Comparative Ultraviolet Reflectance Spectroscopy of Jupiter’s Icy Moons to Constrain Impurities in the Ice

Abstract:

Ultraviolet observations of Ganymede by the Hubble Space Telescope Cosmic Origins Spectrograph (HST/COS) in 2014 show that Ganymede does not exhibit a sharp increase in reflectance at ~165 nm typically observed in lab measurements of pure water ice and in observations of Saturn’s icy moons and rings. Instead, Ganymede gradually becomes more reflective with increasing wavelength in the ~170-210 nm region. Using spectral reflectance models based on Hapke theory we have found that this spectral shape may be reproduced if the ice grains on Ganymede’s surface contain a small fraction (<1%) of a UV-absorbing contaminant material. The best fit candidates we have identified for this contaminant include silicates, tholins, and hydrogen peroxide. We will present our modeling results for Ganymede, and a comparison with new HST/COS observations of Europa and Callisto obtained in June-August 2020. By comparing the observed reflectance of Jupiter's three icy moons we can investigate whether a single contaminant modifies the spectra of all three satellites, and whether the presence of the UV-absorbing material may be related to exogenous processes such as radiolytic processing of the surfaces by the Jovian plasma.