Coprates Chasma Research Articles

Generation and Optimization of Spectral Cluster Maps to Enable Data Fusion of CaSSIS and CRISM Datasets

Four-band color imaging of the Martian surface using the Color and Stereo Surface Imaging System (CaSSIS) onboard the European Space Agency’s ExoMars Trace Gas Orbiter exhibits a high color diversity in specific regions. Not only is the correlation of color diversity maps with local morphological properties desirable, but mineralogical interpretation of the observations is also of great interest. The relatively high spatial resolution of CaSSIS data mitigates its low spectral resolution. In this paper, we combine the broad-band imaging of the surface of Mars, acquired by CaSSIS with hyperspectral data from the Compact Reconnaissance Imaging Spectrometer (CRISM) onboard NASA’s Mars Reconnaissance Orbiter to achieve a fusion of both datasets. We achieve this using dimensionality reduction and data clustering of the high dimensional datasets from CRISM. In the presented research, CRISM data from the Coprates Chasma region of Mars are tested with different machine learning methods and compared for robustness. With the help of a suitable metric, the best method is selected and, in a further step, an optimal cluster number is determined. To validate the methods, the so-called “summary products” derived from the hyperspectral data are used to correlate each cluster with its mineralogical properties. We restrict the analysis to the visible range in order to match the generated clusters to the CaSSIS band information in the range of 436–1100 nm. In the machine learning community, the so-called UMAP method for dimensionality reduction has recently gained attention because of its speed compared to the already established t-SNE. The results of this analysis also show that this method in combination with the simple K-Means outperforms comparable methods in its efficiency and speed. The cluster size obtained is between three and six clusters. Correlating the spectral cluster maps with the given summary products from CRISM shows that four bands, and especially the NIR bands and VIS albedo, are sufficient to discriminate most of these clusters. This demonstrates that features in the four-band CaSSIS images can provide robust mineralogical information, despite the limited spectral information using semi-automatic processing.

A record of syn-tectonic sedimentation revealed by perched alluvial fan deposits in Valles Marineris, Mars

AbstractOn Mars, basins formed by tectonic processes are rare and mostly have unconstrained subsidence histories. One method for understanding this record of subsidence is through associated alluvial fans, which are sourced from uplifted areas and accumulate in downthrown basins. The source, morphology, and superposition of fan deposits can be used to reconstruct fault kinematics, the relative timing of accommodation space formation, and, in turn, the influence tectonic processes had on Martian fan formation. Here we use high-resolution orbital data sets to characterize sediment fan deposits associated with syn-tectonic sedimentation in two regions of the Valles Marineris canyons: Coprates Chasma and Juventae Chasma. These deposits comprise sediment fans on the current canyon floor and low-gradient surfaces perched several kilometers above the canyon floor. We interpret the low-gradient surfaces as remnant sediment fan deposits, which originally formed at the former canyon floor and have since been offset due to normal faulting. The preservation of vertically offset generations of sediment fan deposits supports a progressive, basinward migration of fault activity into the original hanging wall or repeat activity along a fault zone. Each episode of faulting was followed by a basinward shift in drainages, which led to fault-scarp degradation and formation of a new generation of fans. Multiple episodes of syn-tectonic sedimentation occurred during the evolution of the basins, with fluvial activity sporadically active. Our results demonstrate, for the first time on Mars, that depositional cyclicity was linked to tectonic deformation, possibly representative of regional processes throughout Valles Marineris.

Measuring Ripple and Dune Migration in Coprates Chasma, Valles Marineris: A Source to Sink Aeolian System on Mars?

