IUES VIEW OF CALLISTO - DETECTION OF AN SO2 ABSORPTION CORRELATED TO POSSIBLE TORUS NEUTRAL WIND ALTERATIONS

Observations taken with the International Ultraviolet Explorer (IUE) d etected a 0.28 mu m absorption feature on Callisto's leading and Jupit er-facing hemispheres. This feature is similar to Europa's 0.28 mu m f eature, however it shows no correlation with magnetospheric ion bombar dment. The strongest 0.28 mu m signature is seen in the region contain ing the Valhalla impact. This absorption feature also shows some spati al correlation to possible neutral wind interactions, suggestive of S implantation (rather than S-X) into Callisto's water ice surface. Indi cations of possible temporal variations (on the 10% level) are seen at other wavelengths between the '84-'86 and the '96 observations.