Spatially Resolved Visible Wavelength Spectroscopy of the Galilean Moons With VLT/MUSE

Abstract

We present observations of all four Galilean satellites using spatially resolved visible wavelength (475–935 nm) spectroscopic observations with the ground-based VLT/MUSE + AOF facility in 2019. A range of features in the observed reflectance spectra were compared to laboratory measured spectra, and mapped to understand the spatial distributions (at $\sim$300 km spatial resolution) of different compositional species on the moons' surfaces, providing near-global context for future observations. The $\sim$485 nm spectral slope on Io is consistent with the presence sulfur materials such as $S_8$, with stronger slopes on the trailing hemisphere and at high latitudes. Io's 560 nm absorption band is strongest at high latitudes, and shows an enhancement in the Pele plume deposit, consistent with the presence of $S_4$. The similar 530 nm absorption band on Europa is constrained to the trailing hemisphere, and also appears consistent with sulfur materials. On Ganymede, a proxy for the 577.3 nm molecular oxygen absorption was found to be strongest at low-mid latitudes, particularly around the trailing hemisphere, with a potential slight southward bias. This distribution appears consistent with the hypothesis that Ganymede's closed magnetic field line region allows stable bubbles of $O_2$ to exist in Ganymede's surface ice.