P23A-1085
New Mars Water Cycle Simulations Constrained by Laboratory Microphysical Results
Water ice clouds have an important role within the martian climate. While these clouds in general contain much less water mass and are optically thinner than their terrestrial counterparts, the thin martian atmosphere is very susceptible to their radiative effects. Furthermore, water ice clouds have a critical role in moderating the atmospheric transport of water vapor (via sedimentation of cloud particles) and dust (via scavenging of dust as cloud nuclei). Understanding the current climate of Mars and its water cycle requires a thorough understanding of martian water ice clouds. The microphysical processes that govern their formation and growth largely determine the effective role of water ice clouds. However, in most studies of martian water ice clouds these microphysics have been either neglected or greatly simplified. In those models that do attempt a more detailed treatment of cloud growth, the implementation of the critical parameters most important to martian clouds, namely the critical supersaturation, contact parameter and growth rate, is still handicapped by having only terrestrial analogs and data sets from which to derive constraints. The work presented here utilizes new laboratory measurements of cloud formation and growth in martian conditions to constrain a sophisticated hybrid-moment cloud model incorporated into the NASA Ames GCM. Significant differences are seen between water cycle predictions using traditional assumptions to those using the new cloud model and laboratory constraints. These differences, including changes in total water vapor and cloud column amount and surface frost distribution and seasonality, and the implication to water cycle processes and observations will be discussed.
P23A-1086
Laboratory Measurements for Ice Cloud Nucleation Under Martian Atmospheric Conditions on Various Dust Analogs
Understanding the role of water ice clouds in the Martian water cycle and climate as observed by Mars Global Surveyor, Mars Reconnaissance Orbiter, and Mars Odyssey depends on cloud properties such as particle size and number distributions. These, in turn, depend on poorly understood heterogeneous nucleation properties. Using classical nucleation theory, we have investigated the effect of substrate mineralogy and other parameters on model variables such as the contact parameter, m, and critical saturation ratio, S, defined as the partial pressure at nucleation divided by the equilibrium vapor pressure at the same temperature. Laboratory experiments have been performed under Martian temperature and water partial pressure conditions to determine S as a function of temperature and dust composition. Using infrared spectroscopy to monitor ice nucleation and growth, we find values of S ranging from 1 to 7 for water pressures between 5x10-7 and 1x10-4 torr and temperatures between 155 and 185 K. Below 175 K, significant supersaturation conditions may be required to nucleate water ice clouds on dust particles. The physical basis for this temperature dependence will be discussed, and simulations using the NASA Ames Mars General Circulation Model will examine the implications of these findings for the cycling of water between the surface and atmosphere.
P23A-1087
Geo-Effective Solar Flare Events In December 2006: Space Weather Effect on Mars and Venus Oxygen Loss to Space
In Dec. 2006, single sunspot region produced a series of proton solar flares, up to X9.0 level on 5 Dec 2006 10:35 UT. One unique feature of this X9.0 flare is that MeV particles originated from this proton flare were observed at Venus and Mars by Venus Express (VEX) and Mars Express (MEX), which are respectively located away from Earth by nearly +160° and -160° as viewed from the Sun. On 5 Dec 2006, the plasma instruments ASPERA-3 and ASPERA-4 on board MEX and VEX have detected a large enhancement in their respective background count level, which is a typical signature of intensive MeV particle flux. The timing of these enhancements were consistent with the estimated field-aligned travel time along the Parker spiral from the site of X9.0 flare to Venus and Mars. The Mars Express data indicate a one-order enhancement in the heavy ion outflow from the Martian atmosphere during the SEP period. This is the first observation of the increase of escaping flux at Mars during a violent solar activity. This suggests that the solar EUV flux levels also significantly affect the atmospheric loss from unmagnetized planets.
