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Mars has many well‐exposed fluvial ridges and fluvio‐deltaic basins; in two of these locations, the Curiosity and Perseverance rovers are currently searching for signs of habitability. The distribution of organic carbon that might persist in ancient fluvial deposits present on Mars is not well understood. In this study, we set out to assess the preservation potential of organic carbon in a hyperarid fluvial environment with observations and analyses of the Amargosa River in Death Valley, California (United States). The lower reaches of the Amargosa River in Badwater Basin are nearly devoid of plants and contain low gradient, meandering channels, making them a valuable terrestrial analog for early martian fluvial systems. We analyzed sediment taken from fluvial deposits exposed in cutbanks of two bends of a meandering channel. We found total organic carbon abundances that were on average 0.15% up to a meter below the surface. X‐ray diffraction and electron microscopy analyses revealed a suite of high redox potential mineral phases (including iron and manganese oxides) mixed with detrital and authigenic silicates, carbonate, and sulfate salts at or close to redox equilibrium with pore fluids in contact with the atmosphere. This finding highlighted that organic carbon can persist in fluvial deposits at low abundance despite oxidizing conditions and saturated sediments and suggested that ancient fluvial deposits on Mars may retain traces of organics in fine‐grained deposits if they are present during deposition. Plain Language Summary: The Curiosity and Perseverance rovers are currently searching for signs of past life on Mars in two ancient river deposits. Since carbon on the surface of Mars could originate from meteorites and chemical reactions in addition to potential Martian life forms, it is valuable to identify sedimentary environments that best preserve biospheric organic carbon. The distribution of organic carbon that might persist in ancient river deposits on Mars is not well understood, yet this understanding is crucial to detecting evidence of past life on Mars. In this study, we set out to determine the preservation potential of organic carbon in an arid, fluvial environment. The Amargosa River (Death Valley, California, United States) hosts a low gradient single threaded river, running through a catchment with little vegetation and making it a valuable terrestrial analog for early Martian fluvial systems. We found that organic carbon can persist in fine‐grained river sediments at low abundance despite oxidizing conditions in the subsurface. Our results suggested that fine‐grained ancient river deposits on Mars are the most promising sampling targets for the Perseverance rover to detect traces of potential life on Mars. Key Points: The hyper‐arid deposits of the ephemeral, alluvial Amargosa River are a useful process analog for ancient Martian river depositsX‐ray diffraction and electron microscopy analyses revealed that the subsurface remains highly oxidizing despite saturated sedimentsFluvial deposits had low abundances of organic carbon in fine‐grained sediment; mudstones are the most promising targets for sampling on Mars [ABSTRACT FROM AUTHOR]

Sedimentary fans are key targets of exploration on Mars because they record the history of surface aqueous activity and habitability. The sedimentary fan extending from the Neretva Vallis breach of Jezero crater's western rim is one of the Mars 2020 Perseverance rover's main exploration targets. Perseverance spent ∼250 sols exploring and collecting seven rock cores from the lower ∼25 m of sedimentary rock exposed within the fan's eastern scarp, a sequence informally named the "Shenandoah" formation. This study describes the sedimentology and stratigraphy of the Shenandoah formation at two areas, "Cape Nukshak" and "Hawksbill Gap," including a characterization, interpretation, and depositional framework for the facies that comprise it. The five main facies of the Shenandoah formation include: laminated mudstone, laminated sandstone, low‐angle cross stratified sandstone, thin‐bedded granule sandstone, and thick‐bedded granule‐pebble sandstone and conglomerate. These facies are organized into three facies associations (FA): FA1, comprised of laminated and soft sediment‐deformed sandstone interbedded with broad, unconfined coarser‐grained granule and pebbly sandstone intervals; FA2, comprised predominantly of laterally extensive, soft‐sediment deformed laminated, sulfate‐bearing mudstone with lenses of low‐angle cross‐stratified and scoured sandstone; and FA3, comprised of dipping planar, thin‐bedded sand‐gravel couplets. The depositional model favored for the Shenandoah formation involves the transition from a sand‐dominated distal alluvial fan setting (FA1) to a stable, widespread saline lake (FA2), followed by the progradation of a river delta system (FA3) into the lake basin. This sequence records the initiation of a relatively long‐lived, habitable lacustrine and deltaic environment within Jezero crater. Plain Language Summary: Fan‐shaped deposits are important exploration targets on Mars because they record the activity of water and conditions suitable for life on the surface. The Mars 2020 Perseverance rover has been exploring an ancient impact crater, named Jezero, since landing on Mars in February 2021. One of the rover's main exploration targets is a fan‐shaped deposit of sedimentary rock extending into the crater from the western rim. One Earth year into the rover's mission, Perseverance arrived at the eroded edge of this fan, exploring several outcrops of sedimentary rock, and collecting seven rock samples. This study uses images and data from Perseverance to describe these rocks, called the "Shenandoah" formation, and to determine how they formed. The oldest rocks of the Shenandoah formation contain sand and pebbles that were likely laid down at the edge of an alluvial fan. Next, very fine‐grained rocks were deposited in a lake setting. Dipping layers of alternating sand and pebbles represent a change in the local setting and the growth of a delta into a relatively long‐lived lake. The rover's observations of the Shenandoah formation show that past conditions in Jezero crater were capable of hosting and preserving signs of ancient life. Key Points: The Shenandoah formation is a sand‐dominated, clastic, sedimentary sequence comprising the lower 25 m of Jezero's western fanThis sequence records the transition from an alluvial fan setting to a lacustrine setting, followed by progradation of a river deltaThis sequence records evidence for a warm, wet, habitable depositional setting with the potential to preserve biosignatures [ABSTRACT FROM AUTHOR]

