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During the Noachian period, 4.1-3.7 Gys ago, the Martian environment was moderately similar to the one on present Earth. Liquid water was widespread in a neutral environment, volcanic activity and heat flow more vigorous, and atmospheric pressure and temperature were higher than today. These conditions may have favoured the spread of life on the surface of Mars. The recognition that different planets and moons share rocky material cast in space by meteoroid impact entails that life creation is not necessary for each single planetary body, but could travel through the Solar system on board of rock fragments. Studies conducted on the past forms of Martian life have already highlighted possible positive matches with microbialite-like structures, referable to the geo-environmental conditions in the Noachian and Hesperian. However, by necessity, these studies are on predominantly micro and meso-scopic scale structures and doubts arise as to their attribution to the biogenic world. We suggest that in the identification of Martian life, we are currently in a position similar to the one of Kalkowsky who in 1908, based solely on morphological and sedimentological arguments, hypothesized the (now accepted) view of the biotic origin of stromatolites. Our analysis of thousands of images from Spirit, Opportunity and Curiosity has provided a selection of images of ring-shaped, domal and coniform macrostructures that resemble terrestrial microbialites such as the ring-shaped stromatolites of Lake Thetis, and stacked cones reminiscent of the group of terrestrialConophyton. Notably, the latter were detected by Curiosity in the mudstone known as ‘Sheepbed’, the same outcrop where past organic molecules have been detected and where the occurrence of microbial-induced sedimentary structures (MISS) and of many more microbialitic micro, meso and macrostructures has already been hypothesized. Some of the structures discussed in this work are so complex that alternative biological hypotheses can be formulated as possible algae. Alternate, non-abiotic explanations are examined but we find difficult to explain some of such structures in the context of normal sedimentary processes, both syngenetic or epigenetic.

Orbital and surface observations can shed light on the internal structure of Mars. NASA’s InSight mission allows mapping the shallow subsurface of Elysium Planitia using seismic data. In this work, we apply a classical seismological technique of inverting Rayleigh wave ellipticity curves extracted from ambient seismic vibrations to resolve, for the first time on Mars, the shallow subsurface to around 200 m depth. While our seismic velocity model is largely consistent with the expected layered subsurface consisting of a thin regolith layer above stacks of lava flows, we find a seismic low-velocity zone at about 30 to 75 m depth that we interpret as a sedimentary layer sandwiched somewhere within the underlying Hesperian and Amazonian aged basalt layers. A prominent amplitude peak observed in the seismic data at 2.4 Hz is interpreted as an Airy phase related to surface wave energy trapped in this local low-velocity channel., Nature Communications, 12 (1), ISSN:2041-1723

During an extensive grid-mapping campaign on Mars' southern hemisphere, we have detected so-called sheet flow deposits, which are defined by distinctive lobate fronts, thin and mostly planar layers, and faint sub-parallel surface lineations. They originate from the high plains of Dryas Mons, a large massif of impact and tectonic origin, and follow the topographic gradient down into the adjacent basins. Their sources often coincide with steep and multi-layered outcrops. This work addresses the formation of these deposits, and if they are related to the unique tectonic and endogenic environment of Dryas Mons. We applied photogeological mapping, age determinations by crater counts, as well as topographic and heat flow measurements in order to reconstruct the evolution of these landforms. The calculated ages (mid to late Amazonian), their location in the mid-latitudes, as well as some specific morphologies like lobate flow fronts are typical for standard viscous-flow features on Mars. In contrast to such viscous-flow features, the sheet flow deposits occur isolated in the Dryas Mons region. Aside from that, major other landforms, typical for viscous-flow features are lacking, like sublimation pits, brain terrain or arcuate deformations. Considering these similarities and differences, we suggest that both landforms viscous-flow features and the sheet flow deposits of Dryas Mons were formed by the involvement of volatiles; however, at varying amounts and by different emplacement processes. As the formation of the sheet flow deposits is located in an area with one of the highest heat flow values planet-wide, we calculated if the heat flow could be a potential trigger for the release of outcropping volatile-rich layers. However, our calculations have shown that the heat flow is still insufficient to enable near-surface melting. Instead, we suggest that the volatiles originate from deep layers affected by magmatic and/or intrusive activities in a tectonically active environment. These conditions led to melting and mobilisation of the volatiles by advective hydrothermal processes. Thus, the volatiles migrated upward along hypothesized deep-seated fault systems, formed by Noachian/Hesperian impacts and tectonic activities, until they reached conductive layers outcropping at Dryas Mons, enabling them to drain. After their release to the cold Amazonian atmosphere, this mixture of volatiles and solids partially froze, resulting in a comparatively high viscosity, and hence, a slow and laminar (non-turbulent) movement. This flow behaviour might have led to the formation of the lineations at their surface along shear zones. Hence, we suggest that the sheet flow deposits may have originally formed as slowly moving icy debris flows during the mid- to late Amazonian. This work has been funded by the Research Fellowship Program through the European Space Agency. J. Ruiz contribution was supported by the project PGC2018-095340-B-I00 from the Spanish Ministry of Science, Innovation and Universities.

Gorgonum Chaos is part of the Eridania paleolake in Terra Sirenum and displays a number of prominent light‐toned morphological features that bear a record of the regional climatic conditions throughout most of Martian history. Based on an intergrated analysis of orbital data, we mapped a 1,500 km2 area in the southeast of Gorgonum Chaos. Morphologic, spectroscopic, and stratigraphic analyses were used to determine age and composition of the main geological units in the area. We identified four major geological units with decreasing content of hydrated minerals from the oldest to the youngest units, which were completely free of hydrated minerals. In the study area, phyllosilicate‐rich Noachian units compose the majority of the basin floor. Deposits enriched with evaporites were formed around the Noachian/Hesperian transition and erosion created prominent inverted morphologies. Loess‐like material without significant amounts of hydrated minerals was deposited until the late Hesperian. The youngest unit is an Amazonian layer free of hydrated minerals that originated from volcanic activities. This succession of minerals reflects the transition from more humid climatic conditions with the ability to sustain liquid water on the planet's surface during the Noachian to the hyper‐arid Amazonian environment we observe currently on Mars., Plain Language Summary: Gorgonum Chaos is part of the Eridania basin, which is a former lake system located at the southern hemisphere of Mars. The landforms observed in this area and their variable brightness suggest dramatically changing climatic conditions during the history of Mars. In an area of 1,500 km2 in size, we analyzed different landforms, their spectral characteristics, and their temporal sequences to determine the age and composition of the geological units. We found four major geologic units whose content of hydrated minerals decreased dramatically from oldest to youngest. The oldest unit with a high content of clay minerals is about 3.7 billion years old and formed the former lake bottom. This is followed by a younger unit with a mineral composition that indicates desiccation of the lake and erosion by wind. These, in turn, were largely covered by materials deposited by wind, which show very little evidence of liquid water. The last and youngest unit is volcanic in origin and completely devoid of minerals indicative of water. This succession of minerals reflects the evolution of the Martian surface, which was capable to sustain liquid water in the early stages of the planet and lost this ability during a drastic climate change., Key Points: We produced a geological map of southeastern Gorgonum Chaos. We find a succession from phyllosilicates to olivine in aeolian deposits. The presence of water decreases over time.

