Search for Temperature-Related Albedo Changes in Nightside and Posteclipse Images of Io

Abstract

Using an image-summing process that increases the visibility of Jupiterlit surface features in Voyager images, we have produced the best-ever violet-filter image of the nightside of Io and the best-ever nightside/dayside brightness ratio map of this jovian moon. The ratio map shows no convincing evidence, on either global or local scales, of diurnal temperature-dependent albedo variations. We have also taken an image-ratioing technique developed by O'Shaughnessy et al. (1989, Lunar Planet. Sci. 20, 812-813), which those authors applied to Voyager violet-filter observations of one Io eclipse reappearance, and extended it to two other, higher-resolution Voyager posteclipse imaging sequences. In none of the three imaging sequences do we find any isolated surface regions that convincingly exhibit posteclipse temperature-related albedo variations. These negative results suggest that on Io, pure cyclo-octasulfur (S 8), and transient nighttime or in-eclipse deposits of SO 2 frost, are at best limited to isolated areas smaller than the resolution of the images in use (i.e., smaller than a few tens of kilometers in size). Such limits are consistent with (1) the negative results reported by the majority of telescopic observers who have searched for posteclipse brightening of Io, (2) indications that physical processes in the ionian surface environment will change any S 8 into other allotropes of sulfur, and (3) suggestions that Io's atmosphere is too thin to allow the deposition of transient, optically thick SO 2 frost layers at nighttime or during eclipse.