P23A-1088
CRISM Retrieval of Surface Lambert Albedos for Multispectral Mapping of Mars with DISORT- based Radiative Transfer Modeling: Phase 1 -- Using Historical Climatology for Temperatures, Aerosol Optical Depths, and Atmospheric Pressures
The CRISM hyperspectral imaging spectrometer has been acquiring data from its platform on MRO since September 2006. The data consists of both: (i) hyperspectral targeted observations with high spatial resolution (~18 m/pixel) and 544 spectral channels, and (ii) multispectral (MSP) mapping strips with lower spatial resolution (~200 m/pixel) and 72 spectral channels. The spectral coverage for both modes of observation is 0.362-3.920 μm. Herein, we discuss the DISORT-based radiative transfer system ('CRISM_LambertAlb') for atmospheric and thermal correction of the MSP mapping-mode data, which is being processed in a pipeline fashion. Currently, in this phase-one version of the system, we use aerosol optical depths, surface temperatures, and lower-atmospheric temperatures, all from climatology based upon data from MGS/TES, and surface altimetry from MGS/MOLA. The DISORT-based model takes as input the dust and ice aerosol optical depths (scaled to the CRISM wavelength range), the surface pressures (computed from MOLA altimetry, TES lower-atmospheric thermometry, and Viking-based pressure climatology), the surface temperatures, the reconstructed instrumental photometric angles, and the measured I/F spectrum, and then it outputs a Lambertian albedo spectrum. After discussing the capabilities and limitations of the pipeline software system CRISM_LambertAlb, we then demonstrate its application on several multispectral data cubes -- particularly, the outer reaches of the northern ice cap, the interior layered deposits in Juventae Chasma, the Tyrrhena Terra area northeast of the Hellas basin, and the Phoenix landing site area in the northern plains. For the icy spectra near the northern ice cap, aerosols need to be included in order to properly correct for the CO2 absorption in the H2O ice bands at wavelengths near 2.0 microns. For the ILDs in Juventae Chasma, the correction of the photometric angles for slopes is necessary in order to more robustly locate hydrated sulfates with the spectral summary parameters. In future phases of software development, we intend to use CRISM data directly in order to retrieve the climatological spatiotemporal maps of aerosol optical depths, surface pressure and surface temperature. This will allow a second level of refinement in the atmospheric and thermal correction of CRISM multispectral data.
P23A-1089
MRO CRISM Investigation of Hydrated Materials Associated with the North Polar Residual Cap on Mars
The Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) onboard the Mars Reconnaissance Orbiter (MRO) is a visible and near infrared hyperspectral imager with 544 channels from 362 to 3920 nm (6.55 nm spectral sampling). The MRO Primary Science Phase (PSP) began at Ls ~ 130° allowing for significant data acquisition at the high northern latitudes in advance of deteriorating atmospheric and illumination conditions. As a result of early PSP science priorities and operational constraints, the majority of the CRISM data acquired at that time were multispectral survey (200 m/pxl; 73 channel) observations. This allowed for the generation of a nearly complete multispectral survey mosaic for the north polar residual cap and the surrounding terrain (>75° latitude). CRISM multispectral mapping mosaics and related analysis products for the high northern latitudes reveal hydrated material within a number of troughs and reentrants into the north polar residual cap, as inferred from the depth and spatial coherency of the 1.9 μm absorption band. Although the identification of the hydrated exposures occurred after the close of the observing season at the high northern latitudes, a limited number of CRISM hyperspectral targeted (20-40 m/pxl; 544 channel) and multispectral window (100 m/pxl; 73 channel) observations were acquired at these locations. The higher spatial resolution data show that the hydration signature is most commonly associated with low-albedo dunes -- both in the vicinity of the Olympia Undae gypsum-bearing sand sea, and in distant polar cap reentrants. The ongoing evaluation of high resolution hyperspectral CRISM observations of hydrated materials in the vicinity of the north polar residual cap and further refinement of the multispectral mapping spectral analysis products will contribute to the targeting and acquisition of additional CRISM observations in the upcoming full northern summer of MRO operations.
P23A-1090
Mobility and topographic effects for large Valles Marineris landslides on Mars
Recent experiments on dry granular flows over horizontal plane bare some similarities with large Martian landslides observed in Valles Marineris (VM). However, Martian normalized runout are twice as large as those that observed in dry granular flow experiments. Numerical simulations on theoretical 2D and real 3D topographies reconstructed from remote sensing data show that slope effects significantly reduce the shift between experimental results and Martian observation. However, topography effects are not strong enough to explain the high mobility of Martian landslides. As a result, other physical and/or geological processes should play a key role into the dynamics of Martian landslides. A new mobility is defined that makes it possible to characterize the dynamics of the flow regardless of the geometry of the released mass and of the underlying topography.
