Valles Marineris Research Articles

A new model to simulate the Martian mesoscale and microscale atmospheric circulation: Validation and first results

The Laboratoire de Météorologie Dynamique (LMD) Mesoscale Model is a new versatile simulator of the Martian atmosphere and environment at horizontal scales ranging from hundreds of kilometers to tens of meters. The model combines the National Centers for Environmental Prediction(NCEP)‐National Center for Atmospheric Research (NCAR) fully compressible nonhydrostatic Advanced Research Weather Research and Forecasting (ARW‐WRF) dynamical core, adapted to Mars, with the LMD‐general circulation model (GCM) comprehensive set of physical parameterizations for the Martian dust, CO2, water, and photochemistry cycles. Since LMD‐GCM large‐scale simulations are also used to drive the mesoscale model at the boundaries of the chosen domain of interest, a high level of downscaling consistency is reached. To define the initial state and the atmosphere at the domain boundaries, a specific “hybrid” vertical interpolation from the coarse‐resolution GCM fields to the high‐resolution mesoscale domain is used to ensure the stability and the physical relevancy of the simulations. Used in synoptic‐scale mode with a cyclic domain wrapped around the planet, the mesoscale model correctly replicates the main large‐scale thermal structure and the zonally propagating waves. The model diagnostics of the near‐surface pressure, wind, and temperature daily cycles in Chryse Planitia are in accordance with the Viking and Pathfinder measurements. Afternoon gustiness at the respective landing sites is adequately accounted for on the condition that convective adjustment is turned off in the mesoscale simulations. On the rims of Valles Marineris, intense daytime anabatic (∼30 m s−1) and nighttime katabatic (∼40 m s−1) winds are predicted. Within the canyon corridors, topographical channeling can amplify the wind a few kilometers above the ground, especially during the night. Through large‐eddy simulations in Gusev Crater, the model describes the mixing layer growth during the afternoon, and the associated dynamics: convective motions, overlying gravity waves, and dust devil–like vortices. Modeled temperature profiles are in satisfactory agreement with the Miniature Thermal Emission Spectrometer (Mini‐TES) measurements. The ability of the model to transport tracers at regional scales is exemplified by the model's prediction for the altitude of the Tharsis topographical water ice clouds in the afternoon. Finally, a nighttime “warm ring” at the base of Olympus Mons is identified in the simulations, resulting from adiabatic warming by the intense downslope winds along the flanks of the volcano. The surface temperature enhancement reaches +20 K throughout the night. Such a phenomenon may have adversely affected the thermal inertia derivations in the region.

New evidence for a magmatic influence on the origin of Valles Marineris, Mars

In this paper, we show that the complex geological evolution of Valles Marineris, Mars, has been highly influenced by the manifestation of magmatism (e.g., possible plume activity). This is based on a diversity of evidence, reported here, for the central part, Melas Chasma, and nearby regions, including uplift, loss of huge volumes of material, flexure, volcanism, and possible hydrothermal and endogenic-induced outflow channel activity. Observations include: (1) the identification of a new > 50 km-diameter caldera/vent-like feature on the southwest flank of Melas, which is spatially associated with a previously identified center of tectonic activity using Viking data; (2) a prominent topographic rise at the central part of Valles Marineris, which includes Melas Chasma, interpreted to mark an uplift, consistent with faults that are radial and concentric about it; (3) HiRISE-identified landforms along the floor of the southeast part of Melas Chasma that are interpreted to reveal a volcanic field; (4) CRISM identification of sulfate-rich outcrops, which could be indicative of hydrothermal deposits; (5) GRS K/Th signature interpreted as water–magma interactions and/or variations in rock composition; and (6) geophysical evidence that may indicate partial compensation of the canyon and/or higher density intrusives beneath it. Long-term magma, tectonic, and water interactions (Late Noachian into the Amazonian), albeit intermittent, point to an elevated life potential, and thus Valles Marineris is considered a prime target for future life detection missions.

