Melas Chasma Research Articles

Silica‐Bearing Mounds and Strata in the Southwest Melas Basin, Valles Marineris, Mars: Evidence for a Hydrothermal Origin

AbstractA small basin on the Southwest (SW) margin of Melas Chasma in Valles Marineris, Mars, hosts a variety of previously identified sedimentary fans and layered strata hypothesized to have been formed by one or more paleolakes. This basin also contains light‐toned layered mounds that have distinct spectral absorption bands consistent with amorphous hydrated silica (e.g., opal). While the general morphology and mineralogy of these features and the basin itself have been previously characterized, the formation mechanism of the hydrated silica features and their temporal relationships with the proposed paleolake remain to be determined. We use Compact Reconnaissance Imaging Spectrometer for Mars visible through short‐wave infrared reflectance spectra (0.35–2.65 μm) and High Resolution Imaging Science Experiment digital terrain models and images to analyze the stratigraphic location and morphology of the opaline silica‐bearing features in the SW Melas basin. We find that the basin hosts fourteen high‐relief “mounds,” eight low‐relief “patches,” and two extended layers within the sedimentary strata that are light‐toned, fractured, and often exhibit hydrated silica‐like spectral signatures. We hypothesize that the mounds are spring deposits formed by sub‐aerial hydrothermal activity, while the patches and layers correspond to sub‐lacustrine hydrothermal activity. The varied elevations of the mounds and patches indicate at least one fluctuation of lake level in the basin during its history. The combination of contemporaneous hydrothermal and lacustrine activity to form silica‐cemented sedimentary deposits in a nutrient‐rich subaqueous environment would have been conducive to forming and preserving biosignatures in the SW Melas basin.

Lighting and illumination investigation of long-term residence on Mars for the case of a set of designed Martian Habitat Units (MHUs)

Addressing the subject matter of human missions on Mars, the Martian Habitat Units (MHUs) are presented as a comprehensive solution. MHUs are designed in clusters of 10 units, each capable of serving as long-term habitats for 9 scientific crew members. The life-style requirements of the units are targeted at an imitation of cultural thriving life we all know of, and not a mere survival-type shelter expecting the first people to step foot on Mars. One aspect of many challenging issues to be addressed in such complex settings is the lighting and illuminance condition of the said habitats, which in the context of Mars, and generally deep space missions being far from the sun will certainly lead to an arduous task. To check the validity of the argument and assess the extent to which the natural light level available on the surface of Mars will be sufficient for the daily requirements of the crew and mission in terms of illuminance, the current manuscript presents thorough and detailed simulations and analyses on the availability evaluation of natural lighting in the site location of MHUs, namely Valles Marineris, Melas Chasma. In this paper solar irradiation parameters on Mars are calculated based on the previous research which resulted in global, direct and diffuse irradiance at 12 different Martian solar times. The simulations are distributed over the Martian year and its day-time, and for two extreme orientations of MHUs in their circular surrounding cluster, namely East/West and South/North units. The distribution of illuminance for each case, and trend comparison studies are then accompanied by numerical values and analyses on the percentage to which the natural lighting conditions on Mars have been shown to be sufficient as a fraction of the whole lighting load of the habitats, which is to be compensated using artificial sources. The corresponding values are shown to fall well in the range of 35–45% of the total lighting loads. Also, as the results of the simulations show, due to the consistency of the glazed parts of the designed façade through all exterior surface of the MHU, natural lighting sufficiency percentage does not show a significant difference between two simulated orientations. This fact further approves the circular orientation premise of the MHUs in their cluster.

