Erosion of Frozen Sulfur Dioxide by Ion Bombardment: Applications to Io

The erosion of frozen SO 2 due to bombardment by both light and heavy ions (He and F) was measured for bombarding energies of 0.08 to 1.3 MeV/amu. The number of SOp molecules ejected from the target per incident ion (i.e.• the sputtering yield) was 50 for 1.5 MeV He ions and ?300 for 6 MeV F ions. Ion bombardment followed by heating produced an oxygen/sulfur residue which was much more stable against subsequent ion bombardment than the initial frozen SO 2. The erosion rate of SO 2 frost on Jupiter's moon Io depends strongly on the elemental composition and energy spectra of the magnetospheric ion flux which bombards the surface. The combined effects of ion bombardment and heating which produced residues on our target substrates may also occur on Io from magnetospheric ion bombardment and heating by volcanism. Our experimental results compare favorably with a new model of the sputtering process which considers the energy loss of the incident ion to electronic excitation in the target. the existence of an enhanced sputtering effect when insulating targets are bombarded with ions having energies in the 0.1-1.0 MeV/amu range (Biersack and Santner• 1976; Brown et al.• 1980; Griffith et al.• 1980; Qiu et al.• 1982; DHck et al.• 1980; Oilerhead et al.• 1980). The effect has been observed in diver'se materials including CsI• ergosterol• UFk• KCi• H20 • A1203• LiNb03• X% S i02• and CaF 2. Frozen S02 also shows this effect and the data presented here are of interest for comparison with the previously studied materials to help in deciding which of several proposed models best describes the enhanced high energy sputtering process. Fluorine was chosen for J-he heavy ion bombardment in the present study to enable direct comparison with sputtering data on other materials• especially UFk and H20. In addition• fluorine is close in mass and nuclear charge to oxygen which is a major component of the Jovian magnetospheric ion flux. II. EXPERIMENTAL PROCEDURE