Powering Ganymede’s dynamo with protracted core formation

Ganymede is the only known moon with an active dynamo today. Previous studies interpret Ganymede’s dynamo as arising from convection in a metal core that formed billions of years ago. However, Ganymede likely accreted too cold to form with a metal core, which confounds interpretations of Ganymede’s magnetic field as a constraint on the moon’s broader history. Here, we reevaluate the thermal evolution of Ganymede’s rock-metal interior from a cold start. Our models show that Ganymede’s observed dynamo is consistent with ongoing core formation, a process not yet observed elsewhere. If Ganymede has an Fe-FeS core with a sub-eutectic composition, then gradual mantle warming may expel dense Fe melt onto the growing protocore and stir liquid metal, sustaining a dynamo for billions of years.