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Last Updated: Monday, 14 June, 2004, 10:58 GMT 11:58 UK
Cassini pass reveals moon secrets

The Cassini spacecraft, which is en route to Saturn, has made a close pass of the planet's mysterious moon Phoebe.

The US-European spacecraft made its closest approach to the moon on Friday at 2156 BST at a distance of 2,078km.

Images show a scarred moon pounded by massive impacts that tossed building-sized rocks out on to its surface.

The pictures have already revealed exciting clues to Phoebe's history, including alternating layers of bright and dark material around its craters.

The evidence so far points to an ice-rich cosmic body overlain with a thin layer of dark material. One sharply defined crater exhibits two or more layers of alternating bright (icy) and dark material.

Mission scientists think space impacts heaved out debris over an existing surface, to create the alternating layers.

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
Researchers do not know how the dark material forms, but one theory is that exposure to cosmic rays can blacken the original surface.

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. The presence of a huge 50km-wide crater on Phoebe would seem to support this view.

Scientists also want to know where Phoebe came from in the first place.

Its darkness and retrograde orbit have led some scientists to wonder if Phoebe is a Centaur: an object that migrated from the outer Solar System.

Objects of this type - from the region known as the Kuiper Belt - are thought to have served as the building blocks of the outer planets.

"The scenario is probably that this is an object captured early on in the history of the Solar System," imaging scientist Dr Carl Murray, of Queen Mary, University of London, UK, told BBC News Online.

Nasa/JPL/Space Science Institute
Taken from less than 100,000km (Nasa/JPL/SSI)
"What's usually invoked to explain this is a drag process in the early Solar System, when there is gas and dust left over from the formation of the planets."

If this is the case then observations of Phoebe will provide valuable information about how the various worlds that inhabit the cold, outer reaches of our planetary system were formed.

By determining the mass and volume of the 220km-wide Saturnian satellite, its density can also be determined, telling the researchers whether the body is predominantly rocky or icy.

"By measuring the sizes and numbers of [craters] you can try to work out an age for the surface," said Dr Murray.

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 small moon was discovered in 1898 by the US astronomer William Henry Pickering.

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

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