For decades the scientific community has been trying to reconcile abundant evidence for fluvial activity on Noachian and early Hesperian Mars with the faint young Sun and reasonable constraints on ...

Juventae Chasma, a canyon peripheral to the Valles Marineris trough system, is the product of a unique merger of a Martian chaotic terrain depression and an extensional (tectonic) chasm. The chaotic terrain portion is set along a NNE-SSW axis, while a linear trough (the chasm component) is present along a WNW-ESE axis. Within the chasm are four discrete bodies of layered, light-toned rocks of sedimentary origin, Features A–D. The chaotic terrain component formed during the Early Hesperian as a result of expulsion of liquid water along pre-existing planes of weaknesses resulting from a WNW-ENE compression. The chasm component formed in response to NE-SW extension during the Middle to Late Amazonian. The light-toned Features A–D are layered at

We have utilized THEMIS, MOLA, CTX and HiRISE data sets to investigate the morphologic and topographic characteristics of the northeast flank of Apollinaris Mons, a region with evidence for significant erosion of volcanic deposits. Using ArcGIS software, we mapped the geology of the northeast flank and obtained age estimates using crater size-frequency distributions from crater counts. Of the eight mapped units, three (Apollinaris Mons upper, mid-, and lower flank units) represent volcanic flank materials in various states of preservation. The Late Noachian-Early Hesperian upper flank unit is massive and contains joint-like vertical structures along the scarp faces of two large plateaus whose upper surfaces likely represent the original surface or near-surface deposits of Apollinaris Mons. Downslope-facing plateau scarps have up to 500 m of relief. Positioned below this unit are the mid-flank and lower flank units, each representing eroded surfaces within Apollinaris Mons flank materials that have stabilization ages between the Late Hesperian to Early Amazonian. Mantling material in the region contains cavities and boxwork-like textures that resemble patterns in terrestrial eroded pyroclastic flow deposits. Dome-shaped mounds west of the mapping region have morphologic similarities to terrestrial fumarolic mounds or possibly inverted impact craters. Both the texture and features suggest widespread pyroclastic deposits adjacent to Apollinaris Mons. Using MOLA gridded topography, we estimate that ~308 km3 of materials have been eroded along the flanks of the volcano. Statistical and histogram data from the thickness values of eroded materials shows that up to ~300 m of material has been removed from the majority of flank surfaces. Assuming steady state erosion of flank surfaces, we estimate an area-normalized loss rate of ~0.859 nm/yr for northeast Apollinaris Mons. This erosion rate is within the long-term range for Mars (~0.01–10 nm/yr) as estimated from MER landing site geology by Golombek et al. [2006].

We use spacecraft measurements to determine the inventory of CO2 gas on Mars. We estimate the amounts of gas from an early, thicker Martian atmosphere that have been lost to space by impact ejection and by solar and solar-wind stripping, and that have been removed to non-atmospheric reservoirs of CO2 ice, clathrate hydrate, adsorbed gas, or carbonate minerals. Loss to space has removed 1–2 bars of CO2. Deeply buried carbonates may contain up to the equivalent of a bar of CO2, with the other sinks likely containing no more than ∼90 mbar of CO2. These sinks can readily account for loss of the bulk of an early CO2 atmosphere thought to be necessary to provide early greenhouse warming. Loss of CO2 from the atmosphere to these sinks is the likely explanation for the transition in climate from an early, warmer atmosphere to the cold, dry atmosphere that has been present since early in the Hesperian epoch.

The Navua Valles are comprised of a system of channels and valleys on the inner Northeastern rim of Hellas Basin, which is a 1500-km-long sloping terrain. Drainage systems and regional geology in this unique setting were not previously mapped in detail. We mapped this region using CTX (6 m/px) as the base map and assessed surface unit ages resulting from our crater counting. We found that the timing of the deposit-forming episodes in this region during the Hesperian and Early to Middle Amazonian largely correlated to active phases of the Hadriacus Mons volcanic center. We found evidence for several episodes of fluvial activity Hesperian to the Amazonian with declining intensity, and transitioning to ice-dominated processes. The channels in the Navua Valles region erode into deposits dating from the Noachian to Early Amazonian, including the Noachian highlands, Noachian to early Amazonian crater ejecta, and likely volcanic plains formed from the Hesperian to the Hesperian–Amazonian transition. Channels directly originating from Hadriacus Mons are younger, while precipitation-fed channels at larger distance from the volcanic center are older, indicating different triggers for fluvial activity. Crater counting results indicate that almost all channel floors were at least partially resurfaced during the Amazonian and that several channel deposits formed during the last 0.5 Gyr. Water pathways likely included surface channels, lakes, and subsurface flow. The Navua Valles channel system is discontinuous, and the number of terminal deposits (sink locations) is almost as high as the number of channel sources, which is unusual for valley networks elsewhere on Mars. Interior channels formed only in the major Navua channels, they are even more fragmented than their parent channels, but occur along their entire length. Channels and valley systems within the Navua Valles are potential targets for in situ astrobiological studies, as they could have provided potential habitats at least periodically, from the Late Hesperian to the Late Amazonian.

This work presents a structural analysis of the southern half of Ulysses Fossae, a transtensional mega-structure located in the Martian Western Hemisphere, at the western slope of the Tharsis rise, Mars. The analysis includes a structural mapping, a determination of the trend of the main fracture systems, an approximate determination of their age, the construction of five topographic profiles, and an estimation of the elongation amount verified along each profile direction. The structures of the studied zone are typically normal faults and grabens affecting Hesperian units (3.6–3.5 Ga according to crater-counting techniques). The structure is N-S-oriented, although later structures show NW-SE azimuths. These results have been checked against the predictions of a simple kinematic model of transtension, which suggest pure-shear dominated flow. The obtained values of kinematic vorticity (0.1–0.65) vary along and across the structures. The presence of a large-scale shear zone at depth is the explanation preferred in this work.

