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For more than a decade, the CheMin X-ray diffraction instrument on the Mars Science Laboratory rover, Curiosity, has been returning definitive and quantitative mineralogical and mineral–chemistry data from ~3.5-billion-year-old (Ga) sediments in Gale crater, Mars. To date, 40 drilled rock samples and three scooped soil samples have been analyzed during the rover's 30+ km transit. These samples document the mineralogy of over 800 m of flat-lying fluvial, lacustrine, and aeolian sedimentary rocks that comprise the lower strata of the central mound of Gale crater (Aeolis Mons, informally known as Mt. Sharp) and the surrounding plains (Aeolis Palus, informally known as the Bradbury Rise). The principal mineralogy of the sedimentary rocks is of basaltic composition, with evidence of post-depositional diagenetic overprinting. The rocks in many cases preserve much of their primary mineralogy and sedimentary features, suggesting that they were never strongly heated or deformed. Using aeolian soil composition as a proxy for the composition of the deposited and lithified sediment, it appears that, in many cases, the diagenetic changes observed are principally isochemical. Exceptions to this trend include secondary nodules, calcium sulfate veining, and rare Si-rich alteration halos. A surprising and yet poorly understood observation is that nearly all of the ~3.5 Ga sedimentary rocks analyzed to date contain 15–70 wt.% of X-ray amorphous material. Overall, this >800 m section of sedimentary rock explored in lower Mt. Sharp documents a perennial shallow lake environment grading upward into alternating lacustrine/fluvial and aeolian environments, many of which would have been habitable to microbial life. [ABSTRACT FROM AUTHOR]

To accurately determine the chronological framework of climatic variations recorded by various Martian terrains, the absolute ages of Martian events and cratering rate need to be constrained by either in situ dating or returned samples. In situ K-Ar dating is currently a more plausible dating technique as compared with sample return. Jarosite (KFe3[SO4]2[OH]6) is the only confirmed K sulfate mineral that is widely present on Mars, as indicated by in situ detection, orbital remote sensing, and meteorite studies. Jarosite can be used for precise K-Ar and 40Ar/39Ar dating. The preservation of jarosite on Mars provides information about the nature and duration of aqueous processes on the Martian surface. Different ages of Martian jarosite represent the key to constraining the transition from Martian surface water activity to arid climatic conditions. This paper summarizes recent advances in our knowledge of the spatial distribution of Martian jarosite, its mineralogical properties and stability on Mars, the Ar diffusion kinetics of jarosite, and the current status of in situ K-Ar dating. Moreover, we examine the key scientific issues to be addressed for in situ K-Ar dating of jarosite and Martian sample return missions, and discuss future research directions. [ABSTRACT FROM AUTHOR]

The MSL Curiosity rover investigated dark, Mn-P-enriched nodules in shallow lacustrine/fluvial sediments at the Groken site in Glen Torridon, Gale Crater, Mars. Applying all relevant information from the rover, the nodules are interpreted as pseudomorphs after original crystals of vivianite, (Fe2+,Mn2+)3(PO4)2·8H2O, that cemented the sediment soon after deposition. The nodules appear to have flat faces and linear boundaries and stand above the surrounding siltstone. ChemCam LIBS (laser-induced breakdown spectrometry) shows that the nodules have MnO abundances approximately twenty times those of the surrounding siltstone matrix, contain little CaO, and have SiO2 and Al2O3 abundances similar to those of the siltstone. A deconvolution of APXS analyses of nodule-bearing targets, interpreted here as representing the nodules' non-silicate components, shows high concentrations of MnO, P2O5, and FeO and a molar ratio P/Mn = 2. Visible to near-infrared reflectance of the nodules (by ChemCam passive and Mastcam multispectral) is dark and relatively flat, consistent with a mixture of host siltstone, hematite, and a dark spectrally bland material (like pyrolusite, MnO2). A drill sample at the site is shown to contain minimal nodule material, implying that analyses by the CheMin and SAM instruments do not constrain the nodules' mineralogy or composition. The fact that the nodules contain P and Mn in a small molar integer ratio, P/Mn = 2, suggests that the nodules contained a stoichiometric Mn-phosphate mineral, in which Fe did (i.e., could) not substitute for Mn. The most likely such minerals are laueite and strunzite, Mn2+Fe3+2(PO4)2(OH)2·8H2O and –6H2O, respectively, which occur on Earth as alteration products of other Mn-bearing phosphates including vivianite. Vivianite is a common primary and diagenetic precipitate from low-oxygen, P-enriched waters. Calculated phase equilibria show Mn-bearing vivianite could be replaced by laueite or strunzite and then by hematite plus pyrolusite as the system became more oxidizing and acidic. These data suggest that the nodules originated as vivianite, forming as euhedral crystals in the sediment, enclosing sediment grains as they grew. After formation, the nodules were oxidized—first to laueite/strunzite yielding the diagnostic P/Mn ratio, and then to hematite plus an undefined Mn oxy-hydroxide (like pyrolusite). The limited occurrence of these Mn-Fe-P nodules, both in space and time (i.e., stratigraphic position), suggests a local control on their origin. By terrestrial analogies, it is possible that the nodules precipitated near a spring or seep of Mn-rich water, generated during alteration of olivine in the underlying sediments. [ABSTRACT FROM AUTHOR]

NASA's Mars Science Laboratory mission, with its Curiosity rover, has been exploring Gale crater (5.4° S, 137.8° E) since 2012 with the goal of assessing the potential of Mars to support life. The mission has compiled compelling evidence that the crater basin accumulated sediment transported by marginal rivers into lakes that likely persisted for millions of years approximately 3.6 Ga ago in the early Hesperian. Geochemical and mineralogical assessments indicate that environmental conditions within this timeframe would have been suitable for sustaining life, if it ever were present. Fluids simultaneously circulated in the subsurface and likely existed through the dry phases of lake bed exposure and aeolian deposition, conceivably creating a continuously habitable subsurface environment that persisted to less than 3 Ga in the early Amazonian. A diversity of organic molecules has been preserved, though degraded, with evidence for more complex precursors. Solid samples show highly variable isotopic abundances of sulfur, chlorine, and carbon. In situ studies of modern wind-driven sediment transport and multiple large and active aeolian deposits have led to advances in understanding bedform development and the initiation of saltation. Investigation of the modern atmosphere and environment has improved constraints on the timing and magnitude of atmospheric loss, revealed the presence of methane and the crater's influence on local meteorology, and provided measurements of high-energy radiation at Mars' surface in preparation for future crewed missions. Rover systems and science instruments remain capable of addressing all key scientific objectives. Emphases on advance planning, flexibility, operations support work, and team culture have allowed the mission team to maintain a high level of productivity in spite of declining rover power and funding. [ABSTRACT FROM AUTHOR]

On Earth, salt environments are the most relevant analogues to Martian salt deposits with regard to the search for preserved biomolecules. In this study, we analyzed the distribution of lipids in salt samples from Dalangtan and Qarhan in the Qaidam Basin, which is the most important area to carry out analog research of Mars in China. Furthermore, we compared the lipids distribution results with that from other typical Martian analog sites such as the Atacama Desert. Salt samples in the Qaidam Basin are enriched with fatty acid compounds, including normal fatty acids, unsaturated fatty acids, branched fatty acids, and acyclic isoprene acids. In addtion, glycerol dialkyl glycerol tetraethers (GDGTs) and archaeol compounds were also detected in these salt samples. Compared with the hypersaline samples, the clay samples not only have higher abuandance of fatty acids, GDGTs and archaeol compounds, but also a more diversed lipids composition. Only a few lipids were detected in the pure-saline samples, such as archaeol compounds. These clues suggest that content of salt minerals is the major factor affecting the distribution of lipids. This study provides an analogy for detecting and evaluating the traces of life that may exist on Mars. [ABSTRACT FROM AUTHOR]

The Mars 2020 mission will seek the signs of ancient life on Mars and will identify, prepare, document, and cache a set of samples for possible return to Earth by a follow-on mission. Mars 2020 and its Perseverance rover thus link and further two long-held goals in planetary science: a deep search for evidence of life in a habitable extraterrestrial environment, and the return of martian samples to Earth for analysis in terrestrial laboratories. The Mars 2020 spacecraft is based on the design of the highly successful Mars Science Laboratory and its Curiosity rover, but outfitted with a sophisticated suite of new science instruments. Ground-penetrating radar will illuminate geologic structures in the shallow subsurface, while a multi-faceted weather station will document martian environmental conditions. Several instruments can be used individually or in tandem to map the color, texture, chemistry, and mineralogy of rocks and regolith at the meter scale and at the submillimeter scale. The science instruments will be used to interpret the geology of the landing site, to identify habitable paleoenvironments, to seek ancient textural, elemental, mineralogical and organic biosignatures, and to locate and characterize the most promising samples for Earth return. Once selected, ∼35 samples of rock and regolith weighing about 15 grams each will be drilled directly into ultraclean and sterile sample tubes. Perseverance will also collect blank sample tubes to monitor the evolving rover contamination environment. In addition to its scientific instruments, Perseverance hosts technology demonstrations designed to facilitate future Mars exploration. These include a device to generate oxygen gas by electrolytic decomposition of atmospheric carbon dioxide, and a small helicopter to assess performance of a rotorcraft in the thin martian atmosphere. Mars 2020 entry, descent, and landing (EDL) will use the same approach that successfully delivered Curiosity to the martian surface, but with several new features that enable the spacecraft to land at previously inaccessible landing sites. A suite of cameras and a microphone will for the first time capture the sights and sounds of EDL. Mars 2020’s landing site was chosen to maximize scientific return of the mission for astrobiology and sample return. Several billion years ago Jezero crater held a 40 km diameter, few hundred-meter-deep lake, with both an inflow and an outflow channel. A prominent delta, fine-grained lacustrine sediments, and carbonate-bearing rocks offer attractive targets for habitability and for biosignature preservation potential. In addition, a possible volcanic unit in the crater and impact megabreccia in the crater rim, along with fluvially-deposited clasts derived from the large and lithologically diverse headwaters terrain, contribute substantially to the science value of the sample cache for investigations of the history of Mars and the Solar System. Even greater diversity, including very ancient aqueously altered rocks, is accessible in a notional rover traverse that ascends out of Jezero crater and explores the surrounding Nili Planum. Mars 2020 is conceived as the first element of a multi-mission Mars Sample Return campaign. After Mars 2020 has cached the samples, a follow-on mission consisting of a fetch rover and a rocket could retrieve and package them, and then launch the package into orbit. A third mission could capture the orbiting package and return it to Earth. To facilitate the sample handoff, Perseverance could deposit its collection of filled sample tubes in one or more locations, called depots, on the planet’s surface. Alternatively, if Perseverance remains functional, it could carry some or all the samples directly to the retrieval spacecraft. The Mars 2020 mission and its Perseverance rover launched from the Eastern Range at Cape Canaveral Air Force Station, Florida, on July 30, 2020. Landing at Jezero Crater will occur on Feb 18, 2021 at about 12:30 PM Pacific Time. [ABSTRACT FROM AUTHOR]

