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

Amy J Williams,Kathleen L Craft,Roger E Summons,Melissa Floyd,Daniel P Glavin,Caroline Freissinet,Cyril Szopa,Arnaud Buch,Mary Beth Wilhelm,Shane S O’reilly ,A C Mcadam ,C A Knudson ,C A Malespin ,S S Johnson ,Rafael Navarro‐González ,R Williams ,J L Eigenbrode ,Kathleen C Benison ,J M T Lewis ,M Millán ,S Andrejkovicôvá ,P R Mahaffy

doi:10.1089/ast.2018.1819

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.

Highlights

tetramethylammonium hydroxide (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
Thermal volatilization gas chromatography mass spectrometry (TVGC-MS) techniques limited to 500°C without wet chemistry capabilities have been utilized in the past by the Viking landers (Biemann et al, 1977), and TV-MS and wet chemistry experiments limited to 500°C were utilized by the Phoenix lander (Boynton et al, 2001)
A mixture of 0.2 mL MTBSTFA in 500 mL TMAH was introduced into the py-GC-MS system with the goethite sample PS5G to identify any pyrolysis by-products

Summary

Introduction

The ongoing exploration of Gale Crater by the NASA Mars Curiosity rover has modernized our understanding of martian geochemistry (Glavin et al, 2013; McLennan et al, 2014; Ming et al, 2014; Anderson et al, 2015; Freissinet et al, 2015; Mangold et al, 2016; Eigenbrode et al, 2018), mineralogy (McAdam et al, 2014; Vaniman et al, 2014; Sutter et al, 2017), sedimentology (Williams et al, 2013; Edgar et al, 2017; Mangold et al, 2017), modern aeolian processes (Bridges et al, 2017; Cousin et al, 2017), planetary evolution (Atreya et al, 2013; Mahaffy et al, 2015), and potential for the presence of habitable environments (Grotzinger et al, 2014). One of the promising techniques for life detection is wet chemistry pyrolysis gas chromatography mass spectrometry (py-GC-MS), which is available on the Curiosity rover as part of the Sample Analysis at Mars (SAM) instrument. Thermal volatilization gas chromatography mass spectrometry (TVGC-MS) techniques limited to 500°C without wet chemistry capabilities have been utilized in the past by the Viking landers (Biemann et al, 1977), and TV-MS and wet chemistry experiments limited to 500°C were utilized by the Phoenix lander (Boynton et al, 2001). The SAM Sample Manipulation System (SMS) carousel contains 74 individual sample cups housed within two concentric rings of the carousel Nine of these cups are slated for wet chemistry experiments and contain reagents for either thermochemolysis (two cups with tetramethylammonium hydroxide [TMAH] in methanol) or derivatization (seven cups with N-methyl-N-tertbutyldimethylsilyl-trifluoroacetamide [MTBSTFA] in dimethylformamide [DMF], hereafter referred to together as MTBSTFA)

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