DISK-RESOLVED IMAGING OF IO WITH THE HUBBLE-SPACE-TELESCOPE

The first disk-resolved image of lo'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. lo 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 SO2 gas, if present, allowing tighter constraints to be placed on both the surface characteristics and the atmospheric gas content. The gas abundance over SO2 frost-rich regions depends on the assumed surface albedo, since the optical effects of the frost and gas may be competing at 2850 angstrom. 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 SO2 patchy atmosphere with a thick gas patch over the SO2 frost-poor region where Pele is located. An estimate of the SO2 gas flow rate from this volcano required to produce the observed patch is in good agreement with previous estimates based on Voyager data. (C) 1994 Academic Press, Inc.