Geological context of water‐altered minerals in Valles Marineris, Mars

Greater than 15,000 km2 of the layered deposits within Valles Marineris are associated with water‐altered minerals, yet their origin and history of alteration remain a mystery. There are numerous competing hypotheses for the formation of the interior layered deposits including aeolian, lacustrine, and volcanic. Recent orbiter spectroscopic data have indicated that water has played a role in their geological history. Thermal Emission Spectrometer (TES) measurements have revealed significant crystalline hematite‐bearing deposits within Valles Marineris, typically related to interior layered deposits. These hematite deposits, found with a wide range of albedo values, are associated with relatively steep bedrock exposures but can also be seen downslope on flat surfaces where they may be a lag deposit. More recently, Observatoire la Minéralogie, l'Eau, les Glaces, et l'Activité (OMEGA) data have shown hydrated sulfates covering more than 13,000 km2 area of Valles Marineris. Sulfates are found in numerous topographic settings and geological units, but are typically located along the flanks of interior layered deposits and nearby low‐lying floor units. Here we study the detailed morphologies of hematite and sulfate‐bearing units such as mantled wall units, mass‐wasting blocky deposits, massive floor deposits, and tectonically altered floor units. All of these terrains have diverse erosional styles and varied crater populations. In both hematite‐ and sulfate‐bearing units, occasionally found in conjunction with one another, formation processes require contributions from water. The results indicate a wide range of diversity within an individual mineral class, between mineral classes, and also among morphological types. The diversity of geological settings and properties suggest that any single, unified formation mechanism is improbable.

Evidence for extensive olivine‐rich basalt bedrock outcrops in Ganges and Eos chasmas, Mars

Several localized outcrops of olivine‐enriched bedrock have been previously identified in the Ganges and Eos Chasma area on the eastern end of Valles Marineris with the Thermal Emission Imaging System multispectral images. These outcrops form a layer in the walls of Ganges Chasma and appear to be the remnants of a once continuous unit, which was mapped over ∼100 km. In this study we further characterize the composition (forsterite content of ∼0.68), olivine abundance (10 to >15%), thermal inertia (>600 JK−1 m−2 s−1/2, consistent with in‐place rocky material), vertical dimension (∼60 to ∼220 m), extent (>1100 km laterally), volume (∼9.9 × 104 km3), dip (∼0.013°NE), and continuity of this layer utilizing Thermal Emission Spectrometer hyperspectral, Thermal Emission Imaging System multispectral, and Mars Orbiter Laser Altimeter elevation data. Morphologic data from high‐resolution imagery display a relatively unmantled, rough, and pitted surface associated with the olivine‐enriched material, consistent with thermal inertia data. Four possibilities for the origin of the olivine‐enriched unit are (1) volcanism associated with tectonic rifting of the Valles Marineris system, (2) a volcaniclastic flow deposit, (3) an intrusive mafic sill, or (4) a discrete episode in Martian history during which flood lavas were erupted onto the surface. The most likely origin is an eruptive event consisting of compositionally uniform flood lavas originating from a primitive mantle source region, possibly associated with the initiation of Tharsis volcanism. This unit is one of the largest continuous compositional units found on Mars and is strikingly similar to other olivine‐enriched deposits identified in previous studies where compositional, morphologic, and thermophysical similarities are observed. These similarities may indicate that there was a period in early Martian history, where compositionally uniform and extensive olivine‐enriched flood basalts were erupted on the Martian surface.

