Artificial Maturation of Iron- and Sulfur-Rich Mars Analogues: Implications for the Diagenetic Stability of Biopolymers and Their Detection with Pyrolysis–Gas Chromatography–Mass Spectrometry

Abstract

Acidic iron-and sulfur-rich streams are appropriate analogues for the late Noachian and early Hesperian periods of martian history, when Mars exhibited extensive habitable environments. Any past life on Mars may have left behind diagnostic evidence of life that could be detected at the present day. For effective preservation, these remains must have avoided the harsh radiation flux at the martian surface, survived geological storage for billions of years, and remained detectable within their geochemical environment by analytical instrument suites used on Mars today, such as thermal extraction techniques. We investigated the detectability of organic matter within sulfur stream sediments that had been subjected to artificial maturation by hydrous pyrolysis. After maturation, the samples were analyzed by pyrolysis-gas chromatography-mass spectrometry (py-GC-MS) to determine whether organic matter could be detected with this commonly used technique. We find that macromolecular organic matter can survive the artificial maturation process in the presence of iron-and sulfur-rich minerals but cannot be unambiguously distinguished from abiotic organic matter. However, if jarosite and goethite are present in the sulfur stream environment, they interfere with the py-GC-MS detection of organic compounds in these samples. Clay reduces the obfuscating effect of the oxidizing minerals by providing nondeleterious adsorption sites. We also find that after a simple alkali and acid leaching process that removes oxidizing minerals such as iron sulfates, oxides, and oxyhydroxides, the sulfur stream samples exhibit much greater organic responses during py-GC-MS in terms of both abundance and diversity of organic compounds, such as the detection of hopanes in all leached samples. Our results suggest that insoluble organic matter can be preserved over billions of years of geological storage while still retaining diagnostic organic information, but sample selection strategies must either avoid jarositeand goethite-rich outcrops or conduct preparative chemistry steps to remove these oxidants prior to analysis by thermal extraction techniques.

Transformation of Cyanobacterial Biomolecules by Iron Oxides During Flash Pyrolysis: Implications for Mars Life-Detection Missions

“…The moieties most effected by the presence of the iron oxide(oxy)hydroxide substrate are the fatty acids, as these go from being from some of the most relatively abundant pyrolysis products to complete or almost complete loss as A. platensis content is decreased. Previous work by this group (Royle et al, 2021;Tan et al, 2021) has demonstrated that the presence of the same iron oxide/(oxy)hydroxide mineral substrates used here greatly promoted the cyclization, aromatization and oxidative polymerization of fatty acids. This suggests that the main alteration products detected from A. platensis pyrolysed on the iron oxide/(oxy)hydroxide substrates originate from fatty acid parent molecules.…”

“…With the lack of recent plate tectonics (Sleep, 1994), and a lower thermal gradient on Mars (Borlina et al, 2015), preservation in buried (and thereby shielded) sediments could be greatly enhanced (Royle et al, 2018b;Tan et al, 2021), therefore the detectability of biomarkers in the presence of Mars-relevant minerals is of particular interest. 25 However, the current study has ascertained that a larger microbial population/biomass is necessary in iron oxide/(oxy)hydroxide-rich sediments to be able to ascertain a biological source due to the iron-promoted alteration/transformation of biomolecules.…”

“…As well as the oxidizing perchlorate and sulphate salts, the crust of Mars has abundant iron (Taylor and McLennan, 2009) and its oxides are ubiquitous on the martian surface. Iron oxides promote pyrolytic reactions in industrial processes (Minicò et al, 2000;Gooßen et al, 2011;Bukur et al, 2016) and previous work has shown that they also can have transformatory effects on the pyrolytic products of Mars analogue samples (Royle et al, 2021;Tan et al, 2021). The nanophase, hydrated iron oxide ferrihydrite (5Fe2O3•9H2O) is believed to be widespread on the martian surface as a major component of the iron-rich amorphous soil component (Dehouck et al, 2017), along with the more crystalline oxyhydroxide goethite (FeO(OH)) and the iron (II, III) and iron (III) oxides magnetite (Fe3O4) and haematite (Fe2O3).…”

“…Previously, we have shown that sediments rich in iron oxides have obfuscating effects on the detection of organic molecular pyrolysis products in both natural Mars analogue samples 7 (Tan et al, 2021) and on fatty acid standards (Royle et al, 2021). It is necessary to carry out analogue experiments to determine the effect of Mars-relevant minerals on the thermal decomposition of Mars-relevant organisms as this will inform what a successful detection of life, by similar techniques, in situ at Mars could look like.…”

Experimental investigations of the preservation/degradation of microbial signatures in the presence of clay minerals under Martian subsurface conditions

“…A few experimental studies have dealt with the preservation/degradation of organic-clay mixtures under simulated hydrothermal Martian conditions. Yet, these hydrous pyrolysis experiments have been conducted either using single molecules, such as RNA (Viennet et al, 2019) or glycine (Dalai et al, 2017;Gil-Lozano et al, 2020), or using complex natural samples (Tan et al, 2021), rather than using model microorganisms experimentally mixed within a pure synthetic mineral matrix, as previously done by some authors to document the effect of diagenesis under terrestrial conditions (Oehler and Schopf, 1971;Li et al, 2014;Picard et al, 2015;Alleon et al, 2016;Miot et al, 2017;Igisu et al, 2018). Here, we report results of laboratory experiments conducted using cells of a model microorganism (i.e., Escherichia coli cells) and pure saponite (a Mg-rich trioctahedral smectite) synthesized in the lab.…”