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

The first disk-resolved image of lo's trailing hemisphere in the ultra violet, as well as a visible image of the same hemisphere, has been ob tained with the Faint Object Camera (FOC) of the Hubble Space Telescop e on March 15, 1992, at a resolution of 210 km. lo shows no large-scal e changes in the visible over the 13 years elapsed since the last disk -resolved imaging by the Voyager missions in 1979, despite the high vo lcanic resurfacing rate estimated from Voyager data. The UV image prov ides a measure of the combined effects of intrinsic surface albedo and absorption by atmospheric SO2 gas, if present, allowing tighter const raints to be placed on both the surface characteristics and the atmosp heric gas content. The gas abundance over SO2 frost-rich regions depen ds on the assumed surface albedo, since the optical effects of the fro st and gas may be competing at 2850 angstrom. Assuming a surface compo sition 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-p oor 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 a greement with previous estimates based on Voyager data. (C) 1994 Acade mic Press, Inc.