Abstract:
The equations of gas dynamics are extensively used to describe atmospheric loss from solar system bodies and
exoplanets even though the boundary conditions at infinity are not uniquely defined. Using molecular-kinetic
simulations that correctly treat the transition from the continuum to the rarefied region, we confirm that the energylimited
escape approximation is valid when adiabatic expansion is the dominant cooling process. However, this
does not imply that the outflow goes sonic. Rather large escape rates and concomitant adiabatic cooling can produce
atmospheres with subsonic flow that are highly extended. Since this affects the heating rate of the upper atmosphere
and the interaction with external fields and plasmas, we give a criterion for estimating when the outflow goes
transonic in the continuum region. This is applied to early terrestrial atmospheres, exoplanet atmospheres, and the
atmosphere of the ex-planet, Pluto, all of which have large escape rates.