AbstractActive aeolian systems are present across the martian surface, with active dune fields commonly found in depressions such as craters and valleys. There are many dune fields within the equatorial region of Mars, including Valles Marineris, but the effects of topography on wind regimes and consequently dune migration in Valles Marineris is poorly understood. We investigated both the ripple and dune migration in a dune field in Coprates Chasma using High Resolution Science Experiment Images (HiRISE) and Context Camera (CTX) images. Migration rates of dune brinks and ripples were measured over varying time scales, between Earth years 2007–2014. The dunes here are some of the tallest on Mars, with heights of up to 182 m. The dune brinks are migrating eastwards at a rate of 0.1–0.3 m/EY through the valley due to topographical influences on the local winds. Potential sediment sources for the dune field were identified and investigated by studying thermal inertia and mineralogy. The topographic slope‐related katabatic winds travel down the valley walls and converge with the dominant winds traveling through the center of the valley, causing overall eastwards dune migration. Topography is likely the dominant control on the local wind regime; slope winds travel down the sides of the valley walls and are funneled through the center of the valley. These local winds subsequently facilitate the migration of the large dunes in Coprates Chasma, thus expanding our understanding of local winds in the Martian environment.

Local topography effects on the surface temperatures on Mars – Application to the case of Recurring Slope Lineae (RSL)

Recurring Slope Lineae (RSL) are dark linear recurrent features which incrementally lengthen and fade each year on Martian slopes. Their activity during warmest seasons rises the question of modern martian water related process. To investigate this question, this study focuses on the thermal context of slopes hosting RSL. The paper presents a numerical study of surface and subsurface temperatures at high spatial resolution (1 m/pix) of three well-described RSL sites: Palikir Crater, Coprates Chasma and Rauna Crater. We then compare our local thermal simulations to quantitative measurements of the length and timing of RSL lengthening and fading in these three sites. We underline that RSL sites surface temperatures are poorly correlated with RSL morphology's changes. Indeed, lineae lengthening occurs while surface temperature are under the melting point of pure water (273 K) for at least two sites: Coprates Chasma and Rauna Crater, for two different sets of surface physical properties. We also highlight the significant differences in temperature range while lengthening occurs for all the sites, from ~220 K in Coprates Chasma site to a maximum of ~300 K in Palikir Crater. RSL are sometimes associated with signals in topography, involving significant heating variations of the surface, which can yield to the formation of local winds. Facing the thermal inconsistency between wet triggering processes and RSL reported activity, we suggest that RSL are more probably dry processes.

Quantitative assessment of uncertainties in modeled crater retention ages on Mars

With increasing high-resolution coverage of Mars' surface, crater count analysis is being used to estimate the formative ages of small depositional features (<1000 km2) to constrain timing of climate-driven events. We introduce a probabilistic cratering model that quantifies how modeled age estimates vary strongly with crater obliteration rates (β, defined as the combined rate of erosion and infilling), minimum crater diameter counted, and surface area size. Model results show that crater obliteration introduces significant uncertainty on small surfaces, where moderate obliteration rates (β ~ 25 nm a−1) require surface areas of about 10,000 km2 to date 2 to 3 Ga surfaces accurately, and strongly obliterated surfaces (β = 200 nm a−1) require surface areas up to 100,000 km2. In practice, smaller areas can nonetheless be analyzed probabilistically by estimating a range of likely obliteration rate and modeled age combinations and optimizing these values relative to observed crater count data, resulting in a distribution of possible modeled surface ages and associated β. We demonstrate this method using data from the Coprates Chasma landslide (~200 km2) and numerous fan and delta deposits in the Gale crater region. Although the deposits in and around Gale are <1000 km2, a probabilistic analysis enables us to suggest that deltaic deposits in and near Gale have a higher probability of being older (~>3 Ga) than nearby fans (~1–3 Ga). This analysis provides a simple quantitative framework for assigning probable surface age ranges for small features on Mars.