P23A-1091
Characterization of Sulfate Minerals in Juventae Chasma from CRISM images
We are investigating the mineralogy of the Juventae Chasma region that lies north-east of Valles Marineris. The OMEGA investigation on the Mars Express spacecraft identified sulfates in the light-toned outcrops inside Juventae Chasma (Gendrin et al 2005). In particular they observed spectral features attributed to gypsum and kieserite in the mound labelled B by Catling et al (2006). This study analyzed the geomorphology using MOC, MOLA, TES and THEMIS data and formulated hypotheses on the formation of light-toned layered outcrops labeled A to D from the South-West to the North rim of Juventae Chasma. The order of magnitude increase in spatial resolution of the CRISM instrument on Mars Reconnaissance Orbiter over OMEGA is used here to assess mineralogic diversity and to analyze stratigraphic relationships between the mounds and the surrounding Chasma walls. CRISM data show evidence for diagnostic sulfate absorptions in a few locations. These are consistent with monohydrated sulfates such as kieserite and szomolnokite as well as polyhydrated sulfates such as starkeyite. The strongest occurrences of sulfates appear at the location occupied by the light-toned outcrops, in particular mound "A". A mobile basaltic sand sheet covers much of the chasma floor. Patches of olivine and pyroxene are observed in the chasma wall rock but require further spectroscopic study to assess their extent and stratigraphic relationship with the mounds. The detection of sulfate bands in the exposed light-toned material in the mounds, which date back to late – noachian / early hesperian (Catling et al. 2006), indicates the presence of an early acidic environment in the Chasma and probable evaporitic processes. The "sulfate" layered mounds present evidence of high erosion and in some regions of exhumation from the west wall of the chasma and the hummochy terrains in the north, as visible from ConText Imager (CTX) and HiRISE data. References: Gendrin A., et al.(2005). Science, 307, 1587-1591. Catling, D.C. et al. (2006). Icarus, 181,26-51.
P23A-1092
Evidence of a Paleolake in the central Valles Marineris
The Valles Marineris (VM) canyon system of Mars is closely related to large flooding channels, some of which emerge full born from chaotic terrain in canyon floors. Coprates Chasma, one of the largest VM canyons, is connected at its eastern end to chaotic terrain Capri Chasma. Despite the likely large volumes of groundwater that discharged from Capri Chasma [1], no evidence of related fluvial activity in Coprates Chasma has thus far been reported. We present an analysis of the regional topography which, together with photogeologic interpretation of available imagery, suggests that ponding due to late-stage discharge of water from Capri Chasma chaotic terrain produced a shallow lake spanning parts of Melas, Coprates, Capri and Eos Chasmata (MCCE). Overflow of this lake at its eastern end resulted in delivery of water to downstream chaos regions and outflow channels. Our ponding hypothesis is motivated primarily by the identification of scarp and terrace features which, despite a lateral spread of about 1500 km, have similar elevations. Furthermore, these elevations correspond to the maximum ponding elevation of the region (-3562 m). Mean lake depth is 842 m. Simulated ponding in the MCCE system yields an overflow point at its eastern extremity, in Eos Chasma. The neighborhood of this overflow point contains clear indicators of fluvial erosion in a consistent east-west orientation (Figure 4). Specifically, scour marks suggest an eastward convergence of flow lines. Downstream of the overflow point, the direction set by the scour marks is paved by a smooth deposit leading directly to a scoured channel entering the next major region of chaotic terrain, Aurorae Chaos. The smooth deposit is likely made up of remnants of the interior deposit breached by the MCCE paleolake overflow. The next region of chaotic terrain downstream of Aurorae Chasma is Hydraotes Chaos, which lies in a relatively deep depression and would be the next significant ponding location for flow originating in the MCCE region. Crucially, it is also the only other Chryse Planitia chaos to exhibit terraces, which have been attributed a lacustrine origin by other authors [2]. References: [1] Carr M. H. (1979) JGR, 84, 2995-3007. [2] Ori G. G. and Mosangini C. (1998) JGR, 103, 22713-22724.
