Understanding the interior magnetic fields of Ganymede using flybys of the Europa Clipper and JUICE missions

The interiors of the icy moons of Jupiter hold a key to understanding habitability in the Solar System and beyond. They could serve as prototypes to comprehend similar bodies that might have the potential to sustain life. The magnetic field observations from the Galileo mission between 1996 and 2003 suggest large oceans below the icy crusts of Europa and Callisto and a probable subsurface ocean at Ganymede. It also discovered that Ganymede has an intrinsic magnetic field and a dynamic magnetosphere.

NASA’s Europa Clipper and ESA’s Jupiter ICy moons Explorer (JUICE) missions have been designed to better understand and characterise these icy moons. They aim to confirm the existence of a subsurface ocean at the three moons, in particular, Ganymede and Europa, and constrain their internal structure. The missions would also explore the magnetosphere of Jupiter and its interactions within the Jovian system. Ganymede is the largest moon of our Solar System, capable of producing its own dynamo field and possibly possessing an ocean beneath its surface. However, separating the intrinsic field from the induced field is a difficult problem. Galileo measurements provided two models for Ganymede’s overall internal field- a dipole and quadrupole model or a dipole and induction model. Both the quadrupole and induction signals are quite small and well represent the observations together with the dipole field.

Latest trajectory information for Europa Clipper assuming an October 2024 launch, and the predicted JUICE trajectory following the successful launch in April 2023 show initial close Ganymede flybys. In this study, we use them to understand their trajectories and highlight their importance in confirming the induced signal and thereby the ocean as well as for modelling the dynamo field. The first 2 Clipper and the first 3 JUICE flybys occur within an altitude of 500 km from Ganymede’s surface and are hence useful for overall internal field modelling. We predict the measurements that would be observed from the two internal sources as well as the external magnetospheric source to better understand the signals and decipher their differences. For the intrinsic and external fields, we use dynamo and magnetohydrodynamic models respectively while for the induced field, we use Jupiter’s background field along with the induction equation at the spacecraft locations.

The 5 flybys independently as well as together with the data from the Galileo flybys would enhance our understanding of the different magnetic sources at Ganymede and the fields they produce. These joint early flyby observations will enable us to be better equipped to model the magnetic field components near Ganymede in the orbital phase of the JUICE mission.