  Accurately determining escape rates from a planet's atmosphere is critical
for determining its evolution. Escape can be driven by upward thermal
conduction of energy deposited well below the exobase, as well as by
non-thermal processes produced by energy deposited in the exobase region.
Recent applications of a model for escape driven by upward thermal conduction,
called the slow hydrodynamic escape model, have resulted in surprisingly large
loss rates for the thick atmosphere of Titan, Saturn's largest moon. Based on a
molecular kinetic simulation of the exobase region, these rates appear to be
orders of magnitude too large. Because of the large amount of Cassini data
already available for Titan's upper atmosphere and the wealth of data expected
within the next decade for the atmospheres of Pluto, Mars, and extrasolar
planets, accurately determining present escape rates is critical for
understanding their evolution. Therefore, the slow hydrodynamic model is
evaluated here. It is shown that such a model cannot give a reliable
description of the atmospheric temperature profile unless it is coupled to a
molecular kinetic description of the exobase region. Therefore, the present
escape rates for Titan and Pluto must be re-evaluated using atmospheric models
described in this paper.
