Earth-based spectroscopy of Jupiter’s Galilean moons

This thesis uses spatially resolved spectral observations, primarily from the ground based Very Large Telescope (VLT), to model and understand the surface composition of the Galilean moons. Previous studies have proposed various compositional hypotheses, so I developed a Markov Chain Monte Carlo (MCMC) spectral inversion routine which allows detailed investigation of the different compositional mixtures which are consistent with an observed spectrum. I use this MCMC routine to quantify uncertainties on fitted abundances from infrared spectral observations of the icy moons Europa and Ganymede, and quantitatively demonstrate how the hydrated salt mixture on these moons cannot be fully constrained using near-infrared spectroscopy alone.

I produced compositional mapping datasets by analysing observations from the VLT/SPHERE, VLT/MUSE and Galileo/NIMS instruments to study the abundances and spatial distributions of ices, volatiles, and contaminants on the surfaces of the Galilean moons. On Europa, I mapped the contrasts between the high acid abundance (∼ 60%) near the trailing apex and high water ice abundances (∼ 50%) at high latitudes, and constrained the possible spatial distributions of hydrated salts. Modelling results for Ganymede were dominated by water ice in younger areas (abundances up to 50%) and a spectrally flat darkening agent in older terrain (up to 60% abundance). The water ice grain size on Ganymede was found to have strong latitudinal and longitudinal contrasts, driven by global scale trends in temperature and plasma bombardment. Mapping of the 590nm SO2 absorption band on Io identified stronger absorptions at mid latitudes and in the Pele ejecta blanket. The 577.3nm O2 band on Ganymede was found to be spatially constrained to the closed magnetic field line region, with the strongest absorptions on the trailing hemisphere. Comparison of the datasets is used to demonstrate how ground-based telescopes can carry out compositional mapping which was previously only possible using spacecraft. These datasets fill spatial and spectral gaps in observations of the moons and will help to prepare and guide the next decade of exploration of these worlds by the JUICE and Europa Clipper spacecraft. 

I led a successful observing campaign of Ganymede with VLT/SPHERE in 2021, and work from this thesis is published in King et al. (2022) and King and Fletcher (2022).