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D. P. Cruikshank, T. L. Roush, M. J. Bartholomew, Y. J. Pendleton, S. M. White, M. P. Bernstein, C. M. Dalle Ore, B. N. Khare (NASA Ames), T. R. Geballe, J. K. Davies (JAC Hawaii), T. C. Owen, D. J. Tholen (IFA Hawaii), C. de Bergh (Obs. Paris), J. F. Bell, III (Cornell U.), R. H. Brown (LPL Arizona), K. A. Tryka (NAU/JPL)
New spectra and models of the surface of Centaur 5145 Pholus suggest that this object is the equivalent of a giant comet nucleus that is not, and may have never been, active. We have used published spectra and our own data, plus the radiometrically estimated diameter of 190 km, to compile the reflectance spectrum from 0.4 to 2.4 \mum. The photovisual spectrum slopes steeply upward toward the red, a characteristic matched by some refractory organic solids. The spectrum shows two absorption bands of H2O ice (1.5 and 2.0 \mum), and a third clear absorption near 2.27 \mum, at the position of two absorption bands in solid CH3OH and other solid hydrocarbons (e.g., (CH2)6N4 = hexamethylenetetramine). We computed Hapke scattering models with the ices of H2O and CH3OH (although we do not claim a firm identification of CH3OH), plus the refractory organic solid known as Titan tholin (B. N. Khare et al. 1984, Icarus 60, 127), plus amorphous carbon to adjust the albedo. An additional component is required to fit the spectrum at 1-1.4 \mum; olivine (Fo 82) provides exactly the absorption needed. In this two-terrain model, 38.5 % of Pholus is covered with an intimate mixture of 55 % olivine (grain size 20 \mum), 15 % Titan tholin (1 \mum), 15 % H2O ice (10 \mum), and 15 % CH3OH ice (10 \mum); the second terrain material, amorphous carbon, covers 61.5 % of the surface. The spectrally active components in our model are the principal constituents of a comet nucleus. We suggest that if Pholus were to approach the Sun and begin sublimating, it would show the compositional characteristics of a comet. This work is in Press in Icarus.