Quantifying widespread aqueous surface weathering on Mars: The plateaus south of Coprates Chasma

Pedogenesis has been previously proposed on the plateaus around Coprates Chasma, Valles Marineris to explain the presence of widespread clay sequences with Al-clays and possible hydrated silica over Fe/Mg-clays on the surface of the plateaus (Le Deit et al., 2012; Carter et al., 2015). We use previous observations together with new MRO targeted observations and DEMs to constrain the extent and thickness of the plateau clay unit: the Al-clay unit is less than 3 m thick, likely ∼1 m, while the Fe/Mg-clays underneath are few tens of meters thick. We also refine the age of alteration by retrieving crater retention ages of the altered plateau and of later deposits: the observed clay sequence was created by surface pedogenesis between model ages of 4.1 Ga and 3.75 Ga. Using a leaching model from Zolotov and Mironenko (2016), we estimate the quantity of atmospheric precipitations needed to create such a clay sequence, that strongly depends on the chemistry of the precipitating fluid. A few hundreds of meters of cumulated precipitations of highly acidic fluids could explain the observed clay sequence, consistent with estimates based on late Noachian valley erosion for example (Rosenberg and Head, 2015). We show finally that the maximum quantity of sulfates potentially formed during this surface weathering event can only contribute minimally to the volume of sulfates deposited in Valles Marineris.

Amazonian volcanism inside Valles Marineris on Mars

The giant trough system of Valles Marineris is one of the most spectacular landforms on Mars, yet its origin is still unclear. Although often referred to as a rift, it also shows some characteristics that are indicative of collapse processes. For decades, one of the major open questions was whether volcanism was active inside the Valles Marineris. Here we present evidence for a volcanic field on the floor of the deepest trough of Valles Marineris, Coprates Chasma. More than 130 individual edifices resemble scoria and tuff cones, and are associated with units that are interpreted as lava flows. Crater counts indicate that the volcanic field was emplaced sometime between ∼0.4 Ga and ∼0.2 Ga. The spatial distribution of the cones displays a control by trough-parallel subsurface structures, suggesting magma ascent in feeder dikes along trough-bounding normal faults. Spectral data reveal an opaline-silica-rich unit associated with at least one of the cones, indicative of hydrothermal processes. Our results point to magma–water interaction, an environment of astrobiological interest, perhaps associated with late-stage activity in the evolution of Valles Marineris, and suggest that the floor of Coprates Chasma is promising target for the in situ exploration of Mars.

Valles Marineris tectonic and volcanic history inferred from dikes in eastern Coprates Chasma

AbstractMagmatic dikes have been proposed to have weakened and fractured the crust, allowing the formation of Valles Marineris. Hence, dikes were studied in the region of eastern Coprates Chasma in an area that includes a major transition between Hesperian‐aged volcanic deposits in the western walls and pristine Noachian crust in the eastern walls. Over a hundred dikes were identified. Dike widths are 13 m on average. Estimation of magma eruption rates are comparable with previous estimates for Hesperian lava flows on Mars (105 to 106 m3 s−1). Dikes dips range from 55° to 90°; orientations record two distinct main tectonic stress fields (90° and 70°) different from the Chasmata. Dikes striking 90° are only observed at elevations below 1500 m. Dikes striking 70° are observed at elevation below 0 m and are therefore considered older. However, linear features are also observed on the late Noachian/early Hesperian surrounding plateaus and could be related to the 70° dike group. In the western part of our study area, dikes (~10% of the total amount of dikes mapped) strike 110, subparallel to Valles Marineris, and suggest a relationship between dike emplacement and graben formation. The presence of preexisting faults in the two directions (90° and 70°) could explain the shape of eastern Valles Marineris and chaotic terrains, which have a different general orientation than the Valles Marineris main rift. Our results suggest a complex relationship between dike emplacement and the formation of Valles Marineris.

