Hybrid fluid/kinetic modeling of Pluto's escaping atmosphere

TL;DR

This paper develops a hybrid fluid/kinetic model to accurately predict Pluto's atmospheric escape rate and structure, revealing a more extended and hotter atmosphere than previous models, with implications for planetary and exoplanet studies.

Contribution

It introduces a time-dependent hybrid modeling approach that improves upon previous static models, providing more accurate atmospheric escape predictions for Pluto.

Findings

The model predicts a more extended, hotter atmosphere consistent with recent CO observations.

Escape rates are similar to previous estimates but with a significantly larger atmospheric extent.

Results have implications for understanding exoplanet atmospheres and the limitations of energy-limited escape models.

Abstract

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 use a time-dependent solver to allow us to extend those simulations to higher heating rates and we examine fluid models in which Jeans-like escape expressions are used for the upper boundary conditions. We compare these to hybrid simulations of the atmosphere under heating conditions roughly representative of solar minimum and mean 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 have previously shown that such models can be scaled, these results have implications for modeling exoplanet atmospheres for which the energy limited escape approximation is often used.