LATITUDE VARIATIONS OF THE POLAR CAPS ON GANYMEDE

We have modeled Ganymede's polar regions as a frost layer overlying ty pical non-polar materials using a two-layer photometric model. The opt ical depth of the overlying layer is found to increase from a few tent hs at the cap boundary to more than four at the poles. Such an increas e is consistent with the predictions of Johnson's (1985) sputtering mo del but not with those of thermal migration models of the formation of Ganymede's polar caps. If the frost forms a distinct layer, it must b e physically thin-on the order of millimeters or less-particularly nea r the cap boundary. Therefore, the mechanism responsible for its forma tion must be ongoing since impact gardening would destroy such a thin layer on short time scales. Interestingly, the photometric properties of the layer are similar to those of the surface of Europa. (C) 1996 A cademic Press, Inc.