Global color variations on Io

Visible and near-infrared images of Io from the Galileo spacecraft reveal a surface more colorful than previously thought. Red, yellow green, white, a nd black hues decorate the satellite, presumably caused by a varied composi tion of sulfur compounds and silicates. Almost a third of Io is covered by red and orange materials, particularly at polar latitudes above +/-30 degre es. These red regions were scarcely distinguishable in the shorter waveleng th Voyager observations. Bright red pyroclastic deposits mark the locations of many hot spots, plumes, and visible surface changes, providing a promin ent flag of recent volcanic activity. Io's equatorial regions are dominated by yellow materials, which occupy about 40% of the satellite's surface. Wh ite and gray materials cover about 27% of Io, primarily in equatorial areas and in localized deposits at high latitudes. These are identified with mod erate-to-coarse-grained SO2 as mapped by the NIMS instrument on Galileo (R. W. Carlson et al, 1997, Geophys, Res. Lett. 24, 2479-2482). Greenish-yello w materials in small isolated spots on Io's anti-Jupiter hemisphere were re cently discovered in 3 km/pixel color imaging from orbit 14. Unlike other i onian terrains, these regions have a negative near-infrared spectral slope, suggesting contamination by a nonsulfur component. Only about 1.4% of Io's surface is occupied by dark materials, which display a variety of visible colors ranging from black to red and green. Most dark spots have a shallow spectral absorption feature at 0.9 mu m, suggesting magnesium-rich silicate s rather than black sulfur. Little large-scale alteration in the global col or and albedo pattern has occurred between the Voyager and Galileo eras; 90 % of the surface appears unchanged despite the vigorous volcanic activity w hich must have taken place in the intervening 17 years. This suggests that over a time scale of decades, the bulk of Io's resurfacing is restricted to a few small but persistently active areas. (C) 1999 Academic Press.