Disk-Resolved Imaging of Io with the Hubble Space Telescope

Abstract

The first disk-resolved image of Io's trailing hemisphere in the ultraviolet, as well as a visible image of the same hemisphere, has been obtained with the Faint Object Camera (FOC) of the Hubble Space Telescope on March 15, 1992, at a resolution of 210 km. Io shows no large-scale changes in the visible over the 13 years elapsed since the last disk-resolved imaging by the Voyager missions in 1979, despite the high volcanic resurfacing rate estimated from Voyager data. The UV image provides a measure of the combined effects of intrinsic surface albedo and absorption by atmospheric SO 2 gas, if present, allowing tighter constraints to be placed on both the surface characteristics and the atmospheric gas content. The gas abundance over SO 2 frost-rich regions depends on the assumed surface albedo, since the optical effects of the frost and gas may be competing at 2850 Å. Assuming a surface composition in agreement with that inferred from Voyager data and previous spectroscopic measurements, the data are consistent with the presence of a SO 2 patchy atmosphere with a thick gas patch over the SO 2 frost-poor region where Pele is located. An estimate of the SO 2 gas flow rate from this volcano required to produce the observed patch is in good agreement with previous estimates based on Voyager data.