The Brightness of Jupiter's Satellite Io Following Emergence from Eclipse: Selected Observations, 1981-1989

We have used two-dimensional imaging systems to monitor the brightness of Jupiter's satellite Io relative to the brightness of a comparison object following Io's emergence from eclipse by Jupiter on 14 occasions from 1981 to 1989. We find no instances in which Io appeared more than ∼2% brighter than nominal following eclipse reappearance. Although a 2% posteclipse brightening effect cannot be ruled out, the effects seen are of the same size as the 2% error associated with each observation on a given night. We have found no posteclipse brightenings of the ⋧ 15% size that have been reported in the literature. If such posteclipse brightenings are indeed real, then they must be intermittent. If they are caused by condensation followed by evaporation of SO ₂ frost, then removal of the SO ₂ frost by solar heating alone is a very marginal explanation of even a 5% post-eclipse brightening event. The thickness of the frost layer required to provide an optically thick layer may be so large that solar insolation alone may not provide sufficient energy to remove this layer with sufficient speed. An additional source of energy may be required in order to remove the SO ₂ frost in the requisite amount of time.