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 [[micro]meter], GRN, 0.56 [[micro]meter]; and 1MC, 0.99 [[micro]meter]) 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 [[micro]meter] 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 ( Key Words: Europa; photometry; regolith; spectrophotometry; ices; albedo; spectra; geological processes; Galileo; Voyager.