Assessing the formation and preservation of organic signatures in extreme environments in the context of the ExoMars 2020 rover mission

2019 | thesis; doctoral thesis

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​Assessing the formation and preservation of organic signatures in extreme environments in the context of the ExoMars 2020 rover mission​
Reinhardt, M. ​ (2019)

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Reinhardt, Manuel 
Available under: The search for extraterrestrial life is one of the greatest scientific quests of our time. The ESA/Roscosmos ExoMars 2020 rover mission seeks to find evidence for past or modern life on Mars by investigating (sub-)surface sediments at Oxia Planum for molecular biosignatures. To accomplish this goal, the rover is equipped with a variety of extremely sensitive analytical instruments that allow for the identification and characterization of organic matter (e.g., a Raman spectrometer and the Mars Organic Molecule Analyzer, MOMA). A drill allows to obtain samples from up to 2 m depths. The validation and interpretation of any data on potential organic matter produced during this mission, however, requires a sound understanding on possible accumulation and preservation pathways of organic matter at Oxia Planum. This is non-trivial to accomplish, as organic matter on Mars has been exposed to a variety of degradative processes over billions of years through the planet’s history (e.g., radiative and oxidative destruction, thermal alteration by volcanism and impacts). This thesis aims at facilitating the validation and interpretation of potential organic signatures, including specific molecules indicative for biology that might be detected during the ESA/Roscosmos ExoMars 2020 rover mission. More specifically, the thesis provides a detailed picture of organic signature formation and preservation in Oxia Planum-relevant analog environments on Earth (hydrothermal, anoxic iron-rich). The main objectives of this thesis are (i) the assessment of organic sources (abiotic vs. biological), (ii) the discrimination of unambiguous molecular biosignatures, and (iii) the evaluation of organic matter preservation pathways (bitumen vs. kerogen) in the analog environments. These studies are complemented by systematic tests on the detectability of molecular biosignatures in the analyzed materials with MOMA flight-like pyrolysis gas chromatography–mass spectrometry (GC–MS). The first study focusses on organic matter contained in modern hydrothermal cherts from the Pleistocene Lake Magadi (Kenya). The bitumens were dominated by immature archaeal and bacterial “biolipids” (e.g., glycerol mono- and diethers), as well as mature “geolipids” like hopanes, n-alkanes and polycyclic aromatic hydrocarbons (PAHs). Several independent molecular maturity indices from bitumens suggested that parts of the organic matter has been hydrothermally altered. Maturity parameters were also inconsistent for the kerogens, probably reflecting the synsedimentary introduction of pre-altered macromolecules into the depositional environment. However, despite in-situ hydrothermal alteration (particularly defunctionalization) specific molecular fingerprints, such as archaeal isoprenoids, were still incorporated into kerogen. These findings demonstrate that lipid biomarkers may survive syndepositional hydrothermal alteration by rapid sequestration into macromolecular networks (i.e., proto-kerogen and kerogen). This is of great relevance for the preservation of molecular biosignatures on Mars, as such networks are thought to efficiently shield bound compounds against degradative processes like radiation, oxidation and thermal maturation. The second study centers on the analysis of kerogen enclosed in an Archean hydrothermal chert vein (ca. 3.5 Ga, Dresser Formation, Pilbara Craton, Western Australia). While the material experienced lower greenschist metamorphism (ca. 300 °C), the HyPy kerogen pyrolysate still yielded n-alkanes (up to n-C22) that showed a distinct distribution pattern (sharp decrease in abundance > n-C18). A similar chain-length preference was also detected in HyPy pyrolysates of modern bacterial biomass (Anabaena cylindrica), but never in abiotic organic products obtained via Fischer–Tropsch-type synthesis. These findings suggest that the n-alkanes yielded from the Dresser kerogen derive from a biological source. At the same time, the study shows that kerogen can facilitate a preservation of molecular biosignatures over billions of years, even if the organic matter has been subjected to degradative processes such as biodegradation and thermal maturation. It therefore appears possible that the ExoMars 2020 rover may detect biosignatures from the early history of the planet in Noachian-Hesperian (ca. 3.9–3.0 Ga) sediments on Oxia Planum. The third study focusses on the preservation of aromatic carotenoids (pigments from anoxygenic phototrophs) in iron- and sulfur-rich shales from Lower Jurassic anoxic environments (Bächental oil shale, Posidonia Shale). The preservation of organic molecules in such settings is commonly aided by the formation of macromolecules, like kerogen, through early diagenetic sulfurization. Despite high sulfur contents (up to 4.6 wt.%), however, the samples contained only low amounts of sulfurized compounds. Furthermore, aromatic carotenoid biomarkers, including cyclized derivatives, were almost completely found in the bitumens rather than the corresponding kerogens. The results suggest that sulfur crosslinking was probably inhibited by (i) fast defunctionalization of the carotenoid molecules due to cyclization processes and (ii) hydrogenation and/or buffering of sulfide by excess of reduced iron (pyrite formation). This is highly relevant for the ExoMars2020 rover mission as Oxia Planum contains iron-rich sediments (Fe/Mg-smectite clays). It may therefore be possible that crosslinking of organic molecules, and thus the formation of macromolecules, has been suppressed at this site, decreasing their preservation potential over large geological time scales. In the fourth study, one hydrothermal chert (Lake Magadi, Kenya, first study) and one iron-rich shale (Bächental, Austria, third study) were analyzed via MOMA flight-like pyrolysis GC–MS to assess pyrolytic effects on organic signatures. The pyrolysis outcome was mainly driven by the type of organic matter rather than differences in mineral composition (iron-rich smectite vs. opaline silica). Hydrocarbon biomarkers like phytane and arylisoprenoids stayed intact during stepwise pyrolysis (300 °C, 500 °C, 700 °C). Additionally, however, artificial products (e.g., PAHs) were formed during pyrolysis, especially in the 500 °C and 700 °C runs. The pyrolysates from these temperature steps were additionally blurred through carryover effects (i.e., by compounds from previous runs). These findings demonstrate that only the combined application of all techniques available on MOMA (including LDI–MS and derivatization/thermochemolysis GC–MS) will allow for a thorough characterization and interpretation of organic matter. In summary, the results presented in this thesis set important benchmarks for the validation and accurate interpretation of potential data obtained during the ESA/Roscosmos ExoMars 2020 rover mission. At the same time, the studies highlight important limitations and possibilities of the planned analyses. For instance, in-situ hydrothermal defunctionalization and iron-buffering may have hindered the formation of protective macromolecules in potential iron-rich hydrothermal environments at Oxia Planum during Noachian-Hesperian times. This would decrease the preservation potential, as macromolecules effectively shield incorporated compounds against destructive processes like UV-radiation, oxidation by perchlorates, volcanism and impacts. On the other hand, the formation of kerogen-like structures may have not completely been inhibited, and the preservation of organic molecules may have additionally been facilitated by further parameters such as mineral matrix effects and high sedimentation rates. Regardless of these issues, it will clearly require the full set of MOMA’s analytical payload to validate and interpret any potential organic signature on Oxia Planum. Perspectively, a more detailed knowledge on the formation and preservation of organic (bio-)signatures will be essential for the realization of future missions aiming at the detection of organic signatures on planetary bodies beyond Earth.
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