Recovery of Fatty Acids from Mineralogic Mars Analogs by TMAH Thermochemolysis for the Sample Analysis at Mars Wet Chemistry Experiment on the Curiosity Rover

Abstract

The Mars Curiosity rover carries a diverse instrument payload to characterize habitable environments in the sedimentary layers of Aeolis Mons. One of these instruments is Sample Analysis at Mars (SAM), which contains a mass spectrometer that is capable of detecting organic compounds via pyrolysis gas chromatography mass spectrometry (py-GC-MS). To identify polar organic molecules, the SAM instrument carries the thermochemolysis reagent tetramethylammonium hydroxide (TMAH) in methanol (hereafter referred to as TMAH). TMAH can liberate fatty acids bound in macromolecules or chemically bound monomers associated with mineral phases and make these organics detectable via gas chromatography mass spectrometry (GC-MS) by methylation. Fatty acids, a type of carboxylic acid that contains a carboxyl functional group, are of particular interest given their presence in both biotic and abiotic materials. This work represents the first analyses of a suite of Mars-analog samples using the TMAH experiment under select SAM-like conditions. Samples analyzed include iron oxyhydroxides and iron oxyhydroxysulfates, a mixture of iron oxides/oxyhydroxides and clays, iron sulfide, siliceous sinter, carbonates, and shale. The TMAH experiments produced detectable signals under SAM-like pyrolysis conditions when organics were present either at high concentrations or in geologically modern systems. Although only a few analog samples exhibited a high abundance and variety of fatty acid methyl esters (FAMEs), FAMEs were detected in the majority of analog samples tested. When utilized, the TMAH thermochemolysis experiment on SAM could be an opportunity to detect organic molecules bound in macromolecules on Mars. The detection of a FAME profile is of great astrobiological interest, as it could provide information regarding the source of martian organic material detected by SAM.

Inhabited subsurface wet smectites in the hyperarid core of the Atacama Desert as an analog for the search for life on Mars

“…For the GC-MS technique used in this study, samples require specific pre-treatment after organic solvent extraction and prior to analysis, such as separation of the total lipid extract into different polarity fractions (e.g., acidic, apolar and polar) or derivatization with trans-esterification (e.g., BF3) and tri-methylsilylation (e.g., BSTFA) reagents to convert the acidic and alcohol moieties into GC-MS amenable (i.e., volatile enough) compounds (FAMEs and trimethylsilyl ethers, respectively). SAM can perform wet chemistry experiments with tetramethylammonium hydroxide (TMAH) to make lipids sufficiently volatile to detect with GC-MS 103 and with N-methyl-N(tertbutyldimethylsilyl) trifluoroacetamide (MTBSTFA) to detect amino and carboxylic acids 96 . In fact, the first wet chemistry experiment was recently performed on active eolian Martian sand and results are being interpreted 104 .…”

Fatty Acid Preservation in Modern and Relict Hot-Spring Deposits in Iceland, with Implications for Organics Detection on Mars

“…Simply pyrolyzing a sample at high temperature (>650°C) results in fragmentation of the parent molecule due to thermal stress, which limits molecule identification. To cleave these refractory molecules at a milder temperature (e.g., £600°C), thermochemolysis with TMAH allows the analysis of refractory matter in complex matrices containing low amounts of organic material with minimal destruction of the organic material (Geffroy-Rodier et al, 2009;He et al, 2019;Williams et al, 2019).…”

“…TMAH has been used for several decades in a variety of terrestrial fields of study, including petroleum geochemistry (Larter and Horsfield, 1993), soil science (Schulten, 1996;Chefetz et al, 2000;Deport et al, 2006), and sedimentology (Pulchan et al, 1997;Guignard et al, 2005). TMAH is currently available on the Curiosity rover SAM instrument (Mahaffy et al, 2012;Williams et al, 2019) and will also be used on the ExoMars rover MOMA instrument (Goesmann et al, 2017). Fortunately, perchlorate does not substantially affect the recovery of fatty acids liberated with TMAH thermochemolysis (He et al, 2019), making TMAH an ideal reagent to use in the search for select organics on Mars.…”

“…Abiotic fatty acid patterns will favor shorter carbon chain lengths with no carbon preference (Bray and Evans, 1961;McCollom et al, 1999), although it is important to note that very short-chain carboxylic acids can represent metabolic by-products that were not incorporated into cellular membranes. Therefore, within a certain level of uncertainty, characteristics of a fatty acid methyl ester (FAME) profile could be used to deconvolve the origin of fatty acids detected on Mars (Williams et al, 2019).…”

“…Extensive work has been performed, which documents the extraordinary preservation of microbial cell structure in sinter deposits by entombment in a very fine matrix of silica particles (Walter and Des Marais, 1993;Cady and Farmer, 1996;Walter, 1996;Renaut et al, 1998;Jones et al, 2000;Konhauser et al, 2001;Tobler et al, 2008;Guido et al, 2010;Brasier et al, 2011;Campbell et al, 2015;Westall et al, 2015;McCollom et al, 2016;Munoz-Saez et al, 2016;Tarhan et al, 2016). Few studies, however, have focused on the preservation of organic matter in modern and relict (up to 900 year old) siliceous sinter deposits (Pancost et al, 2005(Pancost et al, , 2006Kaur et al, 2008Kaur et al, , 2011Kaur et al, , 2015Gibson et al, 2014;Chaves Torres et al, 2019;Teece et al, 2019), and even fewer studies have explored organic preservation potential in Mars-analog hot-spring environments (e.g., Williams et al, 2019).…”