Satellites in the Jovian and Saturnian system exhibit differences in reflectivity between their ''leading'' and ''trailing'' surfaces which can affect the local vapor pressure. Since these differences are thought to be due to differences in the flux of bombarding magnetospheric ions, we studied the influence of ion impact on the UV-visible reflectance of water ice surfaces (20--90 K) by keV ion bombardment. An observed decrease in reflectance in the UV is attributed to rearrangement processes that affect the physical microstructure and surface ''roughness.'' The ratio in reflectance of bombarded to freshly deposited films is compared to the ratio of the reflectance of the leading and trailing hemispheres for Europa and Ganymede. ¿ American Geophysical Union 1991