Composite images of Saturn's rings, taken by the Hubble Space Telescope, have revealed mysterious colours that suggest the rings could be made of material from the outer Solar System.

The images, captured between 1996 and 2000, show Saturn's rings at their most dispersed, allowing astronomers to look into them and determine their properties.

The rings have a salmon colour when viewed at an angle

Composed of a myriad of small chunks of rock and ice, their origin has always puzzled astronomers. They were thought to be either a shattered moon or material that never got to form a moon at all.

The new observations suggest that they may be the remnants of a shattered body from the most distant parts of the Solar System.

Complex molecules

"The colour of the ring material can help tell us what the rings are made of and will help decipher their origin," said Dr Jeff Cuzzi of the American space agency (Nasa).

"Most people don't know that Saturn's rings aren't white but have a faint salmon colour, which hints that a few percent of complex organic molecules are mixed in with the water ice the rings are mostly made of," he added.

The observation that is intriguing astronomers is that, unlike the rings, Saturn's seven small icy moons do not have a reddish colour. However, many icy objects in the distant, frozen outer reaches of the outer Solar System do.

This had led some scientists to speculate that, unlike its moons, the rings were formed from an outer Solar System object that broke up when it passed too close to Saturn.

To make the composite image, over 100 Hubble images were analysed in eight different colours that cover, and go beyond, the range of human vision. They include violet, blue, green and red in the visible range and ultra-violet and infrared in the non-visible range.

Unknown materials

As well as casting light on the origin of the rings, the observations have shown researchers just how little they understand Saturn's ring system.

Detailed analysis suggests that there may be at least two unknown materials mixed with the rings' water ice, and that the way these materials are distributed in the rings is unlike anything seen on the surfaces of nearby planets or their moons.

The Hubble data also shows that the ring colour changes with viewing angle. The best explanation, says Dr Francois Poulet of Nasa, is that the ring particles are actually aggregates of particles, with many more deep shadows than the relatively smooth surface of a moon. The rings appear redder as more shadows are seen.

Astronomers are looking forward to a much closer view of Saturn in a few years' time when the Cassini spacecraft, which recently passed Jupiter, reaches the ringed planet in 2004.