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Sunlit Saturn shows off its rings

October 12, 2006 By Nicola Jones This article courtesy of Nature News.

Cassini snaps the perfect photo-shot.

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This September brought a fantastic photo opportunity for Cassini, the spacecraft in orbit around Saturn. Its trajectory brought it into the ringed planet's shadow for about 12 hours, giving it a striking view with the Sun lying directly behind.

With the glare of the Sun blocked, Cassini's cameras could pick up light glinting from particles just 1-10 micrometres across. The effect is similar, says Cassini imaging scientist Joe Burns of Cornell University in New York state, to the way that stray hairs are highlighted when a person is illuminated from behind.

The light show revealed several ringlets that haven't been seen before, the team reported at the American Astronomical Society's Division for Planetary Sciences Meeting in Pasadena, California, this week. One particularly bright ringlet, they say, may be the remains of a recently shattered moon.

Some of the other ringlets have no obvious source. Team members are now on the hunt for as-yet-undiscovered moonlets that are probably lurking in the rings: the particles highlighted in this snapshot exist for just 1,000 years before being blasted into individual ions, says Burns, so they must be coming from somewhere. "Just like the old maxim that says where there's smoke, there's fire; at Saturn, where there's a new ring, there's bound to be a moon," says Jeff Cuzzi, Cassini interdisciplinary scientist at NASA's Ames Research Center in California.

True colours

The enhanced snapshot also reveals distinct colour differences within rings, which are probably due to differences in particle size. "This is a little bit surprising," says Burns. The team suspects that particles may be sorted within a ring by the differing effects of gravity and the magnetic field on particles of different sizes.

The view was cobbled together from 165 images taken by Cassini's wide-angle camera over 3 hours on 15 September. The colour differences have been exaggerated.

Cassini was about 15 degrees above the plane of the rings and 2.2 million kilometres from Saturn at this point that makes each pixel in the 1,000 by 1,000 pixel photo about 260 kilometres wide. The great distance meant that Cassini could stay in shadow for a prolonged time. "Normally we're in this shadow for an hour or less," says Burns.

Close-up

The craft has had a much closer view of Saturn before. In 2004, Cassini's trajectory took it between two quite dense rings in its closest pass to the planet: "That was a nail-biting time," says NASA headquarters programme scientist Denis Bogan in Washington DC. The team turned the craft's main antenna dish to act as a shield as it passed through the rings, as an object the size of a marble could easily destroy a more vulnerable part of the craft.

The Cassini-Huygens mission, which is a project of NASA, the European Space Agency and the Italian Space Agency, will keep going on its pre-planned trajectory around Saturn at least until 1 July 2008, when the team is likely to apply for an extension.

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