Compositions of the Amazonian-aged Tharsis and Elysium volcanic provinces of Mars have been largely obscured in Mars-orbital remote sensing data by windblown dust. Fresh impact craters formed within the last few years have disturbed surface dust, providing unique windows to explore these regions' relatively dust-free mineralogic compositions. Such fresh craters, plus other small exposures of less dusty materials, are resolved spatially in high-resolution targeted observations of visible/short-wave infrared spectral reflectance by the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM). Analysis of CRISM observations of small, relatively dust-free locations in the Tharsis and Elysium regions shows that these provinces are dominated by high-Ca pyroxene and olivine, similar to volcanic materials of Hesperian age. Thus, the mafic mineral compositions of Hesperian and Amazonian volcanic materials appear similar to each other, but distinct from the olivine- and low-Ca pyroxene-rich compositions that dominate Noachian rocks. In the core regions of both provinces, where thermal infrared data indicate the thickest dust cover, the dust is sufficiently thick that few fresh craters penetrate to expose the underlying volcanics. These results may be consistent with low Si contents of surface materials in Elysium and western Tharsis measured by gamma-ray spectroscopy resulting from a thick cover of dust depleted in Si-rich phases, possibly due to eolian sorting.

Ancient stratigraphy on Isidis Basins western margin records the history of water on early Mars. Noachian units are overlain by layered, basaltic-composition sedimentary rocks that are enriched in polyhydrated sulfates and capped by more resistant units. The layered sulfates – uniquely exposed at northeast Syrtis Major – comprise a sedimentary sequence up to 600-m thick that has undergone a multi-stage history of deposition, alteration, and erosion. Siliciclastic sediments enriched in polyhydrated sulfates are bedded at m-scale and were deposited on slopes up to 10o, embaying and thinning against pre-existing Noachian highlands around the Isidis basin rim. The layered sulfates were modified by volume-loss fracturing during diagenesis. Resultant fractures hosted channelized flow and jarosite mineral precipitation to form resistant ridges upon erosion. The structural form of the layered sulfates is consistent with packages of sediment fallen from either atmospheric or aqueous suspension; coupling with substantial diagenetic volume-loss may favor deepwater basin sedimentation. After formation, the layered sulfates were capped by a "smooth capping unit" and then eroded to form paleovalleys. Hesperian Syrtis Major lavas were channelized by this paleotopography, capping it in some places and filling it in others. Later fluvial features and phyllosilicate-bearing lacustrine deposits, sharing a regional base level of ~-2300 m, were superimposed on the sulfate-lava stratigraphy. The layered sulfates suggest surface bodies of water and active groundwater upwelling during the Noachian–Hesperian transition. The northeast Syrtis Major stratigraphy records at least four distinct phases of surface and subsurface aqueous activity spanning from late Noachian to early Amazonian time.

International audience; Recent research suggests that major meteorite impact events into a Late Hesperian/Early Amazonian ocean likely produced a mega‐tsunami that would have resurfaced coastal areas in northwestern Arabia Terra. The orientations of the associated lobate deposits, a conspicuous type of landforms called Thumbprint Terrain, suggests that if an impact event triggered the mega‐tsunami, the most likely location of the source crater is within the northern plains regions situated north of Arabia Terra. This study focuses on the identification of impact craters that impacted into the ocean and are likely to have produced the tsunami. We selected 10 complex impact craters, based on their diameters, location, and geomorphic characteristics. Of those, the Late Hesperian ~120‐km‐diameter Lomonosov crater exhibits a unique topographic plan view asymmetry (compared to other similar‐sized and similar‐aged craters in the northern plains such as Micoud, Korolev, and Milankovic). We attribute its broad and shallow rim, in part, to an impact into a shallow ocean as well as its subsequent erosion from the collapsing transient water cavity. The likely marine formation of the Lomonosov crater, and the apparent agreement in its age with that of the Thumbprint Terrain unit (~3 Ga), strongly suggests that it was the source crater of the tsunami. These results have implications for the stability of a late northern ocean on Mars.

Extensional and compressional structures are globally abundant on Mars. Distribution of these structures and their ages constrain the crustal stress state and tectonic evolution of the planet. Here in this paper, we report on our investigation over the distribution of the tectonic structures and timings of the associated stress fields from the Noachis-Sabaea region. Thereafter, we hypothesize possible origins in relation to the internal/external processes through detailed morphostructural mapping. In doing so, we have extracted the absolute model ages of these linear tectonic structures using crater size-frequency distribution measurements, buffered crater counting in particular. The estimated ages indicate that the tectonic structures are younger than the mega impacts events (especially Hellas) and instead they reveal two dominant phases of interior dynamics prevailing on the southern highlands, firstly the extensional phase terminating around 3.8 Ga forming grabens and then compressional phase around 3.5–3.6 Ga producing wrinkle ridges and lobate scarps. These derived absolute model ages of the grabens exhibit the age ca. 100 Ma younger than the previously documented end of the global extensional phase. The following compressional activity corresponds to the peak of global contraction period in Early Hesperian. Therefore, we conclude that the planet wide heat loss mechanism, involving crustal stretching coupled with gravitationally driven relaxation (i.e., lithospheric mobility) resulted in the extensional structures around Late Noachian (around 3.8 Ga). Lately cooling related global contraction generated compressional stress ensuing shortening of the upper crust of the southern highlands at the Early Hesperian period (around 3.5–3.6 Ga). Keywords: Martian dynamics, Southern highlands, Extensional tectonics, Compressional tectonics, Age of structures, Buffer crater counting

Martian lobate scarps are prominent tectonic structures interpreted to be the topographic expression of large thrust faults generated under compression. Here, we present a structural modeling performed on three large lobate scarps (Ogygis Rupes, Bosporos Rupes and Phrixi Rupes) located in the heavily cratered highlands of Mars, specifically in Aonia Terra, between Thaumasia Montes and Argyre impact basin. These lobate scarps, formed in the Late Noachian/Early Hesperian, strike parallel to the edge of the Thaumasia Montes, and were generated by ESE-vergent thrust faults. Structural analysis of craters deformed by these lobate scarps gives minimum estimates for the faults slip of ∼1700–3100 m. We applied two structural methods in order to constrain the geometry of these thrust faults at depth, area balanced cross sections and forward mechanical dislocation modeling, obtaining a depth of faulting in this area between ∼18 and ∼45 km, and dip angles between 23° and 35°. These results are consistent with previous studies of lobate scarps on Mars. The depth of faulting gives an estimation of the depth of the brittle-ductile transition at the time of its formation giving a range of depth in which the state of the lithosphere change from brittle to ductile-dominated deformation. The heat flow values calculated from the obtained depths of the brittle–ductile transition range from 25 to 51 mW m−2. We show that the brittle-ductile transition depth in Aonia Terra is set in 18–27 km at a larger distance from the basin center, while it is deeper closer to the Argyre rim (∼33–45 km). This difference seems to indicate a thickening of the brittle domain under Argyre main rim with respect to the external area but, attending to regional geology and heat flow values calculated, this high value (∼33–45 km) might be an overestimation of the depth of faulting caused by the presence of the crater rim elevation before the formation of the lobate scarps.