Ancient Martian Aeolian Sand Dune Deposits Recorded in the Stratigraphy of Valles Marineris and Implications for Past Climates

AbstractAeolian sediment transport, deposition, and erosion have been ongoing throughout Mars's history. This record of widespread aeolian processes is preserved in landforms and geologic units that retain important clues about past environmental conditions including wind patterns. In this study we describe landforms within Melas Chasma, Valles Marineris, that occur in distinct groups with linear to crescentic shapes, arranged with a characteristic wavelength; some possess slope profiles analogous to modern sand dunes yet show evidence for lithification. Based on the features' dimensions, asymmetry, and spatial patterns relative to modern equivalents, we interpret these landforms to be two classes of aeolian bedforms: decameter‐scale megaripples and sand dunes. The presence of superposed erosional features and depositional units indicates that these landforms were cemented and likely ancient. Melas paleodunes are found atop Hesperian‐aged layered deposits, but we estimate them to be younger, likely lithified in the Amazonian period. Although a range of degradation was observed, some paleodunes are >10 m tall and maintain steep lee sides (>25°), an uncommon scenario for terrestrial examples as other geologic processes lead to dune obliteration. The preserved paleobedform geometries are largely consistent with those of modern aeolian indicators, suggesting no major shifts in wind regime or contributing boundary conditions. Finally, we propose that their appearance and context require sequential periods of dune migration, stabilization following catastrophic burial, cementation, differential erosion, exposure, and burial. The presence of wholly preserved duneforms appears to be more common on Mars compared to the Earth and may signal something important about Martian landscape evolution.

TARGET TRANSFORMATION CONSTRAINED SPARSE UNMIXING (TTCSU) ALGORITHM FOR RETRIEVING HYDROUS MINERALS ON MARS: APPLICATION TO SOUTHWEST MELAS CHASMA

Abstract. Quantitative analysis of hydrated minerals from hyperspectral remote sensing data is fundamental for understanding Martian geologic process. Because of the difficulties for selecting endmembers from hyperspectral images, a sparse unmixing algorithm has been proposed to be applied to CRISM data on Mars. However, it's challenge when the endmember library increases dramatically. Here, we proposed a new methodology termed Target Transformation Constrained Sparse Unmixing (TTCSU) to accurately detect hydrous minerals on Mars. A new version of target transformation technique proposed in our recent work was used to obtain the potential detections from CRISM data. Sparse unmixing constrained with these detections as prior information was applied to CRISM single-scattering albedo images, which were calculated using a Hapke radiative transfer model. This methodology increases success rate of the automatic endmember selection of sparse unmixing and could get more accurate abundances. CRISM images with well analyzed in Southwest Melas Chasma was used to validate our methodology in this study. The sulfates jarosite was detected from Southwest Melas Chasma, the distribution is consistent with previous work and the abundance is comparable. More validations will be done in our future work.

Spectral and stratigraphic mapping of hydrated minerals associated with interior layered deposits near the southern wall of Melas Chasma, Mars

Orbital remote sensing data acquired from the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) onboard Mars Reconnaissance Orbiter (MRO), in conjunction with other datasets, are used to perform detailed spectral and stratigraphic analyses over a portion of south Melas Chasma, Mars. The Discrete Ordinate Radiative Transfer (DISORT) model is used to retrieve atmospherically corrected single scattering albedos from CRISM I/F data for mineral identification. A sequence of interbedded poly- and monohydrated sulfates associated with interior layered deposits (ILDs) is identified and mapped. Analyses from laboratory experiments and spectral unmixing of CRISM hyperspectral data support the hypothesis of precipitation and dehydration of multiple inputs of complex Mg-Ca-Fe-SO4-Cl brines. In this scenario, the early precipitated Mg sulfates could dehydrate into monohydrated sulfate due to catalytic effects, and the later-precipitated Mg sulfates from the late-stage “clean” brine could terminate their dehydration at mid-degree of hydration to form a polyhydrated sulfate layer due to depletion of the catalytic species (e.g., Ca, Fe, and Cl). Distinct jarosite-bearing units are identified stratigraphically above the hydrated sulfate deposits. These are hypothesized to have formed either by oxidation of a fluid containing Fe(II) and SO4, or by leaching of soluble phases from precursor intermixed jarosite-Mg sulfate units that may have formed during the later stages of deposition of the hydrated sulfate sequence. Results from stratigraphic analysis of the ILDs show that the layers have a consistent northward dip towards the interior of the Melas Chasma basin, a mean dip angle of ∼6°, and neighboring strata that are approximately parallel. These strata are interpreted as initially sub-horizontal layers of a subaqueous, sedimentary evaporite deposits that underwent post-depositional tilting from slumping into the Melas Chasma basin. The interbedded hydrated sulfate units and jarosite-bearing units, which have distinct stratigraphic relationships, are indicative of a complex sedimentary and aqueous history in south Melas Chasma.

