NEAR-INFRARED SPECTROSCOPY OF TROJAN ASTEROIDS: ASSESSING COMPOSITIONAL BIMODALITY AT SMALL SIZES

The Trojan asteroids orbit the sun within Jupiter’s gravitationally stable Lagrange points (L4 and L5). Their unique orbits make them prime targets for study to further our understanding of the evolution of the Solar System. The formation scenario most consistent with their orbital properties comes from the Nice Model, which predicts that the Trojan asteroids originally formed in the Kuiper Belt and were trapped in Jupiter’s Lagrange regions during the period of giant-planet-migration-induced instability. Analysis of the surface composition of Trojans asteroids offers a test of this prediction. Although the featureless spectra observed previously in the near-infrared (NIR, 0.7–2.5 μm) preclude compositional determination, further analysis of the spectral slopes for these data revealed two distinct spectral groups. The bimodality of Trojan spectra is defined by the degree of spectral reddening. Two-thirds of Trojans exhibit redder spectra, whereas the remaining one-third exhibit less-red spectra. Previous NIR observations of Trojan asteroids to assess compositional groups focused on the largest objects (diameter >75 km), with only an incomplete census at smaller sizes. This study will investigate whether the spectral slope bimodality observed for the large Trojans is also present for small Trojans (<75 km), and if so, whether there exists any discernable change in the population ratio of redder vs less-red objects at those sizes. Using the SpeX spectrograph at NASA’s Infrared Telescope Facility, we observed 33 small Trojans residing within Jupiter’s L4 and L5 Lagrange points whose diameters range from ~ 39 km to ~ 75 km. We have reduced these data and will analyze the spectral slopes of these objects. We will present the results of this color analysis in the context of previous results. Such analysis will further our understanding of the origins of Trojan asteroids and thereby provide key constraints for interpreting the dynamical evolution of the Solar System.