Comparison of the relative amplitudes and shapes of opposition surges for different terrains at ␭ ϭ 0.56 Ȑ m. The angular widths of the opposition surges for europan terrains are all broadly similar; however, the amplitudes are systematically larger for darker terrains than for brighter ones. (a) SSI GRN-filter amplitudes. The pre-Galileo Hapke model of Domingue et al. (1991) reasonably represents the opposition surge angular width but significantly underestimates its total strength, probably because telescopic data on which the model is based extends to phase angles no smaller than Ͱ ϭ 0.2 Њ . The Moon’s opposition surge (corrected for the smaller angular size of the Sun at Europa’s orbital distance) is significantly broader than Europa’s, consistent with the interpretation that europan regolith is more porous than the Moon’s (Domingue et al. 1991) or, alternatively, that lunar regolith particles are more opaque than grains on Europa’s surface. (b) SSI VLT-filter amplitudes, (c) SSI 1MC- filter amplitudes. 

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... Opposition Effect (SHOE) and the Coherent Backscatter Opposition Effect (CBOE). In SHOE, the brightness of the surface increases with decreasing phase angle as shadows cast by particles become increasingly occulted by the grains themselves (see Hapke 1986, 1993). Because multiply scattered photons in bright surfaces tend to illuminate particle shadows and reduce contrast, the strength of SHOE should decrease with increasing surface albedo. CBOE occurs because of the preferential construc- tive interference of light at small phase angles from multi- ple wavefronts that are scattered in conjugate directions by particles or surface crenulations (see Kravstov and Saichev 1982, Shkuratov 1985, 1988, Van Albada 1985, Hapke 1990, Shkuratov et al. 1991, Mishchenko 1991, 1992a, 1992b, Mishchenko and Dlugach 1992, 1993, Hapke et al. 1993, 1997). Because the efficacy of CBOE depends on multiply scattered photons, it should be present in bright surfaces even when SHOE cannot be detected. By comparison to the lunar photometric function, where both SHOE and CBOE have been observed in lunar shadow-point observations (Buratti et al. 1996, Helfenstein et al. 1997a, see also Shkuratov et al. 1997) and lunar sample data (Hapke et al. 1998), we anticipate that CBOE may form a very narrow contribution to Europa’s total opposition effect relative to that of SHOE. In the lunar case, the angular width of CBOE is about 2 Њ compared to over 8 Њ for SHOE. The three radiometrically calibrated opposition G7ESVLOFOT images (VLT, GRN, 1MC) are shown in Fig. 1. During the 17-sec interval over which the G7ESVLOFOT images were acquired, the spacecraft shadow point tra- versed 70 km from west to east. In each frame, a localized concentric brightening reveals the location the shadow point (see also Fig. 1g) and confirms the presence of a very narrow angular component of the opposition effect. Using the terrain map of Fig. 2 as a guide, we measured the absolute disk-resolved spectral reflectances of separate terrain types as a function of photometric geometry from the G7ESLVOFOT and G7ESLOWFOT images. Of the four terrain materials (Fig. 2), IR-bright icy and IR-dark icy are the most widely distributed and consequently the most uniformly sampled with phase angle. Sampling of dark lineaments and dark spots is less continuous in phase angle and local differences in the albedos of particular examples introduce greater scatter in the phase curve data (Fig. 3). All of the major terrain classes exhibit qualitatively similar opposition behavior (Fig. 4). In the GRN filter, the behaviors of IR-bright icy and IR-dark icy materials are nearly identical at all phase angles shown. Europan terrains typically exhibit a gradual increase in surface reflectance as phase angle decreases from 5 Њ to about 0.3 Њ . Then, at 0.04 Յ Ͱ Յ 0.3 Њ , a more extreme nonlinear surge in reflectance takes place. At 0.04 Њ , all terrains display a conspicuous flattening toward Ͱ ϭ 0 Њ . The observed flattening near 0 Њ , common among all of our phase curves, is predicted as a