Planetary magnetic fields: Observations and models

The state of knowledge and understanding of planetary magnetic fields is reviewed. All of the planets, with the possible exception of Venus, have had active dynamos at some time in their evolution. The properties and characteristics of the dynamos are as diverse as the planets themselves. Even within the subclasses of terrestrial and giant planets, the contrasting compositions, sizes, and internal pressures and temperatures of the planets result in strikingly different dynamos. As an example, the dynamos in Mercury and Ganymede are likely driven by compositional buoyancy distributions different from that in the Earth’s core. Dynamo models operate far from the parameter regimes appropriate to the real planets, yet provide insight into the dynamics of their interiors. While Boussinesq models are generally adequate for simulating terrestrial planet dynamos, anelastic models that also account for large density and electrical conductivity variations are needed to simulate the dynamos in giant planets. Future spacecraft missions to planets with active dynamos are needed to learn about the character and temporal variability of the planetary magnetic fields.