A two-stage mechanism for escape of Na and K from Io

It is generally accepted that Io is the source of S, O, Na and K which, after ionization, form the constituents of the Io plasma torus. The escape of S and O from Io can be understood in terms of the photochemistry of a predominantly SO_2 atmosphere created by the high vapour pressure of SO_2 (refs 1, 15). However, the vapour pressures of Na_2S, K_2S and other common compounds containing Na and K are negligible at the surface temperatures of Io. This has given rise to the suggestion that over part of Io's surface (the nightside) the atmosphere is thin enough so that surface sputtering by co-rotating ions can eject Na and K directly into the Io torus. The main objection to this idea is that it implies a ‘Sun-locked’ source for Na and K, while observations of the Na and K clouds around Io indicate a ‘Jupiter-locked‘ ejection mechanism. We propose here that Na and K escape from Io in two stages. Atoms of Na and K are first sputtered into the atmosphere from the surface by high-energy magnetospheric ions. Atmospheric sputtering by low-energy co-rotating ions then removes these constituents (along with others present) out of Io's gravitational field. We suggest that the observed Na and K ejection asymmetry is due to preferential sputtering of atmospheric particles on the hemisphere of Io facing Jupiter. The estimated injection rates are sufficiently large to maintain the observed K, Na, and O clouds observed around Io