Jovian Jewelry

If the solar system has a glamour world, Jupiter--with its brilliant colors, vast size and fruit fly-like swarm of 16 moons--has always been it. The planet appeared more elegant still in 1979, when the Voyager space probes discovered that it is circled by a fine set of nested rings.

No one knew the origin of the Jovian rings, but astronomers assumed they were either the pulverized remains of a small moon that had been destroyed by a collision or the raw material of an incipient moon that had never had the gravitational muscle to pull itself together. Last week they reached a different conclusion. New images returned by the Galileo spacecraft reveal that the fairy-dust bands are debris blasted into space when the planet's four innermost moons were struck by meteors.

The quartet of small satellites ought to have been elusive targets for incoming debris. Orbiting so close to Jupiter, however, they lay in the path of any projectiles drawn in by the planet's gravity. When a rock hits one of the moons, it releases dust that follows the moons like smokestack exhaust.

While such a bombardment process had been considered by scientists, it was accepted only after recent pictures were analyzed. They showed that the ring material indeed appears to flow from the rump end of the moons, and that the moons and rings orbit the planet at identical angles. "We have a definitive answer to the origin of this ring system," says Michael Belton, leader of the Galileo imaging team.

That answer has meaning not only for Jupiter but also for Uranus and Neptune, which have their own satellites and their own faint bands. Around those planets, little moons have likely taken a similar pounding and decorated their parent worlds in a similar way.

--By Jeffrey Kluger