Monte Carlo calculations are used to describe the kinetics of a column of SO2 gas (10(16) cm(-2)) On Io, which is bombarded by a flux of ions from the Io plasma torus, transferring 8 x 10(11) eV cm(-2) s(-1) 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, similar to 1.12-1.13 Io radii and 0.8-1.2 x 10(8) 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.
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