Pyrolysis of Oxalate, Acetate, and Perchlorate Mixtures and the Implications for Organic Salts on Mars

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 CO₂ 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 CO₂ and CO, while pure acetates evolved CO₂ and a diverse range of organic products dominated by acetone and acetic acid. Dispersal within silica caused minor peak shifting, decreased the amounts of CO₂ 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 CO₂ peaks to lower temperatures, whereas with Ca and Mg oxalate, a weaker CO release was observed but the initial CO₂ evolutions were largely unchanged. The perchlorates induced a stronger CO₂ release from acetates at the expense of other products. Oxalates evolved ∼47% more CO₂ and acetates yielded ∼69% more CO₂ when the perchlorates were abundant. The most compelling fits between our organic salt data and SAM CO₂ and CO data included Martian samples acquired from modern eolian deposits and sedimentary rocks with evidence for low temperature alteration.