Formation and preservation of abiotic organic signatures vs. lipid biomarkers—experimental studies in preparation for the ExoMars 2020 mission
by Helge Mißbach
Date of Examination:2018-05-30
Date of issue:2018-06-22
Advisor:Prof. Dr. Volker Thiel
Referee:Prof. Dr. Volker Thiel
Referee:PD Dr. Walter Goetz
Referee:Prof. Dr. Francois Raulin
Persistent Address:
http://dx.doi.org/10.53846/goediss-6939
Files in this item
Name:Mißbach_ediss_opt.pdf
Size:51.1Mb
Format:PDF
The following license files are associated with this item:
Abstract
English
Molecular organic biomarkers have widely been used to track life through Earth’s history, and they became increasingly important in the search of potential (remnants of) life on Mars. The Mars Organic Molecule Analyzer (MOMA) instrument will be the key instrument onboard the ExoMars rover (launch in 2020), with the goal to characterize the organic inventory of martian sediments/rocks and the search for signs of life in the form of organic biomarkers. However, this task is potentially facing a series of problems and challenges including, for example, the degradation of organic biomarkers (e.g., by UV and/or particle radiation, thermal stress); mixing of biologically with abiotically derived organic matter (e.g., from meteorites/comets, abiotic synthesis) which requires careful source discrimination; limited capabilities of in situ gas chromatography–mass spectrometry (GC–MS) techniques (as used by MOMA) compared to conventional bench-top methods; the presence of perchlorates in martian soils which can lead to degradation of organic molecules upon heating. Therefore, pre-flight tests and experiments with analog materials can help to enhance the later evaluation of potential biosignatures. This thesis combines a series of mainly experimental studies aimed at (i) assessing the diversity of biomarker-like lipids from abiotic Fischer–Tropsch-type (FTT) synthesis, (ii) determining the impact of thermal stress on biologically and abiotically derived lipids, (iii) providing reference data to differentiate between biologically and abiotically synthesized lipids in sediments and rocks and (iv) identifying analytical limitations and potential pitfalls of MOMA GC–MS techniques. In the first study, experimental maturation in gold capsules (300/400 °C, 2 kbar, 2 – 2400 h) was performed on isolated kerogen from the Eocene Green River formation to determine the impact of thermal maturation on biological n-alkane distribution patterns and selected lipid biomarkers (pristane, phytane, steranes, hopanes, cheilanthanes). The study revealed major differences in their thermal degradation behavior. Furthermore, it was demonstrated that n-alkane distribution patterns and respective biomarkers withstand thermal maturation at 300 °C for 2400 h, while they quickly degraded at 400 °C (< 48 h, corresponding to a vitrinite reflectance of 1.83% RO). The second study addressed abiotic FTT synthesis which yielded a variety of solvent extractable biomarker-like lipids (e.g., linear and methyl-branched alkanes and alkanols, n-alkanoic acids). These showed a unimodal distribution of homologous compounds in contrast to uneven distributions of biologically derived lipids. Thus, a discrimination of abiotically (FTT synthesis) and biologically derived lipids based on their primary distributions is principally possible. However, primary distributions change in the course of thermal maturation which can complicate lipid source discrimination. In the third study, kerogen from an Archean hydrothermal chert vein (ca 3.5 Ga Dresser Formation, Pilbara Craton, Western Australia) was investigated. Organic matter, cracked from the kerogen via catalytic hydropyrolysis revealed n-alkane homologs with a distinct decrease > n-C18. The same n-alkane pattern was observed in recent bacterial biomass from Anabaena cylindrica, whereas abiotically derived organics from FTT reactions show unimodal distributions. Based on these observations, a biological origin of the kerogen was inferred. This is furthermore consistent with a low d13CTOC value of – 32.8 ± 0.3 ‰ and stable carbon isotope values of n-alkanes ranging from – 29.4 ‰ to – 33.3 ‰. Moreover, case studies using MOMA-like GC–MS techniques were carried out to test the applicability to different sample types and to assess the impact of perchlorates on these techniques. The studies included pyrolysis, in situ derivatization and thermochemolysis GC–MS analyses on a synthetic and a natural sample with and without Mg-perchlorate (0 wt%, 1 wt%, 10 wt%). It was demonstrated that not every MOMA-like GC–MS technique is applicable to every sample type or organic compound class. For example, pyrolysis was largely affected by perchlorates, while thermochemolysis with tetramethylammonium hydroxide (TMAH) appears to be perchlorate resistant. This underlines that perchlorates in martian soils do not necessarily hamper MOMA-like GC–MS analysis. Finally, it was demonstrated that the discrimination of biologically from abiotically derived organic materials is principally possible with MOMA-like GC–MS techniques. However, advantages and disadvantages of each technique must be carefully weighed up and complementary analyses of a given sample using different techniques should be considered to minimize method dependent biases. Nonetheless, these studies emphasize that MOMA is principally able to detect potential biomarkers in martian soil or rock samples.
Keywords: geobiology; astrobiology; biogeochemistry; biomarker ratios; closed system pyrolysis; gas chromatography–mass spectrometry; Green River Shale; maturity indicators; vitrinite reflectance; thermal degradation; Fischer–Tropsch-type synthesis; abiotic synthesis; lipids; experimental maturation; Dresser Formation; Archean kerogen; ExoMars; Planetary instrumentation; thermochemolysis; perchlorate; Mars Organic Molecule Analyzer; life detection
