Constraints on a Water Layer in Jupiter's moon Ganymede from Tidal Deformation and Forced Librations

One of the major scientific objectives of ESA's JUpiter ICy moons Explorer (JUICE) mission, scheduled for launch in 2022 and planned to arrive at the Jovian system in 2030, is to characterize the hypothesized internal water ocean and overlying ice shell of Jupiter's largest moon Ganymede. As part of the strategy developed to realize this objective, the tidal response of Ganymede's interior will be constrained by JUICE's measurements of surface displacements by the Ganymede Laser Altimeter (GALA) instrument and variations in the gravitational potential by the 3GM radio science package due to the acting diurnal tides. Here we determine, using a viscoelastic Maxwell rheological model, the tidal response at the surface of Ganymede for several plausible internal configurations in order to analyse the relation between the tidal response and the geophysical parameters that characterize Ganymede's interior. Similarly to the case of Jupiter's smallest icy satellite Europa, the tidal response of Ganymede in the presence of a subsurface ocean mostly depends on structural (thickness, density) and rheological (rigidity, viscosity) properties of the ice-I shell. Tidal-induced vertical surface displacements could be as large as 3.5 m if there is an internal water layer. Nevertheless, the dependence of the tidal response on several geophysical parameters of the interior, in particular thickness and rigidity of the ice-I shell, does not allow for an unambiguous determination of depth and thickness of the water layer from tidal measurements alone. Additional constraints could be provided by measuring forced longitudinal librations at the surface, as their amplitudes are more sensitive to ice rigidity than to thickness of the ice shell.