March 10, 2004. Copyright 2004. Graphic News. All rights reserved.

Tilted poles of Uranus and Neptune explained

By Iain Nicolson

LONDON, March 10, Graphic News: Ever since the Voyager 2 spacecraft flew past Uranus and Neptune in the late 1980s, planetary scientists have been struggling to understand the perplexing magnetic fields that it revealed. Harvard planetary scientists Sabine Stanley and Jeremy Bloxham, whose research is published in this weekÕs issue of the scientific journal Nature, may have solved the mystery at last.

Like the giant planets Jupiter and Saturn, the Earth has a magnetic field which is aligned fairly closely with its axis of rotation. As a result our magnetic north pole lies quite close to the geographic North Pole. Magnetic lines of force -- which determine the direction along which compass needles point -- emerge from one hemisphere, loop round, and re-enter the other, making up a pattern which is symmetrical around the north-south magnetic axis. In effect, our planet behaves as if it had a bar magnet embedded in its central core.

In contrast, Uranus and Neptune have complex magnetic fields which are tipped over at large angles so that, in each case, the magnetic poles lie much closer to the equator. The magnetic axis does not pass through the centre of the planet, nor does the magnetic field display the same kind of symmetry as that of the Earth.

Planetary magnetic fields are generated by circulatory motions in electrically conducting fluids, motions which are sustained by convection -- the mechanism which causes hot liquids to rise and cool liquids to sink. The process is called dynamo action. EarthÕs dynamo operates in the liquid-iron outer core, which surrounds our planetÕs solid iron inner core. Inside Uranus and Neptune, the process takes place in an electrically conducting mixture of water, methane and ammonia -- substances which, in planetary scientistsÕ parlance, are known as ÒicesÓ.

Sabine Stanley and Jeremy Bloxham have developed simulations of planetary dynamos. They find that the details of a planetÕs magnetic field depend on the thickness of the layer in which convection is taking place and on the nature of the underlying region. If the dynamo is operating in a thick shell surrounding a solid conducting inner core, the magnetic field matches that of the Earth. If instead the circulation that generates planetary magnetism is confined to a narrow shell surrounding a fluid interior, the resulting magnetic fields are remarkably similar to those of Uranus and Neptune. 

If their theory is correct, it should usher in a new way of investigating the insides of planets. ÒOn Earth, we have seismology to study whatÕs happening in the interior of the planet, but when we send spacecraft to other planets, itÕs difficult to get direct information about the interiorÓ, says Stanley. ÒOur modelling shows that the magnetic field we observe outside a planet can tell us a lot about the structure and composition of a planet's interiorÓ.

Data from the Cassini spacecraft, which is due to enter orbit round Saturn on July 1 this year, are expected to provide an early test of some aspects of the theory.
/ENDS

Sources: Nature, Sabine Stanley