Morphology, stratigraphy, and mineralogical composition of a layered formation covering the plateaus around Valles Marineris, Mars: Implications for its geological history

An extensive layered formation covers the high plateaus around Valles Marineris. Mapping based on HiRISE, CTX and HRSC images reveals these layered deposits (LDs) crop out north of Tithonium Chasma, south of Ius Chasma, around West Candor Chasma, and southwest of Juventae Chasma and Ganges Chasma. The estimated area covered by LDs is ∼42,300 km². They consist of a series of alternating light and dark beds, a 100 m in total thickness that is covered by a dark unconsolidated mantle possibly resulting from their erosion. Their stratigraphic relationships with the plateaus and the Valles Marineris chasmata indicate that the LDs were deposited during the Early- to Late Hesperian, and possibly later depending on the region, before the end of the backwasting of the walls near Juventae Chasma, and probably before Louros Valles sapping near Ius Chasma. Their large spatial coverage and their location mainly on highly elevated plateaus lead us to conclude that LDs correspond to airfall dust and/or volcanic ash. The surface of LDs is characterized by various morphological features, including lobate ejecta and pedestal craters, polygonal fractures, valleys and sinuous ridges, and a pitted surface, which are all consistent with liquid water and/or water ice filling the pores of LDs. LDs were episodically eroded by fluvial processes and were possibly modified by sublimation processes. Considering that LDs correspond to dust and/or ash possibly mixed with ice particles in the past, LDs may be compared to Dissected Mantle Terrains currently observed in mid- to high latitudes on Mars, which correspond to a mantle of mixed dust and ice that is partially or totally dissected by sublimation. The analysis of CRISM and OMEGA hyperspectral data indicates that the basal layer of LDs near Ganges Chasma exhibits spectra with absorption bands at ∼1.4 μm, and ∼1.9 μm and a large deep band between ∼2.21 and ∼2.26 μm that are consistent with previous spectral analysis in other regions of LDs. We interpret these spectral characteristics as an enrichment of LDs in opaline silica or by Al-phyllosilicate-rich layers being overlain by hydroxylated ferric sulfate-rich layers. These alteration minerals are consistent with the aqueous alteration of LDs at low temperatures.     

Melas Chasma: morphology and tectonic patterns

The central Valles Marineris is the widest part of the equatorial trough system of Mars. Melas Chasma and parts of Coprates and Candor Chasmata provide some of the clearest clues on the relationships between erosional landforms, deposits and various volcanic and tectonic features. A detailed geomorphic study of the troughs allows the identification of faults and other structures in most parts of this area, in spite of local obliteration by erosional and depositional processes. Tectonic control on erosional landforms appears mainly in the northern walls of Melas Chasma and in the edge of the inner plateau above the trough floor. Longitudinal major faults are identified only along the northern wall. However the trough may not be a simple half graben: another fault line is inferred inside Melas Chasma southern walls along the edge of a wide bench of layered deposits. A deep and relatively narrow graben linking those of Ius and Coprates Chasmata appears to be downfaulted inside a wider basin with eroded sides. Transverse or oblique faults control some outlines of these erosional landforms, whereas a few monoclines or faults restricted to the basin beds reveal compressional stresses or differential vertical movements related to the basin development.     

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.     