Stratigraphy and structure of interior layered deposits in west Candor Chasma, Mars, from High Resolution Stereo Camera (HRSC) stereo imagery and derived elevations

Interior layered deposits (ILDs) within western Candor Chasma were studied by mapping lithologies, measuring layer attitudes and comparing the stratigraphy of two adjacent mounds. Layering tends to dip in the same direction as the local topographic slope, although at different angles. Regionally consistent attitudes do exist, suggesting postdepositional block rotations. The stratigraphy of two adjacent mounds correlates, but the thicknesses of the units differ. Most layered material appears to have been deposited conformably, with one late major unconformity. Several fault populations are identified and correlate well with regional faults associated with the formation of Valles Marineris. The data suggest that here the ILDs predate the faulting and may be early basin fill. According to our model for ILD formation, ILDs are deposited syntectonically during early basin collapse. Subsequent subsidence of surrounding areas, accompanied by little or no sedimentation, left the early deposits as individual mounds, remnants of the former subbasins. Stratigraphic differences between mounds resulted from different subsidence rates of subbasins. A significant change in depositional environment, from depositional to a near cessation of deposition and the onset of a major erosion event, possibly coincided with the opening of the main Valles Marineris troughs. We further suggest that groundwater played an important role in the formation of sulfates. The youngest unit identified, not including surficial deposits, is likely the result of a posttectonic, regionally limited volcanic event. If basin collapse continued following the cessation of deposition, this model can also account for mounds within closed basins, such as Hebes.

Light‐toned strata and inverted channels adjacent to Juventae and Ganges chasmata, Mars

Light‐toned layered deposits on the plains adjacent to Juventae and Ganges Chasmata have been identified and analyzed with several instruments onboard the Mars Reconnaissance Orbiter, including HiRISE, CTX, and CRISM. Beds exhibit variations in brightness, color, polygonal fracturing, and erosional properties that are not seen in light‐toned layered deposits within the chasmata. At both Juventae and Ganges, the light‐toned layered deposits along the plains have associated features we interpret as fluvial, including sinuous ridges (inverted channels) that exhibit light‐toned layering and an extensive valley system for the Ganges deposit. The strong correlation between the inverted channels and light‐toned layered deposits suggests an association with fluvial‐lacustrine processes, although pyroclastic and eolian activity may also be involved in their emplacement. The identification of potential fluvial‐lacustrine layered deposits and associated valley networks suggests surface water flow in the Valles Marineris region that occurred for sustained periods during the Hesperian (3.7–3.0 Gyr).

Evidence for ponding and catastrophic floods in central Valles Marineris, Mars

The Valles Marineris canyon system of Mars is closely related to large flood channels, some of which emerge full born from chaotic terrain in canyon floors. Coprates Chasma, one of the largest Valles Marineris canyons, is connected at its west end to Melas Chasma and on its east end to chaotic terrain-filled Capri and Eos Chasmata. The area from central Melas to Eos Chasmata contains a 1500 km long and about 1 km deep depression in its floor. Despite the large volumes of groundwater that likely discharged from chaotic terrain in this depression, no evidence of related fluvial activity 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 Hesperian discharge of water possibly produced a lake (mean depth 842 m) spanning parts of the Valles Marineris depression (VMD). 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 (−3560 m). Simulated ponding in the VMD 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.

Large‐scale spring deposits on Mars?

We present a large‐scale spring hypothesis for the formation of various enigmatic light‐toned deposits (LTDs) on Mars. Layered to massive LTDs occur extensively in Valles Marineris, chaotic terrains, and several large craters, in particular, those located in Arabia Terra. Most of these deposits are not easily explained with either a single process or multiple ones, either in combination or occurring sequentially. Spring deposits can have a very wide range of internal facies and exhibit complex architectural variations. We propose the concept of large‐scale spring deposits for explaining LTDs on Mars. Stable volcano‐tectonic settings, such as the ones typical on Mars, are compatible with a large‐scale, long‐term, multistage formation of spring deposits. The large‐scale spring deposit model can explain the formation of LTDs with a common process, although active in different times and locations, compatible with coeval local or regional processes and deposits, such as volcaniclastic ones. LTDs, if formed as spring deposits derived from subsurface fluids, could potentially offer favorable conditions both to life and to the fossilization of past life forms.

