An 'oddball' moon of Saturn captivates astronomers

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?

One possibility is that Phoebe is the offspring of a collision between two comets that occurred in Saturn's neighborhood as it formed, says Dr. Nesvorny. But a more plausible explanation, he continues, is that as Saturn gathered gas from the solar nebula and grew around its rocky core, the planet's gravity strengthened. Eventually, it in essence reached out and grabbed Phoebe, the planet's outermost moon.

Regardless of whether or not it represents a sample of the pristine material from which planets formed, the moon can serve as a powerful tool for shedding light on Saturn's early environment.

By examining the size, number, and relative ages of the moon's craters, Nesvorny says, it's possible to reconstruct a picture of the nebula of gas from which Saturn formed. That gas would have exerted aerodynamic drag on the full range of irregular satellites thought to have surrounded the planet at the time, affecting the collision rate with other objects orbiting a young Saturn.

Beyond Saturn, moons that share many of Phoebe's characteristics have been spotted around Jupiter and other outer planets. While only 10 of these irregular moons were known in 1998, astronomers have now identified about 80. They have come to represent the majority of the solar system's moons.

The nonirregular moons, some 50 in all, comprise three families. Massive moons, like Earth's or Pluto's, are thought to have resulted either from collisions between the planet and another object (as in Earth's case) or gravitational capture (such as Pluto's Charon).

Large planetary moons, like Jupiter's Io or Saturn's Titan, are believed to have evolved from the early solar nebula through mechanisms that mimic the formation of their parent planets. Moons found in planetary rings are among the smallest and are likely to be collision debris. But Phoebe and her counterparts appear to resemble objects found in the most distant reaches of the solar system.

The Cassini flyby represents the only foreseeable opportunity to study an irregular moon up close. The spacecraft is set for a total of 52 "close encounters" with seven of Saturn's 31 moons. The number of encounters will grow if, as expected, the craft continues to operate smoothly and the mission is extended beyond it's nominal four years.

Over the next two weeks, the spacecraft will cruise through a gap in Saturn's rings to take up its orbit of the planet on July 1. What some see as the mission's highlight comes at the end of the year, when Cassini will release the Huygens probe, which will land on the moon Titan in January after relaying information on the moon's atmosphere and chemistry.

Titan is of particular interest because its hydrocarbon-rich composition is thought to resemble that of a young Earth before life took hold to provide the planet's oxygen.

The mission is a joint project between NASA, the European Space Agency, and the Italian Space Agencies.