Diagenetic haematite and sulfate assemblages in Valles Marineris

Previous orbital mapping of crystalline gray haematite, ferric oxides, and sulfates has shown an association of this mineralogy with light-toned, layered deposits on the floor of Valles Marineris, in chaos terrains in the canyon’s outflow channels, and in Meridiani Planum. The exact nature of the relationship between ferric oxides and sulfates within Valles Marineris is uncertain. The Observatoire pour la Mineralogie, l’Eau, les Glaces et l’Activite (OMEGA) spectrometer initially identified sulfate and ferric oxides in the layered deposits of Valles Marineris. The Thermal Emission Spectrometer (TES) has also mapped coarse (gray) haematite in or at the base of these deposits. We use Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) spectra and Context Camera (CTX) and High Resolution Imaging Science Experiment (HiRISE) imagery from the Mars Reconnaissance Orbiter (MRO) to explore the mineralogy and morphology of the large layered deposit in central Capri Chasma, part of the Valles Marineris canyon system that has large, clear exposures of sulfate and haematite. We find kieserite (MgSO₄·H₂O) and ferric oxide (often crystalline red haematite) in the lower bedrock exposures and a polyhydrated sulfate without ferric oxides in the upper bedrock. This stratigraphy is duplicated in many other basinal chasmata, suggesting a common genesis. We propose the haematite and monohydrated sulfate formed by diagenetic alteration of a sulfate-rich sedimentary deposit, where the upper polyhydrated sulfate-rich, haematite-poor layers either were not buried sufficiently to convert to a monohydrated sulfate or were part of a later depositional phase. Based on the similarities between the Valles Marineris assemblages and the sulfate and haematite-rich deposits of Meridiani Planum, we hypothesize a common evaporite and diagenetic formation process for the Meridiani Planum sediments and the sulfate-bearing basinal Interior Layered Deposits.     

Hydrated mineral stratigraphy of Ius Chasma, Valles Marineris

New high-resolution spectral and morphologic imaging of deposits on walls and floor of Ius Chasma extend previous geomorphic mapping, and permit a new interpretation of aqueous processes that occurred during the development of Valles Marineris. We identify hydrated mineralogy based on visible-near infrared (VNIR) absorptions. We map the extents of these units with CRISM spectral data as well as morphologies in CTX and HiRISE imagery. Three cross-sections across Ius Chasma illustrate the interpreted mineral stratigraphy. Multiple episodes formed and transported hydrated minerals within Ius Chasma. Polyhydrated sulfate and kieserite are found within a closed basin at the lowest elevations in the chasma. They may have been precipitates in a closed basin or diagenetically altered after deposition. Fluvial or aeolian processes then deposited layered Fe/Mg smectite and hydrated silicate on the chasma floor, postdating the sulfates. The smectite apparently was weathered out of Noachian-age wallrock and transported to the depositional sites. The overlying hydrated silicate is interpreted to be an acid-leached phyllosilicate transformed from the underlying smectite unit, or a smectite/jarosite mixture. The finely layered smectite and massive hydrated silicate units have an erosional unconformity between them, that marks a change in surface water chemistry. Landslides transported large blocks of wallrock, some altered to contain Fe/Mg smectite, to the chasma floor. After the last episode of normal faulting and subsequent landslides, opal was transported short distances into the chasma from a few m-thick light-toned layer near the top of the wallrock, by sapping channels in Louros Valles. Alternatively, the material was transported into the chasma and then altered to opal. The superposition of different types of hydrated minerals and the different fluvial morphologies of the units containing them indicate sequential, distinct aqueous environments, characterized by alkaline, then circum-neutral, and finally very acidic surface or groundwater chemistry.     

Evidence for subaqueously resedimented sulphate evaporites on Mars

This article documents the clastic nature of sulphate evaporite beds in the Tithonium Chasma located in the Valles Marineris region of Mars. These beds form a stratified succession characterised by very thick interbedded channel-fill breccia bodies. We infer that the bouldery channel-fills were deposited by voluminous mass-flow processes occurring in a relatively deep subaqueous environment. The redeposition of the coarse-grained evaporite would have responded to phases of high denudation rates in rapidly uplifting hinterlands. Tectonic activity also caused the diapiric uprise and exhumation of evaporite diapirs within the Valles Marineris chasmata, where the apparently young and well development karstic landforms probably formed during the late Amazonian age. These new data strongly suggest the deposition of both primary and resedimented evaporites in a marginal basin area, which effectively restricted ocean access through the proposed “proto-Valles Marineris Strait”. The associated ocean may be the “Ocean Borealis” of Late Noachian-Early Hesperian age.     