Geomorphic study of fluvial landforms on the northern Valles Marineris plateau, Mars

Fluvial landforms are observed on the plateau near Echus Chasma and other locations on the Valles Marineris plateau using high resolution (10 to 50 m/pixel) Mars Express images and topography. Branching valleys have a 20‐ to 100‐m deep V‐shape profile typical of fluvial processes. Their incision occurred in a thin (<150 m) and weak, dark unit that overlies the plateau basement. The valleys are distributed close to watershed boundaries as expected for overland flows, and different from pure glacial or hydrothermal processes. The 2D geometry of valley networks show drainage densities reaching 1.3 km−1that indicate a strong 2D extension stage that usually requires a minimum of thousands of years, as established from terrestrial examples. However, the 3D valley geometry shows a limited incision, and a lack of concavity, suggesting a limited development in time (millions of years of evolution are unlikely). Many outlets connect to the heads of canyons of Echus Chasma that might involve sapping processes. These canyons might have formed coevally with shallow valleys: their difference in geometry is a consequence of the difference in lithology which induced a difference in the erosion capacity, and an enhanced infiltration conducing to sapping. Valleys are submitted to modification by aeolian processes, sometimes leading to the formation of inverted channels as observed more broadly through the region. These landforms formed late in the Early Mars history and might be considered as examples of episodic fluvial events due to short‐term climate changes and/or regional fluvial activity after the Noachian.

Ferric oxides in East Candor Chasma, Valles Marineris (Mars) inferred from analysis of OMEGA/Mars Express data: Identification and geological interpretation

The mineralogical composition of the Martian surface is constrained by analyzing the data of the OMEGA visible and near infrared imaging spectrometer onboard Mars Express. Ferric signatures had previously been reported in Valles Marineris, Margaritifer Terra, and Terra Meridiani. Here we use three independent data reduction methods (Spectral Angle Mapper, a modified Spectral Mixture Analysis and Modified Gaussian Model) to detect and map ferric oxides in East Candor Chasma, a part of Valles Marineris. Ferric oxides in East Candor Chasma are concentrated in scattered formations. MOLA altimetry indicates that the ferric oxides are preferentially located in topographic lows. THEMIS, HRSC and MOC images show that the ferric oxide spectral signatures are systematically correlated with superficial deposits of low albedo, located at the foot of, or resting on Interior Layered Deposits (ILDs). This spatial distribution suggests that ferric oxides are genetically linked to ILDs. Gravity and wind‐driven remobilization of ferric oxides previously formed in the ILDs can explain their accumulation around the ILDs.

Two geologic systems providing terrestrial analogues for the exploration of sulfate deposits on Mars: Initial spectral characterization

