All the planets in our solar system, except Mars and Venus, have magnetic fields which are actively maintained by fluid dynamos in their interiors. There is strong evidence that Mars once had a dynamo that is no longer active. These dynamos are believed to be driven by the convection that takes the heat out of the planet as it gradually cools down. The deep interiors of planets are electrically conducting fluids, and the convection stirs the fluid and generates the magnetic fields by dynamo action. The convection is much slower than the rotation rate, so the fluid motion is strongly influenced by Coriolis effects, which play a key role in generating large scale magnetic fields.

The wave motions induced by the convective stirring can be seen in the measurements of the Earth’s magnetic field, and they also lead to small but measurable changes in the length of the day. Observations of torsional oscillations in particular can be used to probe the magnetic field inside the Earth’s core, and provide a test of current dynamo theories.

To date, dynamo models have focussed on dynamos in liquid metal cores, where the fluid is virtually incompressible, but new compressible dynamo models for the giant planets are now being developed. Models of Jupiter’s dynamo hold out the hope that we will be able to use the magnetic data being gathered by the Cassini mission (arrives at Jupiter in 2016) to investigate its deep interior.