Radiative transfer models of water plumes in Ganymede's atmosphere Preparing SWI/JUICE observations

Journal article, 2026

Context. Detecting and characterising plumes that may insert subsurface water into the tenuous atmospheres of the icy Galilean moons is a key objective for the Submillimetre Wave Instrument (SWI) on board ESA's JUpiter ICy moons Explorer (JUICE) mission. However, faint spectral signatures and complex observational geometries during distant moon monitoring pose significant challenges.
Aims. Our aim with this study is to assess the feasibility of detecting water plumes on Ganymede with SWI during the planned moon monitoring observations while orbiting Jupiter. We investigated how observational geometry and plume physical properties might influence the spectral signatures of a gas-phase water plume observed by SWI.
Methods. We used the LIne Modelling Engine (LIME) software package, a 3D non-local thermodynamic equilibrium radiative transfer code, to simulate and analyse the ortho-H2O spectral line at 557.936 GHz for different observational geometries. We focused on an observing distance of approximately 1.2 & times; 10(6) km from Ganymede, corresponding to the moon monitoring phase planned for SWI as part of the JUICE mission. We modelled key plume parameters, including temperature, production rate, and velocity fields, along with different SWI viewing geometries, to assess their impact on plume detectability and to develop strategies for optimising observations.
Results. Simulations indicate that SWI can detect water plumes on Ganymede under favourable conditions, even for water production rates as low as 10(27) s(-1). Detection is most likely if the plume is observed at the limb of the moon or when located at 90 degrees from the subsolar point. In these geometries, the plume contributes with strong emission line wings with temperature contrasts up to similar to 4 K for production rates around 10(29) s(-1). Such line wings can be detected with a signal-to-noise ratio of 7 with a 10-minute integration using the high-resolution Chirp Transform Spectrometer backend of SWI during the modelled moon monitoring observations. Increased plume temperatures and velocity distributions enhance line intensity and broadening, potentially enabling plume identification even at lower production rates.