Evolution of Pluto’s Interior During Charon’s Orbital Migration

Abstract:

We investigate the link between Pluto's thermal evolution and the orbital evolution of the Pluto-Charon system towards dual-synchronous orbit. This work builds on the ideas and methods of Barr and Collins (2015) but explicitly models the feedback between the evolving temperature structure inside Pluto and the rate at which tidal energy is dissipated as Charon’s orbit migrates. We examine how ice shell properties (ice grain size, ammonia concentration) and initial conditions (temperature, initial Pluto-Charon distance), influence the thermal evolution of Pluto and the orbital evolution timescale. The model assumes an initially differentiated Pluto with radiogenic heating in the rocky core, and tracks conduction in the core, melting or freezing of an ocean, heat conduction or parameterized convection in the ice shell using a composite rheology, and Charon’s orbital distance. The k2 Love number is calculated from the tidal frequency and internal viscosity structure of Pluto at any point in the model, which in turn controls the dissipation of tidal heat within Pluto and the migration of Charon. The general model behavior shows that orbital migration is extremely slow until radiogenic heating can warm the interior of Pluto to the point where the ammonia in the internal ocean is diluted significantly below the eutectic, and then a positive feedback quickly softens the ice and expands the ocean until Charon reaches its present distance. We calculate orbital evolution times ranging from less than a million years to more than 4.5 billion years. Rapid evolution occurs if initial core temperature is high, or if Charon starts very close to Pluto, or if ice grain sizes are small, or bulk ammonia concentration is low. Models taking longer than the age of the solar system are obviously disallowed by observations, and occur with cm-scale ice grains and ammonia concentrations above 10%. In these scenarios Pluto has a large, ammonia-rich ocean, but insignificant tidal dissipation occurs in the cold, stiff, conductive ice shell overlying the ocean and Charon never reaches its present orbit. For grain sizes of less than 0.2 mm it is possible for Charon to reach its present location without an ocean forming inside Pluto, but in the vast majority of initial conditions we explored, a relatively warm and ammonia-poor ocean was generated inside Pluto.