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R. T. Pappalardo, K. K. Khurana (Brown), T. Denk (DLR), W. Ip (Max Plank Inst.), T. Rosanova (USGS, Flagstaff), A. S. McEwen (LPL), T. V. Johnson (JPL), P. E. Helfenstein (Cornell), J. W. Head, R. Goyal (Brown), J. Warnecke, M. G. Kivelson (UCLA), G. Neukum (DLR), L. Gaddis, T. Becker (USGS, Flagstaff), Galileo Imaging Team, Galileo Magnetometer Team
Galileo's discovery of a magnetosphere at Ganymede lends support to the hypothesis that charged particle bombardment creates and maintains Ganymede's bright polar caps. Based on modeling of the internal magnetic field of Ganymede, we have obtained the surface location of the boundary between those field lines open to jovian magnetospheric plasma (allowing particles to precipitate onto Ganymede's surface poleward of ~30 to 45 degrees latitude), and field lines which are closed (effectively shielding Ganymede's equatorial regions). We use the violet/green ratio of global-scale Galileo color images to map the location and latitudinal structure of the polar caps; a ratio of ~0.84 is chosen as defining visible cap boundaries, typically between ~30 and 45 degrees latitude. We find good first-order correspondence between the inferred latitudinal extents of the polar caps and the surface regions modeled as open to the Jovian plasma. This suggests that Ganymede's polar caps may owe their existence at least in part to charged particle bombardment, as through sputter-redistribution of water molecules. High-resolution images imply that redistributed water molecules are preferentially cold-trapped on local slopes. We find that the modeled 10 degree tilt of Ganymede's dipolar field, however, is not reflected in the global-scale color image data. Possible explanations are: 1) induction from a conductor inside Ganymede was responsible for the observed 10° tilt of Ganymede's dipole as measured by the Galileo magnetometer; 2) an unmodeled magnetic quadrupole field component exists at Ganymede causing differences between the polar cap and the inferred open/closed field lines boundaries; 3) wander of Ganymede's dipolar field occurs on a time scale shorter than that of cap destruction. Refinements in magnetospheric modeling and image-based cap boundary definition are expected to provide additional insights.