Constraining Ganymede’s hydrosphere structure and composition with magnetic induction and tidal Love numbers

ESA’s Jupiter Icy Moons Explorer (JUICE), equipped with a highly capable suite of geophysical instruments—including the J-MAG magnetometer, the 3GM radio-science instrument, and the GALA laser altimeter—will enable accurate measurements of Ganymede’s magnetic induction and its tidal Love numbers k2 and h2 (Van Hoolst et al., 2024). As part of a prospective study for this upcoming exploration of Ganymede, we compute the moon’s magnetic induction and tidal response across a wide range of interior structure models.

 

Ganymede’s hydrosphere is modeled using equations of state for pure water ice and NaCl aqueous solutions with varying concentrations, following the SeaFreeze formulation (Journaux et al., 2020). The ice shell is assumed to be either fully conductive or convective, depending on its thickness and the adopted viscosity assumptions. By modeling the heat flux through the ice shell, we determine the range of plausible shell thicknesses and thermal properties compatible with the estimated radiogenic power and tidal heating.

 

Using the magnetic induction efficiency recently re-estimated by Jia et al. (2025), we constrain the NaCl content of Ganymede’s ocean to values significantly lower than those explored in most previous studies. We also determine the maximum efficiency of NaCl transport from the silicate mantle to the ocean. Results could be adapted to any other salt for which conductivity measurements are available at the appropriate pressures and temperatures.

 

For each plausible hydrosphere configuration, we explore all possible structures of the silicate mantle and liquid core so that our models’ moment of inertia is within the acceptable range of values (Gomez Casajus et al. 2022). Anelasticity of Ganymede’s interior is modeled with the Andrade rheology following the approach of Amorim and Gudkova (2025) and the moon’s Love numbers and tidal heating are computed for hundreds of thousands of models.

 

The influence of each parameter on the magnetic induction response, tidal Love numbers, as well as on the phase lags of k2 and h2, is analyzed. This approach aims to determine how measurements of these quantities by Juice can provide constraints on Ganymede's interior structure and thermal state.