Past life on Mars, should it have existed, would have left behind fossilised organic biosignatures. Sample selection strategies for current and future life detection missions to Mars require an understanding of the processes that control the preservation, degradation, and detection of diagnostic organic fossil biosignatures in Martian environments over geological time. These include the deleterious effects of radiation, the increased temperatures and pressures associated with burial in the Martian subsurface, and the interactions between organic matter and any oxidising minerals in the depositional environment. In particular, iron minerals dominate the Martian surface environment, and thus any sample selection strategies must consider the effects of iron minerals on the preservation of any potential biosignatures.
In this thesis, the preservation of lipid biosignatures are investigated in a variety of iron-rich environments through the use of terrestrial analogues, including iron-rich acid streams, and circumneutral iron-rich bog systems. It was revealed that lipid biosignatures survive the initial stages of diagenesis in acid stream environments. Lipids were found to be concentrated in mineral layers that are indicative of high water availabilities such as goethite, as these conditions are indicators of more habitable environments.
To study the later stages of diagenesis and the chemical effects of long-term storage in iron-rich environments, a novel technique was developed that involved artificial maturation via hydrous pyrolysis and computer-based kinetic modelling. It was discovered that if any solvent-extractable lipids survived late stage diagenesis, they would record biogenic patterns in their lipid profiles. However, the kinetics of the lipid degradation reaction suggested that lipids were not expected to survive for billions of years necessary for detection at the present day. This quantitative nature of this artificial maturation technique also allows for a quantitative means by which the preservation potential of various Mars-analogue environments can be compared.
Thermal extraction experiments found that insoluble, macromolecular organic matter could survive long-term diagenesis and be detected, as long as oxidising minerals such as goethite and jarosite could be removed from the samples prior to analysis.
The findings of this thesis can inform sample selection strategies on Mars, both in terms of the iron-rich environments in which lipids are most likely to be preserved, as well as the type of techniques that might be the most appropriate life detection.