This paper provides an overview of the Curiosity rover's exploration at Vera Rubin ridge (VRR) and summarizes the science results. VRR is a distinct geomorphic feature on lower Aeolis Mons (informally known as Mount Sharp) that was identified in orbital data based on its distinct texture, topographic expression, and association with a hematite spectral signature. Curiosity conducted extensive remote sensing observations, acquired data on dozens of contact science targets, and drilled three outcrop samples from the ridge, as well as one outcrop sample immediately below the ridge. Our observations indicate that strata composing VRR were deposited in a predominantly lacustrine setting and are part of the Murray formation. The rocks within the ridge are chemically in family with underlying Murray formation strata. Red hematite is dispersed throughout much of the VRR bedrock, and this is the source of the orbital spectral detection. Gray hematite is also present in isolated, gray‐colored patches concentrated toward the upper elevations of VRR, and these gray patches also contain small, dark Fe‐rich nodules. We propose that VRR formed when diagenetic event(s) preferentially hardened rocks, which were subsequently eroded into a ridge by wind. Diagenesis also led to enhanced crystallization and/or cementation that deepened the ferric‐related spectral absorptions on the ridge, which helped make them readily distinguishable from orbit. Results add to existing evidence of protracted aqueous environments at Gale crater and give new insight into how diagenesis shaped Mars' rock record. Plain Language Summary: Vera Rubin ridge is a feature at the base of Mount Sharp with a distinct texture and topography. Orbiter observations showed hematite, a mineral that sometimes forms by chemical reactions in water environments, was present atop the ridge. The presence of both water and chemical activity suggested the area preserved a past habitable environment. In this paper, we detail how the Curiosity science team tested this and other orbital‐based hypotheses. Curiosity data suggested that most ridge rocks were lain down in an ancient lake and had similar compositions to other Mount Sharp rocks. Curiosity confirmed that hematite was present in the ridge but no more abundantly than elsewhere. Larger grain size or higher crystallinity probably account for the ridge's hematite being more visible from orbit. We conclude Vera Rubin ridge formed because groundwater recrystallized and hardened the rocks that now make up the ridge. Wind subsequently sculpted and eroded Mount Sharp, leaving the harder ridge rocks standing because they resisted erosion compared with surrounding rocks. The implication of these results is that liquid water was present at Mount Sharp for a very long time, not only when the crater held a lake but also much later, likely as groundwater. Key Points: We summarize Curiosity's campaign at Vera Rubin ridge (Sols 1726–2302) and the high‐level results from articles in this special issueVera Rubin ridge formed when diagenesis hardened rocks along the base of Aeolis Mons; wind subsequently etched the feature into a ridgeResults add evidence for protracted aqueous environments at Gale crater and give new insight into how diagenesis shaped Mars' rock record [ABSTRACT FROM AUTHOR]

Images from the Mars Science Laboratory (MSL) mission of lacustrine sedimentary rocks of Vera Rubin ridge on "Mt. Sharp" in Gale crater, Mars, have shown stark color variations from red to purple to gray. These color differences crosscut stratigraphy and are likely due to diagenetic alteration of the sediments after deposition. However, the chemistry and timing of these fluid interactions is unclear. Determining how diagenetic processes may have modified chemical and mineralogical signatures of ancient Martian environments is critical for understanding the past habitability of Mars and achieving the goals of the MSL mission. Here we use visible/near‐infrared spectra from Mastcam and ChemCam to determine the mineralogical origins of color variations in the ridge. Color variations are consistent with changes in spectral properties related to the crystallinity, grain size, and texture of hematite. Coarse‐grained gray hematite spectrally dominates in the gray patches and is present in the purple areas, while nanophase and fine‐grained red crystalline hematite are present and spectrally dominate in the red and purple areas. We hypothesize that these differences were caused by grain‐size coarsening of hematite by diagenetic fluids, as observed in terrestrial analogs. In this model, early primary reddening by oxidizing fluids near the surface was followed during or after burial by bleaching to form the gray patches, possibly with limited secondary reddening after exhumation. Diagenetic alteration may have diminished the preservation of biosignatures and changed the composition of the sediments, making it more difficult to interpret how conditions evolved in the paleolake over time. Plain Language Summary: Sedimentary rocks found in deserts on Earth often exhibit striking color differences from red and purple to white, which are caused by groundwater dissolving and reprecipitating iron oxides within the rocks. NASA's Mars Science Laboratory (MSL) mission has observed similar color differences on Mars within the sedimentary rocks of Vera Rubin ridge in Gale crater, which were laid down in an ancient lake. We use color images and spectral data from the Mastcam cameras on MSL to investigate the origin of these color differences and find that they are consistent with changes in iron oxides through the ridge. This variation in iron oxides suggests that groundwater flowed through and altered these rocks multiple times before and after they were buried by later sediments. The MSL mission has shown that habitable lake environments once existed in Gale crater, through detections of the building blocks of life, including organic molecules, and by showing that conditions that existed in the lake were clement for life. However, later alteration by groundwater may have diminished the preservation of organics and changed the composition of these rocks, making it more difficult to interpret the details of how conditions evolved in the lake over time. Key Points: Vera Rubin ridge exhibits strong color differences that crosscut stratigraphy, consistent with postdepositional alteration (diagenesis)Color differences correspond to variations in spectral signatures of nanophase, fine‐grained, and coarse‐grained hematiteBased on terrestrial analogs, these variations can be explained by early oxidation and later coarsening of hematite by diagenetic fluids [ABSTRACT FROM AUTHOR]

Vera Rubin ridge (VRR) is a topographic high within the layers of Mount Sharp, Gale crater, that exhibits a strong hematite spectral signature from orbit. The Mars Science Laboratory Curiosity rover carried out a comprehensive investigation to understand the depositional and diagenetic processes recorded in the rocks of VRR. Sample Analysis at Mars (SAM) evolved gas analyses (EGA) were performed on three samples from the ridge and one from directly beneath the ridge. SAM evolved H2O data suggested the presence of an Fe‐rich dioctahedral smectite, such as nontronite, in the sample from beneath the ridge. H2O data are also consistent with ferripyrophyllite in VRR samples. SAM SO2 data indicated that all samples contained Mg sulfates and some Fe sulfate. Several volatile detections suggested trace reduced sulfur sources, such as Fe sulfides and/or S‐bearing organic compounds, in two samples while significant O2 and NO evolved from one sample indicated the presence of oxychlorine and nitrate/nitrite salts, respectively. The O2 evolution was the second highest to date and the first observed in ~1,200 sols. HCl released from all samples likely resulted, in part, from trace chloride salts. All samples evolved CO2 and CO consistent with oxidized carbon compounds (e.g., oxalates), while some CO2 may result from carbonate. SAM‐derived constraints on the mineralogy and chemistry of VRR materials, in the context of additional mineralogy, geochemistry, and sedimentology information obtained by Curiosity, support a complex diagenetic history that involved fluids of a range of possible salinities, redox characteristics, pHs, and temperatures. Plain Language Summary: The Mars Science Laboratory Curiosity rover conducted a detailed study of the rocks that make up the Vera Rubin ridge (VRR) feature in Gale crater, Mars, to better understand Martian geologic history. The Curiosity rover's Sample Analysis at Mars (SAM), a suite of scientific instruments on the rover, measured several diagnostic gases when it was used to heat samples from on and beneath VRR. These gases provided information about the mineralogy and chemistry of VRR samples that, together with additional information from other instruments on the rover, indicated that several different types of fluids affected the rocks in the ridge over geologic time. These fluids varied in temperature, salt content, and acidity. Key Points: Vera Rubin ridge experienced a complicated diagenetic history involving alteration by fluids with a variety of chemistries and temperaturesSample Analysis at Mars instrument suite data support an Fe‐rich composition for phyllosilicates detected on and near Vera Rubin ridgeSeveral salts were detected, including sulfate, chloride, nitrate, and oxychlorine salts, enabling constraints on diagenetic fluid chemistry [ABSTRACT FROM AUTHOR]

