DISK-RESOLVED PHOTOMETRIC ANALYSIS OF EUROPAN TERRAINS

Using Hapke's photometric model we examined the photometric textures of a representative subset of the geologic units on Europa defined by Lucchitta and Soderblom (1982, Satellites of Jupiter, pp. 521-555, University of Arizona Press, Tucson). The individual units studied possess the same small mean photometric slope angle as that found by Domingue et al. (1991, Icarus 90, 30-42) for the entire disk. With one major exception, the particles composing the surfaces of most of the units have similar scattering properties. However, the single-particle scattering functions of the two bright plains regions examined are very different from each other and from the other units. If the bright plains units are geologically younger than the other units, an aging process that affects the single-particle phase functions and albedos is inferred. In our previous study we concluded that the disk-integrated photometric properties of Europa could not be explained adequately by ion bombardment. However, a new study on the effects of ion irradiation on the optical properties of materials (Sack et al. 1991, J. Geophys. Res. E 96(2), 17535-17539) renders this conclusion invalid. © 1992.