Abstract
The Mars surface/near-surface is often considered to be biocidal. Here, diverse lines of evidence are presented indicating that some terrestrial microbes can survive the in-situ conditions albeit in an inactive state. For the purposes of planetary protection, it is important to consider what we mean by a planetary 'surface'; this term has qualitatively distinct definitions fordifferent scientific disciplines, and can also have different meanings from a humanviewpoint versus that of a microbial cell. Most microbial cells spores or other cells deposited on Mars, even those that initially fall on the outward-facing part of the absolute surface, will fall within pores of the regolith or become covered by its dust. They are, therefore, protected from ultra-violet radiation. Desiccating conditions and low temperatures (-40 to -70°C) can act to preserve rather than kill all microbes, potentially maintaining cellular viability - especially for certain extremophiles - over geological timescales. Whereas salts are ubiquitous on Mars, many terrestrial microbes are highly tolerant to NaCl and other salts, and these substances (including potentially inhibitory chaotropes such as MgCl2 and perchlorates) cannot access cells in the absence of a liquid milieu. Whereas the Mars regolith is nutrient-deplete and conditions may be acidic in places, oligotrophic conditions per se are not biocidal and many terrestrial microbes can thrive in acidic conditions (some acidophiles can proliferate at or below pH0). The low temperatures of Mars' surface are not conducive to metabolic activity, but the biophysical sophistication and robust stress biology of many terrestrial microbes, and the protection afforded by Martian conditions, are likely to ensure the long-term viability of some extremophilic microbes if transported to Mars.
Highlights
Studies of potential habitability of Mars by terrestrial microbes often focus on substances and parameters that are biocidal
There is no evidence that oligotrophic conditions are directly biocidal and, whereas some papers use the term ‘spacecraft microbes’, any microbe that occurs in Earth’s biosphere could potentially occur as an accidental contaminant of spacecraft (Rummel et al, 2014)
Planetary protection committees usually consist of experts from diverse disciplines including planetary science, geochemistry, engineering, space-exploration policy and microbiology (Kminek et al, 2010; Rummel et al, 2014; Spry et al, 2021) where each person may have a different interpretation of what the Mars surface is
Summary
Studies of potential habitability of Mars by terrestrial microbes often focus on substances and parameters that are biocidal. An argument is presented here that the Mars ‘surface’ can be life-preserving rather than biocidal; to be more precise, the near-surface or beneath dust where cells can be shadowed.
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