This atmosphere is very similar to that of Jupiter's moons Europa and Ganymede.
Thefinding was made by two instruments on Cassini, the Ion and NeutralMass Spectrometer (INMS) and the Cassini Plasma Spectrometer (CAPS)instrument, the latter having a European involvement withco-investigators from US, Finland, Hungary, France, Norway and UK.
Saturn'srings consist largely of water ice mixed with smaller amounts of dustand rocky matter. They are extraordinarily thin: though they are 250000 kilometres or more in diameter they are no more than 1.5 kilometresthick.
Despite their impressive appearance, there is very littlematerial in the rings - if the rings were compressed into a single bodyit would be no more than 100 kilometres across.
The origin of therings is unknown. Scientists once thought that the rings were formed atthe same time as the planets, coalescing out of swirling clouds ofinterstellar gas 4000 million years ago. However, the rings now appearto be young, perhaps only hundreds of millions of years old.
Anothertheory suggests that a comet flew too close to Saturn and was broken upby tidal forces. Possibly one of Saturn's moons was struck by anasteroid smashing it to pieces that now form the rings.
ThoughSaturn may have had rings since it formed, the ring system is notstable and must be regenerated by ongoing processes, probably thebreak-up of larger satellites.
Water molecules are first drivenoff the ring particles by solar ultraviolet light. They are then splitinto hydrogen and atomic oxygen, by photodissocation. The hydrogen gasis lost to space, the atomic oxygen and any remaining water are frozenback into the ring material due to the low temperatures, and thisleaves behind a concentration of oxygen molecules on the ring surfacesand, maybe through ion-neutral chemistry, molecular oxygen is formed,but this is not yet well understood.
Dr Andrew Coates,co-investigator for CAPS, from the Mullard Space Science Laboratory(MSSL) at University College London, said: "As water comes off therings, it is split by sunlight; the resulting hydrogen and atomicoxygen are then lost, leaving molecular oxygen.
"INMS and CAPSsee molecular oxygen ions, and CAPS sees an ‘electron view’ of therings. These represent the ionised products of that oxygen and someadditional electrons driven off the rings by sunlight."
Dr Coatessaid the ring atmosphere was probably kept in check by gravitationalforces and a balance between loss of material from the ring system anda re-supply of material from the ring particles.
Last month,Cassini-Huygens mission scientists celebrated the spacecraft's firstyear in orbit around Saturn. Cassini performed its Saturn OrbitInsertion (SOI) on 1 July 2004 after its six-year journey to the ringedplanet, travelling over three thousand million kilometres.
The Cassini-Huygens mission is a co-operative project of NASA, ESA and ASI, the Italian space agency.
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Thesefindings were published by Coates, A.J., H.J. McAndrews, A.M. Rymer,D.T.Young, F.J. Crary, S. Maurice, R.E. Johnson, R. Baragiola, R.L.Tokar, E.C. Sittler, G.R.Lewis, as 'Plasma electrons above Saturn’smain rings: CAPS observations', in Geophys Res Letters, 32, L14S09,doi:10.1029/2005GL022694, 2005. Published online 18 June and in Cassinispecial issue 23 July 2005.
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