Secrets Of The Rings

WHAT CASSINI DISCOVERED WHEN IT GOT TO SATURN--AND THE WONDERS IT MAY UNCOVER IN THE MISSION TO COME

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Cassini will use an elaborate suite of instruments to determine why all this meteorological hubbub is taking place on a planet that is so cold--with cloudtop temperatures of --218°F--that it shouldn't be able to cook up much weather. The best guess is that internal heat left over from the gravitational collapse that formed the planet in the first place is keeping things warm. Cassini will deploy its cameras, infrared sensors, chemical spectrographs and more to deconstruct the planet's atmosphere and find out for sure. Other instruments will map the planet's magnetosphere and gravitational field, perhaps confirming the theory that even so massive a ball of gas as Saturn has a solid core.

But it is by its rings that Saturn is known, and it was close-up pictures of those rings that stole the show last week. Within minutes of Cassini's arrival, the ship's camera had fired off 61 shots of the rings, and by 10 o'clock the next morning, wide-eyed Cassini scientists were showing them to the press. "I don't think we've ever seen structures like this before," said Porco.

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.

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