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D. P. Simonelli, L. Rossier, P. C. Thomas, J. Veverka (Cornell University), Galileo SSI Team
Prior to the Galileo mission, little was known about the albedos and colors of the small inner jovian satellites Thebe and Metis. The common assumption was that these satellites were similar to Amalthea, in spite of a few telescopic hints that there might be albedo/color differences from moon to moon (e.g., Pascu et al.\ 1992; Icarus 98, 38--42). Images taken by the Galileo SSI camera, however, reveal surprisingly large differences in albedo and color among Thebe, Amalthea, and Metis.
Disk-resolved photometric data sampled from clear-filter Galileo images of these three satellites (effective wavelength 0.64 \mum) have been fit with the Hapke photometric function, and the Hapke solution for each satellite has been used to extrapolate the images to zero phase to allow accurate comparison of albedos. Analysis of images acquired through Galileo's 9th orbit is reported in Thomas et al.\ (1998, in press, Galileo special issue of Icarus); the photometric analysis has since been updated to incorporate images from the spacecraft's 10th and 11th orbits. We conclude that Amalthea (global-average geometric albedo \approx 0.09) is much brighter than either Thebe or Metis (geometric albedos of 0.05--0.06). On all three satellites the leading side is brighter than the trailing side by 25% to 35%; albedo maps illustrating the leading/trailing differences are currently in preparation.
A definite color gradient is observed in the Galileo data, with the satellites closer to Jupiter being redder: the mean violet/green ratio (0.42 \mum/0.56 \mum) decreases significantly from Thebe to Amalthea to Metis. This ratio is also lower for the trailing sides of Thebe and Amalthea than for their leading sides.