Geochemical results are presented from Curiosity's exploration of Vera Rubin ridge (VRR), in addition to the full chemostratigraphy of the predominantly lacustrine mudstone Murray formation up to and including VRR. VRR is a prominent ridge flanking Aeolis Mons (informally Mt. Sharp), the central mound in Gale crater, Mars, and was a key area of interest for the Mars Science Laboratory mission. ChemCam data show that VRR is overall geochemically similar to lower‐lying members of the Murray formation, even though the top of VRR shows a strong hematite spectral signature as observed from orbit. Although overall geochemically similar, VRR is characterized by a prominent decrease in Li abundance and Chemical Index of Alteration across the ridge. This decrease follows the morphology of the ridge rather than elevation and is inferred to reflect a nondepositionally controlled decrease in clay mineral abundance in VRR rocks. Additionally, a notable enrichment in Mn above baseline levels is observed on VRR. While not supporting a single model, the results suggest that VRR rocks were likely affected by multiple episodes of postdepositional groundwater interactions that made them more erosionally resistant than surrounding Murray rocks, thus resulting in the modern‐day ridge after subsequent erosion. Plain Language Summary: Results from the ChemCam instrument on Vera Rubin ridge (VRR) in Gale crater, Mars, are presented and compared with observations from similar rocks leading up to the ridge. VRR is a prominent ridge, flanking the central mound, Aeolis Mons, in Gale crater, Mars. The ridge attracted early attention because it displays strong iron‐oxide spectral signatures. Surprisingly, ChemCam data show that VRR rocks do not show an overall increase in iron abundance relative to the comparable bedrock analyzed for almost 300 m in elevation leading up to the ridge. While similar overall, some notable variations were observed on VRR relative to lower‐lying rocks. In particular, geochemical variations suggest a strong decrease in clay content on the ridge, above which, a notable enrichment in Mn is observed. No single geological process confidently explains all observations on the ridge. Rather, we think that VRR rocks underwent a series of interactions with groundwater that caused the rocks of VRR to become more resistant to erosion than their surroundings, thus emerging as a ridge as the rocks around them eroded. This likely implies that groundwater persisted in Gale crater even long after the disappearance of the ancient lake. Key Points: A decrease in Li and Chemical Index of Alteration, reflecting clay mineral content, is observed across Vera Rubin ridge (VRR)A Mn‐rich interval is observed stratigraphically above the decrease in clay mineral content on VRRVRR likely resulted from increased induration from late‐stage fluid interactions long after the lake environment in Gale crater ceased [ABSTRACT FROM AUTHOR]

Wind erosion is integral to the evolution of arid landscapes on Earth and Mars, but the nature of long‐term wind erosion of bedrock is poorly understood. Here we describe the Salina del Fraile (SdF) depression in the Puna Plateau of the Central Andes, NW Argentina, as a landform excavated by wind over several million years. New structural cross sections and a compilation of chronostratigraphic ages rule out the hypothesis that the depression was created by transtensional tectonics. Dated remnant lacustrine and alluvial deposits in the floor of the depression constrain the rate and timing of erosion. Late Oligocene‐Miocene compressional folding uplifted friable strata that were preferentially eroded, resulting in the high‐relief (900 m) depression. Up to 1.95 km and an average of 1.05 km of strata were eroded during the last 8.2 to 17 Ma, at rates of 0.06 to 0.23 mm/yr. These rates are similar to long‐term average wind erosion rates reported in other regions. Coarse‐grained eolian megaripples, yardangs, and elongated ridges indicate ongoing eolian abrasion and deflation, aided by salt weathering, of the floor of the depression. Megaripple migration across stony lag surfaces exposes fresh bedrock to continued erosion. The SdF also contains kilometer‐scale mesas and ridges that we interpret as erosional remnants. These landforms are similar to megayardangs and erosional topography identified on the lower flanks of Mount Sharp, Gale crater, Mars. In such hyperarid landscapes characterized by lithologic heterogeneities, high‐relief landforms can be generated and sustained by wind erosion, without significant fluvial or glacial incision. Plain Language Summary: We investigated the origin of a large topographic depression in the arid Puna Plateau of the Central Andes, NW Argentina. Previous research interpreted the depression as formed by faulting, but we found that it is likely formed by wind erosion. While rivers and glaciers are usually responsible for erosion on Earth, there is no evidence for river or glacier action in the Salina del Fraile. Rather, much like the surface of Mars, wind was primarily responsible for forming the depression and surrounding landscape over several million years. We show that wind was able to excavate the depression because tectonic folding brought fine‐grained rocks close to the surface, where they were exposed to strong winds and breakdown by salt weathering. Wind erosion was active for the last 8.2 to 17 million years and, as evidenced by large dust storms originating from this region, is ongoing today. Wind is lowering the surface at an average rate of 0.06 to 0.23 mm/yr, similar to previous studies. The Salina del Fraile can help researchers understand the long‐term effects of wind erosion on Earth and Mars. Key Points: The Salina del Fraile depression in the arid Puna Plateau was excavated by wind over the last 17 to 8.2 MaCross sections imply up to 2 km of erosion along fold hinges, with remnant kilometer‐scale ridges and mesas analogous to landforms on MarsWind can generate and sustain high‐relief topography, even without significant fluvial or glacial incision [ABSTRACT FROM AUTHOR]

The Curiosity rover's exploration of rocks and soils in Gale crater has provided diverse geochemical and mineralogical data sets, underscoring the complex geological history of the region. We report the crystalline, clay mineral, and amorphous phase distributions of four Gale crater rocks from an 80‐m stratigraphic interval. The mineralogy of the four samples is strongly influenced by aqueous alteration processes, including variations in water chemistries, redox, pH, and temperature. Localized hydrothermal events are evidenced by gray hematite and maturation of amorphous SiO2 to opal‐CT. Low‐temperature diagenetic events are associated with fluctuating lake levels, evaporative events, and groundwater infiltration. Among all mudstones analyzed in Gale crater, the diversity in diagenetic processes is primarily captured by the mineralogy and X‐ray amorphous chemistry of the drilled rocks. Variations indicate a transition from magnetite to hematite and an increase in matrix‐associated sulfates suggesting intensifying influence from oxic, diagenetic fluids upsection. Furthermore, diagenetic fluid pathways are shown to be strongly affected by unconformities and sedimentary transitions, as evidenced by the intensity of alteration inferred from the mineralogy of sediments sampled adjacent to stratigraphic contacts. Plain Language Summary: The mineralogy of mudstones and sandstones investigated by the Mars Science Laboratory rover illustrates a varied and complex history of aqueous alteration in Gale crater sediments. We present the mineralogy of four rocks determined by the CheMin X‐ray diffraction instrument onboard the rover. The results exhibit evidence of multiple diagenetic events, including aqueous alteration by warm groundwaters and a fluctuation of lake levels and evaporative events. Overall, the mineralogy of rocks sampled from the lowermost ~160 m of Gale crater stratigraphy explored by the Curiosity rover shows a decrease in Mg‐Fe‐silicates (i.e., olivine and pyroxene), a transition from magnetite to hematite, an increase in Ca‐sulfates, and a shift from Mg‐phyllosilicates to Al‐phyllosilicates. These trends imply an intensifying influence from oxic, diagenetic fluids. Furthermore, sites adjacent to unconformities and sedimentary transitions show more intense alteration suggesting that these physical boundaries play a key role in driving the path of diagenetic fluids. Key Points: CheMin‐determined mineralogy indicates pervasive low‐temperature diagenesis and localized hydrothermal alteration events at Gale craterThe diagenetic history of Gale crater is characterized by a diverse range in fluid chemistry, Eh, pH, and temperatureThe distribution of SiO2 and FeOT among crystalline and amorphous materials constrains the temperature and duration of diagenesis [ABSTRACT FROM AUTHOR]

A major debate in Mars science is the nature of the early Mars climate, and the availability of precipitation and runoff. Observations of relict erosional valley networks have been proposed as evidence for extensive surface runoff around the Noachian‐Hesperian boundary. However, these valley networks only provide a time‐integrated record of landscape evolution, and thus, the timing, relative timescales and intensity of aqueous activity required to erode the valleys remain unknown. Here, we investigate an ancient fluvial sedimentary system in western Arabia Terra, now preserved in positive relief. This ridge, "Aram Dorsum," is flat‐topped, branching, ~85 km long, and particularly well preserved. We show that Aram Dorsum was an aggradational alluvial system and that the existing ridge was once a large river channel belt set in extensive flood plains, many of which are still preserved. Smaller, palaeochannel belts feed the main system; their setting and network pattern suggest a distributed source of water. The alluvial succession is up to 60 m thick, suggesting a formation time of 105 to 107 years by analogy to Earth. Our observations are consistent with Aram Dorsum having formed by long‐lived flows of water, sourced both locally, and regionally as part of a wider alluvial system in Arabia Terra. This suggests frequent or seasonal precipitation as the source of water. Correlating our observations with previous regional‐scale mapping shows that Aram Dorsum formed in the mid‐Noachian. Aram Dorsum is one of the oldest fluvial systems described on Mars and indicates climatic conditions that sustained surface river flows on early Mars. Plain Language Summary: The oldest regions of Mars contain ancient valleys, carved by running water, and sinuous ridges, often interpreted as the remnants of former riverbeds, left upstanding by erosion. These "inverted channel" systems provide insight into Mars's ancient climate and hydrologic cycle. We use high‐resolution satellite images to investigate "Aram Dorsum," an 85 km long ridge system in the Arabia Terra region. Our observations show that Aram Dorsum is composed of sedimentary rocks, originally deposited in rivers or their adjacent floodplains. The Aram Dorsum sedimentary material is up to 60 m thick, which, by comparison with similar sedimentary deposits on Earth, suggests that the Aram Dorsum river system was active for between 10,000 and 10 million years. Our study also indicates that Aram Dorsum was formed around 3.9 billion years ago, an age consistent with other evidence for ancient rivers and lakes in this region. The water that flowed within the rivers at Aram Dorsum probably came from both locally and regionally generated rainfall or snowfall, rather than a single point source of water, such as large distant ice sheets. Our observations therefore point to an ancient Martian climate that supported precipitation and river flow for thousands or millions of years. Key Points: Aram Dorsum (AD) is an ~85 km long, branching, flat‐topped ridge with layered flanks in Arabia Terra, MarsAD is an exhumed aggradational river system with floodplains deposits and was probably active for 105–107 years in the mid‐NoachianThe AD rivers were sourced both locally and regionally, indicating that the ancient Martian climate supported precipitation and runoff here [ABSTRACT FROM AUTHOR]

