NEAR infrared spectrometer photometry of asteroid 433 Eros

We present near-infrared spectrometer (NIS) observations (0.8 to 2.4 mum) of the S-type asteroid 433 Eros obtained by the NEAR Shoemaker spacecraft and report results of our Hapke photometric model analysis of data obtained at phase angles ranging from 1.2degrees to 111.0degrees and at spatial resolutions of 1.25 x 2.5 to 2.75 x 5.5 km/spectrum. Our Hapke model fits successfully to the NEAR spectroscopic data for systematic color variations that accompany changing viewing and illumination geometry. Model parameters imply a geometric albedo at 0.946 mum of 0.27 +/- 0.04 which corresponds to a geometric albedo at 0.550 mum of 0.25 +/- 0.05. We find that Eros exhibits phase reddening of up to 10% across the phase angle range of 0-100degrees. We observe a 10% increase in the 1-mum band depth at high phase angles. In contrast, we observe only a 5% increase in continuum slope from 1.486 to 2.363 mum and essentially no difference in the 2-mum band depth at higher phase angles. These contrasting phase effects imply that there are phase-dependent differences in the parametric measurements of 1- and 2-mum band areas, and in their ratio. The Hapke model fits suggest that Eros exhibits a weaker opposition surge than either 951 Gaspra or 243 Ida (the only other S-type asteroids for which we possess disk-resolved photometric observations). On average, we find that Eros at 0.946 P m has a higher geometric albedo and a higher single-scatter albedo than Gaspra or Ida at 0.56 mum; however, Eros's single-particle phase function asymmetry and average surface macroscopic roughness parameters are intermediate between Gaspra and Ida. Only two of the five Hapke model parameters exhibit a notable wavelength dependence: (1) The single-scatter albedo mimics the spectrum of Eros, and (2) there is a decrease in angular width of the opposition surge with increasing wavelength from 0.8 to 1.7 mum. Such opposition surge behavior is not adequately modeled with our shadow-hiding Hapke model, consistent with coherent backscattering phenomena near zero phase. (C) 2002 Elsevier Science (USA).