The central uplift of Elorza Crater: Insights into its geology and possible relationships to the Valles Marineris and Tharsis regions

The majority of hydrated silicate occurrences on Mars are associated with impact craters (Ehlmann et al., 2011; Carter et al., 2013). Three formation mechanisms have been suggested to account for this correlation: (1) aqueous alteration occurred pre-impact, and was subsequently exposed via the impact (pre-impact; Bibring et al., 2006; Ehlmann et al., 2011), (2) heat generated from the impact facilitated the formation of a hydrothermal system, leading to alteration products (syn-impact; e.g. Marzo et al., 2010; Osinski et al., 2013), and/or (3) altered materials were deposited after crater formation, or formed within the crater well after the impact had taken place (post-impact). In this study, we analyze the central uplift of Elorza Crater, a ∼40km diameter impact crater located ∼300km north of Valles Marineris. To determine whether hydrated minerals found within the uplift were generated pre-, syn-, or post-impact, we used a data synthesis approach, utilizing High Resolution Imaging Science Experiment (HiRISE), Compact Reconnaissance Imaging Spectrometer for Mars (CRISM), Context Camera (CTX), and Thermal Emission Imaging System (THEMIS) imagery. Opaline silica is observed in two locations on the southwestern side of the uplift and is interpreted to have been pre-existing or formed via hydrothermal alteration due to stratigraphic relationships with the overlying impact melt unit. Both Fe/Mg smectite and low-calcium pyroxene (LCP) are found throughout the uplift. Bedrock exposures on the northern wall of Coprates Chasma containing Fe/Mg smectite and LCP suggest an uplifted origin for these units. In all cases, although a pre-existing origin is probable, it is difficult to rule out the possibility of an impact-generated hydrothermal origin. Using the observed stratigraphy exposed in Coprates Chasma and bedrock exposures analyzed in nearby craters, we were able to constrain the pre-impact stratigraphy around Elorza. The near-subsurface consists of Hesperian-aged, discontinuous lava/ash deposits that may be interposed with opaline silica-bearing deposits, overlying Noachian basement consisting of smectite-bearing bedrock and LCP- bearing light-toned fractured bedrock.

Compositional and structural constraints on the geologic history of eastern Tharsis Rise, Mars

Identification and mapping of distinct compositional units using high-resolution orbital imagery of the walls of Valles Marineris, Mars, provide key structural and environmental constraints on the geologic history of the eastern Tharsis Rise. Our results corroborate an overall vertical structure of the upper crust consistent with that inferred from previous studies. An upper unit spectrally consistent with basalt that has variable thickness is overlain on an early to possibly pre-Noachian substrate. The pre-/early-Noachian material is heterogeneous, dominated by low-calcium pyroxene-rich material but ranging locally from feldspathic to and olivine-rich. Aqueous alteration of pre-/early-Noachian rock to dominantly Fe/Mg-smectite is widespread and observed along the chasma walls from −4 to 1km. However phyllosilicates outcrop discontinuously, suggesting that they were emplaced via impact excavation of even older altered material, or formed by non-uniform alteration driven by impact- or magma-related hydrothermal activity. A distinct, higher-temperature alteration assemblage identified by the presence of chlorite, zeolite, and associated carbonate (+/−serpentine) occurs on both the north and south walls of eastern Coprates Chasma in a confined longitudinal band, where the eastern margin of the Thaumasia Plateau intersects Valles Marineris. These secondary minerals are concentrated within a heavily fractured band exposed at a regional tectonic boundary along the margin of the Thaumasia Plateau, and likely record a regional zone of elevated crustal heat flow and fluid circulation. The generally north–south trending band exhibiting this distinct assemblage serves as a tracer for tectonic deformation; its continuity across Valles Marineris is incongruous with previously suggested lateral slip along the margin of a ‘megaslide’, and constrains left-lateral transtensional faulting to less than ∼25–50km. Mapping of compositionally distinct outcrops to the west also reveals localized uplift in western Coprates Chasma, suggesting thrust faults underlying wrinkle ridges. In western Valles Marineris, the upper basalt layer thickens dramatically, supporting either occurrence of a large, infilled basin to the west, or down-dropping and volcanic burial of material near Melas/Candor Chasma, the latter consistent with a constructional origin of the Tharsis Rise.

