Water signatures from Ganymede’s atmosphere - Preparing the observations for ESA’s JUICE Mission

Licentiate thesis, 2025

The icy Galilean moons and their potential subsurface oceans are candidates for habitable environments in the Solar System, but still remain largely unexplored. Their tenuous atmospheres are expected to be dominated by water, with enhanced abundance in regions where surface ice sublimates, and may be further enriched by plumes of volatile material escaping through cracks in the ice shell. The Submillimeter Wave Instrument (SWI) onboard ESA’s JUICE mission, launched in 2023, is designed to study rotational water lines in the exospheres of the icy Galilean moons.

This thesis focuses on assessing the feasibility of detecting water plumes on Ganymede, the largest moon of Jupiter, and optimizing observational strate-gies for SWI. The 3D non-LTE radiative transfer code LIME was employed to simulate water spectral line profiles and study how their profiles change by varying plume parameters such as temperature, production rate, and velocity fields. Various observation geometries were tested to account for the planned moon-monitoring phase of SWI, approximately 1.2 million km from Ganymede.

The results demonstrate that SWI can detect plumes with production rates as low as 10^27 1/s under favorable geometries, particularly when the plume is located at the limb of Ganymede. Temperature variations affect the emission signal, while velocity fields introduce distinctive Doppler shifts and line broadening that could serve as diagnostics of plume dynamics. Finally, the thesis discusses the limitations of the current models and outlines future work, including the exploration of lower plume temperatures, more realistic plume dynamics, and additional water transitions and collisional partners. Extend-ing this approach to Europa and Callisto will further enhance the scientific return of JUICE and deepen our understanding of water in the Jovian system.