Detection of Reduced Sulfur on Vera Rubin Ridge by Quadratic Discriminant Analysis of Volatiles Observed During Evolved Gas Analysis

Abstract

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.

Constraints on the Mineralogy and Geochemistry of Vera Rubin Ridge, Gale Crater, Mars, From Mars Science Laboratory Sample Analysis at Mars Evolved Gas Analyses

“…Laboratory pyrolysis involving both Fe and Mg sulfates has demonstrated evolution of low‐temperature SO 2 when organics were adsorbed to sulfates, contained within sulfates, or were physically mixed with sulfates (Francois et al, 2016; Lewis et al, 2019). If organics (S organics and/or organics associated with sulfates) were involved in the low‐temperature SO 2 evolution in RH and HF, these organics could contribute C to the OCS and CS 2 observed at similar temperatures (Wong et al, 2020).…”

“…Although further analyses of CheMin data from CB demonstrated that sulfide was below the detection limit of CheMin in this sample (Morrison et al, 2018), sulfur isotope values were consistent with reduced sulfur in CB (Franz et al, 2017) suggesting that the presence of sulfides in CB can still be considered at least in the subsample of the CB sample analyzed by SAM. Detection of likely OCS and CS 2 evolutions at low temperatures during analyses of both HF and RH may also suggest the presence of sulfide in RH and HF (Wong et al, 2020, and references therein).…”

“…SAM HF EGA data showed no indication of trace or amorphous Fe sulfates and lower abundances of (amorphous/trace) Mg sulfate, as well as lower evolution of HCl, which could imply smaller amounts of chlorides. Lower amounts of these soluble salts are consistent with HF being more leached of salts compared to ST. Low‐temperature SO 2 evolution, and trends in OCS and CS 2 evolution (Wong et al, 2020), suggests that HF possesses tracereduced sulfur. Evolved CO 2 and CO were consistent with the presence of oxidized carbon phases and possible lesser carbonate.…”

“…Chlorides are likely present in all samples, even those that contain other Cl phases (akaganeite and oxychlorine salts). Besides the small amounts of OCS and CS 2 (Wong et al, 2020), no sample‐derived reduced carbon compounds were clearly detected during EGA. It should be noted, however, that small amounts of reduced carbon compounds are best detected using SAM's pyrolysis GC‐MS mode and this mode was not utilized on or near VRR.…”

“…Alternatively, formation of this gray hematite by ripening of red hematite at lower temperatures (Langmuir, 1971;Wang et al, 2015) over long periods of aqueous interaction could have occurred. Also, Wong et al (2020) hypothesize that interaction between sulfite-bearing groundwater (Halevy et al, 2007;Halevy & Schrag, 2009) and hematite could have resulted in precipitation of magnetite that was later reoxidized by materials such as oxychlorine phases (Brundrett et al, 2019;Mitra & Catalano, 2019) and nitrate/nitrite (e.g., Dhakal et al, 2013) in fluids similar to those that precipitated oxychlorine and nitrate/nitrite salts at Rock Hall. This process does not require warm fluids, and the hematite formed would be martite, a pseudomorph of magnetite that is indistinguishable from gray hematite in XRD analysis.…”