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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]

The Curiosity rover investigated a topographic structure known as Vera Rubin ridge, associated with a hematite signature in orbital spectra. There, Curiosity encountered mudstones interpreted as lacustrine deposits, conformably overlying the 300 m‐thick underlying sedimentary rocks of the Murray formation at the base of Mount Sharp. While the presence of hematite (α‐Fe2O3) was confirmed in situ by both Mastcam and ChemCam spectral observations and by the CheMin instrument, neither ChemCam nor APXS observed any significant increase in FeOT (total iron oxide) abundances compared to the rest of the Murray formation. Instead, Curiosity discovered dark‐toned diagenetic features displaying anomalously high FeOT abundances, commonly observed in association with light‐toned Ca‐sulfate veins but also as crystal pseudomorphs in the host rock. These iron‐rich diagenetic features are predominantly observed in "gray" outcrops on the upper part of the ridge, which lack the telltale ferric signature of other Vera Rubin ridge outcrops. Their composition is consistent with anhydrous Fe‐oxide, as the enrichment in iron is not associated with enrichment in any other elements, nor with detections of volatiles. The lack of ferric absorption features in the ChemCam reflectance spectra and the hexagonal crystalline structure associated with dark‐toned crystals points toward coarse "gray" hematite. In addition, the host rock adjacent to these features appears bleached and shows low‐FeOT content as well as depletion in Mn, indicating mobilization of these redox‐sensitive elements during diagenesis. Thus, groundwater fluid circulations could account for the remobilization of iron and recrystallization as crystalline hematite during diagenesis on Vera Rubin ridge. Plain Language Summary: The NASA rover Curiosity investigated Vera Rubin ridge, a specific landform within the Gale crater on Mars. Scientific missions in orbit around the planet had previously discovered high concentrations of hematite on top of the ridge, an iron‐oxide mineral that commonly forms in water. However, it was not clear from orbit if such conditions existed at the time of the deposition of the sediments (around 3.5 billion years ago) or occurred much later during "diagenesis," after deposition of the sediments and up to their transformation into rocks. On the surface, the rover did not observe significant differences between the ridge and the terrains encountered before it, except for small, dark geologic features that formed during diagenesis. Their analysis by the ChemCam instrument revealed that these features are composed of hematite—the same iron‐oxide mineral that was observed from orbit—and, interestingly, that the iron required to form them was removed from the adjacent rocks by groundwaters. As such, it appears that groundwaters played an important role in shaping Vera Rubin ridge, and thus partially obscure interpretations on the environmental conditions that existed on the surface of Mars at the time of sedimentation. Key Points: Images from the Curiosity rover show the presence of dark‐toned diagenetic features at Vera Rubin ridgeChemCam analyses of these features point toward an Fe‐oxide composition, consistent with crystalline hematiteDepletion of Fe and Mn in bleached halos around the Fe‐oxide features indicates mobility of Fe and Mn during the later stages of diagenesis [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]

Apatite, a major hydrous mineral in Martian meteorites, has the potential to reveal history of volatiles on Mars. Here we report the H and Cl isotopic systematics of apatite from the enriched basaltic shergottite Northwest Africa (NWA) 8657 to better understand the processes influencing volatiles at or near the Martian (sub)surface. The apatite in NWA 8657 has the highest reported δD value (up to 6,509‰) of any phosphates in Martian meteorites. The positive correlation between the H2O contents (72–2,251 ppm) and δD values (3,965–6,509‰) as well as between the Cl contents (1.28–6.17wt%) and δ37Cl values (−1.7–4.0‰) in NWA 8657 apatite are consistent with mixing between volatiles derived from the Martian mantle and meteoric water/fluid. The D‐ and 37Cl‐enrichment of apatite in NWA 8657 implies isotopic exchange with subsurficial fluids during postcrystallization hydrothermal event(s). Plain Language Summary: Mars was probably a wet and warm world during its earliest geological history, supported by many lines of evidence. Present‐day Mars is cold and arid although recent remote sensing data support the existence of subsurface glaciers and seasonal fluid activities at some locations. In this study, our aim was to use ground truth about the potential water‐rock interaction on or near‐surface environments on recent Mars via measuring the H and Cl isotopic compositions of apatite from a geologically young meteorite from Mars: the enriched basaltic shergottite Northwest Africa (NWA) 8657. The petrography and mineral chemistry of apatite and other associated phases, for example, merrillite, maskelynite, mesostasis, and pyroxene, were documented prior to in situ H and Cl isotopes analysis. The water contents of apatite, merrillite, maskelynite, mesostasis, and pyroxene are positively correlated with the δD values. All of the data sets can be reconciled in terms of mixing between two end‐members, a D‐poor (~0‰) mantle reservoir and a D‐rich (5,920 ± 500‰) near‐surface/underground water reservoir. Moreover, apatite in NWA 8657 displays 37Cl‐enriched (~1 ± 1‰ in average) characteristics. These signatures are consistent with D‐ and 37Cl‐rich fluid‐assisted isotopic exchange in recent near‐surface environment on Mars. Key Points: First report of H and Cl isotopic systematics of apatite from the Martian meteorite NWA 8657NWA 8657 apatite has the highest reported δD value (up to 6,509‰) of any phosphates in Martian meteoritesThe H and Cl isotopic compositions of apatite in NWA 8657 imply D‐ and 37Cl‐rich fluids on surface of Mars [ABSTRACT FROM AUTHOR]

