**Direct Numerical Simulation of Circumplanetary Winds on Io**

**J. V. Austin, D. B. Goldstein (University of Texas)**

The circumplanetary flow of SO gas on Io is modeled using the
direct simulation Monte Carlo (DSMC) method. The gas sublimates from
SO frost in the warm subsolar region and flows towards the colder
night side where it condenses. The acceleration of the flow due to
vapor pressure difference and the cooling due to expansion causes the
flow to become supersonic. A boundary layer grows along the surface
until the gas passes through a standing shock about 70 degrees away
from the subsolar point. Unlike earlier work (Ingersoll *et al*.
1985, 1989 and Wong and Johnson 1995) which assumed continuum
flow, our simulation correctly models the rarefied effects which
become increasingly important as the gas expands away from the
subsolar point. A second, non-condensible gas is also added to
simulate the possible effects of quantities of H S or O in the
atmosphere. The presence of the non-condensible gas can generate
either a hydraulic jump condition or a blanketing effect. Parameter
studies showing condensation distribution as a function of subsolar
temperature and background gas strength will be presented. The DSMC
method solves fully viscous and compressible, non-LTE, rarefied flow
problems by statistically extrapolating from the motions and
collisions of a relatively small number of representative molecules.

This work has been supported by NASA's Planetary Atmospheres Program.