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

An analysis and interpretation of reflected solar Lyman alpha intensity data acquired with the Hubble Space Telescope (HST) implies an equatorially confined 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 asymptotic polar value of < 10(15) cm(-2) at 45 degrees to achieve the observed 2 kR intensity peaks. The corresponding surface reflectivities must be either a constant 0.047 for higher equatorial SO2 or a variable reflectivity of 0.027 with lower SO2 densities at the equator increasing to a polar value of similar to 0.05. The average residence time for an atmospheric SO2 molecule is similar to 2-3 days for the canonical mass loading rate of the Io plasma torus = 10(30) amu s(-1). With atomic hydrogen in the atmosphere and corona constrained by the HST observations, it is estimated that a pickup proton density ratio of 0.25-0.4% can be sustained by a supply of Io plasma torus protons neutralized in Io's atmosphere/exosphere, if protons constitute 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.