For ~500 Martian solar days (sols), the Mars Science Laboratory team explored Vera Rubin ridge (VRR), a topographic feature on the northwest slope of Aeolis Mons. Here we review the sedimentary facies and stratigraphy observed during sols 1,800–2,300, covering more than 100 m of stratigraphic thickness. Curiosity's traverse includes two transects across the ridge, which enables investigation of lateral variability over a distance of ~300 m. Three informally named stratigraphic members of the Murray formation are described: Blunts Point, Pettegrove Point, and Jura, with the latter two exposed on VRR. The Blunts Point member, exposed just below the ridge, is characterized by a recessive, fine‐grained facies that exhibits extensive planar lamination and is crosscut by abundant curvi‐planar veins. The Pettegrove Point member is more resistant, fine‐grained, thinly planar laminated, and contains a higher abundance of diagenetic concretions. Conformable above the Pettegrove Point member is the Jura member, which is also fine‐grained and parallel stratified, but is marked by a distinct step in topography, which coincides with localized meter‐scale inclined strata, a thinly and thickly laminated facies, and occasional crystal molds. All members record low‐energy lacustrine deposition, consistent with prior observations of the Murray formation. Uncommon outcrops of low‐angle stratification suggest possible subaqueous currents, and steeply inclined beds may be the result of slumping. Collectively, the rocks exposed at VRR provide additional evidence for a long‐lived lacustrine environment (in excess of 106 years via comparison to terrestrial records of sedimentation), which extends our understanding of the duration of habitable conditions in Gale crater. Plain language summary: The primary goal of the Mars Science Laboratory Curiosity rover mission is to explore and assess ancient habitable environments on Mars. This requires a detailed understanding of the environments recorded by sedimentary rocks exposed at the present‐day surface in Gale crater. Here we review the types of sedimentary rocks exposed at a location known as Vera Rubin ridge. We find that the rocks at Vera Rubin ridge record an ancient lake environment and are a continuation of underlying lake deposits. Ancient lake deposits are highly desirable targets in the search for habitable environments, due to their ability to concentrate and preserve organic matter. This study significantly expands the duration of habitable conditions that can be confirmed through ground truth of sedimentary rocks and provides a framework for interpreting strata that lie ahead as Curiosity continues to explore Aeolis Mons. Key Points: Six sedimentary facies were identified at and just below Vera Rubin ridge and comprise three members of the Murray formationVera Rubin ridge records deposition in a lacustrine environment, which expands the duration of habitable conditions observed in GaleThe facies and stratigraphy identified here serve as a framework for interpreting strata within the Glen Torridon region and beyond [ABSTRACT FROM AUTHOR]

Perchlorate (ClO4−) was discovered in Martian soil by the Phoenix lander, with important implications for potential Martian biology, photochemistry, aqueous chemistry, and the chlorine cycle on Mars. Perchlorate was subsequently reported in both loose sediment and bedrock samples analyzed by the Sample Analysis at Mars instrument onboard the Curiosity rover in Gale crater based on a release of O2 at 200–500°C. However, the continually wet paleoenvironment recorded by the sedimentary rocks in Gale crater was not conducive to the deposition of highly soluble salts. Furthermore, the preservation of ancient perchlorate to the modern day is unexpected due to its low thermodynamic stability and radiolytic decomposition associated with its long exposure to radioactivity and cosmic radiation. We therefore investigate alternative sources of O2 in Sample Analysis at Mars analyses including superoxides, sulfates, nitrate, and nanophase iron and manganese oxides. Geochemical evidence and oxygen release patterns observed by Curiosity are inconsistent with each of these alternatives. We conclude that perchlorate is indeed the most likely source of the detected O2 release at 200–500°C, but contend that it is unlikely to be ancient. Rather than being associated with the lacustrine or early diagenetic environment, the most likely origin of perchlorate in the bedrock is late stage addition by downward percolation of water through rock pore space during transient wetting events in the Amazonian. The conclusion that the observed perchlorate in Gale crater is most likely Amazonian suggests the presence of recent liquid water at the modern surface. Plain Language Summary: Perchlorate is a chemical found on Mars that dissolves easily in water, which can keep water from freezing at the cold temperatures found on Mars's surface. It was first discovered in soil, and has been found by the Curiosity rover in rock at Gale crater. Perchlorate should have flowed out of the sandy lake bottom in ancient Gale crater before it turned into rock. Perchlorate is also easily destroyed, so even if it were included in the rock originally, perchlorate should have disintegrated in the following ~3.5 billion years. Because of these dilemmas with finding perchlorate in ancient rocks, we have reevaluated the evidence for perchlorate as observed by Curiosity. Of a number of other chemicals which could explain the data, we find that perchlorate is still the best fit and is probably present. Due to the problems with perchlorate being included in the rock originally and surviving until today, we conclude that it has been added to the rocks recently. This recent addition could happen by perchlorate dissolving in water and flowing into the rock. Such a process suggests that liquid water was present in Gale crater long after Mars became cold and dry. Key Points: Geology suggests that ancient Gale crater was not conducive to perchlorate deposition; radiolytic decomposition would destroy ancient perchlorateAlternatives to perchlorate are evaluated as sources of pyrolytic O2; perchlorate best explains the dataPerchlorate in Gale crater is most likely young, possibly introduced by downward percolation of thin films of brine [ABSTRACT FROM AUTHOR]

Wind interacts with the surface of Mars to create the diverse landforms seen today. The morphology of wind streaks and related surface features has been used to interpret the winds on Mars in regions where actual wind measurement are lacking. Here we compare wind directions interpreted from bright wind streaks observed in orbitally‐acquired images with winds measured at the surface by the InSight lander in Elysium Planitia. The distribution of streak orientations is trimodal, dominantly reflecting winds from the southeast. The distribution of winds measured by InSight was highly variable with time of day but was dominantly characterized by winds from the northwest. The lack of coincidence between the two suggests that wind only intermittently deposits dust on the surface. The persistence of the streak orientations over five Mars years demonstrates that saltation in the region, if occurring, is not sustained long enough to rework these dust depositional surfaces. Plain Language Summary: Wind is responsible for much of the surface geology on Mars. Many previous works have used the surface geology to interpret wind directions in a given region with images taken from orbit. Here we compare winds interpreted from wind streaks seen in orbiter‐acquired images and winds measured by the InSight lander to see how well the two coincide. Over 120 Mars days, the directions interpreted from surface geology and measured at the surface did not match, suggesting that wind alters the Martian surface at some times more than other times. Key Points: Bright wind streaks in record winds dominantly from the southeast that do not coincide with winds measured by the InSight landerThe interpreted and measured winds demonstrate that conditions only intermittently favor dust deposition on the surfaceSaltation has not been sustained for long enough in the region to rework and remove surface dust defining the streaks [ABSTRACT FROM AUTHOR]

Estimates of river paleodischarges have been used to constrain the former climate of Mars. Paleodischarge has been calculated using mechanistic approximations of the channel bed‐shear stress at the threshold of particle motion or correlative width‐discharge relations derived from empirical terrestrial hydraulic geometry data. We apply both these methods to a study set of gaged terrestrial rivers with field‐measured bankfull properties to assess the reliability of discharge estimates given similar uncertainties as ancient martian rivers. We find that the threshold method yields conservative values, provided a reasonable grain size is used. However, we find that digital elevation models of a similar resolution to those constructed from Context Camera (CTX) data may not be of sufficient resolution to be useful for estimating paleohydrologic conditions. Results from our analysis demonstrate the inherent uncertainties when approximating paleodischarges on Mars and stress the need to consider realistic boundary conditions when determining such values. Plain Language Summary: The amount of water, or discharge, that flowed through martian rivers can be used to understand the planet's former climate. Various methods have been used by researchers to estimate martian river discharge, including relating river width to discharge or calculating the water volume required to move sediment in the river, but it is uncertain which of these provide more accurate results. We applied these same methods to rivers on Earth that have field‐assessed flow rates. We find that using grain size yields conservative discharge values, which is generally more desirable, but that there is significant uncertainty associated with both methods. Key Points: Two methods of estimating martian paleoriver discharge are applied to terrestrial rivers with known bankfull dischargesThe threshold approach yields more conservative discharges when used with realistic grain sizes and high‐resolution topographic dataUncertainties inherent in both methods emphasize the need to consider realistic boundary conditions when applied to Mars [ABSTRACT FROM AUTHOR]

