Saturn's dark moon reveals bruised past

The new images taken by Cassini space probe during a close pass by Phoebe, reveals that the moon has been battered for billions of years by interplanetary debris.

Saturn's moon Phoebe has been battered for billions of years by interplanetary debris, and the signs of past violence are clear in images snapped by a spacecraft headed for orbit around the ringed planet.

HT Image

Looking at new pictures taken by the robotic Cassini space probe during a close pass by Phoebe, scientist Torrence Johnson said on Monday he saw "an extremely battered object."

"This thing's been hit by interplanetary debris for probably a couple of eons; that's a poetic way of saying several billion years," Johnson said in a telephone interview from the Jet Propulsion Laboratory, where he monitors images from Cassini.

Phoebe probably was pulled into Saturn's orbit around the time the solar system formed, about 4.5 billion years ago, he said. This means this moon might be related somehow to comets or objects from an area at the outer fringe of the solar system known as the Kuiper Belt.

The latest images were snapped when Cassini drew to within 2,068 km of Phoebe on Friday. That is much closer than the last look taken by earthly instruments, a 1981 glimpse by the Voyager probe in 1981, which viewed Phoebe from 2.2 million km away.

Beyond its shape, the new images show what appear to be layers of dark and light material. This moon, one of Saturn's largest, has previously been known as an extremely dark object that reflects only about 6 per cent of the light that reaches it.

"There's dark material which appears to have light material underneath it," Johnson said. "We have strong suspicions based on telescopic data that the light material is water ice, but there may be other volatiles there, too."

The pass by Phoebe was part of Cassini's mission to take a four-year look at Saturn and the objects around it. The spacecraft has been on its journey for seven years and is scheduled to begin its orbit around Saturn on June 30.

To prepare for orbit, Cassini is set to let some of its scientific instruments go quiet this week as the craft performs a maneuver to correct its trajectory.

Cassini is expected to conduct 76 orbits around the Saturn system and execute 52 close encounters with seven of Saturn's 31 known moons.