We present the Messinian evaporite suite (Mediterranean region) and the Solfatara hydrothermal system (Phlegraean Fields volcanic province, Italy), discuss their implications for understanding the origin of sulfates on Mars and show preliminary sets of VNIR laboratory and in situ reflectance spectra of rocks from these geologic systems. The choice was based on a number of evidence relative to Mars: (1) the chemistry of the Martian sulfates, suggesting fluid interactions with possibly alkali-basaltic rocks and/or regolith; (2) close range evidence of sulfates within sedimentary formations on Mars; (3) sulfate spectral signatures associated to large-scale layered patterns interpreted as thick depositional systems on Mars. The Messinian evaporites comprise three units: primary shallow-water sulfates (primary lower gypsum: PLG), shallow- to deep-water mixed sulfates and clastic terrigenous deposits (resedimented lower gypsum: RLG), and shallow-water associations of primary sulfates and clastic fluvio-deltaic deposits (upper evaporites: UE). The onset of the Messinian evaporites records the transition to negative hydrologic budget conditions associated with the Messinian Salinity Crisis, which affected the entire Mediterranean basin and lasted about 640 kyr. The Solfatara is a still evolving hydrothermal system that provides epithermal deposits precipitated from the interaction of fluids and trachybasaltic to phonolitic rocks. Thermal waters include alkali-chloride, alkali-carbonate and alkali-sulfate endmembers. The wide spectrum of sedimentary gypsum facies within the Messinian formation includes some of the depositional environments hitherto identified on Mars and others not found on Mars. The PLG unit includes facies associations correlated over long distances, that could be a possible analog of the stratified rock units exposed from Arabia Terra at least as far as Valles Marineris. The facies cycles within the UE unit can be compared to the sequences of strata observed in craters such as Holden and Eberswalden. The UE unit records paleoenvironmental changes which are ultimately controlled by terrestrial climatic variations. They can be considered as a reliable climatic proxy and may be useful for the reconstruction of climatic events on Mars. The intermediate Messinian RLG unit has not, at present, a well-defined depositional counterpart on Mars, although there are some similarities with the northern lowlands and Vastitas Borealis Formation. The dramatic variation of hydrologic budget conditions at the onset of the Messinian evaporites may provide criteria for the interpretation of similar variations on Mars. The volcanic rocks at the Solfatara bear some similarities with the “alkaline magmatic province” observed at the Gusev crater on Mars, and the assemblages of hydrothermal phases resulting from the Solfatara's parent rocks could be analogues for processes involving Gusev-type rocks. The Messinian sulfates have a prevalent Ca-sulfatic composition and wide textural variability. Preliminary laboratory reflectance spectra of rock samples in the VNIR region reveal the signature of sulfates and mixtures of several Fe-bearing phases. At the Solfatara, in situ reflectance measurements of epithermal minerals close to active fumaroles showed the presence of Fe-bearing sulfates, hematite, Al- and K-sulfates and abundant amorphous fraction. XRD analysis supported this interpretation. The range of depositional facies observed in the Messinian units and the variety of minerals detected in the Solfatara will be useful for the interpretation of close range data of Mars. The spectral characterization at various scales of the Messinian sedimentary facies and the Solfatara hydrothermal minerals will both help in the exploration of Mars from orbit and with close range inspection.

A glimpse of martian plumbing

Numerous long, wall-like ridges can be observed in the Valles Marineris region of Mars. They probably represent fault zones cemented by water-deposited minerals and are indicative of ancient groundwater flow.

A new hypothesis for the origin and redistribution of sulfates in the equatorial region of western Mars

The formation of sulfates on Mars has been under debate since they were identified by several Mars missions starting from the 1970s. We propose that sulfates formed as evaporites in enclosed standing bodies of water in the Valles Marineris area following the early alteration of Martian basaltic crust, were then elevated by the Tharsis uplift, and transported together with rock materials to Meridiani Planum by periodic outbursts of water, where they were deposited as sediments. The proposed model comprehensively addresses all forms of sulfate occurrences near the equator in the western Martian hemisphere and relates it to physiographic processes (volcanic, tectonic and sedimentary) affecting the Martian surface.

Ancient groundwater flow in the Valles Marineris on Mars inferred from fault trace ridges

Long linear ridges in the Valles Marineris region of Mars most likely represent fault zones that have been cemented by water-deposited minerals. This implies that water in the Martian crust could have traversed long distances via faults and fractures.

Dust haze in Valles Marineris observed by HRSC and OMEGA on board Mars Express

We present analysis of a bright haze observed inside Valles Marineris, which formed in mid northern spring. The data were collected by the High Resolution Stereo Camera (HRSC) and the imaging spectrometer, Observatoire pour la Minéralogie, l'Eau, les Glaces et l'Activité (OMEGA), aboard Mars Express. This study provides a case example of the power of simultaneous multiple emission angle and hyperspectral imaging for study of aerosols and clouds in the Martian atmosphere. The haze appeared thinner after three days and disappeared in nine days. It was limited to a 2‐km layer at the bottom of the canyon. The color was redder than the underlying surface. The analysis of the OMEGA spectra indicates that this haze was composed of dust particles. The dust layer appeared brighter with the HRSC stereo channels than the nadir channel due to longer scattering paths. We have estimated the optical depth of the haze by fitting both HRSC and OMEGA data with radiative transfer calculations. The retrieval of the optical depth is very sensitive to the aerosol scattering model used and the reflectance of the surface. Applying an aerosol scattering model derived from sky surveys at a constant elevation by the Imager for Mars Pathfinder, the optical depth of the haze is estimated from HRSC data to be within 1.7 to 2.3 at the wavelength (λ) of 0.675 μm. The wavelength dependence is obtained from OMEGA spectrum. It increases to 2.2–2.6 at λ = 1.35 μm and moderately decreases to 1.2–1.8 at λ = 2.4 μm.

