Constraints on the surface composition of Trojan asteroidsfrom near infrared (0.8-4.0 μm) spectroscopy and spectral modeling

Abstract

The present surface composition of primitive objects provides clues to understanding the conditions under which the solar system formed. Characterization of the inner solar nebula progresses rapidly due to remote studies of asteroid and planetary surfaces as well as laboratory analyses of meteorites and lunar samples. Because of their atypical orbital position at 5.2 AU, the Trojan asteroids hold the potential to help resolve several problems in planetary science, including conditions in the early solar nebula. The study presented herein was undertaken in order to uncover the nature of these poorly understood asteroids. Near-infrared reflectance spectra are presented over the wavelength range 0.8-4.0 μm. These observations nearly double the number of published 0.8-2.5 μm spectra of Trojans and provide the first systematic study of the L-band (2.8-4.0 μm) region for these distant asteroids. The spectra do not contain any definitive absorption features characteristic of surface composition (e.g. H₂O, organics, silicates) as seen on main-belt asteroids and several Centaur and Kuiper Belt objects. These data are combined with previously published data to construct spectra covering the visible and near-IR (0.3-4.0 μm) for as many objects as possible. The composite spectra are analyzed quantitatively using the formulation for scattering in a particulate medium developed by Hapke. Under this rigorous examination, it is found to be unlikely that the red spectral slope is a result of organics on the surfaces, due mainly to the lack of absorptions in the L-band. These surfaces are compatible with mixtures of anhydrous silicates and carbonaceous material. Upper limits are placed on the amount of water ice and hydrated silicates present on the surfaces. Similar analysis is performed for several other groups of dark solar system objects. Comparison of these results with those for Trojan asteroids indicates that it is likely that the Trojans formed in the solar nebula near 5 AU. If this is true, then the determination that the red slope is probably not due to organic material does not fit with the generally accepted view of trends of composition with heliocentric distance. Implications and possible alternative explanations are discussed.