Estimation of dust variability and scale height of atmospheric optical depth (AOD) in the Valles Marineris on Mars by Indian Mars Orbiter Mission (MOM) data

In this paper analyses of bright hazes observed inside Valles Marineris formed during mid-southern spring of Mars is presented. The analysis is performed by using data collected by Mars Colour Camera (MCC) onboard Indian Mars Orbiter Mission on orbits 34, 49 and 52 corresponding to the observation dates of October 28, December 5 and December 13, 2014. It is found that during all these orbits the valley was hazy. On orbit 34 a thick layer of haze was observed, which became relatively thinner on orbit 49. Thick haze reappeared after eight days on orbit 52. We also measured the optical depth of martian atmosphere as a function of altitude above two opposing walls (northern and southern walls of the Valles Marineris near Coprates Chasma region) of the valley, from stereo images that were taken with MCC on December 5, 2014. The optical depth was measured from contrast comparisons of the stereo images with “stereo method”. In the northern wall of Valles, we estimated the optical depth as a function of altitude (ranging between −6km and 3km) and found values between 1.7 (bottom) and 1.0 (top) in red channel and between 2.1 (bottom) and 1.2 (top) in green channel. A fit to these results yields a scale height for the optical depth of 14.08km and 11.24km in red and green channel, which are more or less in good agreement to the pressure scaled height of martian atmosphere at that time in the region as consulted from Global Circulation Model (GCM). We also estimated optical depth in southern wall of Valles Marineris. However, in this case optical depth remains nearly constant with decreasing altitude. We consulted GCM for wind direction in the region and found strong wind with direction from south-west to north-east intersecting the mountain like structure of the southern wall of Valles Marineris. Our optical depth results and the wind direction suggest the presence of lee-wave cloud above the southern wall of Valles Marineris.

Scoria cones on Mars: Detailed investigation of morphometry based on high‐resolution digital elevation models

AbstractWe analyze the shapes of 28 hypothesized scoria cones in three regions on Mars, i.e., Ulysses and Hydraotes Colles and Coprates Chasma. Using available High‐Resolution Imaging Science Experiment and Context Camera (CTX) digital elevation models, we determine the basic morphometric characteristics of the cones and estimate from ballistic modeling the physical parameters of volcanic eruptions that could have formed them. When compared to terrestrial scoria cones, most of the studied cones show larger volumes (up to 4.2 × 109 m3), larger heights (up to 573 m), and smaller average slopes. The average slopes of the Ulysses, Hydraotes, and Coprates cones range between 7° and 25°, and the maximum slopes only rarely exceed 30°, which suggests only a minor role of scoria redistribution by avalanching. Ballistic analysis indicates that all cones were formed in a similar way, and their shapes are consistent with an ejection velocity about 2 times larger and a particle size about 20 times smaller than on Earth. Our results support the hypothesis that the investigated edifices were formed by low‐energy Strombolian volcanic eruptions and hence are equivalent to terrestrial scoria cones. The cones in Hydraotes Colles and Coprates Chasma are on average smaller and steeper than the cones in Ulysses Colles, which is likely due to the difference in topographic elevation and the associated difference in atmospheric pressure. This study provides the expected morphometric characteristics of Martian scoria cones, which can be used to identify landforms consistent with this type of activity elsewhere on Mars and distinguish them from other conical edifices.

Fresh exposures of hydrous Fe‐bearing amorphous silicates on Mars

AbstractWe have discovered relatively fresh exposures of a hydrated, amorphous material along the wall rock slopes in Coprates Chasma, Mars. Visible and near‐infrared reflectance spectra extracted from the deposits exhibit broad absorptions around 1.42, 1.94, and 2.25 µm that are most consistent with laboratory spectra of nanophase hydrated Fe‐rich allophane and Fe‐rich opal. The three absorptions, especially the 1.4 µm band, have the strongest hydration signatures yet detected on Mars by orbital data, suggesting high‐water content material that is relatively fresh and has not altered or lost water since its formation or exposure. Age dating from crater size‐frequency distributions of the Fe‐rich allophane/opal deposits yields ages of ~50–100 Myr, consistent with a young exposure time and minimal time for dehydration. Either the Fe‐rich allophane/opal represents an older material already contained within the wall rock that has been more recently exposed, or it represents a younger material formed during more recent aqueous activity.

