By Paul Rincon

BBC News science reporter



Many mysteries about the ringed planet remain to be resolved

And Saturn is rotating seven minutes more slowly than when probes measured its spin in the 70s and 80s - an observation experts cannot yet explain.

Cassini-Huygens mission scientists are celebrating the spacecraft's first year in orbit around the ringed planet.

Details were unveiled at the British Festival of Space 2005 in Birmingham.

By making close flybys of the ring system, Cassini has been able to determine that the atmosphere around the rings is composed principally of molecular oxygen (O2).

Welcome confirmation

The finding was made by two experiments on Cassini: the Ion and Neutral Mass Spectrometer (INMS) and Cassini Plasma Science (Caps) instrument.

"The INMS sees the neutral oxygen gas, Caps sees the ionised products of that oxygen and the electrons associated with it. There is an enhancement over the rings," said Dr Andrew Coates, co-investigator for the Caps instrument, told the BBC News website.

Dr Coates, from the Mullard Space Science Laboratory (MSSL) at University College London, said the atmosphere was very similar to that of Jupiter's moons Europa and Ganymede.

"As water comes off the rings, the hydrogen is lost from it, leaving the oxygen," he explained.

Saturn's rings consist largely of water-ice mixed with smaller amounts of rocky matter. Dr Coates said the ring atmosphere was probably kept in check by gravitational forces and a balance between loss of material from the ring system and a re-supply of material from elsewhere.

Scientists admitted they were surprised by the finding that Saturn's rotation was slowing.

"The rotation seems to have slowed down by about seven minutes compared with what was inferred from the Pioneer and Voyager data, but we don't actually understand why," said Professor Michele Dougherty, principal investigator for Cassini's magnetometer instrument.

Surprise finding

Data from the magnetometer and Cassini's Radio and Plasma Wave Science (RPWS) instrument both seem to show the slow down in Saturn's rotation.

"You would expect the rotation of the planet to slow down if its internal dynamo had stopped, but that does not seem to be the case with Saturn," Professor Dougherty, from Imperial College London, told the BBC News website.

The internal dynamo is the source of a planet's magnetic field and requires rotation and a fluid core. Professor Dougherty said it was possible the instruments were observing "rotational regions" closer to the surface of Saturn rather than anything to do with the dynamo itself.

"If you sit down and think about it, it's very difficult to come up with a scenario where the interior of the planet is slowing down," she said.

UK science and innovation minister Lord Sainsbury, who was at the briefing in Birmingham, praised UK scientists involved in Cassini-Huygens as the orbiter celebrated its first year in orbit around Saturn.

"The scientists and engineers in this country have played an integral role, making it the biggest British success story in space of the last 12 months," Lord Sainsbury said.

Cassini performed its Saturn Orbit Insertion (SOI) on 1 July 2004 after a six-year, three-billion-kilometre trek.

In December 2004, it released its piggybacked Huygens probe, which performed a successful touchdown on Saturn's moon Titan in January this year.

The mission is a co-operative venture between the US space agency (Nasa), the European Space Agency (Esa) and the Italian Space Agency (Asi).