email a friend iconprinter friendly iconVoyage to Saturn
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In Saturn's rings today, tiny moons play the role of the planets. Each moon's gravitational tug is minute, Cuzzi says, "about the same effect as a passing truck's gravity has on you." Yet the moons' gravity helps maintain the rings by keeping the particles from straying from their orbits. A moon can also carve a gap between rings, and its gravity can send waves of density rippling through a nearby ring, like traffic speeding and slowing on a crowded freeway.

The Voyager probes glimpsed this dance, but Cassini is adding new detail. During its dash through the rings in June 2004, for example, it spotted evidence of miniature moons in the gauzy A ring, the outermost of the main rings. These moonlets—likely to number in the millions—are only a few hundred yards in diameter, but their feeble gravity is enough to leave wakes in the ring. In the F ring, farther from the planet, Cassini imaged a skein of narrow ringlets, accompanied by moonlets that sweep material into clumps and then break them up again.

"We're seeing ringlets interacting with moons and moonlets sculpting rings," Cuzzi says—and gaining new insights into how solar systems develop. "It helps explain how planets form in protoplanetary disks." The tiny moonlets in the A ring spiral slowly inward as they churn the ring particles, in a process that could also have shaped some of the bizarre solar systems detected around other stars. There, Jupiter-size planets are found right next to their suns, in orbits closer than Mercury's—perhaps because of a similar migration process.

One relic of our own early solar system still orbits Saturn: the moon Phoebe. Phoebe revolves in the opposite direction from most of the other moons, a hint that it has an unusual history. Cassini took a close look on its approach to Saturn in 2004 and found that the 130-mile-diameter moon is a hodgepodge of ice, rock, and carbon compounds—much like the Kuiper belt objects, small, icy bodies in the outer solar system that are thought to be leftover building blocks for the outer planets. As the solar system formed, most of the Kuiper belt worlds were flung far beyond Pluto. But Phoebe could be a Kuiper belt object that got left behind, trapped in orbit about the young Saturn.

Saturn's other major moons probably were born in the same clump of gas, dust, and rock that created the planet itself, but they are a study in diversity. Cassini revealed that some are little more than loose collections of rubble, including Hyperion, a potato-shaped mass 215 miles long. Larger moons are denser and have distinctive surface features, sculpted by accidents of history or by internal heat and the geologic activity it drives.

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