Powering Ganymede's dynamo

It is generally believed that Ganymede's core is composed of an Fe-FeS alloy and that convective motions inside it are responsible for generating the satellite's magnetic field. Analysis of the melting behavior of Fe-FeS alloys at Ganymede's core pressures suggests that, besides the growth of a solid inner core, convection can be driven by two novel mechanisms: Fe snow and FeS flotation. To advance our understanding of magnetic field generation in Ganymede, we construct dynamo models in which deep inner core growth, Fe-snow and FeS flotation drive convection. Although a dynamo can be found in each of these cases, the dynamos have different characteristics. For example, some dynamos are dipole dominant and others are not. It is found that multipole-dominant magnetic fields are generated in all Fe-snow cases, while dipole dominant dynamos are found in FeS flotation cases and in inner core growth cases. Ganymede's present dipole-dominant magnetic field suggests that the Fe-snow process does not play a primary role in driving Ganymede's core convection. The reason that Fe-snow driven convection does not produce a dipole-dominant dynamo can be related to the buoyancy flux. In Fe-snow cases, the buoyancy source is located at the core-mantle boundary (CMB), and the buoyancy flux peaks there, while in the other two cases, the buoyancy source is located at the inner core boundary where the buoyancy flux peaks.