The anatomy of a wrinkle ridge revealed in the wall of Melas Chasma, Mars

Wrinkle ridges are among the most common tectonic structures on the terrestrial planets, and provide important records of the history of planetary strain and geodynamics. The observed broad arches and superposed narrow wrinkles are thought to be the surface manifestation of blind thrust faults, which terminate in near-surface volcanic sequences and cause folding and layer-parallel shear. However, the subsurface tectonic architecture associated with the ridges remains a matter of debate. Here we present direct observations of a wrinkle ridge thrust fault where it has been exposed by erosion in the southern wall of Melas Chasma on Mars. The thrust fault has been made resistant to erosion, likely due to volcanic intrusion, such that later erosional widening of the trough exposed the fault plane as a 70 km-long ridge extending into the chasma. A plane fit to this ridge crest reveals a thrust fault with a dip of 13° (+8°, -7°) between 1 and 3.5 km depth below the plateau surface, with no evidence for listric character in this depth range. This dip is significantly lower than the commonly assumed value of 30°, which, if representative of other wrinkle ridges, indicates that global contraction on Mars may have been previously underestimated.

End‐member identification and spectral mixture analysis of CRISM hyperspectral data: A case study on southwest Melas Chasma, Mars

AbstractWe present spectral unmixing results over the southwest Melas Chasma region, where a variety of hydrated minerals were identified. We use the Discrete Ordinate Radiative Transfer radiative transfer model to simultaneously model Mars atmospheric gases, aerosols, and surface scattering and retrieve the single‐scattering albedos (SSAs) modeled by the Hapke bidirectional scattering function from Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) data. We employ a spectral unmixing algorithm to quantitatively analyze the mineral abundances by modeling the atmospherically corrected CRISM SSAs using a nonnegative least squares linear deconvolution algorithm. To build the spectral library used for spectral unmixing, we use the factor analysis and target transformation technique to recover spectral end‐members within the CRISM scenes. We investigate several distinct geologic units, including an interbedded polyhydrated and monohydrated sulfate unit (interbedded unit 1) and an interbedded phyllosilicate‐sulfate unit (interbedded unit 2). Our spectral unmixing results indicate that polyhydrated sulfates in the interbedded unit 1 have a much lower abundance (~10%) than that of the surrounding unit (~20%) and thus may have been partially dehydrated into kieserite to form the interbedded strata, supporting a two‐staged precipitation‐dehydration formation hypothesis. In the interbedded unit 2 phyllosilicates have an abundance of ~40% and are interbedded with ~20% sulfates. The results, in combination with thermodynamic calculations performed previously, suggest that the interbedded phyllosilicates and sulfates likely formed through coupled basalt weathering and evaporation. The methodology developed in this study provides a powerful tool to derive the mineral abundances, aiming to better constrain the formation processes of minerals and past aqueous environment on Mars.

