Role of the global water ocean on the evolution of Titan's primitive atmosphere

During the accretion of Titan, impact heating may have been sufficient to allow the global melting of water ice and the release of volatile compounds, mainly constituted of CO2, CH4 and NH3. The duration and efficiency of exchange between the primitive massive atmosphere and the global impact-induced water ocean likely play a key role in the chemical evolution of the early Titan's atmosphere. To investigate the atmospheric composition of early Titan for a wide range of global (atmosphere + ocean) composition in volatils, we first developed a gas-liquid equilibrium model of the NH3-CO2-H2O system, where the non-ideal behavior of both gas and liquid phases, and the speciation of volatiles dissolved in the aqueous phase are taken into account. We show that the relative abundance of CO2 and NH3 determine the composition of Titan's atmosphere. For CO2/NH3 < 1, CO2 is massively dissolved in the ocean. On the contrary, for CO2/NH3 > 1, CO2 is the main constituent of Titan's primitive atmosphere while the NH3 atmospheric content is dramatically decreased. We then investigate the conditions for the formation of CH4-rich clathrates hydrates at Titan's surface that could be the main reservoir of methane for the present-day atmosphere. In absence of reliable experimental data in the CH4-CO2-NH3-H2O system, the dissolution of methane in water is included using a simplified Henry's law approach. We find that if the concentration of CH4 in Titan's building block was higher than ~0.1 mol.kg-1 and CO2/NH3 < 3, a large fraction of methane may be stored in the primordial crust, which would form at temperature below ~10°C.