Global Color Variations on Io
P.E. Geissler, A.S. McEwen (LPL, Univ. Arizona), D.P. Simonelli (Cornell), R. Lopes-Gautier, A.G. Davies (JPL), J. Granahan (SETS Technology, Inc. and U. Hawaii), T. Denk (DLR, Berlin, Germany), Galileo Imaging Team
Galileo multispectral imaging of Io augments existing Voyager color data by extending the sensitivity of the observations to near-infrared wavelengths. Global 4-color coverage was obtained during Galileo's first two orbits, at resolutions ranging from 10 to 23 km/pixel and phase angles from 4 to 55 degrees, and again during the 4th orbit with a similar range of resolutions and phase angles. We have used these data to derive the distribution of bright red, presumably pyroclastic deposits such as produced by the currently active plume Pele. This unit is indistinct in Voyager images but can be effectively mapped in the longer wavelength data from Galileo. The bright red materials appear to be associated with high temperature, low albedo edifices which show significant change between the Voyager and Galileo eras. Because of their association with regions of current or recent geologic activity, the bright red deposits are inferred to have been recently emplaced. Preliminary comparisons of Voyager and Galileo data suggest that the red materials fade over relatively short time scales, consistent with their interpretation  as metastable sulfur allotropes. Further analysis should allow us to quantify the rates of optical alteration and compare to possible mechanisms such as phase transformations, sublimation, and sputtering by charged particles.
 Spencer, J., et al., Volcanic resurfacing of Io: Post-repair HST imaging, Icarus, in press, 1997.