Aeolian sediment transport, deposition, and erosion have been ongoing throughout Mars's history. This record of widespread aeolian processes is preserved in landforms and geologic units that retain important clues about past environmental conditions including wind patterns. In this study we describe landforms within Melas Chasma, Valles Marineris, that occur in distinct groups with linear to crescentic shapes, arranged with a characteristic wavelength; some possess slope profiles analogous to modern sand dunes yet show evidence for lithification. Based on the features' dimensions, asymmetry, and spatial patterns relative to modern equivalents, we interpret these landforms to be two classes of aeolian bedforms: decameter‐scale megaripples and sand dunes. The presence of superposed erosional features and depositional units indicates that these landforms were cemented and likely ancient. Melas paleodunes are found atop Hesperian‐aged layered deposits, but we estimate them to be younger, likely lithified in the Amazonian period. Although a range of degradation was observed, some paleodunes are >10 m tall and maintain steep lee sides (>25°), an uncommon scenario for terrestrial examples as other geologic processes lead to dune obliteration. The preserved paleobedform geometries are largely consistent with those of modern aeolian indicators, suggesting no major shifts in wind regime or contributing boundary conditions. Finally, we propose that their appearance and context require sequential periods of dune migration, stabilization following catastrophic burial, cementation, differential erosion, exposure, and burial. The presence of wholly preserved duneforms appears to be more common on Mars compared to the Earth and may signal something important about Martian landscape evolution. Plain Language Summary: Wind‐driven sand dunes are common on modern Mars, and the presence of certain sedimentary rock layers indicates that these landforms occurred there in the past. Here, we explore features in the canyons of Valles Marineris that show characteristics frequently attributed to dunes, yet evidence for their lithification and burial is clear. Their horizontal extent, height, shape, slopes, and collective spatial patterns led to our interpretation of them as lithified dune fields. The heavily eroded appearance along with superposed elements such as craters and boulders led to the conclusion that these are relatively ancient landforms. Despite this erosion, this level of preservation is rare for terrestrial sand dunes due to erosion and tectonics, thus providing an opportunity to reconstruct the various factors contributing to their history. Understanding the circumstances that led to the preservation of these ancient duneforms will yield crucial information regarding planetary sedimentary processes and the geologic history of the region. Key Points: Sedimentary deposits were identified in Valles Marineris and interpreted as ancient aeolian dune fields with a range of preservation statesTheir preservation indicates a complex history with periods of migration, burial, stabilization, cementation, exhumation, and erosionPaleodune morphologies compared with modern indicators suggest no major shifts in wind regime have occurred since their stabilization [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]

The Mars Science Laboratory (MSL) Curiosity rover is exploring the Murray formation, a sequence of heterolithic mudstones and sandstones recording fluvial deltaic and lake deposits that comprise over 350 m of sedimentary strata within Gale crater. We examine >4,500 Murray formation bedrock points, employing recent laboratory calibrations for ChemCam laser‐induced breakdown spectroscopy H measurements at millimeter scale. Bedrock in the Murray formation has an interquartile range of 2.3–3.1 wt.% H2O, similar to measurements using the Dynamic Albedo of Neutrons and Sample Analysis at Mars instruments. However, specific stratigraphic intervals include high H targets (6–18 wt.% H2O) correlated with Si, Mg, Ca, Mn, or Fe, indicating units with opal, hydrated Mg sulfates, hydrated Ca sulfates, Mn‐enriched units, and akageneite or other iron oxyhydroxides, respectively. One stratigraphic interval with higher hydrogen is the Sutton Island unit and Blunts Point unit contact, where higher hydrogen is associated with Fe‐rich, Ca‐rich, and Mg‐rich points. A second interval with higher hydrogen occurs in the Vera Rubin ridge portion of the Murray formation, where higher hydrogen is associated with Fe‐rich, Ca‐rich, and Si‐rich points. We also observe trends in the H signal with grain size, separate from chemical variation, whereby coarser‐grained rocks have higher hydrogen. Variability in the hydrogen content of rocks points to a history of water‐rock interaction at Gale crater that included changes in lake water chemistry during Murray formation deposition and multiple subsequent groundwater episodes. Plain Language Summary: We measured the water content of bedrock targets in the Murray formation, a sequence of mudstones and sandstones part of Gale crater's Mt. Sharp, by applying recent laboratory calibrations to measurements from the Curiosity ChemCam instrument. While most rocks contained 2.3–3.1 wt.% H2O, consistent with measurements by other Mars Science Laboratory instruments, we found some stratigraphic intervals contained high water content (6–18 wt.% H2O) rocks including the Vera Rubin ridge. Based on our analysis of the corresponding major element composition, these are likely clays, Ca sulfates, Mn‐enriched units, and akageneite or other iron oxyhydroxides. The variation in water content indicates a history of water‐rock interaction at Gale crater that included changes in lake water chemistry during Murray formation deposition and multiple subsequent groundwater episodes. Key Points: Murray formation bedrock points measured by ChemCam have an interquartile range of 2.3–3.1 wt.% H2OSpecific intervals like the Vera Rubin ridge contain high H and indicate phases including iron oxyhydroxides, akageneite, and jarositeVariations in water content indicate changes in depositional lake water chemistry and multiple subsequent groundwater episodes [ABSTRACT FROM AUTHOR]

