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, and black hues decorate the satellite, presumably caused by a varied composition of sulfur compounds and silicates. Almost a third of Io is covered by red and orange materials, particularly at polar latitudes above ±30°. These red regions were scarcely distinguishable in the shorter wavelength Voyager observations. Bright red pyroclastic deposits mark the locations of many hot spots, plumes, and visible surface changes, providing a prominent flag of recent volcanic activity. Io's equatorial regions are dominated by yellow materials, which occupy about 40% of the satellite's surface. White and gray materials cover about 27% of Io, primarily in equatorial areas and in localized deposits at high latitudes. These are identified with moderate-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-yellow materials in small isolated spots on Io's anti-Jupiter hemisphere were recently discovered in 3 km/pixel color imaging from orbit 14. Unlike other ionian 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 μm, suggesting magnesium-rich silicates rather than black sulfur. Little large-scale alteration in the global color and albedo pattern has occurred between the Voyager and Galileo eras; 90% of the surface appears unchanged despite the vigorous volcanic activity which 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.