The scale of groundwater upwelling on Mars, as well as its relation to sedimentary systems, remains an ongoing debate. Several deep craters (basins) in the northern equatorial regions show compelling signs that large amounts of water once existed on Mars at a planet‐wide scale. The presence of water‐formed features, including fluvial Gilbert and sapping deltas fed by sapping valleys, constitute strong evidence of groundwater upwelling resulting in long term standing bodies of water inside the basins. Terrestrial field evidence shows that sapping valleys can occur in basalt bedrock and not only in unconsolidated sediments. A hypothesis that considers the elevation differences between the observed morphologies and the assumed basal groundwater level is presented and described as the “dike‐confined water” model, already present on Earth and introduced for the first time in the Martian geological literature. Only the deepest basins considered in this study, those with bases deeper than −4000 m in elevation below the Mars datum, intercepted the water‐saturated zone and exhibit evidence of groundwater fluctuations. The discovery of these groundwater discharge sites on a planet‐wide scale strongly suggests a link between the putative Martian ocean and various configurations of sedimentary deposits that were formed as a result of groundwater fluctuations during the Hesperian period. This newly recognized evidence of water‐formed features significantly increases the chance that biosignatures could be buried in the sediment. These deep basins (groundwater‐fed lakes) will be of interest to future exploration missions as they might provide evidence of geological conditions suitable for life., Key Points Geological evidence supporting Martian planet‐wide groundwater upwellingWater‐saturated zone intercepted by basins reaching more than ‐4000m below the Mars DATUMPutative relations between groundwater‐saturated level (groundwater‐fed lakes) and the ocean shorelines around ‐4000m below the DATUM

We used multiple orbital data sets to analyze the morphology, composition, and stratigraphy of clays and other hydrated deposits within Hydrae Chasma. Fe/Mg-smectites are concentrated along the lower floor of the chasma, adjacent to pit craters. There are at least two distinct smectites that vary in their morphology and spectral properties, with a more Mg-rich smectite associated with a thin brighter upper layered deposit and a more Fe-rich smectite in a lower darker massive deposit. We also identified a younger hydrated unit with spectral features similar to ferrihydrite. The spectra of this unit lack an absorption between 2.28 and 2.31 µm, but display a drop in reflectance near 2.3 µm. The ferrihydrite-type detections correspond to cleaner surfaces of light-toned deposits further north and higher in elevation than the smectites. A few small outcrops in the southern chasma floor appear brighter than the clays and display an additional narrow absorption at 2.4 µm in combination with the smectite features, which may indicate a smectite plus perchlorate mixture. There are no valleys along the plateau that intersect Hydrae Chasma and, consequently, any water that created the Fe/Mg-smectites and ferrihydrite must have been from groundwater in the subsurface or surface water sourced from within the chasma. A terraced fan and associated valley to the north of the clays provide support for melting snow/ice within the chasma and this same water may have also altered materials lower in elevation where the ferrihydrite-bearing unit formed under colder conditions than the smectites. The change in stratigraphy from smectites to ferrihydrite is consistent with a decrease in global temperature during the Hesperian.

Late Hesperian-aged, circum-Tharsis floods interpreted to have formed by catastrophic release of groundwater cut large channels and debouched significant quantities of water into the northern lowlands of Mars. The floods are thought by many to have formed an ocean of significant volume and depth, encircled by contacts that have been interpreted as shorelines. Models of catastrophic groundwater release require a thick and continuous cryosphere with mean annual temperatures well below freezing much like those today. In this environment, the bodies of liquid formed by individual outflow events would have been very short lived, undergoing rapid freezing. We investigate the case where floods repeatedly flowed into the northern lowlands under climatic conditions that resemble those of the present day; the water from each flood froze in a geologically short period of time to form an ice layer. Successive ice layers accumulated to form an ocean-sized body of ice that filled the basin up to the -3650 contour, thereby enclosing a 110 m global equivalent layer (GEL) of water. Subsequent to the filling of the basin the ice slowly sublimated into the atmosphere to be lost to space or to accumulate in various surface and near-surface cold traps, such as the polar layered deposits. Where is this excess water today? The presence of the thick global cryosphere meant that only minor amounts of water were lost from the surface back into the global groundwater system. Approximately 20-30 m GEL of water is estimated to be at or near the surface today and exchanging with the atmosphere on geologic time scales (this includes the polar layered deposits and deposits elsewhere at depths less than approximately 80 m). Below the exchangeable reservoir is a non-exchangeable reservoir of unknown capacity. Present day loss rates to space fall far short of those needed to eliminate the mid-Hesperian ice ocean and those needed to cause the observed doubling of the D/H of the exchangeable reservoir since the mid-Hesperian. The discrepancy implies earlier loss rates must have been higher. Assuming a linear increase in loss rates because of the higher early EUV output of the Sun, we estimate that for a present inventory of 30 m GEL, 42 m GEL remains to be accounted for. Possibilities include greater dependence of losses of EUV than assumed, and enhanced losses during periods of high obliquity. In addition, large volumes of ice may be present near the surface at high latitudes outside the polar layered deposits as indicated by recent discoveries of ice layers in cliffs.

Recent work at Aeolis Dorsa, Mars has identified exposure of fluvial sedimentary outcrop deposited early in martian history, likely during the late Hesperian or earlier. Here, we examine a ∼1200 km2 exposure of sedimentary outcrop in southeast Aeolis Dorsa. Total thickness of the stratigraphic section exceeds 100 m. We identify eight discrete complexes, each of which is primarily composed of branching channel-filling fluvial deposits associated with a primary feeder, or trunk, channel deposit. Relative ages for the branching complexes are derived from observed stacking relationships, and there is a pattern of eastward younging. Using digital elevation models derived from Context Camera (CTX) stereo pair images, we measure the strike and dip of stratal surfaces mapped throughout the study area. There is a trend of increasing dip magnitude from north to south. The branching complexes are interpreted as exhumed remnants of the lobes of an aggrading river delta that shifted its depo-center from west to east over time. The increasing dip magnitude of stratal surfaces towards the south records topset-foreset transitions, an architectural feature associated with river deltas, and indicates paleo-transport was locally southward. The interpreted deltaic lobes indicate that Aeolis Dorsa was a coastal region not presently bound by a modern crater basin, and the studied outcrop is part of a larger set of coastal sedimentary deposits identified in the region.

The martian valley networks are a key piece of evidence for the presence of liquid water on early Mars, and understanding their formation conditions can provide valuable insight into the nature of the early climate. Previous studies have used various methods to estimate the volume of water required to carve the valley networks, with results ranging from 3–5000 m Global Equivalent Layer (GEL). In comparison, other workers have found that the surface/near-surface water inventory was likely to have been ∼24 m GEL at the Noachian-Hesperian boundary. Thus, 3 m GEL may be consistent with recycling in a cold and icy Late Noachian-Early Hesperian climate, while 5000 m GEL may require continuous warm and wet conditions. In this study, we use updated methods and datasets to better constrain the necessary volume of water, finding a conservative lower limit of 640 m GEL. Based on valley network formation timescales, we find that our results do not preclude a cold and icy Late Noachian-Early Hesperian climate. Thus, this updated estimate of the minimum volume of water required to carve the valley networks is consistent with both formation in a cold and icy and warm and wet climate.