P23A-1093
Chemical Composition of Meridiani Sediments: Traces of Aqueous Past on Martian Surface
Measurements of outcrop samples by the Alpha Particle X-Ray Spectrometer (APXS), onboard the NASA Mars Exploration Rover Opportunity at Meridiani, showed strong sulfur peaks in the x-ray spectra. Sulfur concentrations increased from natural (as is) rock surfaces over brushed to abraded rocks that turned out to be sulfur-loaded sediments. Along the 11-kilometer traverse of the rover many abraded surfaces could be measured by the APXS because the softness of the outcrops permitted grinding by the Rock Abrasion Tool (RAT) until today. All outcrop samples exhibited high sulfur concentrations of more than 6 weight percent; some samples exceeded 10 wt-% making S a major element and indicating a special history of these sediments. Element concentrations of all abraded rocks along the traverse were studied as function of sulfur content. A linear relationship with a negative slope was found for the silicon-sulfur pair. A similar relation holds for Al, Na, K, P, Ti, and Cr versus S. Iron shows a weak correlation with S (only a slight negative slope). Constant concentrations are exhibited by Mn and Ni. Calcium, Mg, and Zn, reveal a slight increase with increasing S contents (positive slope). During the first half of the traverse Mg and S are strongly correlated, later almost none. The formation of the sediments can be described by a two-component mixing model, where sulfur is mainly present in one component. The composition of the other component, the siliciclastic material, was extrapolated from above sample compositions to low S contents. The derived siliciclastic composition differs from encountered basaltic material, such as 'Bounce Rock' at Meridiani or the Adirondack Class rocks at Gusev crater, but, is similar to rocks discovered near Home Plate (Gusev). Best compositional matches are found for 'Masada Clod', 'Raquelme3', and others, which are significantly altered from an original basaltic composition. Apparently this composition type is wider spread on the Martian surface. The other mixing component contains various sulfates. Assuming large volcanic exhalations of sulfur, any original aqueous solution became very acidic. 'Normal' rocks were rapidly leached and gradually dissolved to form new compounds and large quantities of sulfates in an aqueous system. To bring the two components together, either wind and/or water did the transport. The small scatter of the concentration data points (mostly around a straight line) suggests that there was a concentration gradient in bodies of standing water on a kilometer-wide scale at least for a short period of time. The concentrations of many elements (Si, Al, Na, K, P, Ti, and Cr) are diluted by increasing sulfur contents. Hence, these elements were mainly part of the siliciclastic component. On the other hand, elements whose concentrations increase with increasing S (e.g. Ca, Mg, and Zn) were part of sulfates and of mafic minerals (in the siliciclastic component). Iron showing some dilution by sulfur was determined by Mössbauer spectroscopy to be present also as ferric sulfate. The above observations reveal that several elements formed sulfates in these sediments: Mg, Ca, Fe, and Zn. An aqueous system existed during the period of sediment formation and left unique traces in the sedimentary composition.
P23A-1094
Evidence for possible widespread alteration of basaltic terrains on Mars
Thermal infrared spectral data returned from the THEMIS instrument on the Mars Odyssey Spacecraft display localized variations in the olivine content in northeast Argyre Planitia. Isolated hills and rough terrain near the crater rim have both high olivine concentrations up to 20% and high thermal inertia values consistent with coarse particulate materials or bedrock exposures. Numerous gullies are often located on the slopes of this rough terrain that appear to have transported material from the olivine rich terrain to the relatively flat surrounding plains. The material within the gullies and plains is significantly depleted in olivine content relative to the olivine rich source terrain. However, the plains do not contain significant alteration products visible in either TES or OMEGA spectral datasets and are consistent with basaltic surfaces. These observations imply that surfaces that have been characterized as relatively unaltered basalts based on orbital spectroscopic observations may instead have been significantly altered since the time of their formation. This region displays this relationship particularly well, but a similar pattern appears to be widespread on Mars. High thermal inertia units associated with rocks or bedrock outcrops, such as Ares Valles, Gusev Crater, and Nili Fossae are typically enriched in olivine relative to the surrounding lower inertia regolith. It is possible that the process responsible for producing the Martian regolith involves a significant amount of compositional alteration through weathering or winnowing. For example, the weathering environment on Mars has been hypothesized to be highly conducive to the dissolution of olivine much more rapidly than other common minerals on Mars such as pyroxene and plagioclase. A possible implication of these observations is that altered surfaces are much more common than previously acknowledged on Mars. In addition, exposures of unaltered igneous compositions may be relatively rare and limited to isolated windows.
