SPUTTERING OF WATER ICE SURFACES AND THE PRODUCTION OF EXTENDED NEUTRAL ATMOSPHERES

Plasma and UV photon bombardment of an icy object in the outer solar s ystem can lead to ejection of atoms and molecules from the surface whi ch can, in turn, produce an extended neutral atmosphere. We present ne w laboratory studies of tile sputtering of water ice by keV ions (H+ t hrough Ne+) made using a sensitive microbalance technique that allows measurements at very low ion fluences. These results for the sputterin g yield of ice by keV O+ ions, the dominant sputtering agents in the S aturnian magnetosphere, are much larger than those used previously to model the neutral cloud associated with the icy satellites. The data p resented are used to recalculate previously published sputtering rates for the icy satellites of Jupiter and Saturn, and for the E-ring grai ns at Saturn. The new results call account, in part, for the discrepan cy between the predicted and observed OH Cloud near Tethys in Saturn's inner magnetosphere. We compare the yields induced by tile incident i ons to the recently measured UV photosputtering yield, and discuss pos sible synergism between UV photon and plasma ion induced erosion.