consequence both of the finite angular radius of the Sun (0.05 Њ at Europa’s mean solar distance) and the contribution of coherent backscatter (Etemad et al. 1987) to the opposition effect. It was first seen on the Moon in Apollo 8 photographs of the command module shadow point (Pohn et al. 1969, Whitaker 1969), but was curiously absent in early shadow point measurements from Clementine images (Nozette et al. 1994). The role of the finite angular size of the Sun in controlling the phase-curve flattening near Ͱ ϭ 0 Њ was considered by Lumme and Bowell (1981). Shkuratov and Stankevich (1995) and Shkuratov et al. (1997) argued that the apparent absence of the solar flattening in Clementine data brought to question the analysis methods applied by Nozette et al. (1994). The combined effects of flattening due to the Sun’s angular size and to coherent backscatter are considered in Shkuratov (1991) and Helfenstein et al. (1997a). Helfenstein et al. (1997a) obtained satisfactory fits to the opposition flattening seen in Apollo 8 shadow-point data by taking into account both coherent backscatter and the angular size of the Sun. The mean albedos of terrains at 5 Њ phase and corresponding extrapolated opposition albedos are given in Table III and Figs. 5 and 6. To a good approximation, the extrapolated opposition albedos increase linearly with their corresponding albedos at 5 Њ (Fig. 6)—a result useful for estimating normal albedos of europan terrains from images obtained at phase angles that are well outside of the opposition effect. The relative amplitudes of terrain opposition surges can be compared by normalizing all of the phase curves to unity at Ͱ ϭ 5 Њ , as is done in Fig. 7. Shown for comparison is a pre-Galileo model phase curve for Europa derived from photometry of Voyager images and Earth-based telescopic observations (Domingue et al. 1991). Pre-Galileo telescopic whole-disk photometry of Europa extended only to Ͱ ϭ 0.2 Њ —adequate to reveal the presence of an ex- tremely narrow opposition effect but insufficient to measure accurately its total strength. As shown in Fig. 7a, the opposition effect for bright icy terrains, which dominate Europa’s surface, is about 1.5 times stronger than predicted from pre-Galileo studies. Figure 7a also shows a model phase curve for average lunar regolith (Helfenstein et al. 1997a). The lunar curve includes the narrow contribution observed in shadow-point observations of the lunar surface (cf. Helfenstein et al. 1997a, Buratti et al. 1996) and is corrected to account for the Sun’s smaller angular size at Europa’s mean solar distance. Even accounting for the Sun’s small angular radius at Europa (0.05 Њ compared to 0.27 Њ at the Earth’s distance), the narrow component of the Moon’s opposition effect is broader than Europa’s consistent with the interpretation that lunar regolith ...

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... more opaque than ice particles in Europa’s regolith (see Section 3.1). Figure 7 suggests a general trend; the opposition surges for relatively dark europan materials (dark lineaments and dark spots) are systematically more intense than for brighter materials (IR-bright icy and IR-dark icy materials). In Fig. 8, the relative amplitude of each opposition surge is plotted as a function of the reflectance measured at Ͱ ϭ 5 Њ . The solid line is locus of relative amplitudes predicted from the linear least-squares fit from Fig. 6. The figure confirms the tendency for opposition amplitude to strengthen primarily as function of decreasing terrain albedo and reveals that shadow-hiding and coherent-backscatter both contribute to Europa’s opposition surge. The presence of the shadow-hiding opposition surge is revealed by the nonlinear decrease in opposition effect strength with increasing albedo 4 (see Verbiscer and Helfenstein 1998; Fig. 4 of Helfenstein et al. 1997a). The presence of coherent backscatter is indicated by (1) the fact that absolute surface reflectances exceed unity at opposition but are generally less than unity at phase angles greater than 0.1 Њ (the theoretical amplification limit from coherent ...

