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Predicting the rate of escape and thermal structure of Pluto's upperatmosphere in preparation for the New Horizons Spacecraft encounter in 2015 isimportant for planning and interpreting the expected measurements. Having amoderate Jeans parameter Pluto's atmosphere does not fit the classic definitionof Jeans escape for light species escaping from the terrestrial planets, nordoes it fit the hydrodynamic outflow from comets and certain exoplanets. It hasbeen proposed for some time that Pluto lies in the region of slow hydrodynamicescape. Using a hybrid fluid/molecular-kinetic model, we previouslydemonstrated the typical implementation of this model fails to correctlydescribe the appropriate temperature structure for the upper atmosphere forsolar minimum conditions. Here we use a time-dependent solver to allow us toextend those simulations to higher heating rates and we examine fluid models inwhich Jeans-like escape expressions are used for the upper boundary conditions.We compare these to hybrid simulations of the atmosphere under heatingconditions roughly representative of solar minimum and mean conditions as thesebracket conditions expected during the New Horizon encounter. Although we findescape rates comparable to those previously estimated by the slow hydrodynamicescape model, and roughly consistent with energy limited escape, our modelproduces a much more extended atmosphere with higher temperatures roughlyconsistent with recent observations of CO. Such an extended atmosphere will beaffected by Charon and will affect Pluto's interaction with the solar wind atthe New Horizon encounter. Since we have previously shown that such models canbe scaled, these results have implications for modeling exoplanet atmospheresfor which the energy limited escape approximation is often used.

Hybrid fluid/kinetic modeling of Pluto's escaping atmosphere