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 H 2 O data suggested the presence of an Fe-rich dioctahedral smectite, such as nontronite, in the sample from beneath the ridge. H 2 O data are also consistent with ferripyrophyllite in VRR samples. SAM SO 2 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 O 2 and NO evolved from one sample indicated the presence of oxychlorine and nitrate/nitrite salts, respectively. The O 2 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 CO 2 and CO consistent with oxidized carbon compounds (e.g., oxalates), while some CO 2 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.

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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.

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The Mars Curiosity rover carries a diverse instrument payload to characterize habitable environments in the sedimentary layers of Aeolis Mons. One of these instruments is Sample Analysis at Mars (SAM), which contains a mass spectrometer that is capable of detecting organic compounds via pyrolysis gas chromatography mass spectrometry (py-GC-MS). To identify polar organic molecules, the SAM instrument carries the thermochemolysis reagent tetramethylammonium hydroxide (TMAH) in methanol (hereafter referred to as TMAH). TMAH can liberate fatty acids bound in macromolecules or chemically bound monomers associated with mineral phases and make these organics detectable via gas chromatography mass spectrometry (GC-MS) by methylation. Fatty acids, a type of carboxylic acid that contains a carboxyl functional group, are of particular interest given their presence in both biotic and abiotic materials. This work represents the first analyses of a suite of Mars-analog samples using the TMAH experiment under select SAM-like conditions. Samples analyzed include iron oxyhydroxides and iron oxyhydroxysulfates, a mixture of iron oxides/oxyhydroxides and clays, iron sulfide, siliceous sinter, carbonates, and shale. The TMAH experiments produced detectable signals under SAM-like pyrolysis conditions when organics were present either at high concentrations or in geologically modern systems. Although only a few analog samples exhibited a high abundance and variety of fatty acid methyl esters (FAMEs), FAMEs were detected in the majority of analog samples tested. When utilized, the TMAH thermochemolysis experiment on SAM could be an opportunity to detect organic molecules bound in macromolecules on Mars. The detection of a FAME profile is of great astrobiological interest, as it could provide information regarding the source of martian organic material detected by SAM.

Organic salts, such as Fe, Ca, and Mg oxalates and acetates, may be widespread radiolysis and oxidation products of organic matter in Martian surface sediments. Such organic salts are challenging to identify by evolved gas analysis but the ubiquitous CO 2 and CO in pyrolysis data from the Sample Analysis at Mars (SAM) instrument suite on the Curiosity rover indirectly points to their presence. Here, we examined laboratory results from SAM-like analyses of organic salts as pure phases, as trace phases mixed with silica, and in mixtures with Ca and Mg perchlorates. Pure oxalates evolved CO 2 and CO, while pure acetates evolved CO 2 and a diverse range of organic products dominated by acetone and acetic acid. Dispersal within silica caused minor peak shifting, decreased the amounts of CO 2 evolved by the acetate standards, and altered the relative abundances of the organic products of acetate pyrolysis. The perchlorate salts scrubbed Fe oxalate CO releases and shifted the CO 2 peaks to lower temperatures, whereas with Ca and Mg oxalate, a weaker CO release was observed but the initial CO 2 evolutions were largely unchanged. The perchlorates induced a stronger CO 2 release from acetates at the expense of other products. Oxalates evolved ∼47% more CO 2 and acetates yielded ∼69% more CO 2 when the perchlorates were abundant. The most compelling fits between our organic salt data and SAM CO 2 and CO data included Martian samples acquired from modern eolian deposits and sedimentary rocks with evidence for low-temperature alteration. Plain Language SummaryIn our efforts to characterize indigenous Martian organic matter, we must contend with a near-surface record that has been substantially altered by radiation and oxidation. Under such conditions, much of the surficial organic record on Mars may have decomposed into organic salts, which are challenging for flight instruments to conclusively identify. If organic salts are widespread on the Martian surface, their composition and distribution could offer insight into the less-altered organic record at depth and they may play an important role in near-surface carbon cycling and habitability. The organic detection techniques employed by the Mars Science Laboratory Curiosity rover include thermal extraction in combination with mass spectrometry. In this work, we used laboratory thermal extraction techniques analogous to those of the rover to examine organic salts as pure standards, as minor phases in a silica matrix, and in mixtures with O 2 -evolving perchlorate salts. When we compared our results with flight data, we found that many of the CO 2 profiles produced by our organic salt samples were similar to the CO 2 evolutions observed by the rover. The best fits with our laboratory data included Martian materials acquired from modern eolian deposits and sedimentary rocks that had evidence for lowtemperature alteration.LEWIS ET AL.

Understanding the potential for biosignature preservation and detection in the context of Mars mineralogy is critical for targeting the best sites for Mars sample return. Sedimentary strata on Mars often contain a mix of sulfates, iron oxides, chlorides, and phyllosilicates, a mineral assemblage unique on Earth to acid brine environments. To characterize their astrobiological potential, sediments from four acidic salt lakes and adjacent mudflats/sandflats in the vicinity of Norseman, Western Australia were collected and analyzed. Lipid biomarkers were extracted and quantified, revealing biomarkers from vascular plants alongside trace microbial lipids. The resilience of lipids from dead organic material in acid saline sediments through the early stages of pervasive diagenesis lends support to the idea that iron oxides and sulfates, in tandem with phyllosilicates,

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