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... effect (CBOE), in which light scattered along oppos- ing paths through small regolith grains constructively inter- feres (Kravstov and Saichev 1982, Shkuratov 1985, Van Albada 1985, Shkuratov 1988, Hapke 1990, Mishchenko 1991, 1992a, 1992b, Mishchenko and Dlugach 1992, 1993, Hapke et al. 1993, 1997). We find that stratigraphically young ridges on Europa have anomalously weak opposition surge behavior. Finally, we offer some preliminary interpretations of regolith physical properties that would be consistent with the observed range of europan opposition effects and consider implications for the geological emplacement and evolution of europan terrains. To investigate Europa’s opposition effect during Galileo’s G7 orbit around Jupiter, coverage was obtained in three bandpasses 1 (VLT, 0.414 Ϯ 0.013 0.018 Ȑ m; GRN, 0.559 Ϯ 0.032 0.035 Ȑ m; and 1MC, 0.990 Ϯ 0.015 0.031 Ȑ m). The images (Table I) show a 162 ϫ 220-km region of Europa’s surface located at 30 Њ N, 162 Њ W (see Fig. 3) and provide the best spatial resolution (404 m/pixel) of any Galileo multispectral coverage for europan surface features yet obtained. Downlink limitations permitted only a portion of each 800 ϫ 800- pixel frame to be transmitted back to Earth. Special efforts were made to accurately determine the camera pointing geometry and perform radiometric calibration. Details are provided in Appendix I. For clarity, we have separated the following presentation into three discussions. We begin in Section 2.1 by defining four 2 important terrain materials (IR-bright icy material, IR-dark icy material, dark lineament material, and dark spot material) on the basis of their colors and albedos and discussing the relationship of these materials to terrains classified by other workers. In Section 2.2, we explore the average opposition surge behaviors of different europan terrain materials. Finally, in Section 2.3 we identify and analyze features (young-appearing ridges) that exhibit anomalously weak opposition effects in comparison to those of typical europan terrains. Our primary data set consists of reflectance measurements extracted from calibrated Galileo frames listed in Table I as well as images constructed by combining individual frames as color composites (Figs. 2a and 2b) or as ratio images (Figs. 1b, 1d, and 1f ). Table II and Fig. 2c identify the four terrain materials we investigate in Sections 2.1 and 2.2. Average opposition surge behaviors of the terrains are illustrated in Fig. 4 as disk-resolved ‘‘phase curves’’—plots showing the reflectance of the different terrains as functions of phase angle. We have fit smooth curves through these data 3 for the purposes of comparing terrain albedos at two specific phase angles ( Ͱ ϭ 0 Њ and Ͱ ϭ 5 Њ ) and to compare the gross shapes of disk-resolved phase curves over this range of phase angles. Note, however, that our results and conclusions do not depend on the physical interpretation of photometric model parameters. Our conclusions rely only on the satisfactory ability of the photometric model to fit accurately the photometric data over a limited range incidence (31–35 Њ ) and emission angles (32–37 Њ ) and phase angles (0–5 Њ ). Average spectral albedos of terrain units evaluated at 5 Њ phase and at opposition, respectively are listed Table III. They are also plotted as a function of wavelength, separately at 5 Њ phase (Fig. 5a) and at opposition (Fig. 5b). Figure 6 demonstrates how the opposition albedos of terrains vary systematically with their corresponding albedos measured at Ͱ ϭ 5 Њ . In Fig. 7, we use model fits to compare the relative shapes of opposition effects for different terrains and show that their relative amplitudes significantly vary. This fact is confirmed by the nonuniform appearance of Figs. 1b, 1d, and 1f which show ‘‘phase- ratio’’ images constructed by dividing opposition frames (G7ESVLOFOT) by their 5 Њ phase counterparts (G7ESLOWFOT). To measure relative amplitudes of opposition surges, we define the ratio of the opposition albedo to that measured at Ͱ ϭ 5 Њ . Figure 8 shows how the relative amplitudes of different europan terrains vary in complicity with their corresponding 5 Њ albedos. Figures 1b, 1d, and 1f reveal linear features with anomalously weak opposition surges in comparison to average terrain materials. The locations of these features are mapped in Fig. 1g. Figure 1h is a G1 image obtained at high incidence angle and shows that the anomalous features correspond to prominent topographic ridges. Disk-resolved ‘‘phase curves’’ of the anomalous ridges are pre- sented in Fig. 9, and spectral albedos are listed in Table ...