Ionic strength is a barrier to the habitability of Mars: Conference abstract

Ionic strength is a barrier to the habitability of Mars Mark G. Fox-Powell1, Claire R. Cousins2, John E. Hallsworth3 & Charles S. Cockell1 1UK Centre for Astrobiology, School of Physics and Astronomy, University of Edinburgh, Mayfield Road, Edinburgh, UK EH9 3JZ; 2University of St. Andrews ………. ; 3Institute for Global Food Security, School of Biological Sciences, MBC, Queen’s University Belfast, Belfast, UK BT9 7BL A considerable body of evidence indicates the existence of hypersaline surface waters throughout the history of Mars. It is therefore assumed that, as on Earth, the thermodynamic availability of water (water activity) is a crucial limiting factor for martian habitability. However, differing geologic histories can drive planetary-scale variations in aqueous chemistry, with as-yet-unknown implications for habitability. As products of different geochemical processes from their terrestrial counterparts, martian brines exhibit unique combinations of stressors not normally encountered in brine environments on the Earth. By studying microbial colonization of simulated martian brines, we show that high ionic strength in martian waters constrains their habitability to a smaller window than is predicted by current paradigms. We demonstrate experimentally that ionic strength, driven to extremes on Mars via enrichment of divalent ions such as Mg2+, Fe2/3+ and SO42-, acts to render environments uninhabitable, even when water activity is deemed permissive. Currently, even worse-case assessments for martian brine habitability are based solely on water activity and thus may not be conservative enough. As the chemical composition of a planet’s water bodies is directly dependent on its geologic evolution, these results provide a case study for how differing planetary-scale geochemistries can drive differential habitability on two neighbouring rocky planets. Planets that follow a Mars-like trajectory during their surface evolution are likely to present previously unrecognised but nevertheless significant challenges to biology. Ionic strength, a hitherto overlooked barrier to life, may define habitability at both a microbial and planetary scale.