|dc.description.abstracteng||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