Localized deposition and sputtering of jovian ionospheric sodium on IO

Because of relative motion between the innermost Galilean satellite Io and Jupiter's ionosphere, a current is drawn from the ionosphere that can be a source of both deposition on, and sputtering from, the surface of Io. We show that the ions in this current strike lo in a localized region in the quadrant bounded by a line connecting lo and Jupiter and a tangent line extended in the direction of Io's orbital motion. If these ions are the principal source of sodium that is sputtered from Io, then this current provides a simple explanation of the observation of a localized area from which sodium ions escape from Io. The geometry of this current may also affect the optical surface of Io. We suggest several experimental tests that can determine the compatibility of this hypothesis with the directly observable properties of Io's surface.