Astrobiological potential of putative aqueous microenvironments on Mars

Abstract

Water is a fundamental requirement for life as we know it. Therefore, one approach to the search for extraterrestrial life is to look for environments where liquid water exists or has existed. This approach has led us to Mars, where liquid water once existed in abundance, but for billions of years has been subject to low pressure and low-temperature conditions, making liquid water on the planet’s surface highly unstable. There are however a number of surface microenvironments that have been proposed as potential habitats. In some of the terrestrial environments considered the closest analogues to Mars there exist complex communities of microorganisms inhabiting microenvironments whose physical and chemical properties buffer the polyextreme environmental conditions. As a result, these environments have been proposed as model systems for Martian surface habitats. This work aims to explore the astrobiological potential of three of these microenvironments; brine evaporites, rock pores and fluid inclusions in ice. Using laboratory based analogue environment simulations this work investigates the formation and volatilisation of liquid water, and the effect of dissolved salts on these processes. Firstly, we observe the formation of evaporites from saturated sulphate, chloride, and perchlorate salt solutions and their binary mixtures under Earth and Martian pressure conditions to qualitatively understand their physical behaviour and how this might influence their potential as habitats. The resultant evaporite morphologies are categorised into ‘creeping’ and ‘crusting’ structures, each with distinct astrobiological implications: self-expansion of habitats and the extension of liquid water lifetime respectively. Secondly, we examine the impact of porous media on liquid water lifetime at low pressures. As a result of decreased evaporative surface area, the introduction of porous media increased the lifetime of liquid water compared to free water, though the difference decreased with decreasing atmospheric pressure. The introduction of dissolved magnesium sulphate salt significantly enhanced the longevity of liquid water within porous media as a result of crystallised salt obstructing the pore openings. Finally, we investigate the ability of solar-heated low-albedo material to melt ice under Martian north polar and Antarctic temperature conditions. Even under ideal Martian temperature conditions, no bulk melting occurs as a result of the solar-heating of a black aluminium disc or Martian analogue regolith. When highly deliquescent magnesium perchlorate salt was incorporated into the analogue regolith, sediment migration into the ice surface is observed. These data show that although the presence of salts in high concentrations can challenge life, they also generate new habitat space and prolong the presence of liquid water with implications for the habitability of Martian environments.