Rarefied gas dynamic simulation of transfer and escape in the Pluto-Charon system

We find results consistent with the NH-determined escape rate, upper atmospheric temperature, and lack of a detectable Charon atmosphere. Gas-transfer structures are noted in a binary atmospheric configuration, including preferential deposition of material from Pluto's escaping atmosphere onto Charon's leading hemisphere that peaks at 315° E on the equator. As the moon gravitationally focuses incident flow, a high density structure forms in its wake. If molecules are permitted to escape from Charon in diffuse reflections from its surface, a returning flux forms to Pluto's exobase, preferentially directed toward its trailing hemisphere. Charon is capable of supporting a thin atmosphere at column densities as high as 1.5 × 1017 m−2 in simulations with a plutonian exobase condition similar to the NH encounter. Results computed from a fit to the NH encounter exobase (Gladstone et al., 2016) predict a system escape rate of 7 × 1025 CH4 s−1 in close agreement with those reported by NH (Bagenal et al., 2016; Gladstone et al., 2016), and a net depositional flux to Charon of 2 × 1024 s−1, of which ∼98% is methane.