durand-Manterola H.J., FORMATION OF MARTIAN GULLIES: MECHANISM SUGGESTED, Lunar and Planetary Science XXXVIII, 2007. Ishii T., Sasaki S, FORMATION OF RECENT MARTIAN GULLIES BY AVALANCHES OF CO2 FROST, Lunar and Planetary Science XXXV, 2004. Ishii T., Miyamoto H., Sasaki S., Tajika T., CONSTRAINTS ON THE FORMATION OF GULLIES ON MARS: A POSSIBILITY OF THE FORMATION OF GULLIES BY AVALANCHES OF GRANULAR CO2 ICE PARTICLES, Lunar and Planetary Science XXXVII, 2006. Formation by melting snow and ice. Bridges N.T., Lackner C.N., AGE-ORIENTATION RELATIONSHIPS OF NORTHERN HEMISPHERE MARTIAN GULLIES AND “PASTED-ON” MANTLING UNIT: IMPLICATIONS FOR NEARSURFACE WATER MIGRATION IN MARS’ RECENT HISTORY, Lunar and Planetary Science XXXVI, 2005. Christensen P.R., FORMATION OF RECENT MARTIAN GULLIES THROUGH MELTING OF EXTENSIVE WATER-RICH SNOW DEPOSITS, Nature 422, 2003. Kossacki K.J., Markiewicz W.J., SEASONAL MELTING OF SURFACE WATER ICE CONDENSING IN MARTIAN GULLIES, Icarus 171:272–283, 2004. Soare R.J., Wan Bun Tseung J.M., Osinski G.R., GULLY FORMATION, PERIGLACIAL PROCESSES AND POSSIBLE NEAR-SURFACE GROUNDICE IN UTOPIA PLANITIA, Lunar and Planetary Science XXXVII, 2006. Spring Water formation Heldmann J.L., H. Johansson H., Carlsson E, Mellon M.T., NORTHERN HEMISPHERE GULLIES ON MARS: ANALYSIS OF SPACECRAFT DATA AND IMPLICATIONS FOR FORMATION MECHANISMS, Lunar and Planetary Science XXXVI, 2005. Heldmann J.L., Mellon M.T., GULLIES ON MARS AND CONSTRAINTS IMPOSED BY MARS GLOBAL SURVEYOR DATA, Lunar and Planetary Science XXXV, 2004.

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Fassett C.I., Head J.W., Dickson J.L., GULLIES WITH CHANGING APPEARANCE ON MARS: REGIONAL CHARACTERISTICS AND GEOLOGICAL SETTING, Lunar and Planetary Science XXXVIII, 2007. Edlund S.J., Heldmann J.L., CORRELATION OF SUBSURFACE ICE CONTENT AND GULLY LOCATIONS ON MARS: TESTING THE SHALLOW AQUIFER THEORY OF GULLY FORMATION, Lunar and Planetary Science XXXVII, 2006. Malin M.C., Edgett K.S., Posiolova L.V., Shawn M., McColley S.M., Eldar Z. Noe Dobrea E.Z., PRESENT-DAY IMPACT CRATERING RATE AND CONTEMPORARY GULLY ACTIVITY ON MARS, Science vol 314:1573-1577, 2006 Heldmann J.L., Carlsson E., Johansson H., Mellon M.T., Toon O.B., OBSERVATIONS OF MARTIAN GULLIES AND CONSTRAINTS ON POTENTIAL FORMATION MECHANISMS II. THE NORTHERN HEMISPHERE, Icarus 188: 324–344, 2007. Malin M.C., Edgett K.S., EVIDENCE FOR RECENT GROUNDWATER SEEPAGE AND SURFACE RUNOFF ON MARS, Science 288: 2330, 2000. Groundwater flor formation Marquez A. De Pablo M.A., Oyarzun R., Viedma C., EVIDENCE OF GULLY FORMATION BY REGIONAL GROUNDWATER FLOW IN THE GORGONUM–NEWTON REGION (MARS), Icarus 179: 398–414, 2005. IMAGE CREDITS Figure 1 South Polar Cap: MOC2-1108, Malin Space Science System, www.msss.com Figure 2 North Polar Cap: MOC2-1478, Malin Space Science System, www.msss.com Figure 3 Channels on Mars: http://wapi.isu.edu/Geo_Pgt/Mod09_Mars/mars.htm Figure 4 Rivers on Earth: http://wapi.isu.edu/Geo_Pgt/Mod09_Mars/mars.htm Figure 5 Gully: Head J.W. et al, Lunar and Planetary Science XXXVIII, 1617.pdf, 2007 Figure 6 Deep hole on Mars: Candidate Cavern Entrance Northeast of Arsia Mons PSP_003647_1745, http://hiroc.lpl.arizona.edu/images/2007/details/cut/PSP_3647_1745_cut_b.jpg Figure 7 Sotano “La Lucha”: http://66.84.43.96/expedicion/html/sotanolalucha.html Figure 8 Sotano “Las Golondrinas”: http://www.jornada.unam.mx/viajera/?destino=san+luis+potosi&seccion=06 Figure 9 Sedimentary rocks: http://photojournal.jpl.nasa.gov/jpeg/PIA08065.jpg

