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B.E. Clark, P. Helfenstein, J.F. Bell III, J. Veverka (Cornell University), N.I. Izenberg, D. Domingue (Johns Hopkins), D. Wellnitz, L. McFadden (University of Maryland)
A few hours before going into orbit on February 14, 2000, the NEAR spacecraft flew through the zero phase point between the Sun and Eros, obtaining spectra of the surface from 0.8 to 2.5 microns, at spatial resolutions of 5.5 to 2.5 km/spectrum, and at phase angles ranging from 1 to 37 degrees. Several weeks later during the orbital phase of the mission, NEAR obtained spectra at comparable spatial resolutions at phase angles from 72 to 112 degrees. These data were combined to derive the average disk-resolved spectral photometric behavior of Eros. For each of 52 wavelengths, we fit a five-parameter Hapke model photometric function to the reflectance data. In this paper we present the results of these models and show that the models can be used to remove spectral variations caused by phase effects and viewing geometry to within ~10%. Our model parameters indicate that Eros' average surface is covered with backscattering, opaque particles, similar to asteroids 951 Gaspra and 243 Ida. On average, Eros has a lower opposition surge amplitude than any asteroid observed by spacecraft thus far. Using our models, we have estimated a value for the visual geometric albedo of Eros to be 0.25 +/- 0.05, close to the values of 0.25 (Hicks et al. 1999), and 0.19 (Harris et al. 1999). With one exception, we find no significant wavelength-dependent trends in the spectra of our Hapke parameters. The exception is a curious inverse wavelength dependence in the parameter which measures the angular width of the opposition surge. According to theory (e.g. Muinonen 1990, Shkuratov 1989), a wavelength or albedo dependence in this parameter is indicative of coherent backscattering phenomena near opposition. We conclude that Eros's opposition effect cannot be entirely due to shadow-hiding. The fact that there appears to be a wavelength and/or albedo dependence to its angular width strongly suggests that a mechanism like coherent backscatter is needed.