Light-toned layered deposits in Juventae Chasma, Mars

We examine hypotheses for the formation of light-toned layered deposits in Juventae Chasma using a combination of data from Mars Global Surveyor's Mars Orbiter Camera (MOC), Mars Orbiter Laser Altimeter (MOLA), and Thermal Emission Spectrometer (TES), as well as Mars Odyssey's Thermal Emission Imaging System (THEMIS). We divide Juventae Chasma into geomorphic units of (i) chasm wall rock, (ii) heavily cratered hummocky terrain, (iii) a mobile and largely crater-free sand sheet on the chasm floor, (iv) light-toned layered outcrop (LLO) material, and (v) chaotic terrain. Using surface temperatures derived from THEMIS infrared data and slopes from MOLA, we derive maps of thermal inertia, which are consistent with the geomorphic units that we identify. LLO thermal inertias range from ∼400 to 850 J m⁻² K⁻¹ s^−1/2. Light-toned layered outcrops are distributed over a remarkably wide elevation range () from the chasm floor to the adjacent plateau surface. Geomorphic features, the absence of small craters, and high thermal inertia show that the LLOs are composed of sedimentary rock that is eroding relatively rapidly in the present epoch. We also present evidence for exhumation of LLO material from the west wall of the chasm, within chaotic and hummocky terrains, and within a small depression in the adjacent plateau. The data imply that at least some of the LLO material was deposited long before the adjacent Hesperian plateau basalts, and that Juventae Chasma underwent, and may still be undergoing, enlargement along its west wall due to wall rock collapse, chaotic terrain evolution, and exposure and removal of LLO material. The new data allow us to reassess possible origins of the LLOs. Gypsum, one of the minerals reported elsewhere as found in Juventae Chasma LLO material, forms only at low temperatures () and thus excludes a volcanic origin. Instead, the data are consistent with either multiple occurrences of lacustrine or airfall deposition over an extended period of time prior to emplacement of Hesperian lava flows on the plateau above the chasm.     

Spectral and thermodynamic constraints on the existence of gypsum at the Juventae Chasma on Mars

The spectral imaging of the Mars obtained with the Mars Express/OMEGA experiment demonstrates that a majority of the sulfates-rich regions are associated with the interior light-toned layered deposits within the canyon system in the equatorial zone of the planet. While all sulfates-rich deposits inside the canyons are characterized by the presence of the kieserite and hydrated magnesium sulfates, the spectral features of gypsum were detected only in the Juventae Chasma and the Iani Chaos. The detection of gypsum in the upper part of the layered deposits, stacking the erosional remnant on the floor of the Juventae Chasma (above the spectral signature of the kieserite and polyhydrated sulfates detected on the flanks of the remnant) represents a more intriguing case. To clarify the question of the presence of gypsum in the Juventae Chasma, we present reanalyzed OMEGA spectra within that area and performed the chemical equilibrium modelling of sulfates precipitation sequence at the freezing and the evaporation of a hypothetical aqueous solution which could have existed within the Chasma in the past. Our results did not confirm the presence of distinct spectral signatures of gypsum. The results of equilibrium modelling also exclude significant precipitation of gypsum during the latest stage of the aqueous sedimentation, responsible for the formation of the upper part of the erosional remnant.     

