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T. Denk, G. Neukum (DLR), J.W. Head, R.T. Pappalardo (Brown), Galileo SSI Team
Ganymede, as observed by Galileo SSI, shows a banded, latitude-dependent color structure which is partly independent of geologic units. Five different color contrasts are observed: (1) dark "regio" versus bright "grooved" areas; (2) bright "whitish" polar frost versus darker "reddish" polar frost; (3) magnetically shielded (equatorial) versus unshielded (mid-latitude to polar) areas; (4) longitudinal variations (leading side versus trailing side); and (5) bright craters (Osiris, Tros, ...) versus dark craters (Khensu, Kittu, ...).
The brighter grooved and darker "regio" terrains are the "classical" geologic units on Ganymede. The long SSI wavelength slopes (from 0.6-1.0 um) of measured spectra correlate very well with these units, dark areas are redder than grooved terrain. However, at short SSI wavelengths (0.4-0.6 um), this correlation doesn't exist anymore. The "global violet absorber" (of so far unknown composition; which reddens all parts of the surface at short SSI wavelengths) is therefore independent of geologic units.
A correlation of surface color with Ganymede's magnetic field in the northern hemisphere is presumably responsible for the banded appearance of Ganymede at global scale. The "global violet absorber" is stronger within shielded terrain than within areas not shielded by Ganymede's own magnetic field against the Jovian "background" field. Contrary to this, spectra of dark, unshielded areas are redder at long SSI wavelengths than those of shielded terrain, implying that charged particles from the Jovian environment caused a reddening of the dark non-ice material on Ganymede.
Only a rather poor color/ magnetosphere correlation has been found for the southern hemisphere. This might be due to magnetospheric modelling constraints, note that the targeted Galileo Ganymede flybys never occured over the southern hemisphere.
A leading/trailing side color dichotomy has been measured within shielded terrain. The "global violet absorber" appears to be stronger on the trailing side.