Thermal evolution and dynamics of Ganymede’s rocky interior

The icy moons of Jupiter are the targets of future planetary missions such as JUICE and Europa Clipper due to their subsurface oceans that are thought to potentially represent habitable environments (Van Hoolst et al., 2024). Among these moons, Ganymede stands out, not just as the largest moon in the Solar System, but as the only moon that possesses an intrinsic magnetic field (Kivelson et al., 2004). JUICE will spend three years performing multiple flybys of the Galilean moons before entering the orbit around Ganymede in 2034. During this phase, the spacecraft will map Ganymede’s global topography using GALA (Hussmann et al., 2025) and measure its gravity field up to degree and order ~40 (De Marchi et al., 2021).

In this study we investigate the rocky interior of Ganymede using the geodynamical code GAIA (Hüttig et al., 2013). We vary the thickness of the silicate layer between 704 km and 1304 km (Rückriemen et al., 2018). Our models use a pressure- and temperature-dependent viscosity following an Arrhenius law (Hirth & Kohlstedt, 2003), core cooling, and the decay of radioactive heat sources. As radioactive heating and the rheology of the rocky interior are key parameters that control its dynamics and cooling behavior, we test different concentrations of radioactive isotopes of U, Th, and K. We vary the reference viscosity between 10¹⁸ Pa s and 10²⁰ Pa s, representative of a hydrated or dry mantle. 

We test the effects of magmatism on the interior evolution by considering partial melting in the silicate layer and instantaneous melt extraction. Since magmatism affects the thermal evolution of the interior, we vary the extrusive to intrusive ratio. We investigate models where the entire amount of melt produced in the interior is extracted to the surface and models where the melt remains trapped in the subsurface beneath the ocean floor at depths between 30-200 km. 

We find that extrusive scenarios are more effective in cooling the moon’s interior during the early evolution, yet present-day average mantle temperatures converge to values between 1200-1250 K, depending on intrusion depth. In cases with efficient melt extraction, a thicker lithosphere is formed that insulates the deep interior and leads to higher core-mantle boundary temperatures. Nevertheless, across all scenarios the CMB heat fluxes remain sub-adiabatic, indicating that a purely thermal dynamo could sustain magnetic field generation only during the early stages of planetary evolution. 

The heat transport in the rocky mantle is critical for the core temperature and CMB heat flux, both important parameters for magnetic field generation. Thus, we select models with core temperature cold enough to allow core crystallization that was suggested to drive a present-day core dynamo (Rückriemen et al., 2018). Moreover, we compute the mass anomalies associated with density anomalies in our models and compare them to those inferred from Galileo’s Radio Doppler data (Palguta et al., 2006). Our models provide an important framework for understanding Ganymede’s internal evolution and will help to enhance the scientific return of JUICE by guiding data analysis and contextualizing future observations.