Sputtering of sulfur by kiloelectronvolt ions: Application to the magnetospheric plasma interaction with Io

Abstract

Measurements of total yields, temperature dependences, mass spectra, and energy spectra of molecules sputtered from condensed sulfur (S 8) at low temperatures by keV ions are reported and results are given for Jovian plasma ion bombardment of Io. A change in the reflectance of the sulfur, which can be removed by annealing, is produced by the most penetrating ions and may be connected with the darker, colder polar regions on Io. The measured sputtering yields are much lower than those estimated earlier for room temperature sulfur films but are comparable to previous measurements of keV ion sputtering of SO 2 at low temperatures. The corrected mass spectrum indicates that ≈66% of the total yield corresponds to S 2 ejection while only 5 and 16% correspond to S and S 3, respectively. Therefore, if ions reach the surface of Io its atmosphere will have a non-negligible sulfur component of primarily S 2. The ejection of S and S 2 is temperature independent for temperatures characteristic of most of the surface of Io. The energy spectrum for S has an approximate 1/ E 2 dependence at high ejection energies, whereas S 2 and S 3 fall off more rapidly. Assuming 50% coverage of both sulfur and SO 2 and a thin atmosphere (e.g., nightside and polar region) the direct sputter injection of sulfur atoms and molecules into the Jovian plasma torus and the indirect injection due to coronal processes are estimated. These injection rates for sulfur are compared to those for SO 2 showing that injection from sulfur deposits contributes 13% to the total mass injection rate of ∼2–3 × 10 29 amu/sec.