The Composition of Centaur 5145 Pholus

We present a new spectrum of the Centaur object 5145 Pholus between 1.15 and 2.4 μm. We model this, and the previously published (0.4- to 1.0-μm) spectrum, using Hapke scattering theory. Seen in absorption are the 2.04-μm band of H₂O ice and a strong band at 2.27 μm, interpreted as frozen methanol and/or a photolytic product of methanol having small molecular weight. The presence of small molecules is indicative of a chemically primitive surface, since heating and other processes remove the light hydrocarbons in favor of macromolecular carbon of the kind found in carbonaceous meteorites. The unusually red slope of Pholus' spectrum is matched by fine grains of a refractory organic solid (tholin), as found previously by M. Hoffmannet al. (1993,J. Geophys. Res.98, 7403-7407) and P. D. Wilsonet al.(1994,Icarus107, 288-303). Olivine (which we model with Fo 82) also appears to be present on Pholus. We present a five-component model for the composite spectrum of all spectroscopic and photometric data available for 5145 Pholus and conclude that this is a primitive object which has not yet been substantially processed by solar heat. The properties of Pholus are those of the nucleus of a large comet that has never been active.