Role of volatile compounds on the thermal evolution of Pluto and Charon

Abstract:

The first images taken by New Horizons suggest that the dwarf planet Pluto and its moon Charon are much more active than previously anticipated, with possible recent resurfacing events. This is unexpected for bodies the sizes of Pluto and Charon several billions after their formation. Radiogenic heating is expected to be rather low at present, and tidal heating due to the interaction between the two companions may have played a role but only during the early stage of the evolution before the system reached dual synchronization (Robuchon and Nimmo, Icarus 2011; Barr and Collins, Icarus 2015). Recent resurfacings may suggests a slow cooling of the interior associated with recent activation of dynamical processes in the interior, possibly helped by the presence of anti-freezing compounds, such as ammonia and methanol, and low-conductivity gas clathrates.

In order to determine in which conditions Pluto and Charon may be active several billions after their formation, we investigate the role of anti-freezing compounds (ammonia and methanol) and gas compounds on the formation and evolution of an internal ocean. Using a parameterized model initially developed for Titan (Tobie et al. Icarus, 2005), we model the thermo-chemical evolution of Pluto’s and Charon’s interior, including the formation of an internal ocean and its subsequent crystallization, the formation/dissociation of gas clathrates and their feedback on the internal cooling rate, by assuming various initial compositions. A particular attention will be paid on the conditions under which convective instabilities may initiate in the outer ice shell, on the coupling between ocean crystallization and surface deformation, and on their consequences for the geological activity of Pluto and Charon.