It is thought planets have dynamos in their centre. This explains the intensity and geometry of the magnetic fields found on the surface. The dynamo is in the form of a fluid sphere of molten iron which is churning around producing electric currents and magnetic fields. But Mars has a strange field. Only the southern hemisphere has a strong magnetic field, emanating from the crust. Secondly, this crustal field is very strong, about ten times what’s found on Earth or other rocky planets. This is one of a range of amazing anomalies with Mars. Saturn has other problems. Its fields are too perfect. Sabine Stanley explains.