Jovian Trojan Asteroids in the Mid-IR

Jupiter’s Trojan asteroids (hereafter Trojans) make up a large group of primitive bodies that carry insight into the formation mechanisms that shaped our Solar System. Previous mid-infrared (MIR; 5-35 μm) spectra of three Trojans exhibit puzzling silicate emission features, similar to spectra of optically thin comet comae – an odd finding because asteroid regoliths are optically thick. This discovery led to the hypothesis that Trojan surfaces may consist of a fine grained, ‘fluffy’ regolith of silicates (hereafter ‘regolith porosity’). By understanding physical properties of the regolith (e.g., particle size and regolith porosity), more accurate compositional interpretations can be made. In this presentation we show Spitzer Space Telescope (SST) MIR spectra of the eleven newly analyzed Trojans as well as the three previously analyzed Trojans, our MIR analysis of silicate features, and discuss our results in the context of Trojan origins. In order to further explore the regolith properties and compositions of Trojans we first reduced and analyzed eleven MIR Trojan spectra taken with the SST. We compared the Trojan spectra to spectra of meteorite powder, comets, and mineral endmembers. Results indicate Trojans are likely covered in a highly porous regolith (greater than about 80% pore space) of fine-particulate (less than about 60 μm) crystalline olivine (primarily forsterite), enstatite, and amorphous olivine, with potential minor amounts of diopside and amorphous pyroxene. The minerals and surface properties on the Trojan surface are consistent with the Nice model, which predicts the Trojans originally formed in the primordial Kuiper Belt.