Formation and Evolution of the Atmosphere on Early Titan

The mass and composition of Titan's massive atmosphere, which is dominated by N2 and CH4 at present, have probably varied all along its history owing to a combination of exogenous and endogenous processes. In a recent study, we investigated its fate during the Late Heavy Bombardment (LHB) by modeling the competitive loss and supply of volatiles by cometary impacts and their consequences on the atmospheric balance. We examine the emergence of an atmosphere as well as the evolution of a primitive atmosphere of various sizes and compositions. By considering an impactor population characteristic of the LHB, we showed that an atmosphere with a mass equivalent to the present-day one cannot be formed during the LHB era. Our calculations indicated that the high-velocity impacts during the LHB led to a strong atmospheric erosion, so that the pre-LHB atmosphere should be 5 to 7 times more massive than at present (depending mostly on the albedo), in order to sustain an atmosphere equivalent to the present-day one. This implies that either a massive atmosphere was formed on Titan during its accretion or that the nitrogen-rich atmosphere was generated after the LHB.To investigate the primitive atmosphere of the satellite, we consider chemical exchanges of volatils between a global water ocean at Titan's surface, generated by impact heating during the accretion and an atmosphere. We are currently developing a liquid-vapor equilibrium model for various initial oceanic composition to investigate how a massive atmosphere may be generated during the satellite growth and how it may evolve toward a composition dominated by N2. More generally, our model address how atmosphere may be generated in water-rich objects, which may be common around other stars.