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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The payoff — for space scientists and curious civilians — could be staggering. Three other planets in the solar system — Jupiter, Uranus and Neptune — have rings, but they are faint and thready things, nothing like the magnificently complex cosmic jewelry that decorates Saturn. Seven of the other nine planets have moons, but none that perform the gravitational dances among themselves and within the rings that Saturn's do. And no planet has a moon anything like Titan, a world with much of the preorganic chemistry that Earth had 4.5 billion years ago — offering scientists a one-of-a-kind window into our vanished past.

The Saturnian system is, in a very real sense, the solar system writ small. And while other spacecraft have glimpsed it before — Pioneer 11 in 1979, Voyagers 1 and 2 in 1980 and 1981--they were mere flybys, quick hits by ships snapping a few pictures before whizzing off into deeper space. Cassini-Huygens — named after 17th century astronomers Jean Dominique Cassini and Christiaan Huygens — is there to stay.

"It really is like coming back to the promised land," says Torrence Johnson, a veteran of Voyager and a member of the Cassini imaging team.

The centerpiece of the Saturnian system is, of course, the planet itself, and plans call for it to get a going-over that it has never had before. The second largest of the solar system's four gas giants, Saturn — like its big brother Jupiter — is sometimes described as a starlike body with a chemistry of hydrogen and helium but without sufficient mass to light a nuclear furnace. That doesn't mean, however, that Saturn isn't roaring with activity.

By far the planet's most dramatic feature is its hellish weather. Winds blow around the Saturnian equator at 1,100 m.p.h.--five to 10 times the speed of the most powerful winds on Earth. Giant hurricanes tear through the planet's atmosphere, often two or more storms at a time, which then meet up before dying out. Displays of light similar to Earth's aurora borealis illuminate Saturn's skies, thanks to charged particles falling in from its moons. And where the auroras aren't flashing, lightning may be striking.

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 --218F — 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.

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