Preservation and Detection of Lipid Biomarkers within Martian Sulfates

The environmental conditions on present-day Mars are detrimental for life as we know it; however cumulative evidence suggests that early Noachian Mars (~ 4 billion Ga) had a warmer climate with a denser atmosphere¹ capable of supporting surficial liquid water, and providing protection from UV and cosmic radiation. It is possible, therefore, that early Mars could have been hospitable for microorganisms.

While cellular degeneration is a rapid process following cell death, microorganisms do leave behind molecular clues as to their existence, or biosignatures, such as the lipid molecules that previously comprised their cell membranes. Each group of microorganisms leaves behind a distinct lipid “fingerprint” that is relatively resistant to harsh environmental conditions and can be preserved over geological timescales².

Various robotic and remote-sensing missions have confirmed the presence of salt deposits (e.g., chlorides, sulfates) at the late Noachian to late Hesperian (~ 4 to 3.5 Ga) boundaries of present-day Martian deposits. In particular, bulk enrichments of calcium and magnesium sulfates have been reported in the Hesperian (~ 3.3–3.7 Ga) sedimentary rocks of Gale crater³, which indicate the previous presence of widespread liquid water on Mars. On Earth, such salts have been found to harbour and protect microbial life for a prolonged period, possibly over millions of years⁴, thus evaporite sequences provide a compelling target for life detection, as putative biosignatures could be preserved⁵.

Currently, the beneficial or deleterious effects of sulfate chemistry on the preservation of organic matter, especially under biologically destructive modern Martian conditions is not well constrained. Given the prominent presence of sulfates on the surface of Mars, this work explores whether sulfate minerals can act as viable substrates for the long-term preservation of lipids, when exposed to Mars-like radiative and atmospheric conditions, and aims to determine whether those signs of life can be explicitly detected.

To carry out this work, we use a combination of analogue fieldwork and laboratory-based simulation studies. Microorganisms isolated from a magnesium sulfate-rich analogue site (Basque Lake) and other terrestrial early Mars analogues, will be entombed within artificial sulfate crystals by evaporating an experimental brine under low atmospheric pressure and UV radiation similar to that encountered on the surface of Mars using The Open University’s Mars chamber facility. The lipids within the samples will be extracted and derivatized to make them amenable to by pyrolysis-gas chromatography-mass spectrometry (py-GC-MS); py-GC-MS is an important component of the Sample Analysis at Mars (SAM) instrument suite onboard NASA’s Mars Science Laboratory (MSL) and of the Mars Organic Molecular Organizer (MOMA) on the ESA’s Rosalind Franklin (ExoMars) rover. This work will aid in characterizing the impacts of the simulated Martian environment and Martian mineralogy on the preservation and detection of lipid biomarkers within sulfitic evaporite deposits and will inform the robotic missions targeting the search for ancient signatures of life on Mars.

References: [1] Carr MH, Head JW (2010) Earth and Planetary Science Letters 294: 185-203 [2] Luo G et al., (2019) Earth-Science Reviews 189: 99-124 [3] Rapin, W et al., (2019) Nature Geosciences 12:889–895 [4] Fendrihan S et al., (2006) Reviews in Environmental Science and Biotechnology 5: 203-218 [5] Johnson SS et al., (2020) Astrobiology 2020 20:167-178