Melting and Tectonics from Coupled Orbital and Thermal Evolution of the Pluto-Charon System

As New Horizons data is returned, we should consider the possibility that Pluto and Charon went through an intense period of activity following the Charon-forming impact. Our prior work (Barr and Collins 2015) shows that if the deep layer of Pluto's ice shell is initially warmer than ~200 K after the impact, energy dissipation within Pluto can lead to melting and rapid thinning of the ice shell and tidal stresses can drive tectonics on both bodies. Here, we build upon our prior work to simulate the coupled thermal and orbital evolution of the Pluto/Charon system after the Charon-forming impact, taking into account the growth/freezing of an ocean, the onset/cessation of solid-state ice shell convection, and how the changing interior state of Pluto changes the magnitude of tidal deformation and dissipation. We track the heat flow in Pluto as a function of time for a variety of starting conditions after Charon is formed. We find that if Pluto has even a small ocean after Charon forms, the system can readily evolve to its present dual synchronous state. A feedback mechanism exists in which tidal dissipation in the basal ice above the ocean increases its temperature, which in turn leads to faster tidal dissipation. This feedback leads to a heat pulse within Pluto and rapid buildup of tectonic stresses on Pluto and Charon. If Pluto never had any liquid at the base of its ice shell, the tidal dissipation is too small to keep Pluto's interior warm. Without a starting ocean, it is difficult to start the thermal-orbital feedback process, and difficult for Charon to evolve to its presently observed orbital state within the timespan of solar system history.