Tides on Jupiter's moon Ganymede and their relation to its internal structure

Abstract

AbstractOne of the major scientific objectives of ESA's JUICE (JUpiter ICy moons Explorer) mission, which is scheduled for launch in 2022 and planned to arrive at the Jovian system in 2030, is to characterise the internal water ocean and overlying ice shell of Jupiter's largest moon Ganymede. As part of the strategy developed to realise 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 calculate 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 characterise 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, which could be as large as about 3.5 m in terms of the induced radial deformation, mostly depends on the structural (thickness, density) and rheological (rigidity, viscosity) properties of the ice-I shell. Nevertheless, the dependence of the tidal response on several geophysical parameters of the interior, in particular on the thickness and rigidity of the ice-I shell, does not allow for the unambiguous determination of the shell thickness from tidal measurements alone. Additional constraints could be provided by the measurement of forced longitudinal librations at the surface, as their amplitude is more sensitive to the rigidity than to the thickness of the shell.