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J.F. Bell III, B.E. Clark (Cornell), N. Izenberg (APL), C. Dodd, R. Clinite, C. Peterson, P. Martin (Cornell), S. Murchie (APL), L. McFadden, D. Wellnitz (U. Maryland), M.J. Gaffey (RPI), P. Lucey, M. Winter (U. Hawaii), C. Chapman (SWRI), J. Veverka (Cornell), NEAR MSI/NIS Science Team
The Near-IR Spectrograph (NIS) on the Near Earth Asteroid Rendezvous (NEAR) mission obtained ~200,000 spatially-resolved 800 to 2500 nm reflectance spectra of 433 Eros between February 13 and May 13, 2000. Spectra were obtained over a range of phase angles from 1 to 129 degrees and at spatial resolutions of the narrow rectangular NIS field of view ranging from 5x10 km down to 200x400 m per spectrum. Here we describe the analysis of the highest spatial resolution NIS data, consisting of more than 22,000 spectra having spatial resolution better than 400x800 m/spectrum. Nearly 80% of the surface of Eros was mapped by NIS at this scale, at an average incidence angle of 65+/-15 degrees and an average emission angle of 38+/-18 degrees.
Spectra were calibrated to radiance factor (I/F) and photometrically corrected to normal albedo using Hapke modeling of the NIS-derived Eros phase function [Clark et al. (2000), submitted]. Standard spectral parameters were derived (band minimum, band depth, etc.) for comparison of the NIS data with previous meteorite spectra and telescopic spectra of S asteroids.
Spectral variations across Eros are subtle (+/- 1 to 3%) but detectable by NIS. The largest observed variations are in 1000 nm band depth, which is stronger in parts of the rims of Psyche, Himeros, and other large craters and along a narrow equatorial zone between 180 and 360 degrees longitude. Weaker but spatially-correlated variations in 2000 nm band depth were also detected. Mapping of the 2000 nm to 1000 nm band area ratio parameter has not revealed statistically significant spatial variations in the olivine to pyroxene abundance ratio on the ~80% of the surface covered at NIS resolution. Eros has near-IR spectral properties consistent with the olivine-rich end of the S(IV) asteroid class and with the LL ordinary chondrite meteorite class. The observed variations in band depth are likely a manifestation of physical (e.g., grain size) rather than compositional variations on the surface of Eros.
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