Hubble Space Telescope observations of the large moons of Uranus and implications for radiolytic processing and radiation darkening

Clues regarding surface composition and surface processing (e.g., from magnetospheric interactions) can be gleaned from reflectance spectra spanning the near-ultraviolet (NUV, 200 – 400 nm) and visible (VIS, 400 – 700 nm) wavelength regions. For example, NUV and VIS spectra collected by the Space Telescope Imaging Spectrograph (STIS) and the (now defunct) Faint Object Spectrograph (FOS) on the Hubble Space Telescope (HST) have detected an absorption feature centered near 260 nm on the Galilean moon Ganymede and the Saturnian moons Dione and Rhea, attributed to O₃ formed by charged particle interactions with H₂O ice on their surfaces. In contrast to the Jupiter and Saturn systems, the extent of interactions between Uranus’ magnetosphere and its moons is largely unknown. Measurements made by Voyager 2’s magnetometer determined that Uranus’ magnetic field axis is offset by 59° from the orbital plane of its major moons. Nevertheless, Voyager 2’s Low Energy Charged Particle detector measured particle depletions associated with the orbits of these moons, indicative of charged particle interactions.

Possible evidence for radiolytic processing of the Uranian moons was identified in spectra collected by FOS on HST (220 – 475 nm), which display absorption features centered near 280 nm on Ariel, Titania, and Oberon, attributed to trapped OH formed by charged particle interactions with H₂O ice. The 280-nm band was found to be strongest on the inner moon Ariel and weaker on the outer moons Titania and Oberon, consistent with higher charged particle fluxes closer to Uranus. However, these prior observations did not sample both the leading and trailing hemispheres of the Uranian moons and did not observe Umbriel, orbiting between Ariel and Titania, reducing the utility of these data for understanding charged particle weathering.

In this study, we are analyzing new HST/STIS spectra (200 – 570 nm) of the leading and trailing hemispheres of Ariel, Umbriel, Titania, and Oberon to assess the influence of charged particle bombardment. Our preliminary analysis indicates that the NUV spectral slopes are redder than the VIS spectral slopes in these STIS data. We also tentatively identify weak bands centered near 255 nm, which might result from O₃. Furthermore, the NUV albedo of Oberon’s leading hemisphere (0.252 ± 0.05) is notably darker than its trailing hemisphere (0.278 ± 0.05), consistent with the accumulation of dark material on its leading side that might have originated as dust grains on Uranus’ retrograde irregular satellites. We will present the six STIS spectra collected so far and our ongoing analyses.