Constraining the UV-absorbing material on Jupiter’s icy moons through comparative reflectance spectroscopy

The far-UV spectrum of water ice contains a sharp absorption edge around 165 nm, becoming highly reflective at wavelengths longer than this. This feature has been observed in lab measurements and in Cassini UVIS observations of Saturn’s moons. However, Ganymede’s FUV spectrumdoes not exhibit a sharp increase in reflectance near 165 nm as expected; instead, the reflectance gradually increases at wavelengths >170 nm, implying that there is a UV-absorbing contaminant present. Observations of Europa just outside of the critical region around 165 nm suggest that the water ice feature may be similarly modified there, and Callisto’s reflectance in this region is currently unknown. We therefore propose to observe Europa and Callisto using COS G140L, to determine whether the spectral reflectance of the two moons close to the 165 nm water ice edge is modified similarly to Ganymede’s. We will use four HST orbits to observe the leading and trailing hemispheres of each satellite. The leading and trailing hemispheres of the Galilean moons experience different radiation environments, with Jovian magnetospheric plasma predominantly impacting the trailing hemispheres. By comparing the observed reflectance of Jupiter’s icy moons, and by assessing differences between the leading and trailing hemispheres, we can investigate whether a single contaminant modifies the spectra of all three satellites, and whether this may be a product of radiolytic processing of the surfaces by the Jovian plasma.