Oxychlorine species are globally widespread across the Martian surface. Despite their ubiquitous presence, the ability of oxychlorine species to serve as oxidants on Mars has largely been unexplored. While perchlorate is kinetically inert, chlorate may be a critical Fe(II) oxidant on Mars. However, the timescale over which chlorate may oxidize Fe(II) and the mineral products formed in Mars‐relevant fluids are unclear. Fe(II) oxidation by chlorate was thus investigated in magnesium chloride, sulfate, and perchlorate fluids under neutral to acidic conditions for different total Fe(II) and background salt concentrations. The results show near‐complete Fe(II) oxidation within approximately 2 to 4 weeks, accompanied by formation of the Fe(III) minerals goethite, lepidocrocite, akaganeite, and jarosite. The Fe(II) oxidation rate and the mineral products depend on Fe(II) concentration, the composition and concentration of the background salt, and the acidity of the solution. Calibration of an existing rate law to lower temperatures well reproduces the observed oxidation kinetics in all fluid compositions and allows prediction of the rate of Fe(II) oxidation by chlorate under diverse Mars‐relevant conditions. Rate comparisons demonstrates that chlorate can oxidize Fe(II) substantially faster than O2 and on similar or shorter timescales than ultraviolet light. Notably, chlorate causes rapid oxidation under acidic conditions, unlike other oxidants. Chlorate may thus represent an important abiotic Fe(II) oxidant on Mars. The expected coassociation of chlorate with perchlorate may allow for its percolation into the subsurface during brine migration, leading to oxidation in regions that are cutoff from ultraviolet radiation and atmospherically derived oxidants. Plain Language Summary: The surface of Mars is replete with iron oxides. Salts of chlorine and oxygen, called oxychlorine species, are globally distributed on Mars and theoretically should react with reduced iron in the Martian crust forming iron oxides. However, the actual capacity of these salts to oxidize iron remains unknown. We conducted laboratory experiments in Mars‐relevant waters to explore whether key oxychlorine species, chlorate, oxidizes iron in weeks or less. This reaction proceeds, fully oxidizing iron and making diverse iron oxides observed on Mars. A geochemical model was created to predict when this process will occur on Mars. Chlorate can oxidize reduced iron faster than oxygen and ultraviolet light in many situations, especially under acidic conditions. Our results show that chlorate may have been an important iron oxidant on Mars when water was present on the surface. It may also be an important oxidant on Mars today, especially below the surface. Key Points: Chlorate oxidizes dissolved Fe(II) in Mars‐relevant fluids at both near‐neutral and acidic pH conditionsDiverse Fe(III) mineral products are controlled by the fluid chemistryChlorate may be an important oxidant on Mars in both ancient and modern systems [ABSTRACT FROM AUTHOR]

The upper bound of 50 parts per trillion by volume for Mars methane above 5 km established by the ExoMars Trace Gas Orbiter, substantially lower than the 410 parts per trillion by volume average measured overnight by the Curiosity Rover, places a strong constraint on the daytime methane flux at the Gale crater. We propose that these measurements may be largely reconciled by the inhibition of mixing near the surface overnight, whereby methane emitted from the subsurface accumulates within meters of the surface before being mixed below detection limits at dawn. A model of this scenario allows the first precise calculation of microseepage fluxes at Gale to be derived, consistent with a constant 1.5 × 10−10 kg·m−2·sol−1 (5.4 × 10−5 tonnes·km−2·year−1) source at depth. Under this scenario, only 2.7 × 104 km2 of Mars's surface may be emitting methane, unless a fast destruction mechanism exists. Plain Language Summary: The ExoMars Trace Gas Orbiter and the Curiosity Rover have recorded different amounts of methane in the atmosphere on Mars. The Trace Gas Orbiter measured very little methane (<50 parts per trillion by volume) above 5 km in the sunlit atmosphere, while Curiosity measured substantially more (410 parts per trillion by volume) near the surface at night. In this paper we describe a framework which explains both measurements by suggesting that a small amount of methane seeps out of the ground constantly. During the day, this small amount of methane is rapidly mixed and diluted by vigorous convection, leading to low overall levels within the atmosphere. During the night, convection lessens, allowing methane to build up near the surface. At dawn, convection intensifies and the near‐surface methane is mixed and diluted with much more atmosphere. Using this model and methane concentrations from both approaches, we are able—for the first time—to place a single number on the rate of seepage of methane at Gale crater which we find equivalent to 2.8 kg per Martian day. Future spacecraft measuring methane near the surface of Mars could determine how much methane seeps out of the ground in different locations, providing insight into what processes create that methane in the subsurface. Key Points: Nighttime SAM‐TLS seasonal cycle enrichment measurements and TGO sunset/sunrise measurements are not in oppositionMicroseepage fluxes must be local to Gale, range from 0.82 to 4.6 kg/sol, and are consistent with a constant source at depthLittle of Mars experiences microseepage unless a fast destruction mechanism exists or Gale is very unusual [ABSTRACT FROM AUTHOR]

Deltaic deposits mapped along the Martian crustal dichotomy boundary scarp have been suggested to delineate an ancient ocean in the northern lowlands of Mars. Using recently acquired orbital data, we have expanded the dichotomy delta inventory and performed an updated analysis of delta front elevations, a proxy for paleo‐water levels. Our analysis focused near Gale crater, home of the Curiosity rover. We found that delta front elevations vary by approximately 2,400 m, but these elevation variations do not correspond to modeled deformation from true polar wander or Tharsis. Locally, delta front elevations vary by ≤60 m, and using present‐day topography, they correspond to distinct enclosed basins. We infer that these deltas formed in paleo‐lakes up to approximately 13,000 km2 and approximately 0.4 km deep, perhaps coeval with paleo‐lakes in Gale. Our results suggest that a northern ocean is not needed to explain the deltaic deposits in the Gale crater region. Plain Language Summary: The climate of early Mars is debated. Climate modeling suggests that it was always cold and dry, while geologic evidence suggests that it was much wetter in the past, possibly supporting a northern ocean. Evidence in support of a northern ocean includes deltaic deposits along the margins of the northern lowlands. However, the elevations of these deltas vary greatly, calling into question whether they were all deposited into the same body of water. To test this hypothesis, we used high‐resolution satellite imagery and topography to investigate a large number of lowland deltas in the vicinity of Gale crater, home to the Curiosity rover. We have found that the variability in delta elevations is not due to postformational deformation, rather most deltas in this region formed into lakes, similar in size to the Great Lakes on Earth. These findings show that a northern Martian ocean is not needed to explain the deltas in the vicinity of Gale crater. Key Points: Twenty‐two candidate deltas occur along the dichotomy boundary in the Gale crater region with delta front elevations spanning approximately 2,400 mFront elevation variations do not correspond to modeled deformation from true polar wander or Tharsis emplacementBased on modern topography, most deltas correspond to enclosed basins, which we infer to be paleo‐lakes, not a northern ocean [ABSTRACT FROM AUTHOR]

The occurrence of past habitable conditions in Gale crater is generally associated with lacustrine and alluvial environments present >3 Ga ago, during the Hesperian Period on Mars. However, later‐occurring aqueous activity is consistent with superposition relations between some alluvial deposits and bounding materials on the crater walls, preservation of fine‐scale morphology on these alluvial deposits, and their superposing crater densities. The alluvial deposits include some not previously considered, and collectively lend confidence to the interpretation that local aqueous activity persisted in Gale crater into the Amazonian, or <2 Ga ago. Our conclusions are generally in accordance with late aqueous activity inferred from geochronology data (Martin et al., 2017, https://doi.org/10.1002/2017JE005445), in addition to late alluvial activity elsewhere. Interpreted late aqueous activity points to possible habitable settings in Gale later than previously recognized. Plain Language Summary: Several fan‐shaped deposits on the walls and floor of Gale crater formed via deposition of water transported sediment. Overall, the fan deposits are fairly well preserved, and the number of impact craters on their surfaces indicate they are much younger (~2 billion years old or less) relative to the older, more widespread water‐related deposits that formed in the crater over 3 billion years ago. The late episode(s) of water‐related activity in Gale crater may be associated with late‐occurring chemical precipitation within and (or) cementation of older rocks within the crater and suggests that habitable conditions persisted inside the crater later than previously thought. Key Points: Local alluvial deposits in Gale crater were likely emplaced less than 2 Ga, postdating more widespread water‐related deposits in GaleWater associated with the younger alluvial deposits may be related to late‐occurring diagenetic activityAlluvial deposits formed when Aeolis Mons achieved its present expression, suggesting late‐occurring habitable conditions within Gale [ABSTRACT FROM AUTHOR]

