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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Iapetus, for example, is a two-toned world, its leading edge dark, its trailing edge white. There are many theories advanced for this — including the possibility that there are hemisphere-wide volcanoes or that the moon is picking up dust as it moves through its orbit, staining its face and leaving the other side clean. "We have all kinds of questions," says Cassini physicist Larry Soderblum. "Were there volcanoes? Were there oceans of some mystical hydrocarbon that froze?"

Enceladus holds mysteries of its own. A bright white world with a relatively smooth face, it appears to have been repeatedly resurfaced by some kind of underground slurry or perhaps by ice volcanoes. In some places, once deep crevasses have been largely filled in and craters have been cut neatly in half, leaving one side deep and raw and the other covered, as if by snowdrifts. The area of the Saturnian ring that follows in the wake of Enceladus is slightly thicker than the rest, as if the moon were pumping out some kind of frozen exhaust, leaving a plume in its wake like the smoke from a steamship.

Other questions should be answered when Cassini flies by Hyperion, a tumbling moon that appears to have been knocked off its pins by a collision eons ago and has never regained its footing; and Tethys, a moon that bears such a massive impact scar that only the barest geological margin keeps it from shattering altogether.

It is Titan, however, that will be the main attraction. One of the largest moons in the solar system — larger than Mercury or Pluto — Titan would be a perfectly good planet if it were orbiting the sun under its own steam. NASA scientists were keenly disappointed when the Voyager 1 spacecraft flew by Titan in 1980. The moon's dense, orange atmosphere completely concealed its surface from view, revealing not a clue about what was happening on the ground.

Scientists speculate that there may be quite a bit happening. Rich in nitrogen as well as ethane, methane and other carbon-based gases, the Titanian air contains the raw chemical material believed to be needed to give rise to life — and just the kind that probably existed on the primordial Earth. Titan's frigid temperature — about --280F — would surely have prevented life from emerging. Nonetheless, over time the candlelike heat of the distant sun may have slow-cooked some of the organic materials, forming more complex molecules. What's more, if there is lightning in Titan's atmosphere, the random jolts could have shocked even bigger molecules into existence.

The Cassini-Huygens mission will investigate Titan from many angles. Of the 59 flybys of the nine selected moons, 45 will be devoted to Titan — most at a distance of just 590 miles. Preliminary images received last weekend revealed a bright cloud pattern about the size of Arizona near the south pole and what appeared to be a massive impact crater.

But there will be much more. Radar will pierce the Titanian cloud cover, mapping plains, mountains and perhaps even lakes of liquid ethane and methane — though early observations last weekend cast new doubt on the existence of the lakes. Spectrometers and other instruments will take the chemical measure of the moon's air, and cameras will again try to photograph Titan from outside in.

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