BBC News Online: Sci/Tech

Jupiter's ring riddle solved

Unlike the famous ring system of Saturn, Jupiter's rings are not visible from the Earth.

They were only detected in 1979 by the Voyager 1 spacecraft, and their origins have been a puzzle to scientists ever since.

Michael Belton, head of the team that made the discovery, said: "We now have a definitive answer to the origin of these rings. We also understand the process that leads to their appearance."

Tiny dust particles

Mr Belton, of Cornell University, Ithaca, New York, says that the three rings consist of tiny dust particles kicked up by space rocks hitting Jupiter's four inner moons and dragged into orbit around the solar system's largest planet.

Joe Veverka, a member of the team, said Jupiter's moons - Metis, Adrastea, Amalthea and Thebe - were constantly being "pelted by storms of comets and asteroids travelling at 40km a second, which is 100 times the speed of a .22 calibre bullet,"

With each collision, thousands of dust particles are thrown up. As the small moons have very little gravity, the particles are dragged into the planet's gravity, where they form the three rings.

The innermost is more like a halo, formed as the particles spread out and drift in towards the planet. It comes within 20,000km of the top of Jupiter's clouds.

The main ring, outside this, is formed from the dust of two moons.

Beyond it is a faint gossamer ring, consisting of two wide circles of dust, extending out more than 220,000km from the planet.

'Less dusty than living room'

Huge as the rings may be, the dust particles are very thinly spread, each being about 30 metres from the other.

"It is much less dusty than your living room," Mr Veverka said.

The team's findings were based on analysis of images and data sent back to Earth by the space probe Galileo, which has been orbiting the planet for more than two years.

It was also discovered that the ring formation is "an ongoing process".

The tiny particles that form the ring have a limited lifespan, but they are constantly being replaced by fresh particles, therefore the rings never increase in size or density.

"The rings are regenerating all the time," Joe Burns, another team member said.

Nor are the rings static in their position.

They "wobble", the astronomers say. So much so, that the outer ring was described as "a hula-hoop around Jupiter's fat waist."

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