Gas transfer in the Pluto-Charon system: A Charon atmosphere

Recent hybrid fluid/molecular kinetic models demonstrated that Pluto's upper atmosphere is warmer and more extended than previously thought (Erwin, J.T., Tucker, O.J., Johnson, R.E. [2013]. Icarus 226, 375-384; Tucker, O.J., Erwin, J.T., Deighan, J.I., Volkov, A.N., Johnson, R.E. [2012]. Icarus 217, 408-415). Here we examine the effect of Charon on the molecular escape rate from Pluto's extended atmosphere, simulate the spatial distribution of Ny in this binary system, and describe the resulting accumulation of N-2 on Charon. These Monte Carlo simulations are carried out for approximate solar medium conditions at similar to 33 AU. Including Charon's gravity and orbital motion, the atmosphere on the Pluto's Charon-facing hemisphere is more strongly bound to the system and is more extended than the atmosphere on Pluto's anti-Charon hemisphere. Accounting for Charon's gravity the net escape from the system is reduced by similar to 5%. Most of the loss is direct from Pluto's exobase region with similar to 1-2% due to scattering by Charon. About similar to 0.2% of the flux from Pluto's exobase impinges on Charon: similar to 5.7 x 10(25) N-2/s at nominal solar medium conditions. This is a source of condensed Ny for Charon's night side and forms a tenuous atmosphere. For the approximate range of surface temperatures, the residence time of N-2 on the surface can range from a fraction of a second to 10s of years with the near surface line of sight column densities varying from similar to 3 x 10(18) N-2/m(2) up to >6 x 10(19) N-2/m(2). Such an 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. (C) 2014 Elsevier Inc. All rights reserved.