FIGURE 1. Magnetic fields and interior structures of the planets.

a, Earth has a predominantly dipolar magnetic field, like that of a bar magnet. Magnetic north and the North Pole do not quite coincide: the magnetic axis is tilted by 11° relative to the axis of rotation of the planet. The field is generated by the dynamo action of convective currents in the molten outer core, around a solid conducting inner core. b, The gas giant Jupiter also has a strongly dipolar field, whose axis is aligned at 10° to the planet's rotational axis. In Jupiter's interior, convection occurs in a thick shell of metallic (dissociated) hydrogen that surrounds the planet's small, rocky core. c, Uranus, in contrast, has comparable quadrupole and dipole components in its magnetic field. Strikingly, the magnetic-field axis is oriented at 59° to the planet's axis of rotation. Using a revised description of the planet's interior — with a much thinner convecting shell around a non-convecting but fluid inner region — Stanley and Bloxham² have devised a dynamo model that generates a magnetic field that is similar in structure to Uranus' (and Neptune's) anomalous field. (Rotation axes are artificially aligned vertically for ease of comparison.)