Two complementary approaches in refining the search for liquid water and habitable environments on present-day Mars

Abstract

All known active life requires liquid water. The correlation between liquid water and the presence of life on Earth has guided the search for life on other planets. For terrestrial-like life to exist in the harsh conditions that dominate the surfaces of other rocky planets, the minimum fundamental requirements of liquid water, nutrients, and chemical energy must be met. Within our solar system, Mars is a strong candidate for hospitable environments able to support life, due to the reservoirs of water within its crust and the strong likelihood of liquid water. The aim of this thesis is to refine the search for liquid water and environments that may be hospitable to life on Mars. Two complementary methodologies are developed and utilised to achieve this aim.

Water requires a relatively narrow range of pressures and temperatures to occur in the liquid phase. The first approach of this thesis compares this range with the pressure-temperature conditions that occur within the Earth, the Earth’s active biosphere, and Mars. Temperature, pressure and water activity are examined to determine the extent to which they restrict life from some liquid water environments. The relevant thresholds are then applied to Mars and compared to models of where liquid water environments are likely to occur under present-day martian conditions. Extensive regions of the Earth may be inhospitable despite lying within the hydrosphere. Life is likely restricted from ~ 81% of the volume of the hydrosphere of Earth due to high temperature and/or low water activity. In contrast, the fraction of Mars that can support liquid water is five times larger than that of Earth, given estimates of an average martian brine. Many environments within the martian crust can potentially support life, with perennially habitable conditions extending from approximately 10 to 37 km beneath the surface. The surface and shallow regolith may also be habitable in the warmest regions of the planet.

The second approach focuses on the shallow subsurface of Mars within the top ~20 m. The thermal behaviour of surface materials determines the occurrence of transient shallow liquid water and habitable temperatures for life. Ten classes of surface materials are identified from analysis of global martian thermal inertia and albedo, through the technique of algorithmic classification. These classes are interpreted as mixtures of dust, sand, duricrust, rocks and ice on the surface, and validated through comparisons with independent datasets. Low latitude locations of dark sand, duricrust and pebbles in Syrtis Major, Oxia Palus, Mawrth Vallis and eastern Meridiani Planum are identified as having high potential for hospitable liquid water environments at < 10 m depth. Dark, coarse, sand dominated surfaces are found in Syrtis Major and Aram Chaos and are predicted to be locations of low volume flows of liquid water, potentially analogous to the observed martian recurring slope lineae.

This thesis identifies where habitable liquid water environments may occur on Mars, strengthening the astrobiological significance of the planet and providing direction for future robotic and satellite missions searching for life.