Post-Voyager brightness variations on Io

Abstract

Imaging of Io with the faint object and planetary cameras of the Hubble space telescope in 1992 and 1993 at wavelengths of ~3450, 3700, and 4100 Å shows two surface areas that have undergone significant, large-scale change in reflectivity since the 1979 Voyager encounters. The first is located in Colchis Regio and covers ~10⁶ km² between longitudes 150°-180° and latitudes -25° to +30° the second is centered at longitude ~130°, latitude ~+30°, and extends for ~10⁵ km². Both areas have darkened by >=45° since 1979. In light of the active volcanism discovered on Io by the Voyager 1 encounter, it seems reasonable to infer that these large-scale changes in surface morphology are due to some type of volcanic activity in the intervening 14 years. We hypothesize two possible causes for these darkenings; either large-scale eruptions of the Pele type have covered areas of existing SO₂ surface frost with new, much darker deposits, or previously active eruptions of the Prometheus type (or smaller-scale venting), which apparently produce SO₂ gas that is bright in the visible when it condenses as surface frost, have become inactive.