The atmosphere of Io: Abundances and sources of sulfur dioxide and atomic hydrogen

An analysis and interpretation of reflected solar Lyman alpha intensity dat a acquired with the Hubble Space Telescope (HST) implies an equatorially co nfined atmosphere with SO2 column densities similar to 1-2 x 10(16) cm(-2). Poleward of 30 degrees the SO2 density must decrease sharply reaching an a symptotic polar value of < 10(15) cm(-2) at 45 degrees to achieve the obser ved 2 kR intensity peaks. The corresponding surface reflectivities must be either a constant 0.047 for higher equatorial SO2 or a variable reflectivit y of 0.027 with lower SO2 densities at the equator increasing to a polar va lue of similar to 0.05. The average residence time for an atmospheric SO2 m olecule is similar to 2-3 days for the canonical mass loading rate of the I o plasma torus = 10(30) amu s(-1). With atomic hydrogen in the atmosphere a nd corona constrained by the HST observations, it is estimated that a picku p proton density ratio of 0.25-0.4% can be sustained by a supply of Io plas ma torus protons neutralized in Io's atmosphere/exosphere, if protons const itute 7% of the total torus ion density, which is close to the Chust et al. (1999) pickup proton density ratio and under the widely quoted 10% proton content of the torus.