Uranus: rings of mystery

Press, 30 August 1986, Page 20

Uranus: rings of mystery

From ‘The Economist,’ London

In an otherwise dismal year for space, America has scored one triumph. In January, Voy-ager-2 sailed past Uranus, beaming some 7000 optical images and a mass of data back to earth.

In the journal “Science,” 11 scientific teams have now disclosed what they learnt from experiments mounted during the flyby.

Voyager did not solve Uranus’s biggest mystery: its lopsided position. The other planets spin around an axis that is perpendicular to the sun. Uranus alone lies on its side, with its axis pointing towards the sun. Uranus was probably heaved on to its side by a collision with a gigantic body — perhaps the size of the Earth — but Voyager’s visit leaves that question open. The encounter did, however, bump the number of rings up from nine to 11; and of moons from five to 15.

The Voyager data has helped scientists learn more about how planetary rings were born and why they survive. Uranus’s are decidedly odd. The inner one appears to contain no particles smaller than boulders. That is a mystery. Frequent collisions between boulders should have smashed at least some of them into dust. There is plenty of dust around Saturn and Jupiter. When Voyager passed Saturn, it counted something like a million particles a centimetre wide for every boulder a metre wide. Another difference is that the Uranian rings are thin discs with sharp edges; Saturn’s were thick and fuzzy. Voyager scientists have come up with several explanations for the absence of dust. Both the rings and the newly discovered moons appear to be made of a dark material, perhaps because

they are coated with organic compounds. Uranus contains large amounts of methane, as well as the waterice which is thought to make up about half its mass. Together, these could generate hydrocarbons. Hydrocarbons are often sticky: they might encourage small particles to clump together into bigger ones. Another theory is that gravity is responsible for the absence of dust. Until Voyager, scientists had not realised that the Uranian atmosphere penetrates right into the ring system, where its drag could trap smaller, lighter particles and allow them to be sucked into the planet by gravity. Before Voyager, scientists expected the planet’s magnetic field to be small. Uranus’s home on the fringe of the solar system keeps the planet cold (its surface is only about 60 degrees above absolute zero), with little likelihood of possessing a hot, metallic core of the kind that gives the

Earth its strong magnetic field. Yet the Uranian magnetic field turned out to be roughly as strong as Earth’s. Odder still is the field’s shape. It is slanted at 60 degrees to the planet’s angle of rotation, and displaced from the centre by a third of the planet’s radius. As a result, it sweeps about wildly as Uranus rotates.

Which leads to a theory. Whenever it passes through the ring belt, the magnetic field could tug small particles out of the plane of the rings, to be sucked by gravity into the planet If Uranus contains little metal, what produces this magnetic field? The best guess at present is that enormous pressure- ; near the planet’s centre compress and heat an ocean of water, and possibly ammonia, to the point at which the molecules break down into a mass of charged particles, generating a magnetic field.

The existence of this internal source of heat could also explain why the planet’s temperature is uniform across its surface despite the fact that one pole points constantly at the sun.

Dr Peter Goldreich of the California Institute of Technology and Dr Scott Tremaine of the University of Toronto have an elegant theory to explain the sharpness of Uranus’s rings. Before the Voyager encounter, they developed the notion of “shepherding” — showing in theory that the presence of two moons, rotating on either side of a planetary ring, could prevent particles from the ring from drifting away into space. Voyager brought good news. Two of the new moons it discovered do appear to be shepherding the neatest of the planet’s rings. Copyright — The Economist