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

Abstract

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.

Organic carbon concentrations in 3.5-billion-year-old lacustrine mudstones of Mars

“…SAM EGA pyrolysis experiments of 13 samples demonstrated that CO 2 evolved below 550 °C had a broad range of δ 13 C (−25 ± 20‰ to +56 ± 11‰) representing multiple carbon sources ( 22 ). In addition to the previously discussed instrument carbon source, carbon could come from oxidation or decarboxylation of Martian organics or exogenous carbon ( 17 , 22 – 28 ). Recent work ( 28 ) showed that SAM-like pyrolysis of metastable organic salts such as acetates and oxalates produce CO 2 and CO below 550 °C, putting upper limits for abundances of these compounds in SAM pyrolysis experiments as high as 2 to 3 wt%.…”

“…In addition to the previously discussed instrument carbon source, carbon could come from oxidation or decarboxylation of Martian organics or exogenous carbon ( 17 , 22 – 28 ). Recent work ( 28 ) showed that SAM-like pyrolysis of metastable organic salts such as acetates and oxalates produce CO 2 and CO below 550 °C, putting upper limits for abundances of these compounds in SAM pyrolysis experiments as high as 2 to 3 wt%. These molecules may be present as radiolytic decomposition products of more complex organics ( 29 , 30 ) or could represent simple molecules formed in situ by photolysis or electrochemistry ( 4 , 22 ).…”

Mission Overview and Scientific Contributions from the Mars Science Laboratory Curiosity Rover After Eight Years of Surface Operations

“… 2017 ; Lewis et al. 2021 ). These indirect measurements of organic molecules and organic salts, along with the direct detections and inferences from laboratory work, suggest that hundreds or even thousands of parts per million (by weight) of organic molecules may have been present in the sediments within Gale crater (Sutter et al.…”

Oxidized and Reduced Sulfur Observed by the Sample Analysis at Mars (SAM) Instrument Suite on the Curiosity Rover Within the Glen Torridon Region at Gale Crater, Mars

“…The phyllosilicate nontronite was chosen because evidence of similar phyllosilicates has been observed in several samples (e.g., Bristow et al, 2015Bristow et al, , 2018McAdam et al, 2022;Thorpe et al, 2022). The carbon-bearing species were chosen to represent various oxidation states of carbon that could potentially be found on Mars and affect EGA results, including contamination within SAM (Eigenbrode et al, 2018;Freissinet et al, 2019;Glavin et al, 2013;Lewis et al, 2021).…”