Monte Carlo calculations are used to describe the kinetics of a column of SO 2 gas (1016 cm -2) on Io, which is bombarded by a flux of ions from the I0 plasma torus, transferring 8 x 10 TM eV cm -2 s -• of kinetic energy to the atmospheric molecules. Both the vertical structure of the atmosphere and the atmospheric loss rate, referred to as sputtering, are calculated for a number of values of the momentum-transfer cross section between the ions and the SO2 molecules. The calculations include UV heating and local thermodynamic equilibrium infrared cooling. It was found that the energy removed by sputtering becomes a significant fraction of the plasma energy deposited for the largest cross sections used, affecting the exobase temperature, but the exobase altitude and density change very little, -1.12-1.13 Io radii and 0.8-1.2 x 108 molecules cm -3. The calculated sputtering yields are compared to results from analytic expressions [Johnson, 1994a]. As the interaction cross section of the incident ion with an atmospheric molecule is decreased, so that the penetration depth increases, the dependence of the yield on the momentum-transfer cross section approaches that calculated using the analytic model. The ejecta were found to have an energy spectra consistent with the analytic model, but the angular distribution of the ejecta and the incident angle dependence can differ significantly depending on cross-section size. The results are used to reevaluate the multiple collision contribution to the sputter loss from Io. Introduction The flow of the solar wind plasma, a plasma trapped in a planetary magnetosphere, or a local pickup ion plasma onto an atmosphere or surface of a planet or planetary satellite can lead to loss of material [Johnson, 1990, 1994a]. For instance, pickup ions impacting the atmosphere of Mars may have played a role in the evolution of its atmosphere [Luhmann et at., 1992; Jakosky et at., 1994; Kass and Yung, 1995], and the Jovian magnetospheric plasma impacting the surface of Europa is probably responsible for the presence of an O: atmosphere on that object [Johnson et at., 1982; Johnson, 1990; Hart et at., 1995]. However, one of the most intriguing problems is the effect of the Jovian plasma on the atmosphere and surface of Io. Energetic ions from the Io plasma torus impact Io's atmosphere [Sieveka and Johnson, 1984; Pitcher et at., 1984] and regions of its surface [Matson et at., 1974; Half et at., 1981; Summers et at., 1983; Sieveka and Johnson, 1985]. Whereas the interaction with the surface produces darkening of the polar regions [Chrisey et at., 1987] and sputtered Na species [Chrisey et at., 1988], the atmospheric bombardment causes loss of material from Io [McGrath and Johnson, 1987], which supplies the neutral Na cloud [Matson et at., 1974; Summers et at., 1983; Smyth and Combi, 1988a, b] and the plasma torus [Huang and Siscoe, 1987; Schneider et at., 1989; Thomas, 1992; Johnson and McGrath, 1993]. The atmosphere near the ½xobas½ is also heated by the plasma [Johnson, 1989], resulting in an ...