Orbiter and rover data have revealed a complex and intermittent hydrological history in Gale Crater on Mars, where habitable environments appear to have endured for at least thousands of years. The intermittency may be the result of a dominantly cold climate punctuated by geologically brief periods of warmth and active hydrology. However, the time required to establish an integrated hydrological cycle in a warming climate is difficult to ascertain and has not been thoroughly investigated. Here we model the transient evolution of groundwater flow and subsurface temperature, the slowest evolving components of the hydrological cycle, during a warm departure from cold conditions. We find that tens of thousands of years are likely required before groundwater could be a meaningful source for large lakes in Gale. With highly favorable conditions, primarily high permeability, significant flow might develop in thousands of years. This implies that surface water dominates during the beginning of a warm phase. Annual mean surface temperatures in Gale below 290 K would likely leave the nearby highlands frozen at the surface. In that case, deep aquifers beneath a highlands permafrost layer could deliver water to Gale, where low temperatures would have reduced evaporation. Plain Language Summary: Satellite imagery and data from the National Aeronautics and Space Administration (NASA)'s Curiosity rover indicate that Gale Crater, on Mars, hosted large lakes more than 3 billion yr ago. The climate during this period is not well understood but may have been mostly frozen, only occasionally warming up to the point where lakes could form. If so, warm periods must have been long enough to thaw frozen sources of water and supply the lakes. The slowest thawing source is groundwater. We simulate the thawing and flow of groundwater in the region surrounding Gale to understand whether it could have been a significant source for lakes during a warm climate period. We find that it could only be significant with the most generous assumptions about flow speed and aquifer recharge. Even with generous assumptions, tens of thousands of years are required to thaw enough groundwater to have an impact on large lakes. After long enough, though, the ground would thaw entirely, possibly enabling a higher rate of groundwater delivery. Key Points: Tens of thousands of years are likely required to generate significant groundwater flow to Gale Crater with a thawing crustSurface water would likely be the dominant source for Gale lakes at the beginning of a warm episodeDeep aquifers may have played a role later on if complete thaw occurred beneath Gale [ABSTRACT FROM AUTHOR]

Understanding magmatic processes is critical to understanding Mars as a system, but Curiosity's investigation of dominantly sedimentary rocks has made it difficult to constrain igneous processes. Igneous classification of float rocks is challenging because of the following: (1) the possibility that they have been affected by sedimentary processes or weathering, and (2) grain size heterogeneity in the observed rock textures makes the small‐scale compositions measured by rover instruments unreliable for bulk classification. We avoid these ambiguities by using detrital igneous mineral chemistry to constrain models of magmatic processes in the source region for the fluvio‐deltaic Bradbury group. Mineral chemistry is obtained from X‐ray diffraction of three collected samples and a new stoichiometric and visual filtering of ~5,000 laser induced breakdown spectroscopy (LIBS) spots to identify compositions of individual igneous minerals. Observed mineral chemistries are compared to those produced by MELTS thermodynamic modeling to constrain possible magmatic conditions. Fractionation of two starting primary melts derived from different extent of adiabatic decompression melting of a primitive mantle composition could result in the crystallization of all minerals observed. Crystal fractionation of a subalkaline and an alkaline magma is required to form the observed minerals. These results are consistent with the collection of alkaline and subalkaline rocks from Gale as well as clasts from the Martian meteorite Northwest Africa 7034 and paired stones. This new method for constraining magmatic processes will be of significant interest for the Mars 2020 mission, which will also investigate an ancient volcaniclastic‐sedimentary environment and will include a LIBS instrument. Plain Language Summary: Using instruments on Mars rovers, classification of rocks as igneous or sedimentary can be ambiguous, as many of the sedimentary rocks are made of barely altered igneous minerals. We avoid this issue by using the chemistry of individual igneous minerals found in both sedimentary and igneous rocks to assess possible magmatic processes. We focus on rocks in the Bradbury group observed by the Curiosity rover, which includes river and lake deposits coming from a source area to the north of Gale crater. We report the chemistry of pure igneous minerals hit by the ChemCam laser. Then, using modeling, we assess several scenarios for magmatic processes to identify conditions that could have reproduced the mineral chemistries observed. This new method shows that fractional crystallization of two compositionally distinct parental melts formed during the ascent of a mantle composition can explain the igneous mineral chemistry observed. These results are consistent with previous works in Gale, and are interesting because (1) both alkaline and subalkaline magmas were likely produced very early in Mars history, and (2) this provides a new way to constrain magmatic processes from observations of sedimentary rocks, which will be important for the Mars 2020 mission. Key Points: Detrital feldspar and pyroxene mineral compositions were estimated for the first time from stoichiometric analyses of ChemCam compositionsMELTS modeling was used to constrain possible ancient magmatic contributions to the Martian surface mineral chemistry without igneous outcropsOur method is important for future missions like Mars 2020 where igneous outcrops may be rare but detrital minerals are preserved [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]

