The ammonia–water system at high pressures: Implications for the methane of Titan

Abstract

The Cassini/Huygens mission will provide an accurate description of Titan's surface features. One important outcome of these data is that it will help for understanding the processes of methane exchange between Titan's interior and its atmosphere. Such a correlation between surface features and internal processes involving methane will be highly simplified if the nature of methane reservoirs is understood. In this paper, the behavior of methane within Titan is investigated using both data on methane clathrate stability and data on the ammonia–water system. A mathematical description of the different liquidus of the ammonia–water system is proposed. It is shown that the low pressure and water rich domain of the system is very well constrained. On the contrary, both high pressure ices and ammonia hydrates domains are still very badly understood because of the lack of experimental data. Nonetheless, several important characteristics of both ices and hydrates stability are described. These data are used for proposing a new model which computes the thermodynamical characteristics of the liquid layer within Titan. This provides new constraints on the temperature and composition fields within the liquid layer of Titan which indicates that the dissociation of methane clathrates in the deep interior is almost impossible. In the last part, the methane clathrate behavior within the different layer of Titan's interior is investigated. Due to the density contrasts between methane clathrates and ices, it will be shown that methane is certainly trapped within large clathrate reservoirs below the upper conductive lid of Titan. Further ascent and dissociation of clathrate into gaseous methane + ice must then be associated with tectonic and/or volcanic processes which allow rapid ascent without cooling of clathrates. Indeed, the dissociation is only possible at very shallow depth only if hot material from the ice layer can reach the surface rapidly.