The nature of comet-asteroid transition object (3200) Phaethon

We studied the surface composition of asteroid (3200) Phaethon, a paradigmatic case of asteroid-comet transition objects, in order to determine its cometary or asteroidal nature. We report visible and near-infrared spectra covering the 0.35-2.4 micron range. The spectrum is compared with that of meteorites samples and man-made mineral mixtures to determine possible components, and modeled using multiple scattering formulations. It is also compared with the spectra of comet nuclei and other comet-asteroid transitional objects. The shape of Phaethon's spectrum is similar to aqueously altered CI/CM meteorites and hydrated minerals. A surface composition of hydrated silicates is also suggested by the models. Phaethon's spectrum shows important differences with the few comet nuclei properly observed at these wavelengths and is similar to the spectra of other peculiar comet-asteroid transition objects. The spectral properties reported in this paper and the dynamical properties of (3200) Phaethon support an asteroidal nature rather than a cometary one. Phaethon is more like an "activated" asteroid (similar to the population of Main Belt Comets reported by Hsieh & Jewitt 2006) than an extinct comet.