One million cubic kilometers of fossil ice in Valles Marineris: Relicts of a 3.5 Gy old glacial landsystem along the Martian equator

Self-consistent landform assemblages suggest that Valles Marineris, the giant valley system that stretches along the Martian equator, was entirely glaciated during Late Noachian to Early Hesperian times and still contains huge volumes of fossil ice. Some of these glacial landform assemblages are illustrated here, with representative examples selected in three regions: Ius Chasma, Central Candor Chasma and the junction between Coprates Chasma and Capri Chasma. A morphological boundary separating an upper spur-and-gully morphology from a smooth basal escarpment has been spectacularly preserved along valley walls throughout Valles Marineris. The boundary winds around topographic obstacles and displays long-wavelength variations in elevation. It is associated with lateral benches, hanging valleys and truncated spurs. Comparisons with terrestrial analogs indicate that it is most reasonably interpreted as a glacial trimline. Chasma floors are covered by various kinds of terrains, including hummocky terrains, platy terrains, lateral banks, layered benches and a draping mantle. Landforms in these terrains and their spatial relationship with the interpreted trimline suggest that they correspond to various disintegration stages of an ancient glacial fill, currently protected by a superficial cover of ablation till. Altogether, these landforms and terrains compose a full glacial landsystem with wet-based glaciers that were able to flow and slide over their beds. It was most probably fed by ice accumulating at low elevations directly from the atmosphere onto valley floors and walls, with only minor contributions from tributary glaciers flowing down from higher elevations. Similar fossil glacial landsystems dating back from the early Martian history are to be expected in many other low-latitude troughs such as chasmata, chaos, valleys, impact craters and other basins.

Karst landforms in an interior layered deposit within the Coprates Chasma, Mars

The Coprates Chasma forms part of the backbone of the Valles Marineris canyon system. In the westernmost part of the chasma in an embayment on the northern wall a mound of layered material rises from the chasma floor and displays a characteristic dome-shaped morphology. The mineralogical characteristics of the dome and its surroundings have been determined by analysis of the CRISM (Compact Reconnaissance Imaging Spectrometer for Mars) data (image HRL00003752). The unit shows the clear signatures of kieserite, an evaporite mineral also found on Earth. Through analysis of the M.RO. HiRISE (Mars Reconnaissance Orbiter High Resolution Imaging Science Experiment images) we have investigated the dome landforms and the possible processes involved in their formation and shaping in great detail. The analysis shows that the landforms observed clearly indicate the presence of solutional processes that made those karst landforms. The results of our observation also suggest that liquid water must have existed on the dome in the past for long enough for the solution features to be formed, and that the karst landforms investigated exhibit an older erosional age or shorter than the same landforms studied in a similar kieserite dome located within Tithonium Chasma, another graben of the Valles Marineris system.

Dikes of distinct composition intruded into Noachian‐aged crust exposed in the walls of Valles Marineris

Valles Marineris represents the deepest natural incision in the Martian upper crust. Previous studies showed that the upper parts of the walls were made of finely layered probable basalts in most of the chasmata, while the base of the stratigraphy reveals primary Noachian crustal blocks. Exposures of pristine Noachian bedrock are rare on Mars, and mostly observed outside of their geological context. The occurrence of well‐preserved and extended outcrops of pristine material in this giant rift could bring valuable information on the early processes that took place at the surface of Mars. Analyses of high resolution data over the best exposures of lower walls in Coprates Chasma, central Valles Marineris, revealed the presence of multiple magmatic intrusions interpreted as dikes. These dikes intrude an old, massive, fractured bedrock interpreted as being preserved ancient Noachian crust. Their composition, determined using CRISM data, and distribution, limited to this ancient crust at the bottom walls, indicate that they might have formed early in the rift formation, and therefore represent exceptionally well‐preserved outcrops of the early history of Mars.

