Rings Too Good for Saturn, Have Own Atmosphere
Spectrum from Cassini instruments indicating atmosphere over rings. The increased activity on the right side of the spectrum indicates atmosphere.
Credit: NASA/JPL/Space Science Institute/SWRI/UCL

New data from the Cassini spacecraft indicates that Saturn's trademark rings have their own atmosphere, separate from the gas around the planet they encircle.

During close fly-bys of the rings, instruments on Cassini detected that the environment around the rings is atmosphere-like. More interestingly, though, is that the ring atmosphere is made up of molecular oxygen - two atoms of oxygen bonded together - like that found in Earth's atmosphere.

The ice that makes up Saturn's rings is also the source of the oxygen that makes up this atmosphere.

"As water comes off the rings, it is split by sunlight; the resulting hydrogen and atomic oxygen are then lost, leaving molecular oxygen," said Cassini investigator Andrew Coates of the Mullard Space Science Laboratory at University College London.

Saturn's rings are made up mostly of water ice along with small amounts of dust and rocky bits. Ultraviolet rays from the Sun pry the water molecules loose from the rings and split them into their building blocks - hydrogen and the two forms of oxygen - by a process called photodissociation.

The ring atmosphere is probably kept in place by gravitational forces, Coates says. The check-and-balance between the loss of material from the ring system and a re-supply from the ring particles also helps.

Although the rings are about 155,343 miles (250,000 kilometers) in diameter, they are actually quite thin, less than a mile (1.5 kilometers). And even though the rings appear gigantic, there actually isn't a whole lot to them. If all the rings were squeezed into one solid ring, it would be no more than 62 miles (100 kilometers) across.

Sky-watchers have gazed at Saturn's rings for centuries, but the rings' origin is still somewhat of a mystery. Initially, scientists thought that the rings formed from swirling clouds of cosmic gas around the same time as the planets about 4 billion years ago. The current belief, however, is that they are only a few hundred million years old.

Other theories suggest that the rings were formed by various asteroid collisions with Saturn's moons or from broken-up comets.

The rings are not stable and are constantly regenerated, most likely from the break-up of Saturn's satellites.

The ring system's oxygen atmosphere differs drastically from the atmosphere of Saturn itself - Saturn's atmosphere is 91 percent hydrogen by mass.

The instruments aboard Cassini that registered the rings' atmosphere were the Ion and Neutral Mass Spectrometer - operated by the United States and Germany - and the Cassini Plasma Spectrometer, which is operated by the US, Finland, France, Hungary, Norway, and the United Kingdom.