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Last Updated: Wednesday, 4 May, 2005, 17:48 GMT 18:48 UK
Phoebe moon may be captured comet
Phoebe, Nasa/JPL/Space Science Institute
Phoebe orbits in the opposite direction to Saturn's regular moons
Saturn's pock-marked moon Phoebe could be a comet that was captured by the gravity of the ringed planet.

Data from the Cassini spacecraft suggests it originated in the frozen outer Solar System region called the Kuiper Belt - a reservoir for comets.

Two studies of Phoebe are carried in this week's issue of Nature magazine.

The tiny satellite is very different in its chemical composition to Saturn's larger moons and circles the planet in the opposite direction to them.

When we finally understand Phoebe, we will also understand the Kuiper Belt objects
Ralf Jaumann, German Aerospace Center
"It could have been a comet," said co-author Ralf Jaumann of the German Aerospace Center (DLR).

"Phoebe has a long journey behind it. It comes from the outer Solar System and probably rounded the Sun a few times before it was captured by Saturn's orbit. But we really don't know."

Phoebe and the objects that populate the Kuiper Belt are remnants of primordial objects that served as the building blocks of planets in our Solar System.

The Saturnian satellite could itself be between 4 and 4.4 billion years old.

Icy bodies

During the formation of the planets, gravitational interactions ejected some so-called icy planetesimals like Phoebe into distant orbits to join a native population of similar cosmic bodies.

This process formed the region we know today as the Kuiper Belt.

Phoebe itself must have migrated inwards and was captured by Saturn's gravity after the ringed planet formed from its planetary nebula.

Analysis of Phoebe's surface shows that it is one of the most complex Solar System objects yet studied.

Phoebe, Nasa/JPL/Space Science Institute
Despite its rough topography, Phoebe is nearly round
Scientists have identified water-ice, possible clays, iron-bearing minerals and organics such as aromatic compounds, alkanes and nitriles on the 220km-wide moon. More complex organics also seem to be there, but scientists are yet to characterise them.

The observations come from Cassini's Visual and Infrared Mapping Spectrometer (Vims).

Dr Jaumann thinks clays could have formed through heating if Phoebe came close to the Sun before being captured by Saturn, forcing water-ice to react with silicates.

"When we finally understand Phoebe, we will also understand the Kuiper Belt objects," Dr Jaumann explained.

Phoebe's surface composition also suggests that chemical activity in the first half billion years of the Solar System may have been more complex than previously thought.

"However, we have only seen the surface and this has probably undergone some alteration. But Phoebe has probably not had much alteration through high pressure or heating," Dr Jaumann added.

Cassini collected data on the moon during a close flyby on 11 June 2004.

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