Spectral Variation on the Trojan Asteroid 4709 Ennomos

The Trojan asteroids are hypothesized to have been emplaced from the Kuiper Belt into Jupiter’s stable Lagrange regions early in Solar System history. They therefore contain information about the materials present in the outer Solar System during planet formation and how those materials are affected by inward migration. A bimodal distribution of visible to near-infrared (VNIR; 0.4 to 2.5 μm) spectral slopes suggests two distinct composition groups, but the actual compositions of those groups remain elusive due to a lack of absorption features at these wavelengths. Evolutionary models of Trojans generally predict the sublimation loss of volatiles from surface layers, creating a refractory mantle. Irradiation of surface layers may induce physical and/or chemical changes that further alter the surface relative to the interior. Impacts could break through surface layers and reveal subsurface materials. The absence of spectral variations on Trojans reported in the literature have suggested either that no exposures of sub-surface materials have been detected or that the sub-surface is spectrally identical to the surface layers. An early report of a possible high albedo of the Trojan asteroid 4709 Ennomos was attributed to possible exposure of ice. Follow-up spectra revealed no signatures of ice, but the asymmetric lightcurve is consistent with a bright spot on the surface. In order to further investigate possible exposure of sub-surface materials on Ennomos, we have measured multiple NIR (0.7 to 2.5 μm) spectra at the NASA IRTF, including two sets that continuously cover half a rotation each. The spectral slopes vary with rotation, from spectra consistent with the less-red Trojan spectral group (analogous to P-type asteroids) to slightly blue-sloped spectra (analogous to C- or B-type asteroids). Nevertheless, all spectra remain featureless, including no indication of the presence of water ice. Analyses of these spectra along with other published data of Ennomos place constraints on compositions responsible for the varying spectra. We will discuss these results in the context of the Trojan population, including predictions for surface variability that the Lucy mission may see on the Trojans it visits.