Role of a global ocean and cometary impacts on the formation and evolution of an atmosphere on early Titan

Events occurred during Titan's primitive history constrained the later evolution of its atmosphere, leading to its present-day mass and a N2-rich composition. The purpose of the presented work is to investigate the fate of Titan's atmosphere during two specific epochs of Titan's primitive history: the formation of a primitive atmosphere at the end of the accretion and its the evolution by impact during the Late Heavy Bombardment (LHB) event. Firstly, we investigate the emergence of the primitive atmosphere on Titan after the formation of the satellite, when the impact heating was strong enough to melt the icy surface of the satellite and form a global ocean in contact with the atmosphere. In order to estimate the size and chemical composition of this atmosphere, we model the vapor-liquid equilibrium for the CO2-NH3-H2O and CH4-CO2-NH3-H2O systems that are representative of Titan's primordial composition. We show that the CO2-NH3 interaction may have played an important role for the Titan's primitive atmosphere composition and the formation of CH4-rich clathrate crust. In a second time, we investigate the atmospheric mass balance during the LHB. We show that the emergence by impact of a N2-dominated atmosphere during this event is improbable. Indeed, the atmospheric erosion by impact is a dominating mechanism during the LHB and pre-LHB atmospheres should be at least 5 times more massive than at present, in order to sustain an atmosphere equivalent to the present-day one.