Phoebe is an ugly duckling among Saturn's natural satellites. Its dark surface is heavily cratered. It orbits the planet backward. And it refuses to swing around the planet in the same orbital plane as other moons do.

But the international Cassini-Huygens spacecraft has quickly turned the 140-mile-wide chunk of rock and ice into the darling of planetary scientists.

Images and data taken at the weekend from 11 of the spacecraft's instruments could open an unprecedented window on the conditions that existed in the early solar system generally and on a young Saturn's environment in particular as it formed some 4.6 billion years ago.

The information also could help astronomers come to terms with a class of moons that only five or six years ago was thought to be a oddity. Today, these "irregular" objects account for the majority of moons detected so far in the solar system.

What little has been gleaned from a Voyager flyby in 1981 and more recent ground-based observations suggests that Phoebe represents the kind of object found in the Kuiper Belt, a broad swath of cosmic rubble orbiting the sun beyond Pluto. If the results indicate that the material on Phoebe hasn't been altered significantly over time, the object could be one of the most primitive ever studied.

"The imaging team is in hot debate at the moment on the interpretations of our findings," notes Carolyn Porco, a researcher at the Space Science Institute in Boulder, Colo., and head of the Cassini imaging team.

Cassini's stunning black-and-white images suggest that the moon hosts an ice-rich material covered with a layer of darker soil and rock some 300 to 500 meters thick. Bright streaks along crater walls also hint at ice underneath the surface. In addition to the ubiquitous craters, Phoebe's surface is etched with grooves, pits, and ridges that are expected to yield insights into the moon's internal structure. Researchers say that some of these features are unique compared with those in similarly detailed photos of large asteroids, also thought to be primitive objects.

"This is unlike any other solar system body I have seen," says David Nesvorny, senior research scientist at the Southwest Research Institute in Boulder, Colo.

To Torrance Johnson, a member of the Cassini imaging team, the photos "are showing us an ancient remnant of the bodies that formed over 4 billion years ago in the outer reaches of the solar system.

A big question looms: How did the object wind up at Saturn at all and assume such oddball orbital traits compared with most of Saturn's other moons?

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