The Potential for Oxalates in the Glen Torridon Region within Gale Crater, Mars

Organic salts (e.g., oxalates and acetates) may be widespread products of the radiolytic and oxidative decomposition of organic matter on the Martian surface and could play an important role in the present-day Martian carbon cycle. The Sample Analysis at Mars (SAM) instrument suite on board the Mars Science Laboratory (MSL) Curiosity rover has investigated the volatile organic and inorganic chemistry of over 20 Martian samples by evolved gas analysis (EGA). Many CO₂ and CO peaks observed in situ are consistent with the peaks evolved by oxalate, acetate, and perchlorate mixtures during SAM-like EGA in the laboratory. However, recent SAM EGA data from the Glen Torridon (GT) region lack O₂ peaks indicative of perchlorates and chlorates. In the absence of such oxychlorines, oxalates would be expected to evolve prominent CO peaks with their CO₂ releases, and acetates would produce acetone and acetic acid peaks.

Remote spectral signatures and X-ray diffraction (XRD) data from MSL's Chemistry and Mineralogy (CheMin) instrument show that the GT sediments are largely clay-mineral rich, which is promising for SAM experiments seeking Martian organic matter, but the GT sediments were also subjected to multiple episodes of diagenesis. MSL acquired several GT drill samples, including Glen Etive (GE) from the Knockfarril Hill Member, Hutton (HU), which was acquired near the unconformity between the Fractured Intermediate Unit (FIU) and the overlying Greenheugh pediment, and Glasgow (GG) from the FIU. SAM analysis of GG revealed a CO₂ peak accompanied by a strong CO peak. These peaks were similar to those produced by Fe(II) oxalate in the absence of oxychlorine phases during laboratory EGA. The HU and GE CO₂ and CO profiles show a reasonable fit with the evolutions from 10:1 Fe(II)-oxalate-Ca-perchlorate mixtures in the laboratory. At this mixing ratio, oxalate CO peaks weakened and perchlorate O₂ was completely consumed by oxidation reactions. No conclusive peaks for acetone or acetic acid were seen in GE, HU, and GG, but it is possible that acetate pyrolysis products were oxidized by interactions with other gases evolved from the complex GT samples (e.g., H₂O). CheMin data for the GT samples are currently being assessed for Fe(II) oxalate. It is hoped that the hints of oxalates in SAM data could lead to a conclusive identification by CheMin.