Aeolis Mons (informally, Mount Sharp) exhibits a number of canyons, including Gediz and Sakarya Valles. Poorly sorted debris deposits are evident on both canyon floors and connect with debris extending down the walls for canyon segments that cut through sulphate‐bearing strata. On the floor of Gediz Vallis, debris overfills a central channel and merges with a massive debris ridge located at the canyon terminus. One wall‐based debris ridge is evident. In comparison, the floor of Sakarya Vallis exhibits a complex array of debris deposits. Debris deposits on wall segments within Sakarya Vallis are mainly contained within chutes that extend downhill from scarps. Lateral debris ridges are also evident on chute margins. We interpret the debris deposits in the two canyons to be a consequence of one or more late‐stage hydrogeomorphic events that increased the probability of landslides, assembled and channelized debris on the canyon floors, and moved materials down‐canyon. The highly soluble nature of the sulphate‐bearing rocks likely contributed to enhanced debris generation by concurrent aqueous weathering to produce blocky regolith for transport downslope by fluvial activity and landslides, including some landslides that became debris flows. Subsequent wind erosion in Gediz Vallis removed most of the debris deposits within that canyon and partially eroded the deposits within Sakarya Vallis. The enhanced wind erosion within Gediz Vallis was a consequence of the canyon's alignment with prevailing slope winds. Plain Language Summary: Debris deposits on the walls and floors of canyons cut into Mount Sharp are a consequence of late stage precipitation, with ground water infiltration that triggered landslides and debris flows of blocky regolith. This is supported by their morphologies, and by calculations of slope stability in the presence of groundwater. The relatively soluble nature of the sulphate‐bearing strata facilitated chemical weathering to produce material conditioned to fail as landslides. Subsequent wind‐driven erosion degraded many of these features, including differential stripping of wall and floor rocks to generate debris ridges. Key Points: Debris deposits on the walls and floors of canyons cut into Mount Sharp were a consequence of late stage, water‐related eventsWind‐driven erosion degraded many of these features, including differential stripping of wall and floor deposits to generate debris ridges [ABSTRACT FROM AUTHOR]

