* Astronomy

Data from the NASA/ESA/ASI Cassini spacecraft indicate that Saturn's majestic ring system has its own atmosphere - separate from that of the planet itself.

During its close fly-bys of the ring system, instruments on Cassini have been able to determine that the environment around the rings is like an atmosphere, composed principally of molecular oxygen.


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This atmosphere is very similar to that of Jupiter's moons Europa and Ganymede.
The finding was made by two instruments on Cassini, both of which have European involvement: the Ion and Neutral Mass Spectrometer (INMS) has co-investigators from USA and Germany, and the Cassini Plasma Spectrometer (CAPS) instrument has co-investigators from US, Finland, Hungary, France, Norway and UK.
Saturn's rings consist largely of water ice mixed with smaller amounts of dust and rocky matter. They are extraordinarily thin: though they are 250 000 kilometres or more in diameter they are no more than 1.5 kilometres thick.
Despite their impressive appearance, there is very little material in the rings - if the rings were compressed into a single body it would be no more than 100 kilometres across.

The origin of the rings is unknown. Scientists once thought that the rings were formed at the same time as the planets, coalescing out of swirling clouds of interstellar gas 4000 million years ago. However, the rings now appear to be young, perhaps only hundreds of millions of years old.
Another theory suggests that a comet flew too close to Saturn and was broken up by tidal forces. Possibly one of Saturn's moons was struck by an asteroid smashing it to pieces that now form the rings.

Though Saturn may have had rings since it formed, the ring system is not stable and must be regenerated by ongoing processes, probably the break-up of larger satellites.
Water molecules are first driven off the ring particles by solar ultraviolet light. They are then split into hydrogen, and molecular and atomic oxygen, by photodissocation. The hydrogen gas is lost to space, the atomic oxygen and any remaining water are frozen back into the ring material due to the low temperatures, and this leaves behind a concentration of oxygen molecules.

"As water comes off the rings, it is split by sunlight; the resulting hydrogen and atomic oxygen are then lost, leaving molecular oxygen. The INMS sees the neutral oxygen gas, CAPS sees molecular oxygen ions and an ‘electron view’ of the rings. These represent the ionised products of that oxygen and some additional electrons driven off the rings by sunlight" - Dr Andrew Coates, co-investigator for CAPS, from the Mullard Space Science Laboratory (MSSL) at University College London.

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 the ring particles.

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Saturn is a source of intense radio emissions, which have been monitored by the Cassini spacecraft. The radio waves are closely related to the auroras near the poles of the planet. These auroras are similar to Earth's northern and southern lights. This is an audio file of Saturn's radio emissions.

The Cassini spacecraft began detecting these radio emissions in April 2002, when Cassini was 374 million kilometres from the planet, using the Cassini radio and plasma wave science instrument. The instrument has now provided the first high resolution observations of these emissions, showing that show an amazing array of variations in frequency and time.
In this example, it appears as though the three rising tones are launched from the more slowly varying narrowband emission near the bottom of this display. If this is the case, it represents a very complicated interaction between waves in Saturn's radio source region, but one which has also been observed at Earth.

Time on this recording has been compressed such that 13 seconds corresponds to 27 seconds. Since the frequencies of these emissions are well above the audio frequency range, we have shifted them downward by a factor of 260.

Click here to play sounds of Saturn's radio emissions, which have changes in frequency (127Kb Wave Sound).

This image of Saturn was taken on June 29, 2005 and received on Earth June 29, 2005 by the Cassini spacecraft.


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The image was taken using the CL1 and CL2 filters.

Saturn's icy moon Enceladus hovers above Saturn's exquisite rings in this colour view from Cassini.


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The rings, made of nearly pure water ice, have also become somewhat contaminated by meteoritic dust during their history, which may span several hundred million years. Enceladus shares the rings' nearly pure water ice composition, but appears to have eluded dust contamination through resurfacing processes that scientists are still trying to understand. Enceladus is 505 kilometres across.

Dust affects the rings' colour, while differences in brightness are attributable to varying particle sizes and concentrations.
The images for this natural colour view were taken with the Cassini spacecraft narrow-angle camera on April 5, 2005, at a distance of approximately 2.2 million kilometres from Saturn through red, green and blue spectral filters.
The image scale is 13 kilometres per pixel.