They're big, they're blue, and they have weird magnetic fields--Uranus and Neptune are often called twins. Now, researchers think they've figured out why the two planets have magnetic fields unlike any others in our solar system. The secret, they say, is the layered liquid core inside each planet.
Earth's magnetic field has a simple arrangement, similar to that of a bar magnet: Field lines poke out of the south pole, curve round the planet, and dive back in at the north pole. That's also true of Saturn, Jupiter, and possibly other planets in the solar system. Not so for Neptune and Uranus, which have magnetic poles tilted 47 and 59 degrees, respectively, from their axes of rotation. Even stranger, their field lines loop in and out in shifting patches.
What generates Earth's magnetic field is the reliable churning of liquid metal in the outer core. It works like a dynamo, a device that converts the energy of motion into an electrical current with a magnetic field. The metal particles in the outer core slowly convect, rising after being heated by the inner core, only to sink as they cool. Earth's rotation spins the particles as well, in a different direction from the convection. Together, this cycling and spinning give the Earth's field its overall shape. The solid inner core of the Earth then holds the magnetic field lines steady, roughly aligned north-south.
The strange fields of Uranus and Neptune result from oddities of their interiors, Sabine Stanley and Jeremy Bloxham of Harvard University report in the 11 March issue of Nature. The two geophysicists describe a model of an alternative kind of dynamo that fits what researchers know of the insides of the twin gas giants. Uranus and Neptune don't have large, solid cores. Instead, they have layered liquid cores that do not convect, surrounded by a thin outer layer that does. This thin outer layer works together with the spin of the planet in a dynamo effect. But instead of getting trapped and stabilized by a solid core, the field lines tangle within the liquid core, wreaking havoc with the field and destroying any chance of a simple bar magnet-like field.
Stanley and Bloxham have shown that the same mechanism that works for Earth can explain the more complex fields on Uranus and Neptune, says Jonathan Aurnou of the University of California, Los Angeles. It's possible that a "dynamo generates all the active fields in the solar system."