Towards global SEM mantle convection simulations on Polyhedral-based grids

A novel approach to the construction of three-dimensional grids in spherical geometries is described. The grids are based on a range of underlying regular polyhedra. The faces of the polyhedra are arranged into a number of diamonds in the latitudinal and longitudinal directions to facilitate the creation of a structured mesh. Each polyhedron is completely characterized in terms of the arrangement of diamonds. The approach is shown to be very flexible in terms of the meshes that can be generated. It also aids comparisons between the grids used in many mantle convection studies. A spectral element discretization of Poisson's equation is performed to demonstrate the efficacy of the grid generation technique. Rapidly convergent approximations are obtained that demonstrate that fewer degrees of freedom are required to obtain a desired level of accuracy compared with low-order finite element approximations

Numerical modelling for the hydrothermal activity & habitability of Mars

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

Experimental study of cellular instabilities in non-premixed flames

A systematic experimental study was performed to elucidate the conditions for which cellular patterns of diluted hydrogen diffusion flames near extinction were observed. The formation of cellular instabilities was studied for several burners: jet burners (axisymmetric jet and two-dimensional jet) and a novel one-dimensional burner. The fuel and oxidizer Lewis numbers and the initial mixture strength (fuel-to-oxygen concentration ratio normalized by the stoichiometric value) were identified as the key governing parameters. The formation of cellular flames occurs for low reactant Lewis numbers (less than one) and near the extinction limit. For the jet burners, the parameter space for cellularity was found to decrease with either decreasing initial mixture strength, either increasing the fuel jet velocity. For a given fuel mixture, the wavelength associated with the cellular instabilities was found to decrease with either decreasing oxygen concentration, or increasing the fuel jet velocity. To study the supression of hydrodynamic effects on the cellular instabilities, a unique burner was constructed to experimentally realize a onedimensional unstrained planar non-premixed flame, previously only considered in idealized theoretical models. The results shos that when the oxidizer diffuses against the bulk flow the propensity of cellular instabilities increases with decreasing the initial mixture strength which is in agreement with the theoretical predicitions for this type of burner as well as experimental results for jet diffusion flames

Tracing back the source of contamination

From the time a contaminant is detected in an observation well, the question of where and when the contaminant was introduced in the aquifer needs an answer. Many techniques have been proposed to answer this question, but virtually all of them assume that the aquifer and its dynamics are perfectly known. This work discusses a new approach for the simultaneous identification of the contaminant source location and the spatial variability of hydraulic conductivity in an aquifer which has been validated on synthetic and laboratory experiments and which is in the process of being validated on a real aquifer