Orbital evidence for clay and acidic sulfate assemblages on Mars based on mineralogical analogs from Rio Tinto, Spain

Outcrops of hydrated minerals are widespread across the surface of Mars, with clay minerals and sulfates being commonly identified phases. Orbitally-based reflectance spectra are often used to classify these hydrated components in terms of a single mineralogy, although most surfaces likely contain multiple minerals that have the potential to record local geochemical conditions and processes. Reflectance spectra for previously identified deposits in Ius and Melas Chasma within the Valles Marineris, Mars, exhibit an enigmatic feature with two distinct absorptions between 2.2 and 2.3µm. This spectral ‘doublet’ feature is proposed to result from a mixture of hydrated minerals, although the identity of the minerals has remained ambiguous. Here we demonstrate that similar spectral doublet features are observed in airborne, field, and laboratory reflectance spectra of rock and sediment samples from Rio Tinto, Spain. Combined visible-near infrared reflectance spectra and X-ray diffraction measurements of these samples reveal that the doublet feature arises from a mixture of Al-phyllosilicate (illite or muscovite) and jarosite. Analyses of orbital data from the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) shows that the martian spectral equivalents are also consistent with mixtures of Al-phyllosilicates and jarosite, where the Al-phyllosilicate may also include kaolinite and/or halloysite. A case study for a region within Ius Chasma demonstrates that the relative proportions of the Al-phyllosilicate(s) and jarosite vary within one stratigraphic unit as well as between stratigraphic units. The former observation suggests that the jarosite may be a diagenetic (authigenic) product and thus indicative of local pH and redox conditions, whereas the latter observation may be consistent with variations in sediment flux and/or fluid chemistry during sediment deposition.

Implications for the aqueous history of southwest Melas Chasma, Mars as revealed by interbedded hydrated sulfate and Fe/Mg-smectite deposits

Using visible and near infrared data from the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM), we identified a sequence of hydrated sulfates and Fe/Mg-smectites in southwest Melas Chasma. Specifically, these hydrated sulfate and semectite deposits are interbedded and have been highly deformed. Equilibrium thermodynamic calculations of coupled basalt weathering and fluid evaporation predict that sequential formation of Fe/Mg-smectites and sulfate evaporites in similar quantities, as observed in the interbedded smectite-sulfate sequences, is chemically plausible. The Fe/Mg-smectite-sulfate deposits may have thus formed through in situ basalt weathering and fluid evaporation, although an origin involving repeated cycles of transport and deposition of detrital clays by a neutral fluid containing Mg and SO4 and subsequent evaporation cannot be ruled out. If the Fe/Mg-smectites are authigenic, the Hesperian (or younger) age Melas Chasma would have experienced prolonged periods with near-surface water, providing potentially habitable conditions.

Reflectance spectroscopy and optical functions for hydrated Fe-sulfates

Visible and near-infrared wavelength (VNIR, λ = 0.35–5 μm) laboratory diffuse reflectance spectra and corresponding optical functions (real and imaginary refractive indices) for several iron sulfates (natural K- and Na-jarosite, szomolnokite, rhomboclase) are presented. On Mars, jarosite has been identified in Meridiani Planum, Mawrth Vallis, Melas Chasma, and Eridania Basin; szomolnokite has been found as distinct layers at Columbus Crater and as outcrops at Juventae Chasma, and rhomboclase has been identified at Gusev Crater. Constraining the mineralogy and chemistry (Fe- vs. Mg-rich) of the sulfates on Mars may contribute to our understanding of the environmental and aqueous conditions present on Mars during their formation. The data presented here will help to constrain the mineralogy, abundance, and distribution of sulfates on the martian surface, which will lead to improvements in understanding the pressure, temperature, and humidity conditions and how active frost, groundwater, and atmospheric processes once were on Mars.

