Planetary magnetism

The planets and moons of our solar system have been investigated with the help of magnetometers on board spacecraft. Measurements show that their magnetic moments are mostly proportional to their angular momenta, a relation known as Busse’s law (Fig. 1). The moments are almost always directed along the axis of rotation which suggests a dynamo effect in a conducting core. The core in the gas giants is likely to be a high-pressure metallic form of hydrogen rather than molten Fe–Ni, as in Earth. The magnetic field at the surface of Jupiter is ten times as strong as that at the surface of Earth, and its magnetic moment is roughly 20 000 times as large. The weak fields of the Moon and Mars are probably related to the absence of any liquid core.

Figure 1: Magnetic moments of planets and moons in the solar system, plotted against their angular momentum. (After P Rochette.)