Numerous sedimentary fans on Mars have been studied to better understand the early martian hydrological system. Of particular interest is to estimate the duration of fluvial activity from alluvial fan size by dividing deposit volume by bankfull sediment flux. However, such a calculation requires an intermittency factor—a parameter relating sediment discharge from channel‐filling (bankfull) floods to the long‐term mean sediment discharge—which is poorly constrained. Here, we investigated fluvial fan deposits from the Oligocene–Miocene Caspe Formation, Spain, as an analog to fans on Mars because it has exhumed channel belts that create sinuous ridges in the modern topography, similar to those observed on Mars that are used for paleohydraulics. We made measurements of the thicknesses of dune and bar cross sets within exhumed channel belts at nine field sites to reconstruct bankfull channel depth (∼1.4 m) and bankfull sediment flux (∼0.48 m3/s). We estimated total bed‐material sediment volume of the fan (362 km3) from stratigraphic thickness, the area containing exhumed channel belts, porosity and sand fraction. Combined with previous constraints on depositional timespan (∼6 Myr), we calculated a sediment‐transport intermittency factor of 0.004 (range: 0.0004–0.04). Our approach can be applied to Mars by using remote sensing measurements of fluvial ridge morphology, which indicates the possibility of long depositional timespans exceeding millions of years for martian fans. Plain Language Summary: We examined Oligocene‐Miocene river deposits in the Guadalope‐Mattaranya fan (Ebro Basin, Spain) as an analog for river deposits on Mars that potentially represent long durations of river activity. Similar to deposits on Mars, the Guadalope‐Mattaranya fan has been differentially eroded creating networks of branching, sinuous ridges that represent former river channel belts. Based on measurements of river bar and dune deposits, we estimated that the ancient rivers were sand‐bedded and 1.4 m deep and that they carried a bankfull sediment discharge of 0.48 m3/s. Using these data, we calculated that the entire fan could have been built in as little as 24,000 years if there was continuous bankfull flow in the rivers. However, in reality, the fan was built over 6 million years, highlighting the vast amount of time that sediment transport was inactive. Applying a similar fraction of intermittent sediment transport to case studies on Mars suggests that martian river systems could have been intermittently persistent for millions of years. Key Points: Guadalope‐Matarranya fan in the Ebro Basin, Spain, is a large fluvial fan with fluvial ridges similar to fans on MarsFluvial ridges are exhumed channel belts from rivers 1.4 m deep, with sediment‐transport intermittency factor of 0.004 (range: 0.0004–0.04)If martian depositional rivers had similar intermittency, some persisted at least millions of years [ABSTRACT FROM AUTHOR]

The timespan recorded in deposits from fluvial activity is a key gap in our understanding of ancient environments on Earth and Mars. Because riverine sediment transport occurs under time‐variable water discharge, common models that represent sediment transport with a single bankfull discharge require an intermittency factor. For this reason, the ability to predict intermittency factor values based on environmental factors would improve estimates of time from fluvial deposits. To address this knowledge gap, we calculated intermittency factors from 201 modern rivers and six fans and deltas with depositional timespans of months to millions of years. Intermittency factors range from 0.0064–0.73; they are uncorrelated with averaging timescale, bed‐material grainsize, or climate aridity, but are larger in river catchments with greater rates of denudation relative to precipitation. Application to ancient fluvial systems on Mars indicates long‐lived depositional river systems for up to 104–106 years. Plain Language Summary: To estimate the lifespan of ancient river courses, geologists estimate the time it took to transport and deposit all of the sediment within a landform, such as an alluvial fan or delta. This method relies on sediment‐transport calculations, which often assume a single reoccurring flood event that fills the river channel, even though river discharge varies in time from low flows to large floods. While the discharge of the channel‐filling flood event can be constrained from channel geometry or deposit characteristics, flow variability is unknown, and an intermittency factor is needed to adjust the sediment transport calculation. In this work, we calculated intermittency factors for 201 rivers and six river deposits on Earth. We also examined correlations between intermittency factor and environmental variables, and found no correlation with averaging timescale, bed‐material grainsize, or climate aridity. However, intermittency factors are larger in catchments that receive a large supply of sediment. Applying the new intermittency factor values to river deposits on Mars suggests that some of those rivers persisted for millions of years. Key Points: Rivers transport sediment intermittently; intermittency factor relates river duration to deposit volume and bankfull river characteristicsIntermittency factors we calculated have a 5–95 percentile range of 0.0064–0.73, independent of timescale, grainsize, and climateApplication to Mars implies fluvial activity over thousands to millions of years [ABSTRACT FROM AUTHOR]

Sedimentary basins are the archives of ancient environmental conditions on planetary surfaces, and on Mars they may contain the best record of surface water and habitable conditions. While erosional valley networks have been mapped, the global distribution of fluvial sedimentary deposits on Mars has been unknown. Here we generated an eight-trillionpixel global map of Mars using data from the NASA Context Camera (CTX), aboard the Mars Reconnaissance Orbiter spacecraft, to perform the first systematic global survey of fluvial ridges-exhumed ancient deposits that have the planform shape of river channels or channel belts, but stand in positive relief due to preferential erosion of neighboring terrain. We used large fluvial ridges (>70 m width) as a conservative proxy for the occurrence of depositional rivers or river-influenced depositional areas. Results showed that fluvial ridges are as much as 100 km long, common across the southern highlands, occur primarily in networks within intercrater plains, and are not confined to impact basins. Ridges were dominantly found in Noachian through Late Hesperian units, consistent with cessation of valley network activity, and occurred downstream from valley networks, indicating regional source-to-sink transport systems. These depositional areas mark a globally distributed class of sedimentary deposits that contain a rich archive of Mars history, including fluvial activity on early Mars. [ABSTRACT FROM AUTHOR]