Ultraviolet Spectroscopy of Lucy Mission Targets with the Hubble Space Telescope

Abstract

The recently launched Lucy mission aims to understand the dynamical history of the solar system by examining the Jupiter Trojans, a population of primitive asteroids co-orbital with Jupiter. Using the G280 grism on the Hubble Space Telescope’s Wide Field Camera 3, we obtained near-ultraviolet spectra of four of the five Lucy mission targets—(617) Patroclus–Menoetius, (11351) Leucus, (3548) Eurybates, and (21900) Orus—to search for novel spectral features. We observe a local reflectance minimum at 0.4 μm accompanied by an increase in reflectance from 0.35 to 0.3 μm in the spectra of Patroclus and Orus. We use the principles of Rayleigh scattering and geometric optics to develop a Hapke optical model to investigate whether this feature can be explained by the presence of submicroscopic grains on Trojan surfaces. The near-ultraviolet “bump” feature can be explained by scattering due to fine-grained opaques (iron, amorphous carbon, or graphite) with grain sizes ranging from 20 to 80 nm.