Color Variations on Eros from NEAR Multispectral Imaging

NEAR multispectral imaging was obtained at seven wavelengths (450-1050 nm) to characterize hemispheric and regional color properties of Eros. The highest-resolution whole-disk data, 180 m/pixel, were obtained during the last pre-orbit insertion sequence on 12 February 2000. The same areas were imaged again in color at 10-20 m/pixel from high orbit in March-April 2000, and selected targets have been studied in color at resolutions as high as 4 m/pixel from low orbit. Whole-disk spectra are in close agreement with ground-based observations. These and the disk-resolved measurements show little variation in visible-wavelength color, but they do reveal spatial variation of several percent in the 950-nm/760-nm reflectance ratio, used here as a proxy for depth of the 1-μm olivine-pyroxene absorption band. After photometric correction to i=30° e=0° using both a Hapke correction and a modified empirical phase function, the disk-resolved images show reproducible spatial variations in albedo and 950-nm/760-nm reflectance ratio. The northern hemisphere exhibits average reflectances at i=30°, e=0° of 0.136±0.007 at 760 nm and 0.115±0.006 at 950 nm, equivalent to geometric albedos of 0.30±0.02 and 0.26±0.02, respectively. There is more than factor of 2 spatial variation in reflectance, but only about 10% variation in the 950-nm/760-nm reflectance ratio. Reflectance and color ratio are highly correlated, with the highest reflectances in discrete, sharp-edged patches on slopes exceeding 20°, consistent with material being exposed by downslope movement. Eros is also conspicuously deficient in small, bright, spectrally distinctive craters which are found on the other two S asteroids, Gaspra and Ida, imaged by spacecraft at close range. Eros exhibits a larger range of albedos than other S asteroids, but its color variations are much more subtle: variation in the 950-nm/760-nm reflectance ratio with 760-nm reflectance is several times less than on those asteroids or in the lunar maria. Of the different mechanisms possibly responsible for reflectance and color differences on Eros, spatial differences in the extent of regolith aging by space weathering are most consistent with Eros's observed properties. However, the effects of this process are both qualitatively and quantitatively different than in the lunar maria.