Galileo Observations of Europa’s Opposition Effect

(17% albedo at 0.56 mm and 58 phase) yet found on icy Galilean satellites. While material of comparable albedo is found on During Galileo’s G7 orbit, the Solid State Imaging (SSI) Ganymede and Callisto, low-albedo europan materials are camera acquired pictures of the spacecraft shadow point on much redder. All europan surface materials exhibit an opposiEuropa’s surface as well as a comparison set of images showing tion effect; however, the strength of the effect, as measured by the same geographic region at phase angle a 5 58. Coverage, the total increase in reflectance as phase angle decreases from obtained at three spectral bandpasses (VLT, 0.41 mm, GRN, a 5 58 to a 5 08, varies among terrains. The opposition effects 0.56 mm; and 1MC, 0.99 mm) at a spatial resolution of 404 m/ of IR-bright icy and IR-dark icy materials which dominate pixel, shows a 162 3 220-km region of Europa’s surface located Europa’s surface are about 1.5 times larger than predicted from at 308N, 1628W. We have used these images to measure the pre-Galileo studies. Low-albedo materials in dark spots exhibit near-opposition spectrophotometric behavior of four primary unusually intense opposition effects (up to four times larger europan terrain materials: IR-bright icy material, IR-dark icy than bright icy europan terrains), consistent with the presence material, dark lineament material, and dark spot material. The of a strong shadow-hiding opposition surge. The strengths of high spatial resolution of the G7 images reveal low-albedo materials in dark spots that are among the darkest features the opposition surges among average europan terrains system-