Abstract
Discovery of a remnant habitable environment by the Mars Science Laboratory in the sedimentary record of Gale Crater has reinvigorated the search for evidence of martian life. In this study, we used a simulated martian mudstone material, based on data from Gale Crater, that was inoculated and cultured over several months and then dried and pressed. The simulated mudstone was analysed with a range of techniques to investigate the detectability of biosignatures. Cell counting and DNA extraction showed a diverse but low biomass microbial community that was highly dispersed. Pellets were analysed with bulk Elemental Analysis – Isotope Ratio Mass Spectrometry (EA-IRMS), high-resolution Laser-ablation Ionisation Mass Spectrometry (LIMS), Raman spectroscopy and Fourier Transform InfraRed (FTIR) spectroscopy, which are all techniques of relevance to current and future space missions. Bulk analytical techniques were unable to differentiate between inoculated samples and abiotic controls, despite total levels of organic carbon comparable with that of the martian surface. Raman spectroscopy, FTIR spectroscopy and LIMS, which are high sensitivity techniques that provide chemical information at high spatial resolution, retrieved presumptive biosignatures but these remained ambiguous and the sedimentary matrix presented challenges for all techniques. This suggests challenges for detecting definitive evidence for life, both in the simulated lacustrine environment via standard microbiological techniques and in the simulated mudstone via analytical techniques with relevance to robotic missions. Our study suggests that multiple co-incident high-sensitivity techniques that can scan the same micrometre-scale spots are required to unambiguously detect biosignatures, but the spatial coverage of these techniques needs to be high enough not to miss individual cellular-scale structures in the matrix.
Highlights
The discovery of an ancient lacustrine environment in Gale Crater by the Mars Science Laboratory (MSL) has implications for the history of habitability on Mars
Once corrected for the full sample volume, these counts gave a wide range of total cell number in each sample of between 105–107 cells ml−1, which is consistent with concentrations considered low – normal biomass for terrestrial soils[15,16]
Given our later analysis, which suggest total biomass far below terrestrial soils, we assume that these numbers are overestimates caused by the effect of small number statistics multiplied by the dilution factors, especially since the detection limit at these dilutions would be 104 cells ml−1
Summary
The discovery of an ancient lacustrine environment in Gale Crater by the Mars Science Laboratory (MSL) has implications for the history of habitability on Mars. Current martian surface conditions are inhospitable to life, but it has been proposed that the Yellowknife Bay sedimentary system was habitable in Mars’ past[5] This raises the question of whether life could be detected in such an environment if it was inhabited in the past. Astrobiology missions to date, from the Viking landers to the Curiosity rover, have relied generally on bulk analysis techniques, but near-future missions will carry instruments designed to measure biosignatures at micrometre scales[11,12] These life detection techniques aim to characterise organic molecules using methods such as mass spectrometry or infrared or Raman spectroscopy. That the biogenicity of some of the oldest known fossils on Earth remains ambiguous[14] brings into question how confidently we might be able to identify biological systems of similar ages on Mars
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