Channel networks in the mid-latitude during the late Noachian to early Hesperian Period have invoked opinions in favor of presence of water on Martian surface. Disregarding the controversy of the presence of water, this study aims to probe into the controlling factors involved in the development of the channel pathways. These apparently curvilinear channels may have certain similarities in map pattern though their origin and development might differ in time and space with subtle differences, cryptically engraved within their geometries. Our study of the geometries of the channels and craters with the aid of appropriate statistical methods help us to group channels with different geometrical attributes, implying the control of pre-existing tectonic lineaments of the terrain on the channel course development. This research uses such high precision statistical analyses on THEMIS-IR Day Global Mosaic and detailed observations from MOLA colorized elevation map, HRSC images, HRSC MOLA Blended DEM within the area, Pyrrhae Fossae region, Noachis Terra.

In the present research, we carried out detailed chronological and compositional analyses along with detailed spectral analysis of three unnamed craters on the surface of Mars. Knowledge on chronology/age analysis and compositional analysis of Mars’ surface is essential for future manned and unmanned missions. The study area is near the landing site of previous landed missions, which could be used for future landing. The area is interesting to be studied because of its high elevation in the northeastern side and low elevation in the southern side, consisting of three major geological boundaries, i.e., Hesperian, Noachian, and Amazonian, which are further subdivided into fourteen units. Chronological investigations were carried out using the active machine learning approach and Craterstats 2.0 software, which revealed the age plot of 3.09 ± 0.04 Ga for Amazonian, 3.63 ± 0.0 Ga for Hesperian, and 3.73 ± 0.0 Ga for Noachian geological units, stating that N(1) craters’ density corresponds to the early Amazonian, early Hesperian, and late Noachian/early Hesperian periods according to the established crater density boundaries, respectively. Compact Reconnaissance Imaging Spectrometer for Mars (CRISM)-derived browse products are used for the compositional study of the surface characteristics of Mars. A spectral investigation was performed on an unnamed crater belonging to the Amazonian period, which showed to be majorly composed of oxides as the primary mineral, indicating the spectra of hematite, boehmite, and akaganeite. A Hesperian unit-unnamed crater shows the signature of monohydrated sulfates, melilite, illite, and kaolinite minerals in the region. For the unnamed crater 3, which belongs to the Noachian period, it has diagnostic absorptions of clay minerals in their extracted spectra, indicating the sign of long-term water–rock interactions in the period. Derived chronology results and compositional studies of craters help in better understanding the geological formation units of Mars’ surface.

Mars’ transition from an early “warm and wet” to a “cold and dry” environment left fingerprints on the geological record of fluvial activity on Mars. The morphological and mineralogical observations of aqueous activity provided varying constraints on the condition and duration of liquid water on the Martian surface. In this study, we surveyed the mineralogy of Martian alluvial fans and deltas and investigated the hydrated silica-bearing deposits associated with some of these landforms. Using CRISM data, we identified 35 locations across Mars with hydrated silica in proximity to fans/deltas, where the spectral characteristics are consistent with immature or dehydrated opal-A. In a few stepped fans/deltas, we find hydrated silica occurs within the bulk fan deposits and form sedimentary layers correlated with elevation. Meanwhile, in the older fans/deltas, silica mostly occurs at distal locations, and the relation to primary sedimentary deposits is more complex. We propose that the hydrated silica-bearing deposits in stepped fans/deltas likely formed authigenically from Martian surface waters, mainly during the Late Hesperian and Early Amazonian. These silica-bearing deposits could be a tracer for the temperature or duration of water involved in the formation of these deposits, given more precise and detailed observations of the sedimentary context, accessory minerals, the concentration of hydrated silica, and sediment-to-water ratio. Therefore, we consider that silica-bearing deposits should be among the most critical samples to investigate for future Mars missions, which are accessible in the landing sites of Mars 2020 and ExoMars 2022 missions.

Additional tables of stratigraphic elements, and close-up figures highlighting more details on RMI images.

International audience; We present a geomorphic map of Chukhung crater (38.47°N, 72.42°W) in central Tempe Terra, Mars. Chukhung crater formed ~3.6–2.1 Ga, between the early Hesperian and early Amazonian periods of Mars' geologic history. It hosts dendritic networks of crater wall valleys, broad crater floor valleys, mid-to-late-Amazonian-aged debris-covered glaciers, moraine-like deposits, and a radial assemblage of sinuous ridge landforms. We explore the origins of landforms in Chukhung crater, focusing in particular upon the sinuous ridges. In northern Chukhung crater, sinuous ridges extend downslope from fluvial valley systems on the northern crater wall. We interpret the northern sinuous ridges as inverted paleochannels: ridges formed by exhumation of resistant and/or indurated fluvial channel fill deposits. The origins of sinuous ridges on the southern floor of Chukhung crater are more ambiguous. They emerge from beneath moraine-like ridges which bound extant debris-covered glaciers extending from the southern wall of the crater. The southern sinuous ridges have numerous morphological and contextual similarities to eskers: ridges of glaciofluvial sediment deposited in meltwater conduits within or beneath wet-based glaciers. The close proximity of the northern and southern sinuous ridges, however, calls into question an interpretation which ascribes a different origin to each set. The similarity in the overarching process between esker and inverted channel formation (i.e., exposure by the removal of a bounding medium, be that ice or sediments/rock) results in convergence of form between eskers and inverted paleochannels. We discuss the esker-like characteristics of the southern sinuous ridges in detail, and argue that one of two ridge populations in southern Chukhung crater is best explained by the esker hypothesis while the other could be explained under either the esker or the inverted paleochannel hypothesis. Regardless of the specific formation mechanism for the southern sinuous ridges, we find that Chukhung crater has undergone significant modification by liquid water since its formation. The northern sinuous ridges and associated crater-wall valleys provide evidence for subaerial drainage of precipitation and/or snowmelt. This suggests that Chukhung crater, and possibly the surrounding region, experienced unusually warm and wet episodes between the early Hesperian and mid Amazonian. If some or all of the southern sinuous ridges are eskers, they could provide evidence for an additional influence of glacial meltwater in Chukhung crater during the mid-to-late Amazonian. If wet-based glaciation did occur in Chukhung crater, the location of the crater between major branches of the Tempe Fossae tectonic rift system would add to the growing body of evidence that elevated geothermal heat flux was an important driver of localized occurrences of recent wet-based glaciation on Mars.