The NASA Curiosity rover's ChemCam instrument suite has detected boron in calcium‐sulfate‐filled fractures throughout the sedimentary strata of Gale crater including Vera Rubin ridge. The presence of elevated B concentration provides insights into Martian subsurface aqueous processes. In this study we extend the data set of B in Ca‐sulfate veins across Gale crater, comparing the detection frequency and relative abundances with Li. We report 33 new detections of B within veins analyzed between Sols 1548 and 2311 where detections increase in Pettegrove Point and Jura members, which form Vera Rubin ridge. The presence of B and Li in the Ca‐sulfate veins is possibly due to dissolution of preexisting B in clays of the bedrock by acids or neutral water and redistribution of the elements into the veins. Elevated frequency of B detection in veins of Gale crater correlates with presence of dehydration features such as desiccation cracks, altered clay minerals and detections of evaporites such as Mg‐sulfates and chloride salts in the host rocks. The increased observations of B also coincide with decreased Li concentration in the veins (average Li concentration of veins drops by ~15 ppm). Boron and Li have varying solubilities, and Li does not form salts as readily upon dehydration as B, causing it to remain in the solution. So the weak negative correlation between B and Li may reflect the crystallization sequence during dehydration on Vera Rubin ridge. Plain Language Summary: Boron and lithium were measured in cracks in the rocks that were filled with whitish, calcium sulfate. These were found in Gale crater using the ChemCam instrument on the Mars Science Laboratory Curiosity rover. Both elements are highly water soluble and may indicate late‐stage ground water activity. Boron's lower solubility compared to that of lithium means it forms salts relatively easily, while Li remains dissolved in the brine with progressive evaporation. In our study we find that on Vera Rubin ridge, boron detection frequency is inversely proportional to the concentration of lithium. This finding suggests that, based on their different solubilities, boron may have precipitated in the fractures before lithium. Lithium possibly moved away with the remaining brine, leaving boron behind as precipitates. Key Points: ChemCam analyses of calcium sulfate veins in Vera Rubin ridge (VRR) show the presence of boron and lithiumBoron detection frequency in the Ca‐sulfate veins increases in the Jura member of VRR and is inversely correlated with lithiumThe relationship between B and Li suggests an evaporative sequence formed during dehydration on higher elevations of VRR [ABSTRACT FROM AUTHOR]

The Mars Science Laboratory mission investigated Vera Rubin ridge, which bears spectral indications of elevated amounts of hematite and has been hypothesized as having a complex diagenetic history. Martian samples, including three drilled samples from the ridge, were analyzed by the Sample Analysis at Mars instrument suite via evolved gas analysis‐mass spectrometry (EGA‐MS). Here, we report new EGA‐MS data from Martian samples and describe laboratory analogue experiments. Analyses of laboratory analogues help determine the presence of reduced sulfur in Martian solid samples, which could have supported potential microbial life. We used evolved carbonyl sulfide (COS) and carbon disulfide (CS2) to identify Martian samples likely to contain reduced sulfur by applying a quadratic discriminant analysis. While we report results for 24 Martian samples, we focus on Vera Rubin ridge samples and select others for comparison. Our results suggest the presence of reduced sulfur in the Jura member of Vera Rubin ridge, which can support various diagenetic history models, including, as discussed in this work, diagenetic alteration initiated by a mildly reducing, sulfite‐containing groundwater. Plain Language Summary: The Mars Science Laboratory studied the chemical composition of Vera Rubin ridge in Gale crater, Mars. The Sample Analysis at Mars, a set of scientific instruments designed to study rock chemistry, observed a number of gases released during the heating of Martian drilled samples. The same gases were observed when Mars‐relevant minerals were analyzed with similar instruments on Earth. From these two sets of data, we applied statistical analyses to determine which Mars samples on Vera Rubin ridge contained important sulfur compounds. Two samples on the ridge showed evidence for these compounds, which could have supported the energetic requirements for life. The results presented here improve both the understanding of the history of Gale crater and the potential for ancient life to have existed. Key Points: The Sample Analysis at Mars instrument suite studied carbon‐sulfur gases released during evolved gas analysis of Vera Rubin ridge samplesQuadratic discriminant analysis compared laboratory and flight data to identify Martian samples that contain reduced sulfurTwo Vera Rubin ridge samples from the Jura member were identified as likely to include reduced sulfur [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]

Ground‐based bedding orientation measurements are critical to determine the geologic history and processes of sedimentation in Gale crater, Mars. We constrain the dip of lacustrine strata of the Blunts Point, Pettegrove Point, and Jura members of the Murray formation using a combination of regional stratigraphic correlations and bed attitude measurements from stereo Mastcam images taken by the Mars Science Laboratory Curiosity rover. In situ bed attitude measurements using a principal component analysis‐based regression method reveal a wide range of dips and dip azimuths owing to a combination of high stereo errors, postdepositional deformation of strata (e.g., fracturing, rotation, and impact cratering), and different primary depositional dips. These constrain regional dips to be within several degrees of horizontal on average. Stratigraphic correlations between targets observed in the Glen Torridon trough and at the Pettegrove Point‐Jura member contact of Vera Rubin ridge (VRR) constrain dips to be between 3°SE and 2°NW, consistent with nearly flat strata deposited horizontally on an equipotential surface. The Jura member is determined to be stratigraphically equivalent to the northern portion of the Glen Torridon trough. Rover‐based dip magnitudes are generally significantly shallower than the orientation of VRR member contacts measured from High Resolution Imaging Science Experiment‐based traces, suggesting the sedimentary strata and VRR member contacts may be discordant. Plain Language Summary: The orientation of sedimentary strata is one of the most fundamental measurements of structural geology because it records information about the processes of deposition and subsequent deformation of those strata. For the last 7 years, the Curiosity rover has traversed predominantly fluviolacustrine (river‐ and lake‐deposited) strata. Recently, the rover traversed the Vera Rubin ridge (VRR), a topographic rise within a larger collection of strata with rock exposures whose orientation can be measured using overlapping (stereo) images taken by cameras aboard the rover. By measuring the orientation of beds in stereo rover images and comparing the elevation of similar rock lithologies found along the traverse, we constrain the strata that comprise the VRR to be horizontal or only shallowly dipping. This result is consistent with the sediment that formed the VRR being deposited on a nearly horizontal surface, suggesting that at least the strata that make up the lower portion of Mount Sharp, the large sedimentary mound in Gale crater that dips more steeply outward, may not have directly contributed to its primary formation. The near‐flat orientation also indicates that some portion of the VRR occurs at the same elevation as the region south of the ridge called Glen Torridon. Key Points: We use stereo rover images and stratigraphic correlations to constrain the dip of Vera Rubin ridge strata to be flat or nearly flatThe Jura member of the Vera Rubin ridge is stratigraphically equivalent to part of Glen TorridonVera Rubin ridge member contacts and the strata that comprise them are likely discordant [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]

