ACTIVE VOLCANISM ON TO AS SEEN BY GALILEO SSI

Active volcanism on Io has been monitored during the nominal Galileo s atellite tour from mid 1996 through late 1997, The Solid State Imaging (SSI) experiment was able to observe many manifestations of this acti ve volcanism, including (1) changes in the color and albedo of the sur face, (2) active airborne plumes, and (3) glowing vents seen in eclips e. About 30 large-scale (tens of kilometers) surface changes are obvio us from comparison of the SSI images to those acquired by Voyager in 1 979, These include new pyroclastic deposits of several colors, bright and dark flows, and caldera-floor materials. There have also been sign ificant surface changes on Io during the Galileo mission itself, such as a new 400-km-diameter dark pyroclastic deposit around Pillan Patera , While these surface changes are impressive, the number of large-scal e changes observed in the four months between the Voyager 1 and Voyage r 2 flybys in 1979 suggested that over 17 years the cumulative changes would have been much more impressive. There are two reasons why this was not actually the case, First, it appears that the most widespread plume deposits are ephemeral and seem to disappear within a few years. Second, it appears that a large fraction of the volcanic activity is confined to repeated resurfacing of dark calderas and flow fields that cover only a few percent of Io's surface. The plume monitoring has re vealed 10 active plumes, comparable to the 9 plumes observed by Voyage r. One of these plumes was visible only in the first orbit and three b ecame active in the later orbits, Only the Prometheus plume has been c onsistently active and easy to detect. Observations of the Pele plume have been particularly intriguing since it was detected only once by S SI, despite repeated attempts, but has been detected several times by the Hubble Space Telescope at 255 nm, Pele's plume is much taller (460 km) than during Voyager 1 (300 km) and much fainter at visible wavele ngths. Prometheus-type plumes (50-150 km high, long-lived, associated with high-temperature hot spots) may result from silicate lava flows o r shallow intrusions interacting with near-surface SO2. A major and su rprising result is that similar to 30 of Io's volcanic vents glow in t he dark at the short wavelengths of SSI, These are probably due to the rmal emission from surfaces hotter than 700 K (with most hotter than 1 000 K), well above the temperature of pure sulfur volcanism. Active si licate volcanism appears ubiquitous, There are also widespread diffuse glows seen in eclipse, related to the interaction of energetic partic les with the atmosphere. These diffuse glows are closely associated wi th the most active volcanic vents, supporting suggestions that Io's at mosphere is dominated by volcanic outgassing. Globally, volcanic cente rs are rather evenly distributed. However, 14 of the 15 active plumes seen by Voyager and/or Galileo are within 30 degrees of the equator, a nd there are concentrations of glows seen in eclipse at both the sub- and antijovian points. These patterns might be related to asthenospher ic tidal heating or tidal stresses. Io will continue to be observed du ring the Galileo Europa Mission, which will climax with two close flyb ys of Io in late 1999. (C) 1998 Academic Press.