International audience; The CheMin instrument on the Mars Science Laboratory rover Curiosity detected ubiquitous high abundances (∼15-70 wt%) of X ray amorphous components (AmCs) in ancient sedimentary rocks of Gale crater. Mechanisms and timing of formation for the AmCs are poorly constrained, and could include volcanic, impact, or aqueous processes. We explore trends in AmC composition and abundance, and look for systematic compositional variation between sites within Gale crater. AmC compositions were estimated indirectly based on bulk chemistry and the nature and abundance of the crystalline phases for 19 sedimentary rock samples. AmC abundances positively correlate with AmC SiO2 contents, and a mixing relationship appears to exist between SiO2 rich and FeOT rich AmC endmembers. Endmember compositions are inconsistent with volcanic or impact glass alone, and so we conclude that the SiO2 and FeOT contents formed largely through aqueous processes. Cross cutting relationships and geologic context provide evidence that the most SiO2 rich AmCs observed in Gale crater thus far may result from interactions with localized fluids during late diagenesis. AmCs with moderate to low SiO2 contents likely formed earlier (before or soon after sediment deposition). Thus, the AmC SiO2 and FeOT contents in Gale crater rocks represent mixtures of sedimentary materials formed over most of the sedimentary history of Gale crater, starting before the first sediments were deposited in the crater (late Noachian), and ending well after the youngest sediments were lithified (at least mid Hesperian). However, it remains unclear how these metastable minerals have persisted through billions of years of diagenesis in Gale crater sediments.

In the Late Noachian to Early Hesperian period, rivers transported detritus from igneous source terrains to a downstream lake within Gale crater, creating a stratified stack of fluviolacustrine rocks that is currently exposed along the slopes of Mount Sharp. Controversy exists regarding the paleoclimate that supported overland flow of liquid water at Gale crater, in large part because little is known about how chemical and mineralogical paleoclimate indicators from mafic‐rock dominated source‐to‐sink systems are translated into the rock record. Here, we compile data from basaltic terrains with varying climates on Earth in order to provide a reference frame for the conditions that may have prevailed during the formation of the sedimentary strata in Gale crater, particularly focusing on the Sheepbed and Pahrump Hills members. We calculate the chemical index of alteration for weathering profiles and fluvial sediments to better constrain the relationship between climate and chemical weathering in mafic terrains, a method that best estimates the cooler limit of climate conditions averaged over time. We also compare X‐ray diffraction patterns and mineral abundances from fluvial sediments in varying terrestrial climates and martian mudstones to better understand the influence of climate on secondary mineral assemblages in basaltic terrains. We show that the geochemistry and mineralogy of most of the fine‐grained sedimentary rocks in Gale crater display first‐order similarities with sediments generated in climates that resemble those of present‐day Iceland, while other parts of the stratigraphy indicate even colder baseline climate conditions. None of the lithologies examined at Gale crater resemble fluvial sediments or weathering profiles from warm (temperate to tropical) terrestrial climates., Key Points Terrestrial sediments and weathering profiles from basaltic terrains display first‐order similarities with mudstones from Gale crater, MarsThe chemical index of alteration can be used as paleoclimate proxy and places constraints on the ancient climate of Gale crater, MarsThe paleoclimate of Gale crater, Mars, was variable and ranged from Icelandic‐like conditions to colder climates

A radiative transfer model was used to reproduce several millions of OMEGA (Observatoire pour la Min\'eralogie, l'Eau, les Glaces et l'Activit\'e) spectra representative of igneous terrains of Mars. This task provided the modal composition and grain sizes at a planetary scale. The lithology can be summarized in five mineral maps at km-scale. We found that the low albedo equatorial regions of the Martian surface (from 60{\deg}S to 30{\deg}N) are globally dominated by plagioclase with average abundance ~50 vol% and pyroxenes with total averaged abundance close to 40 vol%. An evolution of the LCP/(LCP+HCP) ratio is observed with time at the global scale, suggesting an evolution of the degree of partial melting throughout the Martian eras. Olivine and Martian dust are minor components of the modelled terrains. The olivine distribution is quite different from the other minerals because it is found on localized areas with abundance reaching 20 vol%. A statistical approach, to classify the pixels of the abundances maps, using k-means clustering, highlighted seven distinct mineral assemblages on the surface. This classification illustrates that diverse mineralogical units are found in the Noachian and Hesperian terrains, which suggests the presence of various and complex magmatic processes at a global scale during the two oldest eras. The chemical composition was derived from the modal composition maps. The OMEGA-derived chemical composition is quite consistent with several distinctive geochemical characteristics previously considered as fingerprints of the Martian surface. A major discrepancy is in regards to the Fe content that is significantly smaller than soil and rock analyses from GRS and in situ measurements. The discrepancy could be partly explained by the assumptions used for the spectral modelling or could also indicate surface alteration rinds.

The history of rivers on Mars is an important constraint on Martian climate evolution. The timing of relatively young, alluvial fan-forming rivers is especially important, as Mars' Amazonian atmosphere is thought to have been too thin to consistently support surface liquid water. Previous regional studies suggested that alluvial fans formed primarily between the Early Hesperian and the Early Amazonian. In this study, we describe how a combination of a global impact crater database, a global geologic map, a global alluvial fan database, and statistical models can be used to estimate the timing of alluvial fan formation across Mars. Using our global approach and improved statistical modeling, we find that alluvial fan formation likely persisted into the last ~2.5 Gyr, well into the Amazonian period. However, the data we analyzed was insufficient to place constraints on the duration of alluvial fan formation. Going forward, more crater data will enable tighter constraints on the parameters estimated in our models and thus further inform our understanding of Mars' climate evolution., Comment: Accepted by The Planetary Science Journal

International audience; Thumbprint terrain was first recognized in Viking Orbiter data and described as sets of alternating continuous parallel ridges and depressions up to several tens of kilometres in length, with high and low albedo respectively. We performed a geomorphological analysis of these features using both Context Camera and High Resolution Imaging Science Experiment images, as well as topographic profiles based on Mars Orbiter Laser Altimeter data, with the aim to provide an origin for thumbprint terrain and constrain the geological evolution of southeastern Acidalia Planitia. The identification of runup lobate deposits, coupled with the putative presence of a Late Hesperian ocean in the northern lowlands, led to speculations that evidence of tsunamis may be present in the geologic record of the area. There are several hypotheses that have been proposed previously to explain the origin of thumbprint terrain and include ice-related, volcanic-related, liquefaction and mud-volcanism driven processes. However, a tsunami-related origin provides a strong geological framework for the energy required for the associated lobes to flow uphill and leaving peripheral termination ridges as a result of viscosity. Numerical simulations indicate that - prior to their emplacement - the lobes sustained high velocities, thereby lending further support to the tsunami hypothesis and their origin from the Lomonosov crater impact. Wave interference patterns, formed during the propagation of the tsunamis over and around prominent topography, likely explain the thumbprint terrain spatial arrangement. The thumbprint terrain sedimentary cones are, herein, considered mud volcanoes related to a tsunami that occurred in southeastern Acidalia Planitia.

