In contrast, Johnson (1985) suggests that the polar layers are created by ion bombardment. He proposes that ion
We have modeled Ganymede's polar regions as a frost layer overlying typical non-polar materials using a two-layer photo-bombardment effects are dominant at Europa, marginally metric model. The optical depth of the overlying layer is found important at Ganymede where thermal effects overwhelm to increase from a few tenths at the cap boundary to more than their effect at low latitudes but not at the colder higher four at the poles. Such an increase is consistent with the latitudes (resulting in the polar caps), and unimportant predictions of Johnson's (1985) sputtering model but not with at Callisto.
those of thermal migration models of the formation of Gan-
Thus views of how the polar caps on Ganymede are ymede's polar caps. If the frost forms a distinct layer, it must formed range from those that see the caps as ancient relict be physically thin-on the order of millimeters or lessfeatures to those that see them as the result of ongoing particularly near the cap boundary. Therefore, the mechanism active processes.
responsible for its formation must be ongoing since impact Here we perform a photometric analysis of Ganymede's gardening would destroy such a thin layer on short time scales. polar regions to constrain the properties of the caps includ-Interestingly, the photometric properties of the layer are similar to those of the surface of Europa.