Photocatalytic Decomposition of Carboxylated Molecules on Light-Exposed Martian Regolith and Its Relation to Methane Production on Mars

Abstract

We propose that the paucity of organic compounds in martian soil can be accounted for by efficient photocatalytic decomposition of carboxylated molecules due to the occurrence of the photo-Kolbe reaction at the surface of particulate iron(III) oxides that are abundant in the martian regolith. This photoreaction is initiated by the absorption of UVA light, and it readily occurs even at low temperature. The decarboxylation is observed for miscellaneous organic carboxylates, including the nonvolatile products of kerogen oxidation (that are currently thought to accumulate in the soil) as well as alpha-amino acids and peptides. Our study indicates that there may be no "safe haven" for these organic compounds on Mars; oxidation by reactive radicals, such as hydroxyl, is concerted with photocatalytic reactions on the oxide particles. Acting together, these two mechanisms result in mineralization of the organic component. The photooxidation of acetate (the terminal product of radical oxidation of the aliphatic component of kerogen) on the iron(III) oxides results in the formation of methane; this reaction may account for seasonably variable production of methane on Mars. The concomitant reduction of Fe(III) in the regolith leads to the formation of highly soluble ferrous ions that contribute to weathering of the soil particles.

The PROCESS Experiment: Amino and Carboxylic Acids Under Mars-Like Surface UV Radiation Conditions in Low-Earth Orbit

“…Laboratory studies have shown that the evolution of mellitic acid produces a radiotolerant organic compound identified as benzenehexacarboxylic acid-trianhydride (C 12 O 9 ) (Stalport et al, 2009). To study a possible photocatalytic interaction of phthalic acid with martian regolith (Shkrob et al, 2010), this acid was also exposed together with JSC Mars-1, a mineralogical analogue of martian soil that is composed of volcanic ashes collected in Hawaii and exhibits similarities to some bright areas of Mars (Allen et al, 1998).…”

“…Numerical models predict that the surface of Mars is exposed to an energetic UV flux in the 190-400 nm range (Kuhn and Atreya, 1979;Cockell et al, 2000;Patel et al, 2002). Laboratory experiments have been developed to mimic such radiation conditions and evaluate their impact on organic materials likely to be present at the martian surface (Stoker and Bullock, 1997;Gontareva, 2005;Ten Kate et al, 2005Shkrob et al, 2010). Those studies highlighted the degradation of most of the organic molecules.…”

A Microbial Oasis in the Hypersaline Atacama Subsurface Discovered by a Life Detector Chip: Implications for the Search for Life on Mars

“…T he space science community agrees on the need to explore the martian subsurface for evidence of intact organic molecules (Kminek and Bada, 2006;Shkrob et al, 2010). In fact, ESA's ExoMars mission aims to search for life or its remains by analyzing samples taken from a drill hole of at least 2 m in depth (http://www.esa.int/SPECIALS/ExoMars/ SEM10VLPQ5F_0.html ).…”

UV production of methane from surface and sedimenting IDPs on Mars in light of REMS data and with insights for TGO

“…Similarly, if the branching ratio, factor (3), is on the order < 7% instead of 20%, there is no disagreement between the model and observations. Recent work by Wadsworth and Cockell (2017), building on previous work by Shkrob et al (2010), has argued that UVactivation of perchlorates in martian dust (Quinn et al, 2013) could result in enhanced destruction of microorganisms. While it is unclear whether such a process would yield methane, or if the process would degrade the organic carbon compounds found in IDPs, it is an example of a potential secondary process that could reduce the branching ratio to methane of available organic carbon at the surface of Mars.…”