On Ganymede's Magnetic Quadrupolar Strength

Ganymede is the only moon in our solar system known to have a large-scale intrinsic magnetic field, likely generated in the moon's metallic core. Initial analyses of Galileo spacecraft measurements concluded that Ganymede's intrinsic magnetic field is dominated by a magnetic dipole and that quadrupolar contributions are exceptionally weak. These findings have influenced the development of models for Ganymede's core dynamo over the past two decades, some concluding that Ganymede's dynamo is limited to the innermost part of Ganymede's core. Here, we reassess Ganymede's internal field contributions based on the magnetic measurements from close Galileo flybys of Ganymede (G1, G2, G7, G8, G28, G29), adding the recent Juno flyby. We find that presently available data cannot constrain Ganymede's quadrupole moment, as we demonstrate by constructing models with a range of quadrupole moments, including relative values comparable to those at the Earth. As a consequence, global analysis of available data cannot constrain the spatial limits of Ganymede's core dynamo. Incorporating ocean induction for a range of Ganymede ocean models indicates that ocean induction may be present, but that available magnetic data cannot discern between end member cases for Ganymede ocean models.