Origin and evolution of the layered deposits in the Valles Marineris,Mars

Thick sequences of layered deposits are found in the Martian Valles Marineris. They exhibit fine, nearly horizontal layering, and are present as isolated plateaus of what may have once been more extensive deposits. Individual sequences of layered deposits are as thick as 5 km. The greatest total thicknesses of deposits are found in Candor, Ophir, and Melas chasmata. individual layer thicknesses range from about 70 to 300 m. Some tilting of sequences is observed, but at the best image resolutions, no angular unconformities are detectable in the layers. The sequences of events in the canyons, as deduced from morphologic and stratigraphic evidence, was (1) graben formation in response to the tharsis uplift, (2) canyon wall retreat and canyon enlargement, roughly contemporaneous with formation of the layered deposits, (3) deep erosion of the layered deposits, (4) landsliding of the canyon walls, and (5) eolian erosion of the layered deposits, perhaps continuing up to the present. We consider four hypotheses for the origin of the layered deposits: they are eolian deposits, they are remnants of the material that makes up the canyon walls, they are deposits of explosive volcanic eruptions, or they were deposited in standing bodies of water. The rhythmic nature of the layers and their lateral continuity, horizontality, great thickness, and stratigraphic relationships with other units in the canyons all appear most consistent with deposition in an aqueous environment. If standing bodies of water existed in the Valles Marineris, they were almost certainly ice-covered. there are three ways in which sediment could have entered an ice-covered lake: down through the ice cover, up from the lake bottom, or in from the lake margins. Layers of sediment could have been transported downward through an ice cover by foundering or Rayleigh-Taylor instabilities, but it is not clear whether there was a viable mechanism for repeatedly accumulating thick sediment layers on top of the ice cover. Subaqueous volcanic eruption on the lake bottom does not suffer from many of the morphologic arguments that make origin by subaerial volcanism seem improbable. While this mechanism is attractive, there are no eruptive centers observed and there is no other direct evidence to support it. Because canyon enlargement took place at roughly the same time as layer deposition, debris from the canyon walls is an obvious and likely source for some of the material in the layered deposits; however, the volume of material removed from the canyon walls may be insufficient to account for all of the presently observed material. We conclude that there are several geologically feasible, but as yet unproven, mechanisms that could have led to formation of thick deposits in ice-covered paleolakes in the Valles Marineris. Present data are insufficient to choose conclusively among the various possibilities. Several types of data from the Mars Observer mission will be useful in further characterizing the deposits and clarifying the process of their origin. The deposits should be considered important targets for a future Mars sample return mission.     

Morphology and geology of the ILD in Capri/Eos Chasma (Mars) from visible and infrared data

Layered deposits have been observed in different locations at the surface of Mars, as crater floors and canyons systems. Their high interest relies in the fact they imply dynamical conditions in their deposition medium. Indeed, in opposition to most of the rocks of the martian surface, which have a volcanic origin, bright layered deposits seems to be sedimentary outcrops.Capri Chasma, a canyon located at the outlet of Valles Marineris, exhibits such deposits called Interior Layered Deposits (ILD). A large array of visible and infrared spacecraft data were used to build a Geographic Information System (GIS). We added HiRiSE images, from the recent MRO mission, which offer a spatial resolution of 25 cm per pixel. It allowed the mapping and the analysis of morphologies in the canyon. We highlighted that the ILD are several kilometers thick and flat-top stratified deposits. They overlap the chaotic floor. They are surrounded and cut by several flow features that imply that liquid water was still acting after the formation of these stratified deposits. The density of crater on the floor of Capri Chasma was quantified. The current topography was aged to 3 Gyr. All these morphological information allow us to suggest a plausible geological history for Capri Chasma. We propose that the Interior Layered Deposits have formed during the Hesperian, during or after the opening of the canyon. Some observations argue that water discharges have happened at several times before and just after the formation of the ILD. Liquid water must have played a major role in the formation of these deposits after 3.5 Gyr, implying that it was present in surface at least locally and temporarily. If this can be applied to ILD in others canyons of Valles Marineris, it would imply that liquid water was stable in surface or sub-surface during the Hesperian. Or in the actual conditions, with a cold and dry martian surface, long-term standing water bodies are not possible. Thus we suggest that either the climate at the Hesperian was cold, but wetter, or as warm as the Noachian climate, what is less likely. Nevertheless, the global climate change which has occurred at the beginning of Mars history may have been later than announced.     

Layering stratigraphy of eastern Coprates and northern Capri Chasmata, Mars

Distinct competent layers are observed in the slopes of eastern Coprates Chasma, part of the Valles Marineris system on Mars. Our observations indicate that the stratigraphy of Coprates Chasma consists of alternating thin strong layers and thicker sequences of relatively weak layers. The strong, competent layers maintain steeper slopes and play a major role in controlling the overall shape and geomorphology of the chasmata slopes. The topmost competent layer in this area is well preserved and easy to identify in outcrops on the northern rim of Coprates Chasma less than 100 m below the southern Ophir Planum surface. The volume of the topmost emplaced layer is at least 70 km³ and may be greater than 2100 km³ if the unit underlies most of Ophir Planum. The broad extent of this layer allows us to measure elevation offsets within the north rim of the chasma and in a freestanding massif within Coprates Chasma where the layer is also observed. Rim outcrop morphology and elevation differences between Ophir and Aurorae Plana may be indicative of the easternmost extent of the topmost competent layer. These observations allow an insight into the depositional processes that formed the stratigraphic stack into which this portion of the Valles Marineris is carved, and they present a picture of some of the last volcanic activity in this area. Furthermore, the elevation offsets within the layer are evidence of significant subsidence of the massif and surrounding material.     