International audience; Gale crater is a large impact crater with a ca 5 km thick sequence of stratified rocks in it, expressed today as a central eroded mound (i.e., Aeolis Mons informally named Mt. Sharp). A goal of the current Mars Science Laboratory mission in Gale crater is to investigate the processes that deposited, lithified, and eroded this fill. The Light-Toned Yardang Unit (LTYu) unit, the subject of the present research, is one of the geological units of Mt Sharp. Our specific purpose here is to refine and interpret the imaging documentation of the morphologic and stratal components of the LTYu, at large outcrop scale. In combination with established orbital images, we use the Remote Micro-Imager (RMI) of ChemCam, a remote sensing instrument currently operated onboard Curiosity rover, which provides several types of context imaging. RMI capabilities now include “Long Distance” acquisitions of targets several kilometers away. In these new acquisitions, substantial differences are visible in LTYu yardang attitudes from lowest to uppermost elevations allowing tentative subdivision of the LTYu into subunits. Bedding geometries in the lower LTYu are consistent with eolian dune foresets which collectively prograde towards an average N134° direction. Based on stratal architectures, the LTYu is viewed as an amalgamated stack of at least two, and possibly four, ancient erg systems bounded by large deflationary “supersurfaces”. Observations point to a multistory generation of yardangs interpreted to have been successively buried during the stratigraphic building of Mt Sharp. We conclude that the successive sequences of eolian deposition-erosion recorded by the LTYu have been generated by cyclic changes from semi-arid to arid conditions, coupled in climatic cycles, including wind regime change. The regional unconformity that tops the Lower mound formation of Mt Sharp, and the subsequent emplacement of the LTYu, collectively express a clear tendency toward sustained arid environments for this region of Mars around the Early – Late Hesperian transition. Given the large time scale involved (i.e., a few tens of million years as a minimum), we consider it likely that the local evidence for increased aridity in the Mt Sharp stratigraphy is a manifestation of climate change affecting the whole planet.

Volcanic activity on Mars peaked during the Noachian and Hesperian periods but has continued since then in isolated locales. Elysium Planitia hosts numerous young, fissure-fed flood lavas with ages ranging from approximately 500 to 2.5 million years (Ma). We present evidence for what may be the youngest volcanic deposit yet documented on Mars: a low albedo, high thermal inertia, high-calcium pyroxene-rich deposit distributed symmetrically around a segment of the Cerberus Fossae fissure system in Elysium Planitia. This deposit is similar to features interpreted as pyroclastic deposits on the Moon and Mercury. However, unlike previously documented lava flows in Elysium Planitia, this feature is morphologically consistent with a fissure-fed pyroclastic deposit, mantling the surrounding lava flows with a thickness on the order of tens of cm over most of the deposit and a volume of 1.1-2.8E7 cubic meters. Thickness and volume estimates are consistent with tephra fall deposits on Earth. Stratigraphic relationships indicate a relative age younger than the surrounding volcanic plains and the Zunil impact crater (~0.1-1 Ma), with crater counting suggesting an absolute model age of 53 to 210 ka. This young age implies that if this deposit is of volcanic origin then the Cerberus Fossae region may not be extinct and Mars may still be volcanically active today. This interpretation is consistent with the identification of seismicity in this region by the Interior Exploration using Seismic Investigations, Geodesy, and Heat Transport (InSight) lander, and has additional implications for astrobiology and the source of transient atmospheric methane., 41 pages, 11 figures

According to recent mineralogical study, the presence of an iron oxi-hydroxide (akaganeite) which form under specific conditions, has been detected in Robert Sharp crater, located in the equatorial region of Mars (133.59°'E, -4.12°N), implying an acidic and oxidizing environment in this region. These deposits of akaganeite might be the ultimate alteration phase of a drying lake or lagoon within Robert Sharp. Morphological and stratigraphical studies and, as well, age determination has been carried out in order to constrain the geological and hydrological history of Robert Sharp. Robert Sharp has known a varied geological history, including the formation of fretted terrains and an aerial/aeolian filling during the Hesperian epoch. The presence of valleys and fan-shaped deposits, and the detection of several aqueous minerals, in the crater suggest the presence of a fluvio-lacustrine activity phase in the crater during the last period of the Martian chronology, the Amazonian epoch. The existence of a putative paleo-lake should be short-time and estimated between 1.3 Ga and 500 Ma. Thus, this study shows that Mars has known several phases of aqueous activities well after the late Noachian/ early Hesperian period.

In order to assess the sedimentological and stratigraphic history of the Melas Chasma rift basin, Mars, and investigate the possibility of past bodies of water, we have mapped Hesperian stacked sed...

Comprehensive analysis of remote sensing data used to select the Interior Exploration using Seismic Investigations, Geodesy and Heat Transport (InSight) landing site correctly predicted the atmospheric temperature and pressure profile during entry and descent, the safe landing surface, and the geologic setting of the site. The smooth plains upon which the InSight landing site is located were accurately predicted to be generally similar to the Mars Exploration Rover Spirit landing site with relatively low rock abundance, low slopes, and a moderately dusty surface with a 3–10 m impact fragmented regolith over Hesperian to Early Amazonian basaltic lava flows. The deceleration profile and surface pressure encountered by the spacecraft during entry, descent, and landing compared well (within 1σ) of the envelope of modeled temperature profiles and the expected surface pressure. Orbital estimates of thermal inertia are similar to surface radiometer measurements, and materials at the surface are dominated by poorly consolidated sand as expected. Thin coatings of bright atmospheric dust on the surface were as indicated by orbital albedo and dust cover index measurements. Orbital estimates of rock abundance from shadow measurements in high‐resolution images and thermal differencing indicated very low rock abundance and surface counts show 1–4% area covered by rocks. Slopes at 100 to 5 m length scale measured from orbital topographic and radar data correctly indicated a surface comparably smooth and flat as the two smoothest landing sites (Opportunity and Phoenix). Thermal inertia and radar data indicated the surface would be load bearing as found., Key Points The atmosphere, safe surface, and geologic setting of the landing site were correctly predicted by remote sensing data before landingThe modeled atmospheric temperature profiles and surface pressure were within 1 sigma of the measured deceleration profile and surface pressureInSight’s surface is similar to Spirit’s with low rock abundance, low slopes, moderate dust, and is composed of impact regolith over basalt

International audience; Jezero crater has been selected as the landing site for the Mars 2020 Perseverance rover, because it contains apaleolake with two fan-deltas, inlet and outlet valleys. Using the data from the High Resolution Stereo Camera(HRSC) and the High Resolution Imaging Science Experiment (HiRISE), we conducted a quantitative geomorphological study of the inlet valleys of the Jezero paleolake. Results show that the strongest erosion is related to a network of deep valleys that cut into the highland bedrock well upstream of the Jezero crater and likely formed before the formation of the regional olivine-rich unit. In contrast, the lower sections of valleys display poorbedrock erosion and a lack of tributaries but are characterized by the presence of pristine landforms interpreted asfluvial bars from preserved channels, the discharge rates of which have been estimated at 10^3–10^4 m^3s^-1. The valleys’ lower sections postdate the olivine-rich unit, are linked directly to the fan-deltas, and are thus formed inan energetic, late stage of activity. Although a Late Noachian age for the fan-deltas’ formation is not excludedbased on crosscutting relationships and crater counts, this indicates evidence of a Hesperian age with significantimplications for exobiology.

