* Astronomy

Title: Hybrid fluid/kinetic modelling of Pluto's escaping atmosphere
Authors: Justin T. Erwin, O. J. Tucker, Robert E. Johnson

Predicting the rate of escape and thermal structure of Pluto's upper atmosphere in preparation for the New Horizons Spacecraft encounter in 2015 is important for planning and interpreting the expected measurements. Having a moderate Jeans parameter Pluto's atmosphere does not fit the classic definition of Jeans escape for light species escaping from the terrestrial planets, nor does it fit the hydrodynamic outflow from comets and certain exoplanets. It has been proposed for some time that Pluto lies in the region of slow-hydrodynamic escape. Using a hybrid fluid/molecular-kinetic model, we previously demonstrated the typical implementation of this model fails to correctly describe the appropriate temperature structure for the upper atmosphere for solar minimum conditions. Here we used a time-dependent solver to allow us to extend those simulations to higher heating rates and we examined fluid models in which Jeans-like escape expressions are used for the upper boundary conditions. We compare these to our hybrid simulations of the atmosphere under heating conditions roughly representative of solar minimum and medium conditions as these bracket conditions expected during the New Horizon encounter. Although we find escape rates comparable to those previously estimated by the slow-hydrodynamic escape model, and roughly consistent with energy limited escape, our model produces a much more extended atmosphere with higher temperatures roughly consistent with recent observations of CO. Such an extended atmosphere will be affected by Charon and will affect Pluto's interaction with the solar wind at the New Horizon encounter. Since we showed earlier that such models can be scaled, these results have implications for modelling exoplanet atmospheres for which the energy limited escape approximation is often used.

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