Abstract
Impact ejected rocks are targets for life detection missions to Mars. The Martian subsurface is more favourable to organic preservation than the surface owing to an attenuation of radiation and physical separation from oxidising materials with increasing depth. Impact events bring materials to the surface where they may be accessed without complicated drilling procedures. On Earth, different assemblages of organic matter types are derived from varying depositional environments. Here we assess whether these different types of organic materials can survive impact events without corruption. We subjected four terrestrial organic matter types to elevated pressures and temperatures in piston-cylinder experiments followed by chemical characterisation using whole-rock pyrolysis-gas chromatography-mass spectrometry. Our data reveal that long chain hydrocarbon-dominated organic matter (types I and II; mainly microbial or algal) are unresistant to pressure whereas aromatic hydrocarbon-dominated organic matter types (types III and IV; mainly land plant, metamorphosed or degraded, displaying some superficial chemical similarities to abiotic meteoritic organic matter) are relatively resistant. This suggests that the impact excavated record of potential biology on Mars will be unavoidably biased, with microbial organic matter underrepresented while metamorphosed, degraded or abiotic meteoritic organic matter types will be selectively preserved.
Highlights
Impact ejected rocks are targets for life detection missions to Mars
We subjected four terrestrial organic matter types to elevated pressures and temperatures in piston-cylinder experiments followed by chemical characterisation using whole-rock pyrolysis-gas chromatographymass spectrometry
This suggests that the impact excavated record of potential biology on Mars will be unavoidably biased, with microbial organic matter underrepresented while metamorphosed, degraded or abiotic meteoritic organic matter types will be selectively preserved
Summary
Types I and II organic matter produce numerous alkene/alkane doublets that reflect highly aliphatic biopolymers (Fig. 1). Pyrolysis of the type I and II organic matter following pressure-temperature treatment produces only a few products: toluene, decane and phthalates, which are most likely laboratory or storage contaminants. This is in direct contrast to anhydrous confined pyrolysis experiments performed on Type I organic matter at 450 °C/16 hours, which showed the production of alkanes, aromatic and polar compounds[36], a result consistent with similar 72 hour experiments run on Type I organic matter at 400 and 500 °C37. Aliphatic polymers have lower densities, elastic moduli and tensile strength
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