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H2O, CO2, SO2, O2, H2, H2S, HCl, chlorinated hydrocarbons, NO, and other trace gases were evolved during pyrolysis of two mudstone samples acquired by the Curiosity rover at Yellowknife Bay within Gale crater, Mars. H2O/OH-bearing phases included 2:1 phyllosilicate(s), bassanite, akaganeite, and amorphous materials. Thermal decomposition of carbonates and combustion of organic materials are candidate sources for the CO2. Concurrent evolution of O2 and chlorinated hydrocarbons suggests the presence of oxychlorine phase(s). Sulfides are likely sources for sulfur-bearing species. Higher abundances of chlorinated hydrocarbons in the mudstone compared with Rocknest windblown materials previously analyzed by Curiosity suggest that indigenous martian or meteoritic organic carbon sources may be preserved in the mudstone; however, the carbon source for the chlorinated hydrocarbons is not definitively of martian origin.

Results from the Sample Analysis at Mars‐evolved gas analyzer on board the Mars Science Laboratory Curiosity rover constrained the alteration history and habitability potential of rocks sampled across the Siccar Point unconformity in Gale crater. The Glasgow member (Gm) mudstone just below the unconformity had evidence of acid sulfate or Si‐poor brine alteration of Fe‐smectite to Fe amorphous phases, leaching loss of Fe‐Mg‐sulfate and exchange of unfractionated sulfur 34S (δ34S = 2‰ ± 7‰) with enriched 34S (20‰ ± 5‰, Vienna Cañon Diablo Troilite). Carbon abundances did not significantly change (322–661 μgC/g) consistent with carbon stabilization by amorphous Al‐ and Fe‐hydroxide phases. The Gm mudstone had no detectable oxychlorine and extremely low nitrate. Nitrate (0.06 wt.% NO3), oxychlorine (0.13 wt.% ClO4), high C (1,472 μg C/g), and low Fe/Mg‐sulfate concentration (0.24 wt.% SO3) depleted in 34S (δ34S = −27‰ ± 7), were detected in the Stimson formation (Sf) eolian sandstone above the unconformity. Redox disequilibrium through the detections of iron sulfide and sulfate supported limited aqueous processes in the Sf sandstone. Si‐poor brines or acidic fluids altered the Gm mudstone just below the unconformity but did not alter underlying Gm mudstones further from the contact. Chemical differences between the Sf and Gm rocks suggested that fluid interaction was minimal between the Sf and Gm rocks. These results suggested that the Gm rocks were altered by subsurface fluids after the Sf placement. Aqueous processes along the unconformity could have provided habitable conditions and in some cases, C and N levels could have supported heterotrophic microbial populations. Plain Language Summary: The Curiosity Rover investigated the chemistry and mineralogy of rocks in the Glen Torridon region of Gale crater, Mars. Rocks sampled across an unconformity (representative of a time gap between when two sedimentary rocks were deposited) gave insight into how they were altered over time. The Glasgow mudstones, below the unconformity, were formed in a lake environment while the younger Stimson sandstones above the unconformity were formed from ancient sand dunes. Using the Sample Analysis at Mars Evolved Gas Analysis (SAM‐EGA) experiment, solid powdered samples from these rocks were heated and released gases (e.g., water, carbon dioxide, and sulfur dioxide) were tracked. These gases revealed a complex history of alteration through the rocks. The Glasgow mudstones were characterized by acidic or Si‐poor brine alteration, variable sulfur, low amounts of carbon, and no nitrate or oxychlorine. SAM‐EGA results from the Stimson sandstone showed rocks that were less altered, had higher levels of carbon, different sulfur compounds and sources, and some nitrate/oxychlorine. Together, these results suggested that the Glasgow rocks were altered by subsurface fluids after Stimson sandstone placement. Aqueous processes along the unconformity could have provided habitable conditions and in some cases, C and N levels could have supported heterotrophic microbial populations. Key Points: Subsurface silica‐poor brines or acidic fluids altered mudstones just below the Siccar Point group unconformityStimson formation sandstone above the unconformity was exposed to limited aqueous alterationAqueous alteration processes along the unconformity could have provided habitable conditions and in some cases, sufficient microbial C and N [ABSTRACT FROM AUTHOR]

