Monte Carlo calculations of plasma ion‐induced sputtering of an atmosphere: SO2 ejected from Io

Abstract

Monte Carlo calculations are used to describe the kinetics of a column of SO2 gas (1016 cm−2) on Io, which is bombarded by a flux of ions from the Io plasma torus, transferring 8 × 1011 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, ∼1.12–1.13 Io radii and 0.8–1.2 × 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.