Portrait of Phoebe: Cassini images a large Saturn moon

By any standard, Saturn’s moon Phoebe is an oddball. The largest of Saturn’s outer satellites, it’s barely held in place by the massive planet’s gravity. Phoebe is among a handful of so-called irregular moons, which swoop above and below the plane of Saturn’s rings and orbit backward with respect to the rotation of the planet.

On June 11, the Saturn-bound Cassini spacecraft took the first close-up images ever recorded of this maverick moon. Flying within 2,068 kilometers of Phoebe, the spacecraft found a “world of dramatic landforms, with craters everywhere, landslides, and linear structures such as grooves, ridges, and chains of pits,” says Cassini scientist Alfred McEwen of the University of Arizona in Tucson.

“The amount of cratering is much higher than, I think, most anticipated and will tell us a lot about the number and sizes of small objects in the Saturn region of our solar system,” comments Tommy Grav of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass.

Resolving features as small as 25 meters across, the images show that some of the bigger craters have diameters of about 50 km. That supports a prediction made 3 years ago by Brett Gladman of the University of British Columbia in Vancouver and his colleagues. These researchers conjectured that the 220-km-wide Phoebe spawned the nearby, smaller irregular moons. In this scenario, a passing comet or satellite gouged a 50-km-hole in Phoebe. Fragments of this collision formed the other moons, which are typically about 20 km in diameter.

The Cassini images also indicate that Phoebe may contain icy material coated with a layer of dark material 300 m to 500 m thick. Small, bright craters may be the result of recent impacts that punched through the dark surface, exposing ice that lies beneath.

If Phoebe is indeed an icy body, Saturn may have captured it from the Kuiper belt. This reservoir of frozen bodies lies beyond the orbit of Pluto. Another possibility is that Phoebe coalesced near its present location from the nebula of dust, gas, and ice that swaddled the young sun. That scenario would make the large moon “utterly unique,” says Gladman, because it would be the only known moon that’s survived at Saturn’s location for the entire 4.5-billion-year history of the solar system.

Once Cassini settles into an orbit about Saturn on June 30, it will explore the ringed planet and seven of its moons for at least 4 years. But Phoebe won’t be among them. At a distance of 13 million km, it lies too far away from Saturn for Cassini to conduct further explorations.