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The Sample Analysis at Mars-Evolved Gas Analyzer (SAM-EGA) on the Curiosity rover detected hydrogen chloride (HCl) and sulfur dioxide (SO2) gas evolutions above 600 °C and 700 °C, respectively, from several drilled rock and soil samples collected in Gale crater, which have been attributed to NaCl and Mg sulfates. Although NaCl and Mg sulfates do not evolve HCl or SO2 within the SAM temperature range (<~870 °C) when analyzed individually, they may evolve these gases at <870 °C and become detectable by SAM-EGA when mixed. This work aims to determine how Mg sulfate and NaCl interact during heating and how that affects evolved HCl and SO2 detection temperatures in SAM-EGA. Solid mixtures of NaCl and kieserite were analyzed using a thermogravimeter/differential scanning calorimeter furnace connected to a quadrupole mass spectrometer, configured to operate under similar conditions as SAM, and using X-ray diffraction of heated powders. NaCl analyzed individually did not evolve HCl; however, NaCl/kieserite mixtures evolved HCl releases with peaks above 600 °C. The results suggested that kieserite influenced HCl production from NaCl via two mechanisms: (1) kieserite depressed the melting point of NaCl, making it more reactive with evolved water; and (2) SO2 from kieserite decomposition reacted with NaCl and water (i.e., Hargreaves reaction). Additionally, NaCl catalyzed the thermal decomposition of kieserite, such that the evolved SO2 was within the SAM-EGA temperature range. The results demonstrated that SAM-EGA can detect chlorides and Mg sulfates when mixed due to interactions during heating. These phases can provide information on past climate and mineral formation conditions. [ABSTRACT FROM AUTHOR]

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

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]

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 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]

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]

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]

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]

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

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

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

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]