Thermally-Diven Atmospheric Escape

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.