Variability of Ganymede’s ionosphere during its revolution around Jupiter: implications on ionospheric conductances and surface sputtering

Previous flybys by NASA missions, namely Galileo and Juno, revealed that Ganymede, the largest moon of the Solar System, hosts a rather complex, dense ionosphere, more diverse than previously thought. Recent modelling work [1] has highlighted that ion-neutral chemistry (e.g. H₂⁺+H₂ ->H₃⁺+H) occurs and is effective at producing new ion species such as H₃O⁺ and H₃⁺. The latter was detected during the Juno flyby [2].

During its revolution around Jupiter, two main parameters are affecting Ganymede’s ionosphere: Ganymede’s local time and latitude in the Jupiter’s dipole frame (magnetic latitude). On one hand, as Ganymede is tidally locked, the subsolar point, where water is sublimated and plasma is produced in large quantities, drifts westwards in Ganymede's fixed frame when Ganymede revolves around Jupiter. On the other hand, depending on Ganymede’s location in the dipole field from Jupiter, whether Ganymede is above, within or below the plasma sheet, the plasma dynamics within Ganymede’s magnetosphere varies.

In support of the Juice mission, we propose to explore the response of Ganymede’s ionosphere to both independent parameters. Our model will be driven by different exospheric configurations – corresponding to different local times [3] – and by different electromagnetic configurations – corresponding to different locations within Jupiter’s dipole [4]. From this set of simulations, we will evaluate how the ionosphere's structure varies. In particular, we will assess the evolution of the ionospheric electrical conductances and the sputtering rates on the surface by ionospheric ions with these two parameters.

Both quantities are essential to quantify and constrain. The ionosphere (through its conductance) might contribute to the induced magnetic field signal and interfere with that of the subsurface ocean and its characterisation. Ion sputtering might be a significant source of the neutral exosphere [5].

[1] Beth et al., Ion-neutral chemistry at icy moons: the case of Ganymede, MNRAS, 2025

[2] Valek et al., In Situ Ion Composition Observations of Ganymede's Outflowing Ionosphere, GRL, 2022

[3] Leblanc et al., Ganymede's atmosphere as constrained by HST/STIS observations, Icarus, 2023

[4] Jia et al., Properties of Ganymede's magnetosphere inferred from improved three-dimensional MHD simulations, JGR, 2009

[5] Carnielli et al., Simulations of ion sputtering at Ganymede, Icarus, 2020