Europa: Color, Photometry, and the Appearance of Surface Features

C.B. Phillips, A.S. McEwen, P.E. Geissler (LPL), B.E. Clark (Cornell), R. Sullivan, R. Greeley (ASU), Galileo SSI Team

The new Europa images currently being acquired by the Galileo spacecraft, in addition to the Voyager dataset, provide an excellent starting point for understanding the properties of Europa's icy surface. Classification of spectral units on the surface of Europa is important for relative age dating of surface features [1,2], but first we must normalize variations due to photometric angles and global exogenic modification pattern(s). Accurate cartography is also important to compare tectonic features with global-scale stress patterns. We report continuing progress in the production of global-scale merged mosaics of Europa, combining Voyager and Galileo data taken in a variety of colors, resolutions, and phase angles [3]. Due to the limited nature of this dataset, understanding how the appearance of surface features changes due to various perturbations of these parameters is necessary for compositional analysis, geologic mapping and classification of features, and relative age dating. Overlapping datasets can also be merged to produce images which retain characteristics of each individual observation. We will show examples in which low-resolution color, medium-resolution albedo, and high-resolution topography observations have been combined to produce a single image which reveals both spectral information and relative ages of surface features. As described by Geissler et al [1], distinct spectral units of the surface may represent different exposure ages of surface material, which could have important implications for the geologic history of Europa, and by extension the other Galilean satellites. It is our hope that a careful analysis of the merged Galileo-Voyager dataset for Europa will result in an improved understanding of this problem.

[1] Geissler, P.E., et al, LPSC XXVIII, 1997; [2] Clark, B.E., et al, LPSC XXVIII, 1997; [3] Phillips, C.B., et al, LPSC XXVIII, 1997