NEAR-INFRARED SPECTROSCOPY OF TROJAN ASTEROIDS: EVIDENCE FOR TWO COMPOSITIONAL GROUPS

Abstract

The Trojan asteroids remain quite poorly understood, yet their physical\nproperties provide unique perspective on chemical and dynamical processes that\nshaped the Solar System. The current study was undertaken to investigate\nsurface compositions of these objects. We present 66 new near-infrared (NIR;\n0.7 to 2.5 microns) spectra of 58 Trojans, including members of both the\nleading and trailing swarms. We also include in the analysis previously\npublished NIR spectra of 13 Trojans (3 of which overlap with the new sample).\nThis data set permits not only a direct search for compositional signatures,\nbut also a search for patterns that may reveal clues to the origin of the\nTrojans. We do not report any confirmed absorption features in the new spectra.\nAnalysis of the spectral slopes, however, reveals an interesting bimodality\namong the NIR data. The two spectral groups identified appear to be equally\nabundant in the leading and trailing swarms. The spectral groups are not a\nresult of family membership; they occur in the background, non-family\npopulation. The average albedos of the two groups are the same within\nuncertainties (0.051\\pm0.016 and 0.055\\pm0.016). No correlations between\nspectral slope and any other physical or orbital parameter are detected, with\nthe exception of a possible weak correlation with inclination among the\nless-red spectral group. Synthesizing these results with previously published\nproperties, we conclude that the two spectral groups represent objects with\ndifferent intrinsic compositions. We further suggest that while the less-red\ngroup originated near Jupiter or in the main asteroid belt, the redder spectral\ngroup originated farther out in the Solar System. If correct, the Trojan swarms\noffer the most readily accessible large reservoir of Kuiper Belt material as\nwell as a unique reservoir for the study of material from the middle part of\nthe solar nebula.\n