An upper limit to the global SO2 abundance on Io

Gaseous sulphur dioxide has been detected on Io by the Voyager 1 IRIS experimemt 1 . It was associated with a major volcanic plume, and it was observable only because the local underlying surface was warm enough (>210K) to be detected near 7 µm, and thereby reveal the gas in absorption. The estimated column abundance of SO 2 was ∼0.2cm atm. At most other locations on Io, the surface was not warm enough to be detected at this wavelength (S. Kumar, personal communication). A refined Io model for an SO 2 atmosphere in equilibrium with the solid at local surface temperatures predicts a global average column abundance of SO 2 of 0.032cmatm (J. Pearl, personal communication). Furthermore Smythe et al. 2 and Fanale et al. 3 have shown the similarity between ground-based observations of the reflectivity of Io from 1.0 to 4.2 µm and laboratory observations of SO 2 frost. We demonstrate here that the near UV reflectivity of Io, measured from Earth-orbit, demands that the hemispherical average column abundance of SO 2 at the time of our observation was very much less than the local abundance quoted above. Our limit is also less than the average abundance the model would predict, although we do find the observations to be consistent with the presence of condensed SO 2 on Io's surface. A qualitative reconciliation of these results is presented.