Mars Exploration Rover candidate landing sites as viewed by THEMIS

The analysis of six landing sites that were candidates for the two NASA Mars Exploration Rovers (MER) benefited from recently available image data from the Thermal Emission Imaging Spectrometer (THEMIS) onboard the 2001 Mars Odyssey spacecraft. The combination of daytime and nighttime thermal infrared images from THEMIS supplemented by additional data sets has lead to new or expanded insights into the nature of each landing site. In Meridiani Planum, a layer of lighter-toned, higher thermal inertia material is observable just below the hematite-bearing layer. Gusev Crater displays a more complex stratigraphy than previously observed, including an upper layer with lobate margins. The highest inertia unit of southern Isidis Planitia is confined to topographic lows in the rim/basin margin and does not appear to be due to highland material transported onto the basin floor. The enigmatic, ovoid, blocky terrain on the floor of Melas Chasma displays higher thermal inertia than its surroundings, an indication that it contains coarser or more indurated material than the adjacent aeolian bedforms. The myriad channels of Athabasca Valles display distinctive thermal signatures despite the presence of a bright layer of dust covering the region. The presence of alluvial fans produced from spur-and-gulley erosion of the walls of Eos Chasma demonstrates that mass movements have occurred following the canyon scouring floods.     

A formation mechanism for hematite-rich spherules on Mars

Gray crystalline hematite on Mars has been detected in three regions, Sinus Meridiani, Aram Chaos, and Valles Marineris, first by the Thermal Emission Spectrometer (TES) onboard the Mars Global Surveyor (MGS) orbiter, and then confirmed by other instruments. The hematite-rich spherules were also detected by the Mars Exploration Rover (MER) Opportunity at Meridiani Planum (Sinus Meridiani). Formation mechanisms of the hematite-rich spherules have been discussed widely since then. Here, we argue for an alternative formation mechanism, that is, the spherules originally formed at Valles Marineris due to the interaction of volcanic deposits and acidic hydrothermal fluids, and then were transported to and deposited at Meridiani Planum and Aram Chaos as alluvial/fluvial sedimentary deposits with other materials such as sulfates and rock fragments during the wash-out flows from Valles Marineris to Meridiani Planum and Aram Chaos. Diagenesis of the hematite-rich spherules may have also been a possible mechanism following sediment transport and emplacement. The hypothesis is consistent with available relevant information to date and provides an insight into the understanding of Martian surficial processes.     

Structural attitudes of large scale layering in Valles Marineris, Mars, calculated from Mars Orbiter Laser Altimeter data and Mars Orbiter Camera imagery

Valles Marineris, located on the flank of the Tharsis Ridge uplift on Mars, exposes layering within the canyon walls interpreted to be volcanic flood lavas. By combining 1/128°×1/128°Mars Orbiter Laser Altimeter elevation data with wide-angle Mars Orbiter Camera images using Orion structural analysis software, we computed the attitude of some of this large-scale layering. Multilinear regression was used to fit planes to three-dimensional coordinates of points selected along exposed layer traces, giving the plane attitude and various fitting statistics. By measuring the same layer using different images, we found the measurements to be quite reproduceable. Errors in dip angle were typically only a few degrees or less. Analysis of the data indicates that most layers dip gently into the adjacent chasma. We interpret this orientation to be the result of the crustal subsidence, probably related to the formation of the early collapse basins, rather than the result of rotations produced by extensional faulting. Since the dip is consistent far away from the edge of the current chasmata we suggest that the scale of the depressions was on the order of hundreds of kilometers, exceeding the dimensions of the current chasmata.     