P23A-1095
Silica Deposits Within Gusev Crater: Clear Evidence for Martian Water
Out of the nearly 200 samples analyzed by the Alpha Particle X-ray Spectrometer (APXS) onboard the Mars Exploration Rover (MER) Spirit within Gusev Crater, 10 measurements exceed 60 wt% SiO2. The highest measured concentration of SiO2 is in excess of 90 wt%. All of these samples are found along the eastern margin of Home Plate, localized within a region less than 50 meters in diameter. These occurrences include rocks, subsurface soil excavated by the rover wheels, and centimeter-scale nodules. Higher concentrations of Si in these samples correspond to lower Fe, Mg, and Ni as well as generally higher Ti and Cr. In one example, after the basaltic soil contamination is removed, the renormalized composition consists of 95% SiO2, 1.2% TiO2, and 0.3% Cr2O3. Other remaining oxides include small amounts of MgO, Al2O3, and SO3. The elemental chemistry of these samples clearly indicates the presence of abundant silica. Excess silica (up to 35 wt%) is also found in the distinct, but related, light-toned Paso Robles class soils which are dominated by sulfates and likely fumarolic in origin. Silica is readily mobilized in aqueous solutions and its solubility is relatively independent of pH under acid to neutral conditions but strongly dependent on the temperature of the fluid (higher temperatures correspond to greater solubility). Geochemical indicators (Zn, Ni, and Cl trends) in other samples east of Home Plate are consistent with interaction with fluids at elevated temperatures. Thus one possible process for concentrating SiO2 in the measured samples involves the dissolution of silicate rocks in hydrothermal fluids and reprecipitation of silica as the solutions cooled and evaporated. Given the proximity to likely fumarolic deposits (the silica concentrations are found within 100 meters of a Paso Robles class soil exposure), an alternative process may involve interactions with acidic vapors and small amounts of water which leached many elements leaving behind a residual dominated by silica. This alternative is supported by the association with Ti, as acid-sulfate weathered analogs from Kilauea Volcano (Hawaii) are enriched in both amorphous silica and anatase. It is likely that a combination of these processes, both of which involve localized aqueous interactions, have been active along the eastern margin of Home Plate.
P23A-1096
Assessing the Potential for Ancient Habitable Environments in Gusev Crater, Mars
In order to be habitable for microbial life as we know it, an environment must provide nutrient elements, energy and liquid water. We assess the potential for habitable environments in the areas explored by the MER rover Spirit. These areas include the basaltic plains near Columbia Memorial Station, West Spur, Husband Hill, and the inner basin south of Husband Hill. Little aqueous activity apparently occurred in Gusev crater since the basaltic plains were emplaced in Hesperian times, therefore the basaltic plains were highly unlikely to have sustained habitable environments. The Columbia Hills, located ~3 km southeast of the landing site, are older than the surrounding basaltic plains. Aqueous processes have extensively altered bedrock in the Columbia Hills. Ferrous iron in the original, unaltered parent rock of hills materials has typically been oxidized extensively to form ferric oxides, hydroxides, and other ferric minerals. Migrating fluids have removed Ca and other cations, allowing residual Al to become relatively more abundant, and fluids added sulfates and chlorides. In subsurface environments on Earth, microorganisms can obtain key nutrients from the weathering of basalts. Materials examined in the Columbia Hills have comparable or greater abundances of these elements than do MORB. Wishstone rock and Watchtower outcrop have very high contents of phosphorous. Chemoautotrophs ("chemical- feeders" that obtain energy from inorganic chemicals) can thrive in subsurface environments. Mixing oxidized constituents from surface environments with generally more reduced constituents from subsurface rocks and thermal emanations provides energy to sustain microorganisms. Ferrous iron in parent materials in the Columbia Hills has been oxidized to form a variety of ferric minerals. On Earth, microbial processes have been documented to contribute to the production of goethite, hematite and other iron oxides. Observations by Spirit are consistent with the possibility that liquid water, nutrients and sources of chemical energy were simultaneously available to sustain habitable conditions in subsurface Columbia Hills materials at least some time in the distant (Noachian?) past. There is as yet no evidence that these conditions ever existed at the surface. Future research must seek to determine whether ancient migrating fluids in Gusev ever achieved the water activity necessary to sustain life.
