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.