Geophysicists might have solved one of the biggest mysteries in the solar system – why do the magnetic fields of Uranus and Neptune differ from those of the other planets? Computer simulations by Sabine Stanley and Jeremy Bloxham of Harvard University suggest that the two planets have a fluid core that is surrounded by a relatively thin outer layer. The Earth and other planets such as Jupiter and Saturn have an inner rocky core that is surrounded by a thick convecting shell. The results mean it may be possible to use magnetic fields to learn more about the internal structure and composition of planets (Nature 428 151).
The magnetic fields of the Earth, Jupiter and Saturn resemble the field that would be created by a massive bar magnet located at the centre of the planet and roughly aligned with its spin axis. On Uranus and Neptune, on the other hand, the magnetic poles are tilted away from the spin axes and towards the equators. Moreover, the magnetic fields of these planets seem to be produced by two north and two south poles.
The magnetic field of the earth is generated by convection in a thick fluid shell – made of molten iron and nickel – that surrounds a small, electrically conducting solid inner core. Similarly, the magnetic fields on Jupiter and Saturn are produced by a thick layer of convecting metallic hydrogen that surrounds a small rocky centre.
Stanley and Bloxham have developed a different model for Uranus and Neptune. They suggests that convection in these planets is produced in a thin fluid outer shell – probably made of “ice” containing water, methane, ammonia and hydrogen sulphide – that surrounds a non-convecting fluid interior. The simulation generates magnetic fields similar to those observed on the planets by the Voyager 2 spacecraft in the 1980s. “This shows we can learn about a planet’s interior by studying the morphology of its magnetic field,” Stanley told PhysicsWeb.
