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Last Updated: Thursday, 17 June, 2004, 14:58 GMT 15:58 UK
Phoebe probably distant traveller
Nasa/JPL/Space Science Institute

Images of Saturn's moon Phoebe from the Cassini spacecraft suggest it may be a relic of objects that formed billions of years ago in the outer Solar System.

The pictures seem to show ice in its craters, boosting the theory that it is more similar to comets and very distant Solar System objects than to asteroids.

Scientists think Phoebe migrated inwards and was probably captured by Saturn's gravity billions of years ago.

Several tiny Saturn moons may have been blasted out of Phoebe by space impacts.

"Battered and beat-up as [Phoebe] is, it is still giving us clues to its origin and history," said Dr Torrence Johnson, an imaging team member from the US space agency's Jet Propulsion Laboratory (JPL) in Pasadena, US.

Phoebe is thought to have an ice-rich layer overlain with a thin covering of darker material 300-500m (980-1,600ft) thick.

"Based on our images, some of us are leaning towards the view that has been promoted recently, that Phoebe is probably ice-rich and may be an object originating in the outer Solar System, more related to comets and Kuiper Belt objects than to asteroids," said Dr Carolyn Porco, head of Cassini's imaging team.

1. Antennas enabling communication with Earth
2. Boom carrying instrument to measure magnetic fields
3. Two cameras will take 300,000 pictures of the planet
4. Infra-red spectrometer analyses Saturn's temperature and composition
5. Radioisotope thermoelectric generators supply 750W of power
6. Cassini has two engines - one is a back-up
7. Thrusters used for small changes of direction or speed
8. Huygens probe will land on Saturn's largest moon, Titan
9. Plasma spectrometer measures charged particles and solar winds
Objects from the distant Kuiper Belt are thought to have served as the building blocks of the outer planets.

The surface of Phoebe is covered by craters, in which bright streaks and rays are visible. The scientists think this is where the dark surface material gives way to the ice-rich layer beneath.

Cassini imaging team member Dr Alfred McEwen of the University of Arizona in Tucson, US, commented: "Phoebe is a world of dramatic landforms, with craters everywhere, landslides, and linear structures such as grooves, ridges, and chains of pits.

"These are clues to the internal properties of Phoebe, which we'll be looking at very closely in order to understand Phoebe's origin and evolution."

Phoebe orbits Saturn in a direction opposite to that of the larger, and closer, Saturn moons. This has led some scientists to suggest that it may be the parent body to other, smaller moons that circle Saturn in a retograde orbit.

These tiny satellites could be debris ejected into space during the bombardment of Phoebe.

Long-distance images were obtained by the Voyager 2 flyby in 1981, but Cassini's images - with a resolution of a few tens of metres - are far superior.

Cassini is a joint mission of the US and European space agencies and the Italian space agency. The probe will enter orbit around Saturn on 1 July.

Next year, it will deliver the Huygens probe into the atmosphere of Saturn's major moon, Titan.

The BBC's Sue Nelson
"Scientists believe it is an ancient relic from when our solar system formed"

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