Gray hematite distribution and formation in Ophir and Candor chasmata

We have studied eight of the spatially largest locations in Ophir and Candor chasmata that have the highest abundances of coarse‐grained gray hematite, as measured by the Mars Global Surveyor Thermal Emission Spectrometer (TES). All of the eight hematite locations correspond to relatively smooth dark units that overlie or are adjacent to light‐toned rock units. Linear modeling of TES spectra sampled in the eight hematite locations shows mostly mafic associations with the hematite, dominated by pyroxene and feldspar, with a lower abundance of sulfates. The spectral and morphologic similarity between the hematite‐bearing units at Ophir and Candor to those at Meridiani Planum lead us to favor a similar aqueous origin for the hematite in Valles Marineris. Water that may have infiltrated the light‐toned rocks by an increase in the water table or when lakes may have existed within the canyons could have resulted in the formation of hematite‐rich grains in a scenario similar to that proposed for Meridiani Planum. Erosion by wind of the formerly overlying or adjacent friable light‐toned rocks that contain the hematite‐rich grains enabled the more resistant hematite grains to be concentrated in sufficient abundances along the surface to be detected by TES. A more limited supply of hematite grains within the light‐toned rocks, inadequate winds, subsequent burial beneath dust or sand, and fewer low‐lying terrains upon which the grains could form a lag deposit may explain why there are smaller exposures of hematite within Valles Marineris compared to Meridiani Planum and Aram Chaos.

The timing of martian valley network activity: Constraints from buffered crater counting

Valley networks, concentrations of dendritic channels that often suggest widespread pluvial and fluvial activity, have been cited as indicators that the climate of Mars differed significantly in the past from the present hyperarid cold desert conditions. Some researchers suggest that the change in climate was abrupt, while others favor a much more gradual transition. Thus, the precise timing of valley network formation is critical to understanding the climate history on Mars. We examine thirty valley network-incised regions on Mars, including both cratered upland valley networks and those outside the uplands, and apply a buffered crater counting technique to directly constrain when valley network formation occurred. The crater populations that we derive using this approach allow assessment of the timing of the last activity in a valley network independent of the mapping of specific geological units. From these measurements we find that valley networks cluster into two subdivisions in terms characteristics and age: (1) valley network activity in the cratered highlands has an average cessation age at the Noachian–Hesperian boundary and all valleys that we crater counted are Early Hesperian or older. No evidence is found for valley networks in the cratered uplands of Late Hesperian or Amazonian age. The timing of the cessation of cratered upland valley network activity at the Noachian–Hesperian boundary also corresponds to a decline in the intensity of large crater formation and degradation and to the apparent end of phyllosilicate-type weathering. (2) A few valley network-incised regions formed outside of the cratered uplands on volcanic edifices, in association with younger impact craters, and on the rim of Valles Marineris. We applied our buffered crater counting technique to four such valleys, on the volcanoes Ceraunius Tholus, Hecates Tholus, and Alba Patera and on the rim of Echus Chasma, and find that each has distinctive and different Late Hesperian or Early Amazonian ages, indicating that valley networks formed from time to time in the post-Noachian period. Unlike the cratered upland valley networks, these isolated occurrences are very local and have been interpreted to represent local conditions (e.g., snowpack melted during periods of intrusive volcanic activity). In contrast to a gradual cessation in the formation of valley networks proposed by some workers, our new buffered crater counting results indicate a relatively abrupt cessation in the formation of the widespread cratered upland valley networks at approximately the end of the Noachian, followed only by episodic and very localized valley network formation in later Mars history, very likely due to specific conditions (e.g., local magmatic heating). These valley network ages and correlations are thus consistent with a major change in the near-surface aqueous environment on Mars at approximately the Noachian–Hesperian boundary. The Noachian environment supported surface running water and fluvial erosion across Mars in the cratered uplands, enhanced crater degradation, and a weathering environment favoring the formation of phyllosilicates. The Hesperian–Amazonian environment was more similar to the hyperarid cold desert of today, with valley networks forming only extremely rarely and confined to localized special conditions. Sources of water for these latter occurrences are likely to be related to periodic mobilization and equatorward migration of polar volatiles due to variations in spin-axis orbital parameters, and to periodic catastrophic emergence of groundwater.