Branching to sinuous ridges systems, hundreds of kilometers in length and comprising layered strata, are present across much of Arabia Terra, Mars. These ridges are interpreted as depositional fluvial channels, now preserved as inverted topography. Here we use high‐resolution image and topographic data sets to investigate the morphology of these depositional systems and show key examples of their relationships to associated fluvial landforms. The inverted channel systems likely comprise indurated conglomerate, sandstone, and mudstone bodies, which form a multistory channel stratigraphy. The channel systems intersect local basins and indurated sedimentary mounds, which we interpret as paleolake deposits. Some inverted channels are located within erosional valley networks, which have regional and local catchments. Inverted channels are typically found in downslope sections of valley networks, sometimes at the margins of basins, and numerous different transition morphologies are observed. These relationships indicate a complex history of erosion and deposition, possibly controlled by changes in water or sediment flux, or base‐level variation. Other inverted channel systems have no clear preserved catchment, likely lost due to regional resurfacing of upland areas. Sediment may have been transported through Arabia Terra toward the dichotomy and stored in local and regional‐scale basins. Regional stratigraphic relations suggest these systems were active between the mid‐Noachian and early Hesperian. The morphology of these systems is supportive of an early Mars climate, which was characterized by prolonged precipitation and runoff. Plain Language Summary: Landscape features identified as former rivers and lakes are common across ancient Martian surfaces (>3.7 billion years ago), strong evidence for an ancient hydrologic cycle. However, the nature of the ancient climate and environment remains unclear and detailed investigations of Mars's geology are necessary to help constrain this. Using high‐resolution satellite images, we investigate a series of sinuous ridges preserved at the Martian surface throughout the ancient Arabia Terra region. These sinuous ridges often occur downslope of river valleys, commonly as the valleys enter topographic basins. The ridges are usually found on the oldest exposed geological surfaces. The morphology of the ridges and their relationship to river valleys suggests that they are sedimentary rocks, which form in or next to rivers. We interpret the basins as ancient lakes and their associated deposits as lake sediments. These rivers were active ~3.7 billion years ago and transported and deposited sediment through the Arabia Terra region. On Earth, rivers typically take between 50,000 and 1,000,000 years to develop such thick deposits. These river deposits are now exposed as ridges due to erosion. The formation of these rivers and lakes in Arabia Terra was likely due to prolonged and episodic precipitation‐driven erosion. Key Points: Fluvial depositional systems, 50–100 m in stratigraphic thickness, are common across Arabia Terra, MarsThese deposits represent former rivers (at least 100–200 km long), floodplains, and lakes and are found filling fluvial valleys and basinsThese systems are mid‐Noachian to early Hesperian in age and represent significant periods of geologic time [ABSTRACT FROM AUTHOR]

NASA's strategy in exploring Mars has been to follow the water, because water is essential for life, and it has been found that there are many locations where there was once liquid water on the surface. Now perhaps, to narrow down the search for life on a barren basalt‐dominated surface, there needs to be a refocusing to a strategy of "follow the nutrients." Here we model the entry of metallic micrometeoroids through the Martian atmosphere, and investigate variations in micrometeorite abundance at an analogue site on the Nullarbor Plain in Australia, to determine where the common limiting nutrients available in these (e.g., P, S, Fe) become concentrated on the surface of Mars. We find that dense micrometeorites are abundant in a range of desert environments, becoming concentrated by aeolian processes into specific sites that would be easily investigated by a robotic rover. Our modeling suggests that micrometeorites are currently far more abundant on the surface of Mars than on Earth, and given the far greater abundance of water and warmer conditions on Earth and thus much more active weather system, this was likely true throughout the history of Mars. Because micrometeorites contain a variety of redox sensitive minerals including FeNi alloys, sulfide and phosphide minerals, and organic compounds, the sites where these become concentrated are far more nutrient rich, and thus more compatible with chemolithotrophic life than most of the Martian surface. Plain Language Summary: NASA's exploration program has allowed the scientific community to demonstrate clearly that Mars had a watery past, so the search for life needs to move on to identifying the places where water and nutrients coincided. We have investigated the relative abundance of micrometeorites on Mars compared to the Earth because these contain key nutrients that the earliest life forms on Earth used, and because their contained minerals can be used to investigate past atmospheric chemistry. We suggest that micrometeorites should be far more abundant on the Martian surface than on Earth's, and that wind‐driven modification of sediments is expected to concentrate micrometeorites, and their contained nutrients, in gravel beds and cracks in exposed bedrock. Key Points: Micrometeorites are predicted to be far more abundant on Mars than on EarthMicrometeorites are concentrated in the residue of aeolian sediment removal, such as at bedrock cracks and gravel accumulationsMicrometeorite accumulation sites are enriched in key nutrients for primitive microbes: reduced phosphorus, sulfur, and iron [ABSTRACT FROM AUTHOR]

The origin of the sedimentary mound within Gale crater, the landing site for the Mars Science Laboratory rover Curiosity, remains enigmatic. Here we examine the total potential contribution of fluvial material by conducting a volume‐based analysis. On the basis of these results, the mound can be divided into three zones: lower, intermediate, and upper zones. The top boundary of the lowermost zone is defined by maximal contribution of water‐lain sediments, which are ~13% to 20% of the total mound volume. The upper zone is defined by the elevation of the unbreached rim to the north (–2.46 km); sediments above this elevation cannot have been emplaced by flowing water. These volume balance calculations indicate that mechanisms other than flowing water are required to account for the overwhelming majority of the sediments transported into Gale crater. The most likely candidate process is settling from eolian suspension. Key Points: Fluvial processes are responsible for less than 20% of the volume of Aeolis Mons, the central mound in Gale craterThe overwhelming majority of sediments transported into Gale crater were transported by mechanisms other than flowing waterMost of the sediments in Aeolis Mons were likely deposited as eolian airfall deposits [ABSTRACT FROM AUTHOR]

Noachian surfaces on Mars exhibit vertical assemblages of weathering horizons termed as weathering profiles; this indicates that surface water caused alteration of the rocks that required a different, warmer climate than today. Evidence of this early Martian climate with CO2 vapor as the main component causing greenhouse warming has been challenged by the lack of carbonate in these profiles. Here we report the analysis of Compact Reconnaissance Imaging Spectrometer for Mars L‐detector data leading to the detections of carbonates using a spectral signature exclusively attributed to them. The carbonates are collocated with hydroxylated minerals in weathering profiles over the Martian surface. The origin of CO2 for the formation of carbonates could be the atmosphere. The widespread distribution of weathering profiles with carbonates over the surface of the planet suggest global interactions between fluids containing carbonate/bicarbonate ions with the surface of Mars in the presence of atmospheric water until around 3.7 billion years ago. Plain Language Summary: The oldest surface of Mars witnessed an impact of dense and humid atmosphere leading to the formation of hydrated minerals. The analysis of remote sensing data allows identifying carbonates mixed with the hydrated minerals. These associations of carbonates and hydrated minerals are widespread on the surface of the planet. This indicates a planetary‐scale process of formation involving the presence of fluids containing inorganic carbon. This finding allows a better understanding of the environment of Mars during the period suspected to be the most likely to have host habitable environment on the planet. Key Points: Carbonates mixed with clay minerals can be detected with the use of a unique spectral feature in the 3.40 to 3.80 μm rangeCarbonates are detected in the middle part of the weathering profilesMineralogical associations in the weathering profiles are consistent with weathering by water that contained inorganic carbon species [ABSTRACT FROM AUTHOR]

The observed fall and rapid recovery of the Tissint Mars meteorite has provided minimally contaminated samples of the Martian surface. We report analyses of Tissint for organic compounds by pyrolysis‐gas chromatography‐mass spectrometry and for soluble salts by ion chromatography. Pyrolysis‐gas chromatography‐mass spectrometry analysis shows the presence of organic compounds similar to those in the Mars EETA79001 and Nakhla meteorites. The organic profile is dominated by aromatic hydrocarbons, including oxygen and nitrogen‐containing aromatics, and sulfur‐containing species including thiophenes. The soluble salts in Tissint are dominated by sulfate and various oxidation states of chlorine, including perchlorate. The organic compounds and salts in the soils from the Tissint recovery strewn field differ significantly from those found in Tissint suggesting minimal terrestrial contamination. Our results support the hypothesis that the soluble inorganic components of Tissint are most likely a result of indigenous fluid inclusion, thus providing a glimpse into the composition of early Martian fluids. Plain Language Summary: Meteorites that come from Mars usually become contaminated after landing on Earth. One Mars meteorite, named Tissint, was seen falling in the dry Moroccan desert, and many of its pieces were collected within months. We analyzed the meteorite and the local soil for organic compounds and soluble salts. We found differences between the Tissint meteorite and the local soil that suggest that Tissint is uncontaminated. The analyses also showed that the soluble salts in Tissint most likely came from Martian fluids that were similar to terrestrial seawater. Key Points: Results support hypothesis that indigenous fluids incorporated into Tissint fissures on Mars and resulting chemical signatures preservedDifferences between soluble chemistry of Tissint and strewn field soil suggest that terrestrial contamination from recovery site is unlikelyAnalyses provide a glimpse into the chemical composition of a potentially habitable brine on early Mars [ABSTRACT FROM AUTHOR]

