18.14-P

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.