Habitability of aqueous environments on Mars
Fox-Powell, Mark George
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It is clear that a planet's ability to support life is intimately associated with its physical evolution, but many aspects of this link have not been resolved. For example, differing geologic histories have the potential to drive large-scale differences in the chemistry of planets’ waters, with unknown implications for habitability. In this thesis, I link the geochemical evolution of Mars to the habitability of its associated evaporitic or brine environments, which have been widespread throughout the planet's history. Their habitability is compared with the Earth system, where a chloride-dominated chemistry permits the microbial colonisation of brines with extremely low water availability. By assessing the physicochemical environments in martian brines, I present evidence that high ionic strength, driven to extremes on Mars by the ubiquitous occurrence of divalent ions, can influence habitability even if water availability is high. The importance of this parameter has been overlooked in terrestrial microbiology, likely due to the paucity of environments with high levels of di- or multivalent ions, and its possible mechanics and significance for defining habitat space on Earth and other planets are discussed. Additionally, cultivation techniques and next-generation DNA sequencing were used to identify organisms capable of growth in extreme Mars-relevant brines, which contrast with those typically found in NaCl-rich brines on the Earth. The isolation of a novel sulfate-tolerant Marinococcus strain, and its growth response to fluctuating martian brine compositions are reported. These results show that microbial growth kinetics are defined not merely by additive ion effects, but rather by bulk physicochemical conditions defined by complete ion assemblages. Changes to composition driven by evaporation or freezing can therefore push a brine into more biologically clement conditions by altering a brine’s physicochemical profile The data herein present a strong case that geochemical context is essential to understanding habitability in extreme saline environments. A new framework for predicting brine habitability is required, taking into account the geochemical history of the brine as well as the effects different ionic compositions exert on microorganisms. This work is a significant contribution across several fields, and emphasises the value of interdisciplinary science in answering questions of planetary habitability. Furthermore, this thesis provides a case study for exploring the impact of planetary-scale geochemical evolution on the ability of a planet to support life.