title: Numerical modelling for the hydrothermal activity &amp; habitability of Mars
Creators: name: Christou, Evangelos
Publisher: name: University of Glasgow
Year: 2023

description: Modern space and planetary explorations are enthusiastically searching for extraterrestrial biosignatures, and even intelligence in our cosmic neighbourhood. Mars is the epicentre of planetary research and astrobiology, as during ancient geological periods, the Red Planet should have had a thicker atmosphere, and exhibits evidence for ancient aqueous, volcanic and hydrothermal activity. Such physical processes that persist on a planetary body through geological time increase the probability of the emergence and evolution of antediluvian microbial species. However, present-day Mars is a cold and arid desert. So, could the Red Planet host evidence of extinct or/and even extant microbial life? To contribute towards deciphering this mystery, this PhD research focuses on determining the thermodynamic and hydrological evolution, and subsequent habitability of ancient hydrous environments on Mars. Martian habitability, especially during the planet’s ancient geological history, has not been decisively established yet. Moreover, quantitative analyses and models for the ancient or present bioenergetic potential on Mars are scarce. Water – rock interactions enduring in longlived hydrothermal settings on Earth yield appreciable quantities of chemical nutrients that support microbial species under hydrothermal conditions. Through this perspective, the habitability of simulated Martian hydrothermal systems deserves to be computed and analysed. This PhD research explores simulated volcanogenic and impact-induced hydrodynamics on Mars, and the astrobiological potential of such ancient or more recent Martian aqueous environments via computational scenarios. High-resolution numerical simulations for the aqueous circulation and thermodynamics in a variety of putative Martian hydrothermal systems have been constructed and interpreted. Rock permeability, porosity, temperature, pressure, enthalpy, heat capacity, and thermal conductivity comprise governing physical parameters for the duration and mechanics of the hydrothermal cycle in each simulation. Therefore, the presented thermodynamic simulations explore thoroughly the evolution and duration of putative impact-induced or magmatic-induced hydrological systems on Mars from the pre-Noachian to the late Amazonian. The thermodynamic results of these models are then used as input conditions in further computations for Martian water – basaltic rock reaction pathways and their subsequent bioenergetic yield (habitability). Eventually, quantitative habitability assessments are conducted based on the energy – chemical nutrient availability and on the thermal constraints that cumulatively render these environments habitable or uninhabitable for hypothetical lithotrophic microbial species in the Martian subsurface. In parallel, NWA 8159 (shergottite) and Lafayette (nakhlite) Martian meteorite samples were examined through Scanning Electron Microscopy (SEM) analysis to identify their Martian mineralogies, and detect alteration phases – fluid compositions that have affected these basaltic rocks on Mars, or on Earth due to weathering processes after their fall. Petrological analyses provided additional insights into the geochemical composition and evolution of these Martian rocks. Furthermore, image processing on acquired SEM-BSE montage maps of the NWA 8159 and Lafayette samples revealed the porosity of these Martian rocks, and subsequently constrained and enhanced the hydromechanic and habitability models of this PhD research. The hydrothermal and habitability simulations indicate that the Martian basaltic subsurface could have supported hydrogenotrophic microbial life for periods ranging from 0.1 Myr to 3 Myr under preserved hydrothermal conditions. The modelling results additionally suggest that deeper basaltic domains (subsurface depth ≥ 1.5 km) in large impact craters (100-, 200-km diameters) or intrusive volcanic rock settings, could comprise the most promising sites for astrobiological research. The ideal habitable thermal range in which nutrients, and specifically H2, are released in appreciable amounts through ongoing water – rock reactions is from 50 °C to 121 °C. Under such hydrothermal conditions, the Martian subsurface is modelled able to support the survival and growth criteria of hydrogenotrophic life. However, aqueous circulation and geochemical reactions should endure for an average minimum period of 120 Kyr to support microbial growth, and conceivably, the microbial colonization of the Martian subsurface. The numerical simulations of this research support that cold aqueous flows and short-induration hydrological systems on Mars are unable to support the survival of potential microbial species for a period ≥ 2 Kyr. Finally, even in the most optimistic thermodynamic scenarios for Martian habitability, microbial species in the deep Martian subsurface cannot be supported for a period longer than 1 – 2 Myr, after hydrothermal activity has halted. This indicates that any potentially inhabited environments on Mars could have supported microbial life only for an average maximum period of 3 – 4 Myr. Conclusively, planetary environments beyond Earth that may have been hosting hydrothermal or aqueous activity continuously for Myr or even Gyr (i.e.: the Jovian and Kronian moons, beneath their icy crusts) comprise the most habitable extraterrestrial niches of the Solar System, and promising sites for astrobiological findings.