Reconstructing the aqueous history within the southwestern Melas basin, Mars: Clues from stratigraphic and morphometric analyses of fans

New details of the aqueous history in the southwestern Melas Chasma elevated basin have been revealed from analysis of high-resolution image, topographic and spectral datasets. We have identified eleven fan-shaped landforms that reflect various depositional environments. A distinctive marker bed with inferred indurated aeolian bedforms is within the stratigraphic record of presumed lacustrine deposits. This observation, taken together with the stratigraphic succession of fan-shaped deposits indicates fluctuating lake levels with, at a minimum, early and late-stage lake highstands. Tributary drainage pattern in the western valley network changed from a dendritic to a meandering system, recording a shift in fluvial activity that is consistent with fluctuating lake levels. Only a few hydrated minerals have been detected in the study region, the most common being opal which appears to represent younger alteration and deposition within the basin. Landform scale was used to estimate average discharge (∼30m3/s), formative discharge (200–300m3/s), and fan formation timescale, which further inform the duration of lacustrine activity within the basin. Warm surface conditions and precipitation recharge of source basins is required to generate and sustain this long-lived lake over periods of at least centuries to millennia during the Late Hesperian to Early Amazonian.

Mixtures of clays and sulfates within deposits in western Melas Chasma, Mars

We have utilized several data sets from multiple spacecraft that have been acquired over a continuous observation campaign across the southwestern Melas Chasma region of Mars. The blocky deposit observed on the chasma floor and on portions of the southern wallrock consists of mixtures of light- and medium-toned materials that exhibit displacement structures consistent with transport down the wallrock and onto the chasma floor. CRISM visible and near-infrared reflectance spectra of the light-toned blocks suggest mixtures of nontronite, jarosite, Al-clays, hydrated silica, and/or an acid leached clay. The medium-toned blocks typically lack spectral features or exhibit signatures of polyhydrated sulfates. Bright layered mounds embayed by blocky deposit materials display spectra consistent with Ca-sulfates (gypsum and/or bassanite) that may have resulted from precipitation of less soluble Ca-rich minerals during ponding and evaporation of dilute fluids along the western chasma floor.Draping across some wallrock slopes is a light-toned deposit that sometimes overlies a blocky, layered, and spectrally bland medium-toned deposit. The light-toned deposit exhibits spectra consistent with mixtures of materials that may include jarosite, acid-leached clays, silica, poorly crystalline clays, and polyhydrated sulfates. Hesperian-age valleys dissect the medium-toned deposit along some wallrock slopes and may be partially filled in by light-toned deposits. The draping nature of these deposits can best be explained by airfall, either volcanic ash or atmospheric dust, with ice/snow accumulation perhaps controlling sedimentation along the wallrock. An examination of other regional hydrated deposits around southwestern Melas indicates similar draping deposits occur farther east and west along the wallrock, whereas layered sulfates to the southeast consist of monohydrated and polyhydrated sulfates that are similar to interior layered deposits found in many of the chasmata. Small opal and jarosite exposures within the Melas basin and along the nearby plateau are most likely the result of younger localized alteration and deposition. The diversity of hydrated minerals and fluvial features indicates multiple episodes of aqueous activity under distinct environmental conditions occurred throughout southwestern Melas Chasma during the Hesperian to Amazonian that may be contemporaneous with late aqueous activity elsewhere on Mars.

Sublacustrine depositional fans in southwest Melas Chasma

Two depositional fan complexes have been identified on the floor of southwest Melas Chasma. The western fan complex is located near the center of an enclosed basin in southwest Melas Chasma and is composed of multiple lobes with dendritic finger‐like terminations. These fans are very flat and have a morphology unlike any other fan that has been previously identified on Mars. On the basis of the morphologic similarity of the western fan complex to the Mississippi submarine fan complex, we suggest that it may be a deep subaqueous fan depositional system. There are numerous channels on the surface of the western fan complex, and measurements of channel length, width, and sinuosity are consistent with channels observed on terrestrial submarine fans. The eastern Melas depositional fans are less well preserved and may be of deltaic or sublacustrine origin. Recognition of the fans supports earlier suggestions for the presence of a former lake in Melas Chasma and indicates that a significant body of water was present and stable at the surface of Mars for at least 102 to 104 years.