Evolved hydrogen chloride (HCl) detected by the Sample Analysis at Mars (SAM) instrument's evolved gas analysis (EGA) mode on board the Mars Science Laboratory Curiosity rover has been attributed to oxychlorines (i.e., perchlorates and chlorates) or chlorides in Gale crater samples. Previous laboratory EGA studies of oxychlorines have been unable to reproduce the high‐temperature (>600 °C) HCl evolutions observed in most Gale crater samples. The objective of this work was to reproduce these high‐temperature HCl releases from laboratory mixtures of perchlorates and chlorides with phases that evolve water upon heating. Magnesium and sodium perchlorate and chloride were mixed with saponite, nontronite, and a basaltic glass and analyzed in a laboratory thermal evolved gas analyzer configured to operate similarly to the SAM instrument. Na perchlorate and chloride evolved HCl only when mixed with all three water‐producing phases. Mg perchlorate and chloride evolved a mid ‐temperature HCl release (~450–550 °C) and evolved an additional high‐temperature HCl release (~810–820 °C) when mixed with saponite. This work demonstrated that chlorides, either originally present or from perchlorate decomposition, evolved high‐temperature HCl when reacting with water from water‐producing phases. The HCl release temperature was dependent on the mixture's mineralogy and chemical composition. HCl releases detected by SAM were consistent with oxychlorines and/or chlorides in the presence of water‐producing phases. Additionally, this work provided constraints on the presence of oxychlorines or chlorides and their cation types, which has implications for past aqueous and diagenetic processes, the potential for past life, and detection of organics by EGA. Plain Language Summary: The detection of gaseous hydrogen chloride (HCl) on Mars can indicate the presence of oxychlorines (ClO4 or ClO3) or chlorides, which have implications for past physical and chemical changes in the rock, the detection of organics, and the potential that microbial life existed in the past. The Sample Analysis at Mars (SAM) instrument on Curiosity has detected gaseous HCl during the heating of solid rock and soil samples, but its origin is not well understood. Perchlorates and chlorides were mixed with materials that release gaseous H2O upon heating and analyzed in a SAM‐analog instrument. Results indicated that chlorides and/or perchlorates reacting with water caused the HCl releases detected by SAM. The presence of perchlorates is significant because they could have served as fuel for past microbial life but can also hinder the detection of organic material. Perchlorates and chlorides also provide information on the geologic history of the area. Key Points: High‐temperature (>600 °C) HCl evolutions in SAM data were caused by oxychlorine phases or chlorides reacting with waterOxychlorine phases mixed with water‐producing phases evolved oxygen and hydrogen chlorideChlorides mixed with water‐producing phases evolved hydrogen chloride but did not evolve oxygen [ABSTRACT FROM AUTHOR]

