ExoMars analog sample campaign: preparing in situ research of organic matter with Gas Chromatography-Mass Spectrometry at Oxia Planum, Mars, with the MOMA experiment

                    Introduction: Mars is one among the Solar System bodies of highest interest for research about prebiotic chemistry, mainly because early Mars surface environment was similar to the Earth’s one at the time Life arose on our planet [1]. This is one reason why Mars surface has been intensively explored for decades, notably by the use of mobile rover probes, which allowed the first detection of indigenous organic molecules [2].

The Exomars mission has among its main objectives to characterize the organic matter present in its landing site, Oxia Planum, which is characterized by wide plains rich in clays [3]. Clay formation requires prolonged exposure to liquid water. The geochemical conditions necessary for the formation of such clays could facilitate prebiotic activity [4]. Therefore, Oxia Planum deposits could hold a record of formation and evolution of organic matter into molecules of biological interest. To prepare the interpretation of future data to be collected by the different instruments onboard the Rosalind Franklin rover, the Exomars science team organized a campaign of analyses performed on a variety of terrestrial samples, possibly analogs of Oxia Planum soils and rocks, that were distributed among the laboratories participating to the ExoMars project.

Mars Organic Molecule Analyzer (MOMA) is the main instrument dedicated to characterize the organic matter present in the collected samples. It is a dual-mode instrument aboard the rover, designed to detect and characterize organic molecules in subsurface samples down to 2 m depth, where radiation-induced degradation is minimized [5]. MOMA employs a combination of laser desorption (LD) and gas chromatography–mass spectrometry (GC-MS) with complementary sample preparation techniques: pyrolysis (up to 850 °C) which allows organic molecules to evolve (or be fragmented) into volatile species prior to their injection into the GC-MS analyzer. In addition to this vaporization technique, chemical derivatization using tetramethylammonium hydroxide (TMAH), to support molecule vaporization but limiting their fragmentation or N-methyl-N-(trimethylsilyl)trifluoroacetamide (MTBSTFA), and dimethylformamide–dimethylacetamide (DMF-DMA) for polar organic molecules of interest for prebiotic chemistry analyzable by GC-MS, such as amino acids or fatty acids [6].

My work focuses on the GC-MS analysis part. The goal of this study is to replicate MOMA GCMS protocols in the laboratory and asses the best parameters to confirm the detectability of biomarkers then understand the best conditions to operate MOMA. To this aim, each sample was subjected to sequential pyrolysis (250 °C, 400 °C, 550 °C, 700 °C, and 850 °C) and derivatization with TMAH, MTBSTFA, and DMF-DMA. All analyses were performed using laboratory-scale GC-MS systems configured to replicate MOMA operational parameters. Blank runs and procedural controls were conducted to assess contamination levels.

                    Results: We were able to detect organic molecules in all the samples, using MOMA analytical technics, though in various abundances and number, depending on the samples. Although the possibility of contamination cannot be ruled out, it remains minimal. We observed that the thermal treatment (temperature and duration of pyrolysis) had a significant impact on the nature of the organic molecules detected (Fig. 1). These results allowed us to determine that in order to extract as much information as possible from a sample, two pyrolysis temperature should be used successively. A first “low temperature” (550°C) pyrolysis to extract volatile compounds while limiting their alteration, and a “high temperature” (850°C) pyrolysis to extract molecules potentially trapped in refractory minerals that can thermally decompose above 550°C, with the drawback of aromatization processes that occur to organic molecules at such high temperatures.

Fig. 1. Distribution by chemical family of the number of compounds detected by pyrolysis GCMS for successive pyrolysis temperatures of 250°C, 400°C, 550°C, 700°C and 875°C, applied to the same fraction of the Modern Gypsum sample of the ExoMars mission sample exercise.

The derivatization techniques used allowed us to identify important biological molecules like amino acids or fatty acids. It was observed that the MTBSTFA (in large excess comparing to MOMA conditions) derivatization allowed the identification of a higher number of compounds. Results also show that for the same sample the amino acids identified depend on the derivatization agent used. Furthermore, even if DMF-DMA (also in large excess) derivatization does not allow to detect as much molecules as with MTBSTFA, DMF-DMA leaves room for chiral separation of the molecules. This observation demonstrates the derivatization technics are complementary to each other’s and to pyrolysis.

                    Conclusion: We demonstrated that the sample preparation technics used in the MOMA instrument to perform GCMS analyses, allow to detect organic molecules, including species of interest for prebiotic chemistry, in martian analog samples. This works also demonstrated the interest of a two-step pyrolysis when analyzing natural sample to prevent the alteration of the most volatile compounds. In addition, the derivatization technics allowed the identification of building blocks of life, proving the relevance and the complementarity of the technics boarding MOMA.

Though these results focus on laboratory technics reproducing MOMA, the MOMA team works together studying the same sample in more MOMA-like situation (subsystems, sample to reagent ratio …) in order to transpose our results to the actual instrument.

                    Acknowledgments: Thanks are due to the Exomars project team and the teams that provided the variety of samples to be characterized in the frame of the “ExoMars mission samples” campaign. T. Govekar acknowledges financial support from the Centre national d’études spatiales (CNES), France (ROR: https://ror.org/04h1h0y33) within the framework of the ExoMars mission. T. Govekar thanks University of Versailles Saint Quentin and CNES for his PhD grant funding. 

 

                    References: [1] Cockell, et al. (2016), 16(1), 89-117. [2] Freissinet, et al. (2015). Journal of Geophysical Research: Planets. [3] Quantin-Nataf, et al. (2021). Astrobiology, 21(3), 345-366. [4] Pinnavaia, T. J. (1983). Science, 220(4595), 365-371. [5] Vago, et al. (2017). Astrobiology, 17(6-7), 471-510. [6] Goesmann, et al. (2017). Astrobiology, 17(6-7), 655-685.