In deep space, personnel and equipment are exposed to the space radiation environment in the form of energetic particles, specifically galactic cosmic rays and sporadic solar energetic particle events. Radiation fields resulting from these particles are modified by shielding, but most radiation measurements in deep space have been made with detectors that were unshielded or very lightly shielded. In contrast, the space radiation environment on the International Space Station (ISS) is more complicated, with time‐dependent modification of the incident flux by the geomagnetic field and complex bulk shielding distributions; measured particle spectra inside the ISS are affected by both types of shielding. The geomagnetic field is also responsible for the existence of the South Atlantic Anomaly, a region of trapped energetic protons and electrons, and hence enhanced radiation dose, through which the ISS travels several times per day on average. Here our primary aim is to compare charged‐particle spectra at high linear energy transfer obtained by the Anomalous Long‐Term Effects in Astronauts instrument on ISS during high‐latitude portions of the orbit to data acquired at the same time by the Cosmic Ray Telescope for the Effects of Radiation and Radiation Assessment Detector instruments, both in deep space. The hypothesis being tested is that these spectra are the same, modulo shielding differences, since the effects of the geomagnetic field are expected to be minimal at high latitudes. Plain Language Summary: Exposure to highly energetic particle radiation in space is a concern for current and future human missions. To date, only the Apollo astronauts have ventured outside the protective effects of the Earth's magnetic field, but astronauts on future missions back to the Moon, or to Mars or other destinations in deep space, will not have this protection. In the high‐latitude parts of the orbit of the International Space Station, the magnetic shielding is weak, and in principle we expect the radiation environment there to be very similar to that in deep space. Here we present the first direct test of this hypothesis, using data obtained by three different particle detectors, one aboard the ISS, one in lunar orbit, and one that was in interplanetary space between Earth and Mars for part of the time period being studied, and on the surface of Mars for the rest of the period of interest. Key Points: We report comparisons of energetic particle spectra taken in low‐Earth orbit at high geomagnetic latitudes to similar data from deep spaceThree instruments were used in the study, one aboard the International Space Station, one in lunar orbit, and one in interplanetary spaceDespite shielding differences, the spectra have many features in common, but fluxes aboard the Space Station are slightly smaller [ABSTRACT FROM AUTHOR]

Hydrous phases have been identified to be a significant component of Martian mineralogy. Particularly, prehnite, zeolites, and serpentine are evidence for low‐grade metamorphic reactions at elevated temperatures in mafic and ultramafic protoliths. Their presence suggests that at least part of the Martian crust is sufficiently hydrated for low‐grade metamorphic reactions to occur. A detailed analysis of changes in mineralogy with variations in fluid content and composition along possible Martian geotherms can contribute to determine the conditions required for subsurface hydrous alteration, fluid availability, and rock properties in the Martian crust. In this study, we use phase equilibria models to explore low‐grade metamorphic reactions covering a pressure‐temperature range of 0–0.5 GPa and 150–450 °C for several Martian protolith compositions and varying fluid content. Our models replicate the detected low‐grade metamorphic/hydrothermal mineral phases like prehnite, chlorite, analcime, unspecified zeolites, and serpentine. Our results also suggest that actinolite should be a part of lower‐grade metamorphic assemblages, but actinolite may not be detected in reflectance spectra for several reasons. By gradually increasing the water content in the modeled whole‐rock composition, we can estimate the amount of water required to precipitate low‐grade metamorphic phases. Mineralogical constraints do not necessarily require an elevated geothermal gradient for the formation of prehnite. However, restricted crater excavation depths even for large impact craters are not likely sampling prehnite along colder gradients, suggesting either a geotherm of ~20 °C/km in the Noachian or an additional heat source such as hydrothermal or magmatic activity. Plain Language Summary: There is evidence that greater amounts of water were present on a younger Mars as compared to the dry conditions observed today. Water not only shapes the surface and forms river beds, deltas, and lakes but also reacts with the existing rocks to form characteristic minerals. Understanding how these minerals formed gives us useful information about how much water may be present in the Martian crust, which is also important for possible habitable environments. Our knowledge about Mars has increased greatly with the data from rovers and orbiters, but we still only have a limited amount of rocks available. Therefore, we use computer models to simulate how rocks from Mars would behave if they were exposed to higher temperatures, pressures, and water. We can estimate how much water is needed to form certain minerals, and that helps us to understand the conditions on Mars over geologic timescales and how and why it is different from Earth. Key Points: Low‐grade metamorphic phases detected on Mars can be reproduced by phase equilibria modelingProtolith composition and fluid content strongly influence the mineralogy of low‐grade metamorphic assemblagesWarmer geotherms and/or additional heat by hydrothermal or magmatic activity are more favorable for low‐grade metamorphic minerals [ABSTRACT FROM AUTHOR]

Molecular hydrogen (H2) from volcanic emissions is suggested to warm the Martian surface when carbon dioxide (CO2) levels dropped from the Noachian (4100 to 3700 Myr) to the Hesperian (3700 to 3000 Myr). Its presence is expected to shift the conversion of molecular nitrogen (N2) into different forms of fixed nitrogen (N). Here we present experimental data and theoretical calculations that investigate the efficiency of nitrogen fixation by bolide impacts in CO2‐N2 atmospheres with or without H2. Surprisingly, nitric oxide (NO) was produced more efficiently in 20% H2 in spite of being a reducing agent and not likely to increase the rate of nitrogen oxidation. Nevertheless, its presence led to a faster cooling of the shock wave raising the freeze‐out temperature of NO resulting in an enhanced yield. We estimate that the nitrogen fixation rate by bolide impacts varied from 7 × 10−4 to 2 × 10−3 g N·Myr−1·cm−2 and could imply fluvial concentration to explain the nitrogen (1.4 ± 0.7 g N·Myr−1·cm−2) detected as nitrite (NO2−) and nitrate (NO3−) by Curiosity at Yellowknife Bay. One possible explanation is that the nitrogen detected in the lacustrine sediments at Gale was deposited entirely on the crater's surface and was subsequently dissolved and transported by superficial and ground waters to the lake during favorable wet climatic conditions. The nitrogen content sharply decreases in younger sediments of the Murray formation suggesting a decline of H2 in the atmosphere and the rise of oxidizing conditions causing a shortage in the supply to putative microbial life. Plain Language Summary: Climate models are able to warm early Mars when CO2 sources were strong but fail at later times when liquid water still flowed on the surface. A possible solution for the climate puzzle is the presence of abundant H2 arising from volcanic emissions that could have kept the planet from freezing. H2 could have also played a key role in the chemistry of the atmosphere. Curiosity discovered the presence of nitrites and nitrates, forms of fixed nitrogen that are required for the origin and sustainability of life in sediments in Gale crater. Here we present theoretical and experimental data that quantify the conversion of molecular nitrogen into fixed nitrogen in the presence and absence of H2 by the entry shocks of asteroids in the Martian atmosphere and surface. Fixed nitrogen was originally deposited on the surface of Gale crater and then transported to the lake during favorable wet climatic conditions. We found that H2 is required to yield sufficient fixed nitrogen to explain its detection. The levels of fixed nitrogen sharply dropped in younger sediments suggesting a decline of H2 in the atmosphere and the rise of oxidizing conditions causing a nitrogen crisis to putative microbial communities. Key Points: A hydrogen‐rich atmosphere is required to explain the levels of fixed nitrogen that are found in sediments encountered in Gale craterFixed nitrogen was deposited on the surface of the crater and then transported to the lake during favorable wet climatic conditionsThe levels of fixed nitrogen sharply decreased in younger sediments causing a shortage in the supply to putative microbial communities [ABSTRACT FROM AUTHOR]

Oxygen and HCl gas releases detected by the Sample Analysis at Mars (SAM) instrument on board the Mars Science Laboratory Curiosity rover in several Gale Crater samples have been attributed to the thermal decomposition of perchlorates and/or chlorates. Previous experimental studies of perchlorates mixed with Fe‐bearing phases explained some but not all of the evolved oxygen releases and cannot explain the HCl releases. The objective of this paper was to evaluate the oxygen and HCl releases of chlorates and chlorate/Fe phase mixtures in experimental studies and SAM evolved gas analysis data sets. Potassium, magnesium, and sodium chlorate were independently mixed with hematite, magnetite, ferrihydrite, and palagonite and analyzed in a thermal evolved gas analyzer configured to operate similarly to the SAM instrument. Fe phases depressed the chlorate decomposition temperature 3–214 °C and consumed up to 75% of the evolved oxygen from chlorate decomposition. Chlorate/Fe phase mixtures have oxygen and HCl releases consistent with some samples analyzed by SAM. Reported oxychlorine abundances based on calculations using oxygen detected by SAM could be minimum values because Fe phases consume evolved oxygen. The results of this work demonstrate that chlorates could be present in the Martian soil and that oxygen and HCl release temperatures could be used to constrain which chlorate cation species are present in samples analyzed by SAM. Knowledge of which chlorates may be present in Gale Crater creates a better understanding of the detectability of organics by evolved gas analysis, habitability potential, and the chlorine cycle on Mars. Plain Language Summary: The Sample Analysis at Mars (SAM) instrument on board the Curiosity rover heats Martian soil and then detects the gases produced from the breakdown of chemicals. The SAM instrument has detected oxygen and hydrochloric acid (HCl) gas in several Gale Crater samples, which have been attributed to perchlorates (ClO4−) and/or chlorates (ClO3−). In this study, we mixed chlorates with iron minerals that exist on Mars (e.g., hematite). These mixtures and pure chlorates were analyzed in a SAM‐like laboratory instrument. Oxygen and HCl gas releases were compared to SAM data sets. Iron minerals decreased the chlorate breakdown temperature and the amount of oxygen produced. Chlorate/iron mineral mixtures have oxygen and HCl releases consistent with some samples analyzed by SAM. Reported perchlorate/chlorate abundances based on calculations using oxygen detected by SAM could be minimum values because iron minerals decrease the amount of oxygen produced from chlorates. These results demonstrate that chlorates could be present on Mars and that oxygen and HCl release temperatures could be used to constrain which type of chlorates are present in SAM samples. Knowledge of which chlorates may be present in Gale Crater creates a better understanding of the detectability of organic material by SAM, habitability potential, and the chlorine cycle on Mars. Key Points: Fe‐bearing phases catalyze the thermal decomposition of chloratesFe‐bearing phases consume oxygen during thermal decomposition of chloratesChlorates may be present in rocks and soils in Gale Crater [ABSTRACT FROM AUTHOR]

