MOUNTAINS AND CALDERAS ON IO - POSSIBLE IMPLICATIONS FOR LITHOSPHERE STRUCTURE AND MAGMA GENERATION

The combination of Voyager images and newly acquired Galileo images wi th low illumination and resolutions ranging from 2 to 6 km/pixel now a llows determination of the global distribution of mountains and volcan ic centers on Io. The mountains generally do not have characteristics typical of terrestrial volcanic landforms, they are evenly distributed across the surface and show no obvious correlation with known hot spo ts or plumes. Relative elevations, determined by shadow measurements a nd stereoscopy, indicate that mountains in the newly imaged area range in elevation up to at least 7.6 km. The origin of the mountains remai ns uncertain. Some appear to be multitiered volcanic constructs; other s enclosing the partial remains of large circular depressions appear t o be remnants of old volcanoes; yet others show extensive tectonic dis ruption. Volcanic centers also appear to be distributed evenly across the surface except for an apparently somewhat lower density at high la titudes. The low latitudes have one volcanic center per 7 x 10(4) km(2 ), and, on average, the centers are spaced roughly 250 km apart. The g lobal distribution of high mountains suggests that the lithosphere ove r most of Io is thick. Although the thickness cannot be calculated, th e previously suggested 30 km appears reasonable as a lower limit. The high rates of resurfacing combined with the likely dissipation of most of the tidal energy in the asthenosphere and underlying mantle implie s a very low temperature gradient in the upper part of the lithosphere and steep gradients in the lower lithosphere. The slow rate of separa tion of melt from host rock in the magma source regions as a consequen ce of the low gravity on Io, coupled with the high rate of magma produ ction, will likely result in larger melt fractions than is typical for source regions on Earth. The variety of volcanic landforms suggests t hat volcanic products with a range of compositions are deposited on th e surface. This mixture will be carried downward through the lithosphe re as a consequence of the 0.5-1.5 cm/yr resurfacing rates. During des cent, the more volatile components will tend to be driven off early, b ut complete or near-complete melting at the base of the lithosphere ma y result in rehomogenization of the silicate mixture that remains. (C) 1998 Academic Press.