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Figure 10 Close-up sediments: http://www.jpl.nasa.gov/missions/mer/images.cfm?id=1862 Figure 11 Sediments on Earth: unknown (my archive). Figure 12 Depth of ice: http://mars.jpl.nasa.gov/odyssey/gallery/martianterrain/images/PIA09336Bandfield_fig4_rgb_br.jpg Figure 13 Albedo: Elements, V2, N3, 2006. Figure 14 Mineral abundance: http://tes.asu.edu/ Figure 15 Martian gully: http://liftoff.msfc.nasa.gov/Shared/News2000/MarsWater/gully.jpg Figure 16 Terrestrial gully http://liftoff.msfc.nasa.gov/Shared/News2000/MarsWater/gully.jpg Figure 17 Gullies on Mars: http://www.msss.com/mars_images/moc/june2000/sp_pit/sp_pit_100.jpg Figure 18 Gullies orientation: Berman D.C. et al, SURVEY OF CRATERS WITH ARCUATE RIDGES AND GULLIES IN THE NEWTON BASIN REGION ON MARS, Lunar and Planetary Science. Figure 19 New structures: http://www.msss.com/mars_images/moc/2006/12/06/gullies/not_dust/index.html#Fig1b Figure 20 Morphology of gullies: Reiss D., Jaumann R. Kereszturi A., Sik A., Neukum G., GULLIES AND AVALANCHE SCARS ON MARTIAN DARK DUNES, Lunar and Planetary Science XXXVIII, 2007. Figure 21 CO2 frost: Ishii T., Sasaki S, FORMATION OF RECENT MARTIAN GULLIES BY AVALANCHES OF CO2 FROST, Lunar and Planetary Science XXXV, 2004. Figure 22 Snow mantle: Christensen P.R., FORMATION OF RECENT MARTIAN GULLIES THROUGH MELTING OF EXTENSIVE WATER-RICH SNOW DEPOSITS, Nature 422, 2003. Figure 23 New gullies formed: Malin M.C., Edgett K.S., Posiolova L.V., Shawn M., McColley S.M., Eldar Z. Noe Dobrea E.Z., PRESENT-DAY IMPACT CRATERING RATE AND CONTEMPORARY GULLY ACTIVITY ON MARS, Science vol 314:1573-1577, 2006 Figure 24 Alcove diagram: Heldmann J.L., Carlsson E., Johansson H., Mellon M.T., Toon O.B., OBSERVATIONS OF MARTIAN GULLIES AND CONSTRAINTS ON POTENTIAL FORMATION MECHANISMS II. THE NORTHERN HEMISPHERE, Icarus 188: 324–344, 2007.

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Figure 25 Groudwater flow theory: Marquez A. De Pablo M.A., Oyarzun R., Viedma C., EVIDENCE OF GULLY FORMATION BY REGIONAL GROUNDWATER FLOW IN THE GORGONUM–NEWTON REGION (MARS), Icarus 179: 398–414, 2005. Figure 26: H2O derived map: Elements, V2M3, 2006, ISSN 1811-5209 Figure 27 Clusters of gullies: Malin M.C., Edgett K.S., EVIDENCE FOR RECENT GROUNDWATER SEEPAGE AND SURFACE RUNOFF ON MARS, Science 288: 2330, 2000.

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