Interior layered deposits within a perched basin, southern Coprates Chasma, Mars: Evidence for their formation, alteration, and erosion

A basinA¢Â�Â�like area containing three interior layer deposits (ILDs) on the southern margin of Coprates Chasma was studied. We interpret the area as an ancestral basin and demonstrate that ILD deposition postdates the formation of the current wall rock slopes. The geometry of the ILD and the wall rock spurs form a catchment area between each ILD and the plateau to the south. Erosional remnants of extensive ash or dust layers deposited on the plateau south of Valles Marineris also crop out on the southern plateau of Coprates Chasma. A mass balance calculation suggests that the volume of each ILD is compatible with the volume of the ash or dust that would have been deposited within each catchment area. We propose that the ILDs likely formed by episodically washing such aerially deposited material down from chasma walls. Rifting of the IusA¢Â�Â�MelasA¢Â�Â�Coprates graben opened the enclosed basin and removed any standing water. Faults within the ILDs are compatible with this chasm opening. Sulfates are associated with the ILDs and lightA¢Â�Â�toned material on the basin floor. We suggest that they result from water alteration of preexisting deposits, though the timing of that alteration may predate or postdate the breaching of the basin. Scours within one ILD are similar to terrestrial glacial scours. During a period of high obliquity ice would accumulate in this region; hence we argue the scours are Martian glacial scours. A late deposited layer marks the end of the active local geological history between 100 My and 1 Gy.

Structural analysis of interior layered deposits in Northern Coprates Chasma, Mars

Interior layered deposits within an embayment on the northern wall of Coprates Chasma in the Valles Marineris, Mars, are studied using HRSC, CTX, HiRISE and CRISM data. The layered material outcrops in three separate locations. The largest layered deposit within the embayment, a free standing central mound, has an approximate stratigraphic thickness of 2 km. Dip directions change along the central axis of this mound, which is also a zone of deformation. The surface texture of layers within this mound displays polygonal structures at HiRISE scale. By contrast, the western layered deposit abuts directly against the chasm wall and appears to have a relatively uneroded depositional surface approximately 600 m below the current top elevation of the central mound. A basement ridge, exposed by a landslide scar near the eastern portion of the area, is covered with layered material and shows downward displacement. We suggest that the entire embayment originated as a small ancestral basin. The displaced basement ridge is evidence of the early basin collapse. The central mound was most likely deposited on a wall rock spur. Deposition did not fill the basin evenly. The detection of hydrated sulfates attests to alteration or deposition by liquid water. Following an erosion event, which coincided with or postdated Valles Marineris formation, thin mesa-forming materials covered most of the area.

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.

Evidence that floodwaters filled and overflowed Capri Chasma, Mars

Erosional features on a plateau in the Valles Marineris provide evidence that a lake filled Capri Chasma until it overflowed its eastern rim, carving two large spillover channels. The floodwaters surged into the adjacent lowlands of ancestral Ganges Chasma and eastern Eos Chasma. The channel floor elevation and depth of incision at Daga Vallis reveal that a 1200‐m‐deep lake water column was drained by the flooding. The width, depth, and steep energy slope (∼0.02) of Daga Vallis and the existence of several cataracts, including a 500‐m‐high dry falls, demonstrate the power of the floodwaters. We estimate a possible peak discharge rate of 1–6 × 108 m3·s−1. The catastrophic flows may have been triggered by the collapse of topographic barriers in eastern Coprates Chasma, providing a gateway for lake waters in the central Valles Marineris to pour eastward toward Capri Chasma and the lowlands beyond. These may have been among the earliest flows in Simud‐Tiu Valles, contributing discharge to a possible sea in the northern plains of Mars.