THE COMPOSITION OF CENTAUR-5145-PHOLUS

We present a new spectrum of the Centaur object 5145 Pholus between 1. 15 and 2.4 Gem. We model this, and the previously published (0.4- to 1 .0-mu m) spectrum, using Hapke scattering theory. Seen in absorption a re the 2.04-mu m band of H2O ice and a strong band at 2.27 mu m, inter preted as frozen methanol and/or a photolytic product of methanol havi ng small molecular weight. The presence of small molecules is indicati ve of a chemically primitive surface, since heating and other processe s remove the light hydrocarbons in favor of macromolecular carbon of t he kind found in carbonaceous meteorites. The unusually red slope of P holus' spectrum is matched by fine grains of a refractory organic soli d (tholin), as found previously by M. Hoffmann et al, (1993, J. Geophy s. Res. 98, 7403-7407) and P. D. Wilson et al. (1994, Icarus 107, 288- 303). Olivine (which we model with Fo 82) also appears to be present o n Pholus. We present a live-component model for the composite spectrum of all spectroscopic and photometric data available for 5145 Pholus a nd conclude that this is a primitive object which hips not yet been su bstantially processed by solar heat. The properties of Pholus are thos e of the nucleus of a large comet that has never been active. (C) 1998 Academic Press.