Paola Sartoretti

The Pele Plume (Io): Observations with the Hubble Space Telescope

In July 1996, with the Hubble Space Telescope (HST), we observed the Pele plume silhouetted against Jupiter at a wavelength of 0.27µm, the first definitive observation of an Io plume from Earth. The height, 420 ± 40 km, was greater than any plume observed by Voyager. The plume had significantly smaller optical depth at 0.34 and 0.41µm, where it was not detected. The wavelength dependence of the optical depth can be matched by a plume either of fine dust, with minimum mass of 1.2 × 109 g and maximum particle size of 0.08µm, or of SO2 gas with a column density of 3.7 × 1017 cm−2 and total mass of 1.1 × 1011 g. Our models suggest that early Voyager imaging estimates of the minimum mass of the Loki plume [Collins, 1981] may have been too large by a factor of ∼ 100. We may have detected the Pele plume in reflected sunlight, at 0.27µm, in July 1995, but did not see it 21 hours earlier, so the plume may be capable of rapid changes.

Post-Voyager brightness variations on Io

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 A 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.