Saturn: a ringside seat

The Cassini mission has shed new light on one of the great mysteries of the solar system, says Steve Connor

They are one of the most spectacular sights in the solar system yet the visual serenity of Saturn's rings belies the violent destruction that continually takes place to maintain these remarkable planetary structures.

From Earth the distinctive set of rings belonging to the sixth planet from the Sun appear to be tranquil garlands adorning their mother planet but on closer inspection the rings of Saturn – composed of orbiting, ice-strewn debris – are far from peaceful.

Immense tidal forces tear into the materials that make up the rings to prevent them clumping together to form bigger objects and, eventually, a new moon. Giant rocks and meteorites pummel the rings with violent collisions that break apart the orbiting debris still further.

Ever since Galileo first observed the rings of Saturn – which he thought were a pair of twin moons on either side of the planet – they have been an object of fascination. Now scientists working on data sent back from the Cassini space probe believe they are close to answering some of the most intriguing questions of Saturn's defining feature.

After six years of studying the rings, the researchers involved in the £1.6bn Cassini mission believe they can show that the planetary rings are far from a static collection of space debris orbiting their host but rather a highly dynamic interaction of continually colliding objects.

"It has been amazing to see the rings come to life before our very eyes, changing even as we watch," said Jeff Cuzzi of Nasa's Ames Research Centre in Moffett Field, California, and lead author of a study published in the journal Science. "The rings were still a nearly unstructured object in even the best telescopes when I was a student, but Cassini has brought us an intimate familiarity with them," said Dr Cuzzi, one of the scientists in the joint American-European mission named after the great Italian-born astronomer Giovanni Cassini.

From Earth it is only possible to see a couple of the biggest rings but there are many more, each composed of small objects, some the size of a grain of sand, that orbit the planet around its equator. The particles are "shepherded" by some of the 62 known moons of Saturn, covering a distance of 150,000 miles across, but less than a mile thick.

Professor Carl Murray, an astronomer at Queen Mary, University of London, who works on the Cassini mission, said that the most bizarre ring of all, the outer F-ring, is routinely bombarded with cannon-ball like objects that appear to come from nowhere and disappear just as suddenly.

"Rapid change is the order of the day at the F-ring with bizarre structures created by the combined effect of gravity and impacts from nearby objects. Each Cassini image gives us another piece of the F-ring jigsaw puzzle and gradually a complete picture of this strange ring is starting to emerge," Professor Murray said.

The Cassini spacecraft, launched in 1997, arrived at Saturn in 2004 and has since captured some of the most remarkable images of the planet's ring system. It has shown that the brightness of the rings is caused by the purity of the water ice they contain, which reflects sunlight. However, the rings have a reddish tinge caused by an as-yet unidentified contaminant.

Cassini has also monitored the immense weather systems that take place on Saturn, a "gas giant" planet mostly composed of hydrogen and helium. It has monitored the sound of the lightning strikes caused by the electrical storms that take place every few years on the planet. Even bigger equatorial storms erupt and encircle the planet once every 15 or 20 years.

As Cassini approached Saturn in 2004 it released Huygens, a small probe, into the dense atmosphere of Saturn's largest moon, Titan, which is believed to be the most Earth-like world in the Solar System. Cassini revealed that Titan's surface, shrouded in an orange haze, is shaped by rivers of liquid methane and ethane which form clouds that rain down on the moon's surface.

One of the most startling discoveries to emerge from the mission was the discovery of plumes of ice particles and water vapour spouting from "volcanoes" on Saturn's icy moon Enceladus.

In numbers

62 The number of known moons of Saturn, 53 of which have been officially named. Titan is the largest and most interesting, in that it is similar to a rocky, Earth-like planet.



93% The composition of water ice in the rings of Saturn. The high water content explains why the rings are so prominent – they reflect sunlight easily.



7 The number of years it took for the Cassini spaceprobe, below, to arrive at Saturn. Nasa has recently announced a further 7 years of life, which will end with the probe being crashed into the planet.

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