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Near-infrared Spectroscopy of Trojan Asteroids: Evidence for Two Compositional Groups
The Trojan asteroids, a very substantial population of primitive bodiestrapped in Jupiter's stable Lagrange regions, remain quite poorlyunderstood. Because they occupy these orbits, the physical properties ofTrojans provide a unique perspective on the chemical and dynamicalprocesses that shaped the Solar System. The current study was thereforeundertaken to investigate surface compositions of these objects. Wepresent 66 new near-infrared (NIR; 0.7-2.5 ?m) spectra of 58 Trojanasteroids, including members of both the leading and trailing swarms. Wealso include in the analysis previously published NIR spectra of 13Trojans (3 of which overlap with the new sample). This data set permitsnot only a direct search for compositional signatures, but also a searchfor patterns that may reveal clues to the origin of the Trojans. We donot report any confirmed absorption features in the new spectra.Analysis of the spectral slopes, however, reveals an interestingbimodality among the NIR data. The two spectral groups identified appearto be equally abundant in the leading and trailing swarms. The spectralgroups are not a result of family membership; they occur in thebackground, non-family population. The average albedos of the two groupsare the same within uncertainties (0.051 ± 0.016 and 0.055± 0.016). No correlations between spectral slope and any otherphysical or orbital parameter are detected, with the exception of apossible weak correlation with inclination among the less-red spectralgroup. The NIR spectral groups are consistent with a similar bimodalitypreviously suggested among visible colors and spectra. Synthesizing thepresent results with previously published properties of Trojans, weconclude that the two spectral groups represent objects with differentintrinsic compositions. We further suggest that whereas the less-redgroup originated near Jupiter or in the main asteroid belt, the redderspectral group originated farther out in the Solar System. If thissuggestion is correct, the Trojan swarms offer the most readilyaccessible large reservoir of Kuiper Belt material as well as a uniquereservoir for the study of material from the middle part of the solarnebula.
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