Evolved gas analysis (EGA) data from the Sample Analysis at Mars (SAM) instrument suite indicated Fe‐rich smectite, carbonate, oxidized organics, Fe/Mg sulfate, and chloride in sedimentary rocks from the Glen Torridon (GT) region of Gale crater that displayed phyllosilicate spectral signatures from orbit. SAM evolved H2O data indicated that the primary phyllosilicate in all GT samples was an Fe‐rich dioctahedral smectite (e.g., nontronite) with lesser amounts of a phyllosilicate such as mixed layer talc‐serpentine or greenalite‐minnesotaite. CO2 data supported the identification of siderite in several samples, and CO2 and CO data was also consistent with trace oxidized organic compounds such as oxalate salts. SO2 data indicated trace and/or amorphous Fe sulfates in all samples and one sample may contain Fe sulfides. SO2 data points to significant Mg sulfates in two samples, and lesser amounts in several other samples. A lack of evolved O2 indicated the absence of oxychlorine salts and Mn3+/Mn4+ oxides. The lack of, or very minor, evolved NO revealed absent or very trace nitrate/nitrite salts. HCl data suggested chloride salts in GT samples. Constraints from EGA data on mineralogy and chemistry indicated that the environmental history of GT involved alteration with fluids of variable redox potential, chemistry and pH under a range of fluid‐to‐rock ratio conditions. Several of the fluid episodes could have provided habitable environmental conditions and carbon would have been available to any past microbes though the lack of significant N could have been a limiting factor for microbial habitability in the GT region. Plain Language Summary: The Mars Science Laboratory Curiosity rover carried out a comprehensive investigation of the sedimentary rocks in the Glen Torridon (GT) region in Gale crater, Mars, to better understand Martian geologic history. The rover's Sample Analysis at Mars instrument suite was used to heat samples from GT and detect gases that were evolved to help identify the volatile‐bearing mineralogy of the samples. Results indicated the presence of Fe‐rich smectite, carbonate, oxidized organics, Fe and Mg sulfate, and chloride while oxychlorine salts, Mn3+/Mn4+ oxides, and nitrate/nitrite salts were either not detected or occurred in trace amounts. This mineralogy data implied that geochemical conditions and fluid‐to‐rock ratios varied over time and that the conditions of GT depositional and diagenetic environments would have been amenable to microbial life with sufficient carbon but nitrogen abundances could have been limiting. Key Points: Sample Analysis at Mars data indicated Fe‐smectite, carbonate, oxidized organics, Fe/Mg sulfates, and chlorides in Glen Torridon (GT) rocksGT's environmental history involved fluids of variable redox potential and chemistry under a range of fluid‐to‐rock ratiosFluid episodes could have provided habitable environments and carbon was present, but nitrogen could have been a limiting factor [ABSTRACT FROM AUTHOR]

Abstract: The sample analysis at Mars instrument evolved gas analyzer (SAM‐EGA) has detected evolved water, H2, SO2, H2S, NO, CO2, CO, O2, and HCl from two eolian sediments and nine sedimentary rocks from Gale Crater, Mars. These evolved gas detections indicate nitrates, organics, oxychlorine phase, and sulfates are widespread with phyllosilicates and carbonates occurring in select Gale Crater materials. Coevolved CO2 (160 ± 248–2373 ± 820 μgC(CO2)/g) and CO (11 ± 3–320 ± 130 μgC(CO)/g) suggest that organic C is present in Gale Crater materials. Five samples evolved CO2 at temperatures consistent with carbonate (0.32 ± 0.05–0.70 ± 0.1 wt % CO3). Evolved NO amounts to 0.002 ± 0.007–0.06 ± 0.03 wt % NO3. Evolution of O2 suggests that oxychlorine phases (chlorate/perchlorate) (0.05 ± 0.025–1.05 ± 0.44 wt % ClO4) are present, while SO2 evolution indicates the presence of crystalline and/or poorly crystalline Fe and Mg sulfate and possibly sulfide. Evolved H2O (0.9 ± 0.3–2.5 ± 1.6 wt % H2O) is consistent with the presence of adsorbed water, hydrated salts, interlayer/structural water from phyllosilicates, and possible inclusion water in mineral/amorphous phases. Evolved H2 and H2S suggest that reduced phases occur despite the presence of oxidized phases (nitrate, oxychlorine, sulfate, and carbonate). SAM results coupled with CheMin mineralogical and Alpha‐Particle X‐ray Spectrometer elemental analyses indicate that Gale Crater sedimentary rocks have experienced a complex authigenetic/diagenetic history involving fluids with varying pH, redox, and salt composition. The inferred geochemical conditions were favorable for microbial habitability and if life ever existed, there was likely sufficient organic C to support a small microbial population. [ABSTRACT FROM AUTHOR]