The Sample Analysis at Mars (SAM) instrument onboard the Mars Science Laboratory Curiosity rover measures the chemical composition of major atmospheric species (CO2, N2, 40Ar, O2, and CO) through a dedicated atmospheric inlet. We report here measurements of volume mixing ratios in Gale Crater using the SAM quadrupole mass spectrometer, obtained over a period of nearly 5 years (3 Mars years) from landing. The observation period spans the northern summer of MY 31 and solar longitude (LS) of 175° through spring of MY 34, LS = 12°. This work expands upon prior reports of the mixing ratios measured by SAM QMS in the first 105 sols of the mission. The SAM QMS atmospheric measurements were taken periodically, with a cumulative coverage of four or five experiments per season on Mars. Major observations include the seasonal cycle of CO2, N2, and Ar, which lags approximately 20–40° of LS behind the pressure cycle driven by CO2 condensation and sublimation from the winter poles. This seasonal cycle indicates that transport occurs on faster timescales than mixing. The mixing ratio of O2 shows significant seasonal and interannual variability, suggesting an unknown atmospheric or surface process at work. The O2 measurements are compared to several parameters, including dust optical depth and trace CH4 measurements by Curiosity. We derive annual mean volume mixing ratios for the atmosphere in Gale Crater: CO2 = 0.951 (±0.003), N2 = 0.0259 (±0.0006), 40Ar = 0.0194 (±0.0004), O2 = 1.61 (±0.09) x 10‐3, and CO = 5.8 (±0.8) x 10‐4. Plain Language Summary: The atmosphere of Mars is made up of primarily carbon dioxide, and during the Martian year, the barometric pressure is known to cycle up and down substantially as this carbon dioxide freezes out and then is rereleased from polar caps. The Mars Science Laboratory Curiosity rover has now acquired atmospheric composition measurements at the ground over multiple years, capturing the variations in the major gases over several seasonal cycles for the first time. With the Sample Analysis at Mars instrument, the annual average composition in Gale Crater was measured as 95.1% carbon dioxide, 2.59% nitrogen, 1.94% argon, 0.161% oxygen, and 0.058% carbon monoxide. However, the abundances of some of these gases were observed to vary up to 40% throughout the year due to the seasonal cycle. Nitrogen and argon follow the pressure changes but with a delay, indicating that transport of the atmosphere from pole to pole occurs on faster timescales than mixing of the components. Oxygen has been observed to show significant seasonal and year‐to‐year variability, suggesting an unknown atmospheric or surface process at work. These data can be used to better understand how the surface and atmosphere interact as we search for signs of habitability. Key Points: First multiyear in situ measurements of the major components of the Mars atmosphere have been obtained by the MSL/SAM investigationSeasonal variation of CO2, N2, and Ar reveals differences in atmospheric transport and mixing timescalesOxygen varies seasonally and interannually, independently from Ar and N2, on timescales too fast to be explained by known chemistry [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 Sample Analysis at Mars (SAM) experiment on the National Aeronautics and Space Administration Curiosity rover seeks evidence of organic compounds on the surface of Mars. Since the beginning of the mission, various organic molecules have been detected and identified. While several have been demonstrated to be indigenous to the Martian soil and rocks analyzed, others appear to have been produced from sources internal to the experiment. The objective of this study is to build an exhaustive molecular database to support the interpretation of SAM results by identifying all the chemical species produced from Tenax® adsorbents, by determining (1) the thermal degradation by‐products of Tenax®, (2) the effect of Tenax® conditioning on the formation of Tenax® by‐products, (3) the impact of MTBSTFA or a mixture of MTBSTFA and DMF on Tenax® decomposition, and (4) the reaction between Tenax® and calcium perchlorate. Our results indicate that the by‐products of the SAM trap are due to the impact of trap heating, the impact of the derivatization reagent (MTBSTFA) and the presence of perchlorate in Martian soil. Some of these by‐products are observed in the SAM gas chromatograph mass spectrometer data from Mars. Plain Language Summary: The Sample Analysis at Mars (SAM) experiment onboard the Curiosity Rover has a polymer‐based chemical trap (Tenax®) that concentrates the evolved species from the Martian samples. We studied the impact that this trap could have on the SAM results when heated, when exposed to the chemical compounds used for sample processing (derivatization) and when exposed to Martian perchlorates. We conclude by demonstrating that some of the organic compounds detected in the background signal of the SAM chromatograms likely came from the degradation of Tenax®. This study will help to discriminate the endogenous organic compounds detected on Mars by SAM from the contamination. Key Points: In this article, we evaluate the impact of the Tenax® traps on the Sample Analysis at Mars experiment results, with concurrent implications for the future Martian Organic Molecule Analyser experiment resultsTenax® is an adsorbent resin used on SAM as a trap; it is an organic polymer that can be degraded into smaller moleculesBy‐products of Tenax® may contribute to the background of the SAM chromatogram. Here we identify them and the conditions of their production [ABSTRACT FROM AUTHOR]

Rock fractures are indicators of stress release within geologic systems, and fracture morphologies can commonly be used to infer formation conditions. Polygonal fractures are common in isotropic, contractional stress regimes such as in rocks exposed at the surface of a planet undergoing thermal cycling or in sedimentary substrates undergoing repeated wetting and drying. Such polygonal fracture systems, on centimeter to decameter scales, have been widely documented on Mars. Utilizing a combination of orbital‐ and ground‐based images, we report a laterally extensive polygonal fracture network that occurs within siliciclastic rocks of the lowermost Siccar Point group, Gale crater, Mars. The Siccar Point group is exposed over approximately 20 km2 in northwest Gale crater, where it unconformably overlies eroded strata of Mount Sharp (Aeolis Mons) and reflects likely aeolian deposition along the lower flanks of Mount Sharp. Images reveal an extensive network of erosionally resistant polygons, approximately 7.5 m across, that exhibit interior angles (i.e., fracture intersections) with modes at 90° and 120°. Polygon morphology indicates that fractures formed during multiple cycles of expansion and contraction, which is attributed to desiccation and subsequent recharge of near‐surface groundwater. The erosional resistance of preserved fractures is inferred to reflect postfracture diagenetic fluid flow along the sub–Siccar Point group unconformity and cementation. Evidence for multiple fluid events in the relatively young strata of the Siccar Point group requires a protracted history of fluid stability in Gale crater. Plain Language Summary: Deciphering the processes that affect sedimentary rocks after their deposition is critical to understanding the geologic history of a basin. Fractures occur when applied forces exceed the strength of the host material, and we can infer the process by which fracturing occurred by assessing the morphology of fractures. In this study, we analyze a network of polygonal fractures within rocks of the Siccar Point group, a relatively young geologic unit that is exposed over ~20 km2 of Gale crater, the field site of the Curiosity rover. Polygons formed by these fractures are similarly sized and intersect at angles that show dominant modes at 90° and 120°. These observations suggest that fractures formed under conditions of uniform stress and likely result from contractional processes such as climate‐driven wet‐dry cycles. The cementation of fractures by later fluids then imparted erosional resistance that permits these features to be recognized from orbit. The presence of an extensive, fluid‐driven fracture system within aeolian strata highlights the potential complexity of martian climate signals. Key Points: Siccar Point group strata record extensive fracturing and fluid flow, resulting in a regional network of erosionally resistant polygonsPolygons reflect single to multiple cycles of sediment contraction associated with evaporation and recharge of near surface fluidsThe presence of fluid‐related fractures within a predominantly dry aeolian system highlights potential variability in martian climate [ABSTRACT FROM AUTHOR]