Related Magma-Ice Interactions: Possible Origins of Chasmata, Chaos, and Surface Materials in Xanthe, Margaritifer, and Meridiani Terrae, Mars

We examine here the close spatial and temporal associations among several unique features of Xanthe and Margaritifer Terrae, specifically the Valles Marineris troughs or chasmata and their interior deposits, chaotic terrain, the circum-Chryse outflow channels, and the subdued cratered material that covers Xanthe, Margaritifer, and Meridiani Terrae. Though previous hypotheses have attempted to explain the origin of individual features or subsets of these, we suggest that they may all be related. All of these features taken together present a consistent scenario that includes the processes of sub-ice volcanism and other magma/ice interactions, results of intrusive events during Late Noachian to Early Amazonian times.     

Ages of Valles Marineris (Mars) landslides and implications for canyon history

The chronology of landslides of Valles Marineris, the equatorial trough system of Mars, has been investigated by a crater population study. Valles Marineris landslides have widespread debris aprons which offer a remarkable opportunity to study the crater population with high resolution images from Mars Orbiter Camera (MOC) and from Mars Odyssey Thermal Emission Imaging System (THEMIS). Sixty-six ages were determined within Valles Marineris including 56 landslide ages and 10 ages of the canyon floor. Results reveal that landslides of Valles Marineris system of canyons occurred during a widespread period of time between 3.5 Gy and 50 My. In some locations, the canyon floor has an apparent age of 3.5 Gy suggesting that at least locally within Valles Marineris no major refreshing processes have occurred for 3.5 Gy. The temporal repetitivity of landslides implies that the triggering mechanisms of the landslides are reproducible in time. Landslides have the same features whatever their age. The dynamic of these landslides is probably the same either with intervention of water up to recently (the last 100 My) or without water since 3.5 Gy.     

Landslides in Valles Marineris (Mars): A possible role of basal lubrication by sub-surface ice

There is much interest on the occurrence of water and ice in the past history of Mars. Because landslides on Mars are much better conserved than their terrestrial counterparts, a physical examination and morphological analysis can reveal significant details on the depositional environment at the instant of failure. A study of the landslides in Valles Marineris based on their physical aspect is presented and the velocity of the landslides is calculated with a stretching block model. The results show that the landslides were subject to strong basal lubrication that made them travel at high speed and to long distances. We use physical analysis to explore the four alternative possibilities that the natural lubricant of the landslides in Valles Marineris was either ice, deep water, a shallow carpet of water, or evaporites. Examination of the furrows present on the surface of the landslide deposits shows that either sub-surface ice or evaporites were likely present on the floor of Valles Marineris during the mass failures.     

Importance of aeolian processes in the origin of the north polar chasmata, Mars

A detailed examination of the location and orientation of sand dunes and other aeolian features within the north polar chasmata indicates that steep scarps strongly influence the direction and intensity of prevailing winds. These steep scarps are present at the heads and along the margins of the north polar chasmata. Topographic profiles of the arcuate head scarps and equator-facing wall of Chasma Boreale reveal unusually steep polar slopes ranging from ∼6°-30°. The relatively steep-sloped (∼8°), sinuous scarp at the head of two smaller chasmata, located west of Chasma Boreale, exhibits an obvious resistant cap-forming unit. Scarp retreat is occurring in places where the cap unit is actively being undercut by descending slope winds. Low-albedo surfaces lacking sand dunes or dust mantles are present at the base of the polar scarps. A ∼100-300 m deep moat, located at the base of the scarps, corresponds with these surfaces and indicates an area of active aeolian scour from descending katabatic winds. Small local dust storms observed along the equator-facing wall of Chasma Boreale imply that slope wind velocities in Chasma Boreale are sufficient to mobilize dust and sand-sized particles in the Polar Layered Deposits (PLD). Two amphitheater forms, located above the cap-forming unit of the sinuous scarp and west of Chasma Boreale, may represent an early stage of polar scarp and chasma formation. These two forms are developing within a younger section of polar layered materials. The unusually steep scarps associated with the polar chasmata have developed where resistant layers are present in the PLD, offering resistance during the headward erosion and poleward retreat of the scarps. Steep slopes that formed under these circumstances enhance the flow of down-scarp katabatic winds. On the basis of these observations, we reject the fluvial outflood hypothesis for the origin of the north polar chasmata and embrace a wind erosion model for their long-term development. In the aeolian model, off-pole katabatic winds progressively remove materials from the steep slopes below chasmata scarps, undermining resistant layers at the tops of scarps and causing retreat by headward erosion. Assuming a minimum age for the onset of formation of Chasma Boreale (10⁵ yr) results in a maximum volumetric erosion rate of . Removal of this volume of material from the equator-facing wall and head scarps of chasma would require a rate for scarp retreat of .     

