Powering Ganymede's dynamo

Abstract

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.