Secrets Of The Rings

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COME HITHER: Cassini snapped this picture when it was 17.6 million miles from Saturn; each pixel in the image covers 105 miles

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What so moved her and the other scientists was the complexity and dynamism of the rings' internal structures. Scientists have known since Voyager that the seven broad bands that make up the ring system are not undifferentiated masses of material but rather are made up of hundreds of individual strands, like the grooves in a record album. The strands are made up of billions of bits of rubble and ice, some of them crystals smaller than a grain of sand, some of them boulders bigger than a house.

As the pictures from Cassini reveal, the interaction of this orbiting material can create bizarre effects. The edge of one ring shows elegant scalloping, presumably caused by the gravitational wake of a moon cruising alongside it. As the moon sails by at predictable intervals, the random collisions of ring particles become more rhythmic, forming tidy peaks and troughs.

Other images show that the moving moons cause equally graceful formations within the rings by tugging on particles and causing them to pile up and thin out, pile up and thin out, rippling outward in what ring scientists call a density wave. Another kind of wave known as a bending wave is caused by a moon that orbits at an angle inclined to the ring plane, warping or corrugating the ring's edge.

"We knew from Voyager that these phenomena probably existed," says Cassini planetary scientist Josh Colwell, "but there's never been anything this clear. The resolution in these pictures is unprecedented." Says an ecstatic Porco: "I'm surprised at how surprised I am by the beauty of these images."

They'll get better. Although Cassini will never again be as close to the rings as it was last week, it took only black-and-white pictures on the way into orbit. From now on, it will shoot between 100 and 200 images a day, most of them in color. The spacecraft will assemble mosaics of the rings, photographing them section by section and arranging the pictures in sequence from the center of the bands out.

Though no one is sure how the rings formed, some of the material is almost certainly the remains of small pulverized moons that were destroyed either by a cataclysmic meteor hit or when they wandered too close to a gravitational danger zone known as the Roche limit: the altitude above a planet at which the difference in gravity between the end of an object closest to the planet and the end farthest from the planet is great enough to pull the object apart while not pulling the remains out of orbit. Instead, the rubble disperses around the planet. Photographs of the debris could help confirm this phenomenon and could even turn up smaller, still undiscovered moons hiding within the rings.

Scientists already know of some moonlets that orbit inside the rings, sweeping areas clean of debris and accounting for at least two conspicuous gaps. Other tiny moons move along the outer rim of rings; these are the so-called shepherding moons that groom the ring edges and keep them sharp.

If Cassini-Huygens traveled all the way to Saturn and returned nothing but data on the planet and its rings, the mission would probably still be judged a success. Yet the true scientific goods will come when the spacecraft trains its instruments on the swirl of Saturnian moons. It would be nearly impossible for one ship to visit all 31 known satellites in Saturn's litter, so NASA has selected nine of them, both for their scientific promise and their comparatively convenient locations. The exotic names of the chosen moons — Phoebe, Titan, Iapetus, Enceladus, Mimas, Tethys, Hyperion, Dione and Rhea — hint at the exotic science that awaits.

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