Ganymede's Far-Ultraviolet Reflectance: Constraining Impurities in the Surface Ice

摘要

We present reflectance spectra of Ganymede's leading and trailing hemispheres in the wavelength range 138-215 nm, obtained by the Hubble Space Telescope Cosmic Origins Spectrograph (HST/COS) in 2014. The most notable feature of both spectra is the absence of a sharp water absorption edge at similar to 165 nm, seen in laboratory measurements of ice reflectivity and in previous observations of Saturn's icy moons and rings. Rather than displaying a sharp change in the reflectivity at the wavelength of the water ice absorption edge, Ganymede's reflectance gradually increases with wavelength at lambda > 165 nm. We show that the observed shape of Ganymede's UV reflectance is inconsistent with intimate mixture models of pure ice with UV-dark materials including tholins, amorphous carbon, graphite, and silicates. However, we find that intraparticle models, in which a small proportion of a UV-absorbing contaminant is trapped as inclusions within the ice matrix, are able to suppress the 165 nm feature at contaminant concentrations of 165 nm, but additional absorbing surface materials are required to produce a good fit to Ganymede's previously observed near-UV and visible reflectance. Plain Language Summary We use the Hubble Space Telescope to study how Jupiter's moon Ganymede reflects sunlight at ultraviolet (UV) wavelengths. Ganymede's surface is known to contain significant amounts of water ice, which is very reflective at UV wavelengths longer than 165 nm, but reflects very little light at shorter wavelengths. We find that Ganymede does not show the same abrupt change in reflectivity at 165 nm as pure water ice. We use computer models that account for how light interacts with different materials to try and explain why we cannot see evidence of water ice in UV observations of Ganymede, even though we know ice is present there. We find that models that include pure water ice cannot explain our observations, but if we model ice containing a small fraction of impurities, we get a good match with Ganymede's UV reflectance. Models like the ones we produced for this study will be useful for interpreting observations of Ganymede and Jupiter's other icy moons, Europa and Callisto, by the ultraviolet instrument on the upcoming JUpiter ICy moons Explorer (JUICE) mission. Determining which impurities exist in icy moon surfaces helps us to understand the habitability of these worlds-JUICE's main scientific goal.