Abstract
Results of Voyager observations intended to settle the question of the transient brightening of the Io disk after its emergence from the Jovian shadow are presented. Two observation sequences were obtained on each Voyager flight through the violet filters of the Narrow Angle Cameras at frame intervals of 96 sec. No posteclipse was detected in the disk-integrated brightnesses in any of the sequences to within a few percent accuracy for a time span of up to 30 min following eclipse reappearance. Analysis of several representative individual areas on the disks of Io and Europa confirms the full-disk observations and is in contrast to telescopic observations showing a brightening of 10% just after eclipse which disappears on a time scale of 20 to 15 min. A small amount of brightening lasting about 3 min is found in the south polar region, along with some posteclipse darkening in neighboring regions. Observations imply that yellow sulfur (S8), the reflectance of which changes significantly over the eclipse temperature range, can not be a major constituent of the Io surface, and that the atmospheric column density of SO2 must be significantly less than 0.2 cm-atm at local noon.
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