  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.
