- Saturn's vast and majestic ring system has its own atmosphere
- separate from that of the planet itself, according to data from the Cassini
spacecraft.
-
- 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 resupply of material from elsewhere.
-
- Scientists admitted they were surprised by the finding
that Saturn's rotation is slowing.
-
- "The rotation seems to have slowed down by about
seven minutes compared to 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 mission between the US
space agency Nasa, the European Space Agency (Esa) and the Italian Space
Agency (Asi).
-
- © BBC MMV
-
- http://news.bbc.co.uk/2/hi/science/nature/4640641.stm
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