Opaline silica in young deposits on Mars

High spatial and spectral resolution reflectance data acquired by the Mars Reconnaissance Orbiter Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) instrument reveal the presence of H_2O- and SiOH-bearing phases on the Martian surface. The spectra are most consistent with opaline silica and glass altered to various degrees, confirming predictions based on geochernicall experiments and models that amorphous silica should be a common weathering product of the basaltic Martian crust. These materials are associated with hydrated Fe sulfates, including H_3O-bearing jarosite, and are found in finely stratified deposits exposed on the floor of and on the plains surrounding the Valles Marineris canyon system. Stratigraphic relationships place the formation age of these deposits in the late Hesperian or possibly the Amazonian, implying that aqueous alteration continued to be an important and regionally extensive process on Mars during that time.

Correlations between hematite and sulfates in the chaotic terrain east of Valles Marineris

We have identified the presence of polyhydrated sulfates in association with crystalline gray hematite in outcrop units of the chaotic terrain east of Valles Marineris. The hematite is found in abundances of up to ∼18%, and is usually associated with thin (∼10's of meters) cliff-forming layers of intermediate-toned outcrops (albedo ∼ 0.15 – 0.20 ) as well as mantling deposits adjacent to the outcrops. The polyhydrated sulfates are usually restricted to the bedrock unit, and are not found in the adjacent mantling units. In analogy to the observations performed at the Opportunity landing site, we hypothesize that erosion of the sulfate/hematite-bearing outcrops leaves the hematite behind as a lag and breaks the sulfates down to wind-transportable sizes. We also find that the layered outcrops present, for the most part, embayment or on-lap relationships with respect to the hummocks that constitute the chaotic terrain, suggesting that these units were emplaced via subaqueous or aeolian deposition and/or flow after the event that formed the associated chaos. These morphological observations, in conjunction with the correlation between hematite and polyhydrated sulfates also suggest an aqueous genesis for the crystalline gray hematite in these chaotic units, and presents evidence for the action of aqueous processes after the formation of at least some of the chaotic units on Mars.

Evidence for aqueous deposition of hematite‐ and sulfate‐rich light‐toned layered deposits in Aureum and Iani Chaos, Mars

Two new gray hematite‐rich units have been found in Aureum and Iani Chaos, Mars, using data acquired by the Mars Global Surveyor Thermal Emission Spectrometer. These regions correspond with light‐toned sulfate‐rich deposits identified by the European Space Agency's OMEGA visible/near‐IR spectrometer. Much of the light‐toned material in Aureum and Iani Chaos is in the form of a capping unit similar to that seen in Aram Chaos. However, some light‐toned material is also seen within the chaotic mounds, indicating that it was present before the formation of the chaotic terrains. The stratigraphy of the units in Iani and Aureum Chaos suggests that the capping and hematite‐rich units must have been deposited after the formation of the chaotic terrains in the mid to late Hesperian, up to 1 Gyr after the formation of the light‐toned outcrop in Meridiani Planum. The geologic contexts of these and other hematite‐ and sulfate‐rich units in Aram Chaos, Valles Marineris, and Meridiani Planum indicate that they likely formed in similar environments under aqueous conditions. If this is the case, it suggests an active hydrological cycle lasting from the late Noachian to the late Hesperian.