par Fortier, Valentin 
Président du jury Qiuzhen, Yin
Promoteur Dehant, Véronique;Debaille, Vinciane
Publication Non publié, 2023-11-09
Président du jury Qiuzhen, Yin
Promoteur Dehant, Véronique;Debaille, Vinciane
Publication Non publié, 2023-11-09
Thèse de doctorat
| Résumé : | Search for life in the Universe is one of the main reasons for space exploration and has been blooming since the 1960’s, as being one of the most fundamental questions for Humankind: “are we alone in the universe?”. For now, extraterrestrial life has not been detected anywhere in the Universe. Nevertheless, the notion of habitability, i.e. the capacity of an environment to sustain life as we know it, has emerged, with the nearest planetary bodies we have access to being the best candidates, particularly Mars due to its many similarities with Earth, including proficient liquid water activity at some point in its history. Liquid water is the main parameter when considering extraterrestrial life since it is a universal solvent and requires temperatures adequate for biological reactions. One way to investigate extraterrestrial life is to look for molecules that are produced by biological activity. As such, an important component has been detected in the martian atmosphere: methane (CH4). On Earth, it is mainly a product of microorganism activity, thus making it a main element for life consideration on Mars. Methane can be a biological product, but it can also be produced by abiotic reactions, with rock-gas-water interactions without life intervention. On Earth, hydrothermal systems such as the ones observed in the abyss sustain ecosystems based on favorable temperature-pressure-pH conditions, and on local production of dihydrogen (H2) and CH4, both used as an energy source by microorganisms. On Earth, these hydrothermal systems are based on serpentinization, a redox reaction oxidizing Fe2+ in mafic minerals (olivine and pyroxene), and which can form serpentine (reaction’s characteristic mineral product), clays, talc, (hydr)oxides, … and H2. In addition to being a potential energy source for microorganisms, this H2 is a fuel for Fischer-Tropsch-Type (FTT) reactions: abiotic gas-rock reactions using a metallic catalyst present in rocks to sustain H2 interaction with a carbon source (CO, CO2, …) to form hydrocarbons, mainly CH4, and water. Such hydrothermal serpentinizing systems are thus a cradle for life on Earth, and can result in substantial abiotic methane production without intervention of life. What about Mars? Numerous Mars studies tell us that in addition to the CH4 detected today in the atmosphere, liquid water was active in the distant past of the planet, and serpentinization happened in the martian near-surface based on present serpentine detection. Moreover, conditions for stable liquid water could be reached in the present-day subsurface from ~ 20 up to ~ 3 km depth. Overall, serpentinization could be happening in present-day Mars, producing H2 that might fuel FTT reactions and form CH4.The objectives of the present work are to experimentally investigate the potential serpentinization and abiotic CH4 production in present-day subsurface martian conditions. Two different experimental setups, one focusing on hydrothermalism and the other one on water-free gas-rock interactions, were used. In order to ensure sufficient material for experimental work, a shergottite analog was developed to answer at best the petrological requirements of our experiments. Based on the different experiments, it is shown that serpentinization and FTT reactions could currently happen on Mars, thus proving that methane could be produced today abiotically without intervention of biological reactions, and proving that a serpentinizing system could be active today thus providing environments beneficial to life. The potential production of dihydrogen and methane has been evaluated, and the secondary mineralogy produced in our experiments have been characterized in the frame of current martian orbital and in-situ missions. As a by-product of our work, the rock analog and hydrothermal samples experimentally produced mimic martian petrology, and thus can be used to improve ongoing mission interpretations, and to prepare future missions. |
