Erosion and modification of SO2 ice by ion bombardment of the surface of Io

Abstract

We report measurements of total yields, energy spectra, and mass spectra of molecules sputtered from SO2 ice at low temperatures by keV Ar+ ions. These measurements are applicable to corotating sulfur and oxygen ion sputtering of SO2 on Io. We find that the yields are much larger than those estimated earlier for low energy ion sputtering of SO2 and the energy spectra are peaked at lower energies than expected. We observed the irradiation time dependence of the chemical alteration of fresh SO2, leading to the production of new molecular species, and identified masses 32 (O2 or S), 48 (SO), and 80 (SO3 or S2O) as ejected species in addition to SO2. These measurements are used to determine the rate of direct sputter ejection of sulfur into the Jovian plasma torus, the rate of erosion of patches of SO2 ice, and estimates of the surface column densities of SO3 produced in SO2 ice by the corotating and fast ions. We estimate that the direct sputter ejection rate of sulfur into the Jovian plasma is of the order of 1010 atoms/cm2/s, that the erosion rate of fresh SO2 deposits due to sputtering is of the order of 10−3 cm/year, and that only for penetrating ion bombardment can a significant, and possibly observable, column density of SO3 be produced in an SO2 frost.