SEARCH FOR TEMPERATURE-RELATED ALBEDO CHANGES IN NIGHTSIDE AND POSTECLIPSE IMAGES OF IO

Using an image-summing process that increases the visibility of Jupite rlit surface features in Voyager images, we have produced the best-eve r violet-filter image of the nightside of Io and the best-ever nightsi de/dayside brightness ratio map of this jovian moon. The ratio map sho ws no convincing evidence, on either global or local scales, of diurna l temperature-dependent albedo variations. We have also taken an image -ratioing technique developed by O'Shaughnessy et al. (1989, Lunar Pla net. 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. I n none of the three imaging sequences do we find any isolated surface regions that convincingly exhibit posteclipse temperature-related albe do variations. These negative results suggest that on Io, pure cyclo-o ctasulfur (S8), and transient nighttime or in-eclipse deposits of SO2 frost, are at best limited to isolated areas smaller than the resoluti on of the images in use (i.e., smaller than a few tens of kilometers i n size). Such limits are consistent with (1) the negative results repo rted by the majority of telescopic observers who have searched for pos teclipse brightening of Io, (2) indications that physical processes in the ionian surface environment will change any S8 into other allotrop es of sulfur, and (3) suggestions that Io's atmosphere is too thin to allow the deposition of transient, optically thick SO2 frost layers at nighttime or during eclipse. (C) 1994 Academic Press, Inc.