Mars' surface is not universally biocidal

Summary 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, an