P23A-1097
Evidence for a Possible Siliceous Sinter Deposit at Home Plate in Gusev Crater
Instruments from the Athena payload on the Mars Exploration Rover Spirit in Gusev Crater have identified materials remarkably enriched in silica. Spectra from the Miniature Thermal Emission Spectrometer (Mini-TES; ~5-25 microns) and Alpha Particle X-ray Spectrometer have revealed a high-silica phase in light-toned soil and heavily eroded outcrops extending as much as 50m from the low-lying, layered plateau known as Home Plate. The various forms of silica (SiO2) are readily distinguishable in the thermal infrared (TIR) wavelengths as is their purity. The Mini-TES spectra clearly indicate an amorphous phase with silica content >80% that is most similar to opal-A, the least crystalline form. The presence of amorphous silica is consistent with a variety of primary and secondary origins including obsidian, high-silica rhyolite/tuff, acidic alteration/leaching of silicate rocks, and precipitation from silica-saturated fluids. The Mini-TES spectra contain a prominent absorption near 8 microns and an emissivity maximum near 12 microns that depart from typical opal-A. Existing literature and new laboratory measurements demonstrate that these features can arise in certain forms of amorphous silica as a function of both geometric and scattering effects. The feature near 8 microns that typically appears as a shoulder in opal-A becomes a prominent absorption minimum with increasing emission angle as well as with the presence of porosity at the scale of a few 10s of microns. An emission peak near 12 microns is accentuated as a result of scattering due either to porosity or particle-size effects. Both of these features are common to siliceous sinter deposits that form as sedimentary precipitates around hydrothermal springs on earth. The Mini-TES spectra display these features and are best fit by spectra of natural sinter samples that contain microporosity and are measured at the high emission angles (>45 degrees) typical of Mini-TES observations. Primary igneous forms of amorphous silica such as obsidian and high-silica glass found in tuffs do not provide a good fit to the Mini-TES spectra. We have not explored fully the spectral characteristics of amorphous silica produced from acid alteration environments such as fumaroles, so this origin for the high-silica materials remains viable. However, their proximity to the Home Plate structure, which appears to be a depression buried by pyroclastic deposits, leads to the hypothesis that Home Plate represents a buried hot spring with an associated apron of siliceous sinter. Continued exploration of the site by the Spirit rover is intended to further investigate this hypothesis as well as the alternative hypothesis of a fumarolic origin for the high-silica materials.
P23A-1098
Episodic Geologic Evolution Of Mars
After almost 4 years of operation in orbit, the High Resolution Stereo Camera (HRSC) Experiment on ESA's Mars Express Orbiter has covered almost 40% of the surface of Mars at a resolution of 10-20 m/pixel in color and stereo. By now, the international team of Co-Investigators has investigated much of the major structures appearing to have been shaped by volcanic, fluvial, glacial, or hydrothermal activity. Contrary to early Viking-based attempts of understanding the time-stratigraphic relationships on the martian surface by crater-counting techniques and principles of stratigraphic superposition, where most of the geological units and constructs came out as being rather old, in the range of billions of years, the new HRSC-based data tell us that Mars had continued activity throughout its whole history from more than 4 Ga ago until very recently, i.e. millions of years ago or in a few areas ongoing now. The new data, though, show episodic geological activity. There is a striking appearance of peaking of the geological activity or episodicity of resurfacing at certain times: approx. 3.5 Ga, 1 to 1.5 Ga, 300 to 600 m.y., approx. 200 m.y. ago, respectively. Even more striking is that within relatively narrow limits, the cratering ages of the different age groups fall together with the age groups of martian meteorites. The martian meteorite ages reflect both igneous events and aqueous alteration events. So do the cratering ages. There is a remarkable paucity of age occurrences in the 2-3 Ga age range in the cratering data. This corresponds to a paucity of meteorite ages in the same, even somewhat more extended age range. This appears to be a hint to either lower geologic activity in this time frame, or the covering up of more ancient activity by subsequent events <2 Ga ago, with the exception of the residues from the time >3 Ga ago (the peak at approx. 3.5 Ga) when the martian surface was thoroughly shaped at a very high level of activity by gigantic volcanic, fluvial, and glacial events which could not be completely erased by later events. The episodic behavior of martian geologic activity over time can possibly be explained by responses to the evolution of the interior of the planet that has just not reached plate tectonics. At the time of the peak of volcanic and fluvial activity around 3.7 to 3.3 Ga ago, Mars changed its environment from more basic to more acidic, and the large sulfate deposits formed. Then, rapidly, Mars fell dry on a global scale.