The Sciences

Cassini Watch: An Outsider Named Phoebe

Planetary scientists now suspect Phoebe started out in the Kuiper belt.

By Maia WeinstockSep 30, 2004 5:00 AM

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Phoebe, the ninth largest of Saturn’s 31 known moons, always seemed like the black sheep of the Saturn family, traveling in the opposite direction from the other moons and at a highly inclined orbit. On June 11, NASA’s Cassini probe scrutinized the 137-mile-wide moon from only 1,240 miles away and sent back an array of stunning photos that may explain why the small, dark satellite looks so out of place.

Before Cassini’s visit, the best picture of Phoebe was a blurry view snapped by Voyager 2 in 1981 from 1.3 million miles away. Because of its dark color, planetary scientists speculated that Phoebe was a passing asteroid that got caught in Saturn’s gravitational field. “Now the consensus seems to be that it is more like a comet,” says Cassini program manager Robert Mitchell of NASA’s Jet Propulsion Laboratory. Bright, streaky material visible underneath many of the craters all but proves that Phoebe’s dusky surface is in fact a thin layer on top of a mother lode of ice, Mitchell says. “There were a lot of craters, some shiny areas, some darker areas, loose surface material falling into craters—many more features than I would have guessed.”

Planetary scientists now suspect Phoebe started out in the Kuiper belt, a region of icy rocks at the outer edge of the solar system. If so, Phoebe has given astronomers their first opportunity to study an object from distant space at close range. Cassini will soon examine several of Saturn’s other remarkable moons. Next up: two swings by fog-shrouded Titan this fall and a February 2005 encounter with Enceladus, where ice volcanoes may be spewing new material into Saturn’s rings.

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