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.