Authors acknowledge funding from the Science and Technology Facilities Council from the Grant ST/P000657/1. We would also like to acknowledge funding from a Leverhulme Trust Research Project Grant (RPG-2016-153) and thank the Polar Continental Shelf Program (Natural Resources Canada) for logistical field support in Nunavut. The transition of the martian climate from the wet Noachian era to the dry Hesperian (4.1–3.0 Gya) likely resulted in saline surface waters that were rich in sulfur species. Terrestrial analogue environments that possess a similar chemistry to these proposed waters can be used to develop an understanding of the diversity of microorganisms that could have persisted on Mars under such conditions. Here, we report on the chemistry and microbial community of the highly reducing sediment of Colour Peak springs, a sulfidic and saline spring system located within the Canadian High Arctic. DNA and cDNA 16S rRNA gene profiling demonstrated that the microbial community was dominated by sulfur oxidising bacteria, suggesting that primary production in the sediment was driven by chemolithoautotrophic sulfur oxidation. It is possible that the sulfur oxidising bacteria also supported the persistence of the additional taxa. Gibbs energy values calculated for the brines, based on the chemistry of Gale crater, suggested that the oxidation of reduced sulfur species was an energetically viable metabolism for life on early Mars. Publisher PDF

Extraformational sediment recycling (old sedimentary rock to new sedimentary rock) is a fundamental aspect of Earth's geological record; tectonism exposes sedimentary rock, whereupon it is weathered and eroded to form new sediment that later becomes lithified. On Mars, tectonism has been minor, but two decades of orbiter instrument-based studies show that some sedimentary rocks previously buried to depths of kilometers have been exposed, by erosion, at the surface. Four locations in Gale crater, explored using the National Aeronautics and Space Administration's Curiosity rover, exhibit sedimentary lithoclasts in sedimentary rock: At Marias Pass, they are mudstone fragments in sandstone derived from strata below an erosional unconformity; at Bimbe, they are pebble-sized sandstone and, possibly, laminated, intraclast-bearing, chemical (calcium sulfate) sediment fragments in conglomerates; at Cooperstown, they are pebble-sized fragments of sandstone within coarse sandstone; at Dingo Gap, they are cobble-sized, stratified sandstone fragments in conglomerate derived from an immediately underlying sandstone. Mars orbiter images show lithified sediment fans at the termini of canyons that incise sedimentary rock in Gale crater; these, too, consist of recycled, extraformational sediment. The recycled sediments in Gale crater are compositionally immature, indicating the dominance of physical weathering processes during the second known cycle. The observations at Marias Pass indicate that sediment eroded and removed from craters such as Gale crater during the Martian Hesperian Period could have been recycled to form new rock elsewhere. Our results permit prediction that lithified deltaic sediments at the Perseverance (landing in 2021) and Rosalind Franklin (landing in 2023) rover field sites could contain extraformational recycled sediment., With funding from the Spanish government through the "María de Maeztu Unit of Excellence" accreditation (MDM-2017-0737)

Great Salt Lake (GSL), Utah, is a thalassohaline terminal lake that currently occupies the Bonneville Basin, a depression in the larger Great Basin area of the western United States. Natural processes and climate conditions create a dynamic ecosystem with shifting salinity gradients and lake levels. The hypersaline north arm of GSL provides a model for exploring the limits of life on Earth and for potential life on other space bodies, especially the ancient closed-basin systems on Mars. The north arm water features hundreds of species of halophilic microorganisms with cellular strategies that allow them to live in hypersaline environments and high doses of ultraviolet light. These microbes also survive desiccation and can become entrapped in minerals as they are formed. The modern GSL evaporitic environment, generated by halite and gypsum precipitation events, illuminates the initial steps in preservation of biological material over geologic time. These minerals accumulate on the desiccated shores, in the sediment, and in the surrounding evaporite deposits and have been shown to have biopreservation abilities, protecting halophilic cells and their molecules inside brine fluid inclusions within the crystal structure. Entrapment allows in situ analyses of microbial diversity, which can be studied as a function of salt mineral assemblage. Globally across Mars these same types of evaporite precipitation events took place in closed-basin lake systems where surface waters have evaporated, leaving behind mineral vein structures composed of gypsum and other sulfate salts that have been modified or dissolved from later fluid shallow subsurface activity. We have chosen GSL as our analogue for Martian late Noachian/early Hesperian closed basin systems due to the overlapping evaporite mineralogy and fluid activity. Here we explore the transference of biological material and organics from hypersaline GSL brine to the minerals as they form in the water. We draw parallels to the evaporites extensively mapped on Mars, which likely formed in a similar way. These observations and insights, taken together, suggest GSL is an appropriate analogue for the study of ancient salt lakes and evaporites discovered on Mars, and what is more, the halophilic archaea that live in Earth’s salty lake may be good models for life elsewhere in our solar system.

A lithified aeolian dune field in the Apollinaris Sulci region of Mars presents a unique opportunity to study ancient aeolian processes and paleo-wind conditions during an earlier episode of martian climate history. The ancient dunes occur inside an eroded, partly-filled impact structure. The petrified dune field retains its original geomorphology and has south-, south-southeast-, and south-southwest-facing lee slopes, indicating dominance of a north-to-south paleo-wind direction. The preserved dunes have crescentic and marginal barchan morphologies with crestlines ranging from 100 m to 7 km in length. To the north of the preserved dunes, the yardangs of the Apollinaris Sulci, eroded into strata of the Medusae Fossae Formation, have an average orientation (azimuth) of 169°. The yardangs are interpreted to form from reversing winds along this orientation, in contrast with the northerly winds that formed the paleodunes. The observed paleodunes, yardangs, modern bedforms, and abundant dust cover collectively demonstrate that the climate has changed over time. Certain, heavily eroded, slopes on the paleodunes show crest-parallel lineations interpreted as exposed cross strata. Second-order bounding surfaces within the strata provide evidence that the meter-scale, superimposed bedforms migrated across some lee faces when the dunes were active. This potentially reflects preservation of ancient bedforms analogous to the “large martian ripples” commonly observed on active martian dunes today. In the paleo-interdune spaces, linear ridges and grooves are interpreted as preserved paleo-transverse aeolian ridges. The juxtaposition of these three common martian bedforms – dunes, transverse aeolian ridges, and large martian ripples – all preserved in close proximity, suggests Mars in the Hesperian epoch was subject to similar aeolian dynamics as are observed today.