Thermal evolution of an early magma ocean in interaction with the atmosphere : conditions for the condensation of water ocean

The research of new life forms is an exciting quest but requires understandingthe origin of the appearance of a form of life. The only planet that houses life as weknow is the Earth. Understand why the other planets in our solar system do nothouse it, is needed to better target our looking for new lives in other star systems.The objective of this thesis is to provide preliminary answers to this question.We mainly focused on the comparison between thermal evolution of Mars, Earthand Venus to the end of their accretion during their cooling magma ocean. Thethermal evolution of magma oceans produced by collision with giant impactorsduring accretion is expected to depend on the composition and structure of theatmosphere through the greenhouse effect of CO2 and H2O released by the magmaduring its crystallization. In order to constrain the various cooling timescales ofthe system, we developed a 1-D parameterized convection model of a magma oceancoupled with a 1-D radiative-convective model of the atmosphere. We conducted aparametric study and described the influence of several variables such as the initialvolatile inventories, the initial depth of the magma ocean and planet-sun distance.Our results suggest that the presence of a steam atmosphere delays the end ofthe magma ocean phase by about 1 Myr. In addition, we also observe that thewater vapor condenses to an ocean after 0.1, 1.5 and 10 Myr respectively for Mars,Earth and Venus. This time would be virtually infinite for an Earth-sized planetlocated at less 0.66 UA from the sun. In view of these results, we note that for theEarth and Mars, the timescales of the water ocean formation are shorter than timegaps between major impacts. This would imply that successive water oceans mayhave developed during accretion. However, Venus, due to its close proximity to thethreshold distance from the sun (0.66 AU), could have maintained its magma oceanphase longer during accretion. Thereafter, taking into account the hydrodynamicescape permitted us to see that this phenomenon has very little influence on theoverall water tank of a planet during the magma ocean phase. However, we canobserve that after the condensation of the water vapor, the hydrodynamic escapebecomes more efficient and the water tank be completely evaporated shortly beforethe end of the mantle solidification. Finally, we began to study the influence ofother major impacts during the cooling of the magma ocean. The first results showthat in the case of Mars and Earth, the duration of their magma ocean phase isshorter than time gaps between major impacts. In consequently, these planets hadto know an alternation between a phase magma ocean and a ocean water phase. Thisphenomenon does not, however, have taken place on Venus. Indeed, the durationof its magma ocean phase is longer than the time gaps between major impacts.Therefore, the magma ocean phase on Venus had to extend throughout the phaseimpacts and no ocean water has been formed before the end of this period.