POST-VOYAGER BRIGHTNESS VARIATIONS ON IO

Imaging of Io with the faint object and planetary cameras of the Hubbl e space telescope in 1992 and 1993 at wavelengths of similar to 3450, 3700, and 4100 Angstrom shows two surface areas that have undergone si gnificant, large-scale change in reflectivity since the 1979 Voyager e ncounters. The first is located in Colchis Regio and covers similar to 10(6) km(2) between longitudes 150 degrees-180 degrees and latitudes -25 degrees to +30 degrees; the second is centered at longitude simila r to 130 degrees, latitude similar to + 30 degrees, and extends for si milar to 10(5) km(2). Both areas have darkened by greater than or equa l to 45% since 1979. In light of the active volcanism discovered on Io by the Voyager I encounter, it seems reasonable to infer that these l arge-scale changes in surface morphology are due to some type of volca nic 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 d arker deposits, or previously active eruptions of the Prometheus type (or smaller-scale venting), which apparently produce SO2 gas that is b right in the visible when it condenses as surface frost, have become i nactive.