The effects of radiation on the survivability of key biosignatures are a driving factor in exploration strategies throughout the solar system. Ultraviolet (UV) radiation, especially shorter wavelength UVC radiation, is known to be damaging to organisms and to potential organic biosignatures; however, the interaction of UV radiation with minerals and rocks is not well understood. Constraining the survivability of organics and generation of habitable zones requires assessment of physical parameters such as penetration depth of UV photons. This type of information helps to identify to what extent rocks and minerals can provide effective shielding against UV radiation and is especially important on Mars, where the surface chemistry is more oxidizing, and the radiation environment is more extreme than on Earth. Using pressed pellets of natural gypsum, kaolinite, Mars simulant basalt, and welded tuff, we measured the spectral transmittance of each in the wavelength range of 220–400 nm. Although transmittance drops off quickly with depth, detectable levels of UV can penetrate >500 μm in each material. Each substrate allowed higher transmittance of UVC radiation than of longer wavelength UVA/B radiation, possibly as a result of surface reflectance and internal scattering properties. This could result in increased subsurface photolysis of organic compounds and biosignatures. We have used the transmittance data collected herein to constrain the lifetimes of several organic molecules in the Martian subsurface. These results will also have implications for organic analyses to be conducted by Mars 2020 and could be used to better constrain the SHERLOC/Mars 2020 interrogation volume. Plain Language Summary: Some types of radiation, including ultraviolet (UV) radiation, can be damaging to organisms and to organic molecules which could potentially be used as evidence of life. For this reason it is important to understand how rocks and minerals can provide protection against these types of radiation. In this study we prepared pellets of varying thickness made of different types of rocks and minerals relevant to Mars exploration and measured the amount of UV light that was able to pass through them. These data were used in combination with knowledge of the UV environment on Mars in order to determine what the UV radiation dosage would be under various thicknesses of the rocks and minerals we had analyzed. The percentage of UV radiation that was able to pass through the rock and mineral pellets was found to be higher than we expected, which suggests that this type of damaging radiation would be able to penetrate further into rock and mineral environments. This information was then used to determine approximately how long different types of organic molecules could be expected to survive if buried under different depths in rock and mineral materials. Key Points: The interaction of UV photons with rocks and minerals has implications for habitability as well as organic biosignature preservation and detectionTransmittance of UV radiation has been determined for four types of natural rocks and minerals that are relevant for Mars explorationUV photons can penetrate to depths greater than anticipated, resulting in a large effective radiation dosage over geological timescales [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]

This special collection, sponsored by National Aeronautics and Space Administration's Solar System Exploration Research Virtual Institute, includes contributions relevant to the science and exploration of Moon, near‐Earth asteroids, and the moons of Mars. Contributions appear in the Journal of Geophysical Research—Planets, Earth and Space Science, and GeoHealth. Major topics covered by the contributions include, but are not limited to, space weathering, geologic analysis of potential lunar landing sites, field analog investigations, and infrared spectroscopic measurements applied to airless bodies in the solar system. Plain Language Summary: National Aeronautics and Space Administration's Solar System Exploration Research Virtual Institute (SSERVI) promotes the science and exploration of the Moon, near‐Earth Asteroids, and the moons of Mars. This special collection includes contributions relevant to this mission by SSERVI team members and the broader science community. Key Points: SSERVI works to advance basic and applied research fundamental to lunar and planetary science and to advance human exploration of the solar system through scientific discoveryThis special collection includes contributions from both SSERVI team members and nonteam members related to science and exploration of the Moon, near‐Earth asteroids, and the moons of MarsContributions highlight how remote sensing, laboratory geologic analyses, field work, and medical studies contribute to SSERVI's science and exploration plans [ABSTRACT FROM AUTHOR]