Formation of an Hesperian-aged sedimentary basin containing phyllosilicates in Coprates Catena,Mars

The extensive light-toned deposits in canyons and troughs in Valles Marineris provide evidence of formation through water-related processes. As such, these deposits offer a window to past conditions on Mars. We study a small outcrop of light-toned deposits in a closed trough in Coprates Catena, a chain of collapse pits to the south-east of the main Valles Marineris system. A well-exposed sequence of deposits on the base of the north wall of the trough offers a 220 m section for geochemical and morphologic analysis. Using CRISM data we identify the presence of both phyllosilicates and sulfates and/or opaline silica in the light toned deposits, which vary in relative strength with elevation. We observe a trend in the dominant mineralogical signal, with Al phyllosilicates occurring near the base of the deposits, both below and above a band of Fe/Mg phyllosilicates, before a transition to more sulfate- or opaline silica-rich material near the top of the section. This trend likely reflects a change in the chemistry of the water in which the deposits formed. Using a HiRISE Digital Elevation Model, we find that the layers in the light-toned deposits on both sides of the trough dip gently towards the center of the trough, with a dip direction that aligns with the strike of the trough, suggesting that the light-toned deposits formed after the trough. Our general morphologic and mineralogical observations fit well with significant amounts of water in the trough. The deposits are too small to be dated using crater counting techniques, however, our crater analysis suggests that the plains in which the trough formed are probably Late Hesperian in age. If the chemistry of the light-toned deposits reflects the primary depositional mineralogy, then this and other small troughs in Coprates Catena might provide evidence of limited phyllosilicate formation in this region towards the end of the Hesperian era on Mars.     

Morphology and geometry of Valles Marineris landslides

The walls of the Valles Marineris canyons are affected by about 45 landslides. The study of these landslides provides a test of the hypothesis of processes having affected Martian wallslopes after their formation. The dynamics of Valles Marineris landslides are controversial : either the landslides are interpreted as large debris flows or as dry rock avalanches. Their morphology and their topography are basic parameters to understand their dynamics. From topographic MOLA data and remote sensing images acquired with different spatial resolutions (Viking, THEMIS, MOC), the 3D geometry of 45 landslides of Valles Marineris has been studied. The landslides have been classified in 3 geomorphologic classes from the topography of the landslide deposits: the “chaotic” landslides without well identified structures, the “structured deposit without debris aprons” landslides with tectonic structures and small roughness at the deposit front and the “structured deposit with debris aprons” which display circular normal faults at the back of the deposit and several debris aprons at the front of the landslide. The spatial distribution of the three morphological types is in relation with the confinement of the canyons. The initial volume and the total deposited volume were also measured to compute volume balances. The deposited volumes range from 50 to . All volume balances display a maximum deficit ranging from 5% to 70%. The landslides with the largest deficits take place within an enclosed-canyon (Hebes Chasma). Lacking material exportation, these deficits could be interpreted as reflecting the porosity of the landslide source. This fact is in agreement with the hypothesis of a karstic origin of these enclosed-canyons. The Valles Marineris landslides have large mobilities (length/vertical drop) ranging from 1.8 to 12 implying low coefficients of friction and so fluidization mechanisms. The possible filling up of the porosity by volatile could be compatible with the fluidization patterns of Valles Marineris landslides.