Mars Reconnaissance Orbiter observations of light-toned layered deposits and associated fluvial landforms on the plateaus adjacent to Valles Marineris

We have used data from the Mars Reconnaissance Orbiter to study 30–80 m thick light-toned layered deposits on the plateaus adjacent to Valles Marineris at five locations: (1) south of Ius Chasma, (2) south of western Melas Chasma, (3) south of western Candor Chasma, (4) west of Juventae Chasma, and (5) west of Ganges Chasma. The beds within these deposits have unique variations in brightness, color, mineralogy, and erosional properties that are not typically observed in light-toned layered deposits within Valles Marineris or many other equatorial areas on Mars. Reflectance spectra indicate these deposits contain opaline silica and Fe-sulfates, consistent with low-temperature, acidic aqueous alteration of basaltic materials. We have found valley or channel systems associated with the layered deposits at all five locations, and the volcanic plains adjacent to Juventae, Ius, and Ganges exhibit inverted channels composed of light-toned beds. Valleys, channels, and light-toned layering along the walls of Juventae and Melas Chasmata are most likely coeval to the aqueous activity that affected the adjacent plateaus and indicate some hydrological activity occurred after formation of the chasmata. Although the source of water and sediment remains uncertain, the strong correlation between fluvial landforms and light-toned layered deposits argues for sustained precipitation, surface runoff, and fluvial deposition occurring during the Hesperian on the plateaus adjacent to Valles Marineris and along portions of chasmata walls.

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.

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.

Stratigraphic architectures spotted in southern Melas Chasma, Valles Marineris, Mars

The distribution in space and time of liquid water on Mars is a haunting question because of its relevance to astrobiology and astroclimatology. To date, most orbital observations which attest to past fluvial and lacustrine activity on Mars have been dedicated to surficial landforms (i.e., drainage networks and depositional fans). Here we report an unprecedented identification of typical stratigraphic architectures made in Mars Global Surveyor Mars Orbiter Camera images of layered material in southern Melas Chasma. A particular set of strata is arranged in three depositional sequences bounded by stratigraphic unconformities. The sequences show spectacular, steeply-inclined clinoforms, and a broad channel-like depression. There are the same complex stratal patterns in Valles Marineris on Mars as there are in terrestrial deposits. Insights into these three-dimensional deposit geometries are used to tentatively isolate specific depositional processes and point to ancient subaqueous settings. Findings in southern Melas mark out a very attractive spot to which the search for ancient habitable environments on Mars could be directed.

Stratigraphic architectures spotted in southern Melas Chasma, Valles Marineris, Mars: COMMENT: COMMENT

[Dromart et al. (2007)][1] describe a spectacular stratigraphic complex within southern Melas Chasma, Vallis Marineris, Mars. Following a rigorous stratigraphic description of the complex, they proceed to interpret the responsible depositional processes as analogous to subaqueous channel-levee

Fluvial and lacustrine activity on layered deposits in Melas Chasma, Valles Marineris, Mars

Valley networks on Mars are the most obvious features attesting that different geologic processes and possibly climatic conditions existed in the past. THEMIS images reveal valley networks within Melas Chasma, in Valles Marineris, a Hesperian‐age canyon system. The valley networks in Melas Chasma are dense and highly organized, and the heads of the valleys are scattered at different elevations. All these features suggest that the networks were fed by precipitation. The morphological details reveal inner channels on some valley floors, attesting that water flowed within these valleys. On the DEM, the valleys flow into a completely enclosed depression. The edge of this feature follows a MOLA contour line, and the depression shows many sedimentary morphologies suggesting lacustrine environment. These landforms are located on remnants of layered deposits possibly composed of sulfate layers suggesting that fluvial activity could have contributed to the erosion of the layered terrains in Valles Marineris. Collectively, the features in Melas Chasma are a maximum of Hesperian in age. These results suggest that warm, wet environmental conditions on Mars persisted through the Hesperian and were present during the formation of Valles Marineris. The evidence for a paleolake in Melas Chasma attests to adequate environmental conditions for life development through the Hesperian period.

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.