Gas Transfer in the Pluto-Charon System: A Charon Atmosphere

Abstract:

Recent hybrid fluid/molecular kinetic models demonstrated that Pluto’s upper atmosphere is warmer and more extended than previously thought (Erwin et al., 2013; Tucker et al. 2012). This extended atmosphere is a source of molecules to Charon’s surface. For approximate solar medium conditions at ~33AU about ~0.2% of the flux from Pluto’s exobase impinges on Charon at a rate of ~5.7×10²⁵ N₂/s. This is a source of condensed N₂ for Charon’s night side, and molecules re-emitted from warmer (>20 K) surface regions form a tenuous atmosphere. For the approximate range of surface temperatures, the residence time of N₂ on the surface can range from a fraction of a second to 10’s of years with the near surface line of sight column densities varying from ~3×10¹⁸ N₂/m² up to >6×10¹⁹N₂/m². Additional mechanisms that can eject N₂molecules from the surface include cometary impacts (Stern et al. 2014) and electronic sputtering (Johnson et al., 2013). A Charon atmosphere could be detectable during the solar occultation that will occur during the New Horizon encounter providing a measure of the transfer of gas between bodies in this binary system.

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