A denser CO2 atmosphere and higher temperatures than present‐day conditions are frequently invoked as prevailing conditions for the formation of some ancient hydrous mineralogical associations present at the surface of Mars. The environmental conditions are of particular interest to better understand and constrain the weathering processes of the early Martian crust. For this purpose, 6‐month‐long batch weathering experiments on Martian crust simulants and individual Martian mineral analogs were performed at low temperature (45 °C) under a dense CO2 atmosphere (1 atm). Constraints on the weathering conditions are deduced from the solution properties and thermodynamic calculations, as well as mass balance calculations. Experimental solutions vary from mildly acidic to near neutral (4.75–6.48 pH). The Eh‐pH conditions (Eh from 0.189–0.416 V/standard hydrogen electrode) suggest favorable conditions for the formation of ferric minerals despite an anoxic CO2 atmosphere. The chemical weathering appears to be 4 times more intense for Martian simulants under a CO2 atmosphere than under Earth ambient air. The weathering trend under a CO2 atmosphere involves leaching of alkali and alkaline earth elements (Mg, Ca, Na, and K) and Si and enrichments of the solid phases in Al, Fe, and to a lesser extent Si compared to the initial chemical composition of the starting minerals. This geochemical partitioning between solution and solids resembles those deduced from weathering profiles on Earth. Our results strongly support the idea that carbonates could not have extensively formed at the surface of early Mars despite a dense CO2 atmosphere. Plain Language Summary: Mars orbital and landed missions have provided mineralogical, morphological, and field evidence for liquid water at the surface approximately 3.5 billion years ago. The chemical and mineralogical composition of the Martian rocks have potentially been modified by interaction with this liquid water. The purpose of our study is to use laboratory experiments to constrain the physicochemical conditions of water resulting from the chemical weathering of Martian crust simulants under an atmosphere composed of carbon dioxide, as is the case for Mars. The water in contact with simulants is mildly acidic. The partitioning of chemical elements between the solution and minerals is similar to what is observed on Earth, but weathering is more intense. Despite that Mars had a primitive CO2‐dense atmosphere, the conditions were not favorable to the extensive formation of carbonate at the surface. Key Points: Chemical weathering in mildly acidic conditions under a CO2 atmosphere yielded leaching of alkali and alkaline earth elementsMass balance calculations indicated Al, Fe, and Si enrichment in the weathering productsOur results imply unsuitable conditions for carbonate formation despite CO2 in the Martian atmosphere [ABSTRACT FROM AUTHOR]

Martian magmas were likely enriched in S and Cl with respect to H2O. Exsolution of a vapor phase from these magmas and ascent of the gas bubbles through the magma plumbing system would have given rise to shallow magmas that were gas‐charged. Release and cooling of this gas from lava flows during eruption may have resulted in the addition of a significant amount of vapor‐deposited phases to the fines of the surface. Experiments were conducted to simulate degassing of gas‐charged lava flows and shallow intrusions in order to determine the nature of vapor‐deposited phases that may form through this process. The results indicate that magmatic gas may have contributed a large amount of Fe, S, and Cl to the Martian surface through the deposition of iron oxides (magnetite, maghemite, and hematite), chlorides (molysite, halite, and sylvite), sulfur, and sulfides (pyrrhotite and pyrite). Primary magmatic vapor‐deposited minerals may react during cooling to form a variety of secondary products, including iron oxychloride (FeOCl), akaganéite (Fe3+O (OH,Cl)), and jarosite (KFe3+3(OH)6(SO4)2). Vapor‐deposition does not transport significant amounts of Ca, Al, or Mg from the magma and hence, this process does not directly deposit Ca‐ or Mg‐sulfates. Plain Language Summary: The surface of Mars is covered by dust, and this dust will be the most abundant material encountered by future manned missions to the planet's surface. Understanding the mineralogic makeup of this dust is vital to assessing its potential toxicity. This dust also records information on the most recent geological activity and atmospheric conditions on Mars. This work focuses on the potential contribution of micron‐sized particles formed by condensation of gas from young lava flows to the dust. We experimentally simulated a boiling magma and exposed the gas given off to the temperatures that you might see above a lava flow. The results indicate that some of the minerals found in the fine‐grained material of the Martian surface such as chlorides, sulfides, sulfur, and silica could be formed in this way. These precipitated minerals, in turn, can react with the cooling gas or the atmosphere to form a set of secondary minerals, such as maghemite and hematite, and very reactive substances such as iron oxychloride. Iron oxychloride can obliterate traces of the organic material that we have counted on to provide information about organics brought to Mars by meteorites and about potential past life on the planet. Key Points: Gas from Cl‐ and S‐rich, OH‐poor magma readily transports iron and alkalisMagmatic gas produces micron‐sized crystals of molysite, halite, sylvite, hematite, maghemite, silica, pyrrhotite, pyrite, and native sulfurThe surface deposits may react during cooling and hydration to produce secondary phases including iron oxychlorides and hematite [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]

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]

Plain Language Summary: Miller et al. [2015, https://doi.org/10.1002/2015JE004825] described the result of experiments testing the potential of Tenax TA, a polymer used on Sample Analysis at Mars (SAM), as a source of chlorinated benzene. Miller et al. [2015] conclude that the amount of chlorobenzene produced is low and that Tenax TA cannot be the source of the chlorobenzene observed on Mars by SAM. Miller et al. [2015] did not provide the identification of two unknown compounds produced during these pyrolysis experiments, though their abundance is orders of magnitude higher than that of chlorobenzene. Here, we tentatively identify these compounds based on the mass spectra provided by Miller et al. [2015], the most abundant of which is a chlorinated monomer of Tenax TA. This chlorinated monomer is likely to accumulate in the hydrocarbon Tenax trap and in the transfer line between the trap and the mass spectrometer. Further breakdown of these compounds could lead the high background of chlorobenzene observed on Mars. Key Points: Unknown compounds of Miller et al. [2015] are tentatively identified as rearranged monomers and chlorinated monomers of the Tenax TAThe chlorinated monomer is two orders of magnitude more abundant than chlorobenzene produced in the same experimentsChlorobenzene moieties of chlorinated Tenax TA monomers could provide high backgrounds during analysis of Mars samples by SAM's GC‐MS [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]