Sputter Ejection of Matter From Io

The direct collisional interaction of magnetospheric particles with Io will lead to sputtering of atoms and molecules from the satellite into circum-Jovian space. The -•520-eV S (and -•260-eV O) ions composing the Io toms are the most effective agents for net sputter emoval of matter from the satellite. An incident flux of-•10 •ø cm -2 s -• is estimated to provide -•5 x 10 •ø S atoms cm -2 s -• from sputtering of a (dayside) atmosphere with an exobase ata few hundred kilometers and up to -•10 • S atoms cm -• s -• from an atmosphere at 1500øK with an exobase at -•2.2 Rio. The supply of S (and O) required to stabilize the toms has been estimated by others to be from 10 •ø to 10 • cm -• s -•. If Na and K are present in the atmosphere at a concentration level of 1%, the corresponding sputtering rates are calculated to be a few times 108 cm -2 s -• for an exobase at several hundred kilometers. These numbers are large enough to supply the 10 7 cm -2 s -• fluxes required to maintain the Na and K clouds. Sputtering can also remove heavy molecules, like Sn, from the atmosphere. At night, direct S sputtering of the unprotected surface is calculated to eject S and Na (1% concentration) at rates given approximately b -•10 •ø and -•108 cm -• s -•, respectively. All atomic species residing on the surface must be ejected into circum-Jovian space at a rate approximately proportional to their (surface) abundances, if direct surface sputtering occurs, so that the particle content of the inner Jovian magnetosphere should map rather faithfully all species present on Io's surface. The processes ofplume sputtering, avalanche cascading, and ionic saltation may lead to spatial and temporal variations in the number of ejected particles.