CO2-dominated Atmosphere in Equilibrium with NH3-H2O Ocean: Application to Early Titan and Ocean Planets

Abstract:

During the accretion of Titan, impact heating may have been sufficient to allow the global melting of water ice (Monteux et al. 2014) and the release of volatile compounds, with CO₂ and NH₃ as main constituents (Tobie et al. 2012). Thus, on primitive Titan, it is thought that a massive atmosphere was in contact with a global water ocean. Similar configurations may occur on temperate water-rich planets called ocean planets (Léger et al. 2004, Kitzmann et al. 2015).

Due to its rather low solubility in liquid water, carbon dioxide is expected to be one of the major components in the atmosphere. The atmospheric amount of CO₂ is a key parameter for assessing the thermal evolution of the planetary surface because of its strong greenhouse effect. However, ammonia significantly affects the solubility of CO₂ in water and hence the atmosphere-ocean thermo-chemical equilibrium. For primitive Titan, estimating the mass, temperature and composition of the primitive atmosphere is important to determine mechanisms that led to the present-day N₂-CH₄ dominated atmosphere. Similarly, for ocean planets, the influence of ammonia on the atmospheric abundance in CO₂ has consequences for the definition of the habitable zone.

To investigate the atmospheric composition of the water-rich worlds for a wide range of initial compositions, we have developed a vapor-liquid equilibrium model of the NH₃-CO₂-H₂O system, where we account for the non-ideal comportment of both vapor and liquid phases and the ion speciation of volatiles dissolved in the aqueous phase. We show that adding NH₃ to the CO₂-H₂O binary system induces an efficient absorption of the CO₂ in the liquid phase and thus a lower CO₂ partial pressure in the vapor phase. Indeed, the CO₂ partial pressure remains low for the CO₂/NH₃ ratio of liquid concentrations lower than 0.5.

Assuming various initial compositions of Titan's global water ocean, we explore the thermal and compositional evolution of a massive primitive atmosphere using the thermodynamical model. We are currently investigating how a massive atmosphere may be generated during the satellite growth and how it may then evolve toward a composition dominated by N₂. Applications to ocean planets will also be presented at the conference.