Jupiter's rings formed by cosmic crash

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THE RINGS of Jupiter were formed as a result of cosmic collisions between the planet's moons and the space rubble left over from the birth of the Solar System.

Astronomers have discovered how the mysterious rings were generated as a result of analysing data sent back to Earth from the Galileo spacecraft, which has been orbiting Jupiter since the end of 1995.

Collisions between the many meteorites that have pounded Jupiter's moons generated huge amounts of dust which was ejected into space where it became caught in the planet's gravitational pull.

As the dust swirled around the huge planet, it formed an intricate system of rings, which the instruments on board Galileo have now analysed in detail for the first time.

A team drawn from Cornell University in Ithaca, New York and the US National Optical Astronomy Observatories in Tucson, Arizona, announced last night that they can definitively explain the rings' origin.

"We now know the source of Jupiter's ring system and how it works", said Joseph Burns, professor of astronomy at Cornell, who was also one of the first scientists to report the intricate nature of the rings.

Nearly 20 years ago, the two Voyager spacecraft revealed that Jupiter had a flattened main ring and an inner, cloudlike ring called the halo. Both were composed of small, dark particles. More detailed images sent back from Galileo revealed a third ring, known as the gossamer ring. The spacecraft demonstrated that this faint ring was actually made up of two rings, one embedded within the other. Analysis showed that both rings are composed of microscopic debris from two of Jupiter's 16 moons - Amalthea and Thebe.

"For the first time we can see the gossamer-bound dust coming off Amalthea and Thebe and we now believe it is likely that the main ring comes off from two other moons, Adrastea and Metis", Professor Burns said.

Astronomers have long suspected that ring systems are the result of collisions, which generate dust that is then swept into orbit around a planet's equator. Galileo has now confirmed this theory with precise observations that appear to show the act of ring formation in progress.

Jupiter exerts a huge gravitational attraction, which draws in meteorites and other objects floating in space, some of which collide with the four innermost of its 16 moons.

"In these impacts, if the meteoroid is going too fast it buries itself deep in the moon, then vapourises and explodes, causing debris to be thrown off at such high velocity that it escapes the satellite's gravitational field", Professor Burns said. Adrastea is just five miles in diameter and small enough to allow dust to escape easily from its gravity. Its small size and close proximity to Jupiter make it perfectly suited to the job of creating the main ring, Professor Burns said.

"Rings are important dynamic laboratories to look at the processes that probably went on billions of years ago, when the Solar System was forming from a flattened disc of dust and gas", he said.