Ganymede has a subsurface ocean sealed beneath kilometers of ice. We know it's there from tidal flexing models and Hubble observations of its magnetic field. But knowing an ocean exists is different from knowing what it does. The ice is opaque to light and impractical to drill. The ocean's circulation — its currents, jets, convective cells — would seem permanently hidden.
Cabanes, Gastine, and Fournier show that the ocean reveals itself magnetically. Saltwater moving through a magnetic field generates its own magnetic field by kinematic induction — the same physics that drives planetary dynamos. Ganymede is uniquely suited to this detection because it has both a subsurface ocean and its own internal magnetic field. The combination means ocean currents don't need to generate a field from scratch; they modify an existing one.
The team models zonal jet flows from thermal convection simulations and solves the induction equation across the ocean shell. Ocean flows primarily generate toroidal magnetic fields through the omega-effect — differential rotation wrapping field lines into new configurations. Weaker poloidal components leak through the ice shell to the surface. In a deep ocean configuration with elevated magnetic Reynolds number, the surface signal reaches 9 nanoTesla — detectable by a low-altitude orbiter.
The JUICE spacecraft, en route to Jupiter, will orbit Ganymede starting in the 2030s. These predictions tell it what to look for: at higher spherical harmonic degrees, the ocean-induced signal can dominate over the internal dynamo and Jupiter's external field. The ocean's fingerprint hides at small spatial scales where the other sources are weak.
You cannot see through the ice. But the currents beneath it write their signature in the magnetic field above.