X‐ray amorphous materials have been detected in all samples measured by the CheMin X‐ray diffractometer (XRD) on board the Mars Science Laboratory rover in Gale Crater, Mars. The origin(s) of these materials are poorly understood, and there are significant uncertainties on their estimated abundances and compositions. Three methods are used to estimate the bulk amorphous component abundance and composition of Martian samples using XRD and bulk chemical data: (1) Rietveld refinements, (2) FULLPAT analyses, and (3) mass balance calculations. We tested these methods against a quantitative XRD (internal standard) method commonly used in terrestrial laboratories. Additionally, we tested for instrumentation effects by measuring our samples on a laboratory XRD instrument (PANalytical X'Pert Pro) and the CheMin test bed instrument (CheMin IV). We used three natural samples known to contain amorphous materials: glacial sediment, Hawaiian soil, and a paleosol. Our methods resulted in nine amorphous abundances and four amorphous compositions for each sample. For a single sample, amorphous abundance estimates and amorphous compositions are relatively similar across all estimation methods. CheMin analog measurements perform well in our tests, with amorphous abundances and compositions comparable to laboratory quantitative XRD measurements, though slightly underestimated. This suggests that previous amorphous component estimates for Martian samples are relatively accurate. This study highlights the usefulness of the mass balance calculation method for characterizing amorphous materials in terrestrial samples, providing important supplemental information to destructive and time consuming size separation and dissolution procedures. Plain Language Summary: Natural soil and sediment samples on Earth and Mars are commonly mixtures of crystalline and noncrystalline materials. X‐ray diffraction techniques are frequently used to quantify the abundance and composition of crystalline materials, and relatively recent developments in X‐ray diffraction data analysis methods allow noncrystalline materials to also be characterized. Noncrystalline materials are studied using the following methods: (1) noncrystalline X‐ray diffraction peaks modeled with broad peaks, (2) noncrystalline peaks modeled with diffraction patterns of measured noncrystalline materials, and (3) a mass balance calculation that combines chemical and X‐ray diffraction data. However, it is uncertain how accurate these methods are. Here we systematically test these three data analysis methods on complex natural samples. We find that both modeling methods (1 and 2) are capable of providing relatively accurate noncrystalline component abundances. Modeling noncrystalline peaks with patterns of measured noncrystalline materials (method 2) provides the most accurate abundance results but does not necessarily provide accurate compositional information, whereas the mass balance calculation provides accurate noncrystalline material compositions, but not abundances. Our results suggest that a combination of methods (either 1 and 3 or 2 and 3) should be used to more completely characterize the abundance and composition of noncrystalline materials. Key Points: Amorphous abundance estimates from different methods can vary significantly for a single sampleMass balance calculations can help determine the accuracy of mineralogy models and amorphous abundance and composition estimationsAmorphous composition estimates from the CheMin instrument on Mars are likely accurate [ABSTRACT FROM AUTHOR]

The Mars Science Laboratory (MSL) rover Curiosity has documented a section of fluvio-lacustrine strata at Yellowknife Bay (YKB), an embayment on the floor of Gale crater, approximately 500 m east of the Bradbury landing site. X-ray diffraction (XRD) data and evolved gas analysis (EGA) data from the CheMin and SAM instruments show that two powdered mudstone samples (named John Klein and Cumberland) drilled from the Sheepbed member of this succession contain up to ~20 wt% clay minerals. A trioctahedral smectite, likely a ferrian saponite, is the only clay mineral phase detected in these samples. Smectites of the two samples exhibit different 001 spacing under the low partial pressures of H2O inside the CheMin instrument (relative humidity <1%). Smectite interlayers in John Klein collapsed sometime between clay mineral formation and the time of analysis to a basal spacing of 10 Å, but largely remain open in the Cumberland sample with a basal spacing of ~13.2 Å. Partial intercalation of Cumberland smectites by metal-hydroxyl groups, a common process in certain pedogenic and lacustrine settings on Earth, is our favored explanation for these differences. The relatively low abundances of olivine and enriched levels of magnetite in the Sheepbed mudstone, when compared with regional basalt compositions derived from orbital data, suggest that clay minerals formed with magnetite in situ via aqueous alteration of olivine. Mass-balance calculations are permissive of such a reaction. Moreover, the Sheepbed mudstone mineral assemblage is consistent with minimal inputs of detrital clay minerals from the crater walls and rim. Early diagenetic fabrics suggest clay mineral formation prior to lithification. Thermodynamic modeling indicates that the production of authigenic magnetite and saponite at surficial temperatures requires a moderate supply of oxidants, allowing circum-neutral pH. The kinetics of olivine alteration suggest the presence of fluids for thousands to hundreds of thousands of years. Mineralogical evidence of the persistence of benign aqueous conditions at YKB for extended periods indicates a potentially habitable environment where life could establish itself. Mediated oxidation of Fe2+ in olivine to Fe3+ in magnetite, and perhaps in smectites provided a potential energy source for organisms. [ABSTRACT FROM AUTHOR]

The three main requirements for life as we know it are the presence of organic compounds, liquid water, and free energy. Several groups of organic compounds (e.g., amino acids, nucleobases, lipids) occur in all life forms on Earth and are used as diagnostic molecules, i.e., biomarkers, for the characterization of extant or extinct life. Due to their indispensability for life on Earth, these biomarkers are also prime targets in the search for life on Mars. Biomarkers degrade over time; in situ environmental conditions influence the preservation of those molecules. Nonetheless, upon shielding (e.g., by mineral surfaces), particular biomarkers can persist for billions of years, making them of vital importance in answering questions about the origins and limits of life on early Earth and Mars. The search for organic material and biosignatures on Mars is particularly challenging due to the hostile environment and its effect on organic compounds near the surface. In support of life detection on Mars, it is crucial to investigate analogue environments on Earth that resemble best past and present Mars conditions. Terrestrial extreme environments offer a rich source of information allowing us to determine how extreme conditions affect life and molecules associated with it. Extremophilic organisms have adapted to the most stunning conditions on Earth in environments with often unique geological and chemical features. One challenge in detecting biomarkers is to optimize extraction, since organic molecules can be low in abundance and can strongly adsorb to mineral surfaces. Methods and analytical tools in the field of life science are continuously improving. Amplification methods are very useful for the detection of low concentrations of genomic material but most other organic molecules are not prone to amplification methods. Therefore, a great deal depends on the extraction efficiency. The questions "what to look for", "where to look", and "how to look for it" require more of our attention to ensure the success of future life detection missions on Mars. [ABSTRACT FROM AUTHOR]

The acidic sulfate-rich waters of the Meridiani Planum region were potentially a habitable environment for iron-oxidizing bacteria on ancient Mars. If life existed in this ancient martian environment, jarosite minerals precipitating in these waters may record evidence of this biological activity. Since the Meridiani jarosite is thermodynamically stable at the martian surface, any biosignatures preserved in the jarosites may be readily available for analysis in the current surface sediments during the ongoing robotic exploration of Mars. However, thermal decomposition experiments indicate that organic compound detection of sediments containing jarosite may be challenging when using pyrolysis experiments; the instrument commonly used to assess organic matter in martian samples. So, here, we assess if the biogenicity of the Meridiani-type jarosites can be determined using complimentary spectroscopic techniques also utilized during the robotic exploration of Mars, including the upcoming ExoMars2020 rover mission. An abiotic jarosite, synthesized following established protocols, and a biological jarosite counterpart, derived from a microbial enrichment culture of Rio Tinto river sediments, were used to compare four spectroscopy techniques employed in the robotic exploration of Mars (Raman spectroscopy, mid-infrared (IR) spectroscopy, visible near-infrared reflectance (VNIR) spectroscopy and Mössbauer spectroscopy) to determine if the complimentary information obtained using these instruments can help elucidate the biological influence of Meridiani-type jarosites. Raman spectral differences might be due to the presence of unreacted reagents in the synthetic spectra and not biological contributions. Reflectance (IR/VNIR) spectra might exhibit minor organic absorption contributions, but are observed in both sample spectra, and do not represent a biosignature. Mössbauer spectra show minor differences in fit parameters that are related to crystal morphology and are unrelated to the biological (i.e., organic) component of the system. Results of this study suggest that the identification of biosignatures in Meridiani-type jarosites using the in situ robotic exploration on Mars may be possible but will be challenging. Our work provides additional insight into extraterrestrial biosignature detection and data interpretation for Mars exploration and indicates that sample return missions are likely required to unequivocally resolve the possible biogenicity of the Meridiani sediments or other jarosite-containing sediments. [ABSTRACT FROM AUTHOR]

Minerals might have played critical roles for the origin and evolution of possible life forms on Mars. The study of the interactions between the "building blocks of life" and minerals relevant to Mars mineralogy under conditions mimicking the harsh Martian environment may provide key insight into possible prebiotic processes. Therefore, this contribution aims at reviewing the most important investigations carried out so far about the catalytic/protective properties of Martian minerals toward molecular biosignatures under Martian-like conditions. Overall, it turns out that the fate of molecular biosignatures on Mars depends on a delicate balance between multiple preservation and degradation mechanisms, often regulated by minerals, which may take place simultaneously. Such a complexity requires more efforts in simulating realistically the Martian environment in order to better inspect plausible prebiotic pathways and shed light on the nature of the organic compounds detected both in meteorites and on the surface of Mars through in situ analysis. [ABSTRACT FROM AUTHOR]