Evolved Gas Analysis of Oxalates, Acetates, and Perchlorates: Examining Pyrolysis Data from Mars for Evidence of Organic Salts

Missions sent to Mars with the goal of identifying organic matter have relied on evolved gas analysis (EGA) for organic molecular detections. A ubiquitous product of these experiments is carbon dioxide. Potential sources include atmospherically adsorbed CO₂, carbonates, instrument background, and organic matter. Organic molecules can release CO₂ through processes such as decarboxylation or via combustion, if phases that thermally decompose to release oxygen are present in a sample. Such phases include oxychlorine salts, which have been found to be widespread across the martian surface. Thus, understanding how sample chemistry influences CO₂ release during heating is essential for constraining martian carbon sources, including organic molecules.

Mars is bombarded by ionizing radiation that cleaves water molecules and generates oxidising species in martian rocks and sediments. In the presence of these powerful oxidants, the surficial organic record of Mars is expected to include metastable organic salts, such as oxalates and acetates. Oxalates are challenging to unambiguously identify with EGA as the only products are CO₂ and CO. EGA of acetate yields CO₂, CO, acetone, acetic acid, and acetic anhydride, but the latter three products have not yet been detected in Mars data. A range of synthetic oxalate, acetate, and perchlorate standards were studied using a commercial laboratory setup analogous to the EGA experiments carried out by the Sample Analysis at Mars (SAM) instrument suite on board Curiosity rover. Evolved gas analyses were conducted on organic salts on their own and in mixtures with different perchlorate species. Lab EGA release temperatures for CO₂ and CO released from organic salts mixed with perchlorates most closely match SAM-EGA release temperatures of martian samples. CO₂ and CO release temperatures for isolated organic salts did not match SAM-EGA data. On Mars, metastable organic salts in the presence of oxychlorine salts could be responsible for many of the CO₂ peaks observed by SAM-EGA.