Active volcanism on Io: Global distribution and variations in activity

Io's volcanic activity has been monitored by instruments aboard the Galileo spacecraft since June 28, 1996. We present results from observations by th e near-infrared mapping spectrometer (NIMS) for the first 10 orbits of Gali leo, correlate them with results from the Solid State Imaging System (SSI) and from groundbased observations, and compare them to what was known about Io's volcanic activity from observations made during the two Voyager flyby s in 1979. A total of 61 active volcanic centers have been identified from Voyager, groundbased, and Galileo observations. Of these, 41 are hot spots detected by NIMS and/or SSI. Another 25 locations were identified as possib le active volcanic centers, mostly on the basis of observed surface changes . Hot spots are correlated with surface colors, particularly dark and red d eposits, and generally anti-correlated with white, SO2-rich areas. Surface features corresponding to the hot spots, mostly calderas or flows, were ide ntified from Galileo and Voyager images. Hot spot temperatures obtained fro m both NIMS and SSI are consistent with silicate volcanism, which appears t o be widespread on Io. Two types of hot spot activity are present: persiste nt-type activity, lasting from months to years, and sporadic events, which may represent either short-lived activity or low-level activity that occasi onally flares up. Sporadic events are not often detected, but may make an i mportant contribution to lo's heat flow and resurfacing. The distribution o f active volcanic centers on the surface does not show any clear correlatio n with latitude, longitude, Voyager-derived global topography, or heat flow patterns predicted by the asthenosphere and deep mantle tidal dissipation models. However persistent hot spots and active plumes are concentrated tow ard lower latitudes, and this distribution favors the asthenosphere rather than the deep mantle tidal dissipation model. (C) 1999 Academic Press.