Galileo observations of Europa's opposition effect

During Galileo's G7 orbit, the Solid State Imaging (SSI) camera acquired pictures of the spacecraft shadow point on Europa's surface as well as a comparison set of images showing the same geographic region at phase angle alpha = 5 degrees. Coverage, obtained at three spectral bandpasses (VLT, 0.41 mu m, GRN, 0.56 mu m; and 1MC, 0.99 mu m) at a spatial resolution of 404 m/ pixel, shows a 162 x 220-km region of Europa's surface located at 30 degrees N, 162 degrees W, We have used these images to measure the near-opposition spectrophotometric behavior of four primary europan terrain materials: IR-bright icy material, IR-dark icy material, dark lineament material, and dark spot material. The high spatial resolution of the G7 images reveal low-albedo materials in dark spots that are among the darkest features (17% albedo at 0.56 mu m and 5 degrees phase) yet found on icy Galilean satellites, While material of comparable albedo is found on Ganymede and Callisto, low-albedo europan materials are much redder, All europan surface materials exhibit an opposition effect; however, the strength of the effect, as measured by the total increase in reflectance as phase angle decreases from alpha = 5 degrees to alpha = 0 degrees, varies among terrains, The opposition effects of IR-bright icy and IR-dark icy materials which dominate Europa's surface are about 1.5 times larger than predicted from pre-Galileo studies, Low-albedo materials in dark spots exhibit unusually intense opposition effects (up to four times larger than bright icy europan terrains), consistent with the presence of a strong shadow-hiding opposition surge. The strengths of the opposition surges among average europan terrains systematically vary with terrain albedo and can be explained in terms of the simultaneous contributions of shadow-hiding and coherent-backscatter to the total opposition effect. Coherent backscatter introduces a narrow angular contribution (<0.2 degrees wide) to all europan terrains while the presence of a shadow-hiding contribution is revealed by the fact that the opposition surge is especially strong in low-albedo terrain materials. Stratigraphically young ridges with relatively high topographic relief exhibit anomalously weak opposition surges that could be caused by the presence of relatively coarse-grained regolith, highly compacted particulates, exposures of solid ice, or some combination of these. Very-high resolution images (22 m/pixel) of young-appearing ridges suggest that downslope wasting of particulate cover exposes more lithified ice along ridge crests and wall escarpments. We propose that over time, erosion of ridge topography accompanied by accumulation of detritus and mantling by regolith cover results in more mature ridge surfaces of increasingly lower (mature) porosity. We interpret the dark, reddish color of dark ridges and lineaments as a particulate coating of low-albedo materials vented from (or collected adjacent to) reactivated ridge-margins and within lenticulae. (C) 1998 Academic Press.