A very thin atmosphere of molecular oxygen has been found around another one of Saturn's moons.
The atmosphere detected by NASA's Cassini spacecraft around Dione contains one oxygen ion for every 11 cubic centimetres, NASA said in a news release.
That makes it about as dense as the Earth's atmosphere 480 kilometres above the surface — a little bit higher than the altitude of the International Space Station — and five trillion times less dense than the atmosphere at Earth's surface.
The oxygen molecules surrounding Dione are thought to have been kicked off the moon's icy surface by high energy particles from space or energy from the sun, said Robert Tokar, a scientist at Los Alamos National Laboratory and the lead author of the paper.
Oxygen also found on Rhea, rings
A similar thin oxygen atmosphere was found around another of Saturn's 62 known moons, Rhea, in 2010, and oxygen has also been detected from Saturn's rings.
"This shows that molecular oxygen is actually common in the Saturn system and reinforces that it can come from a process that doesn't involve life," Tokar said in a statement.
Cassini detected the oxygen on Dione using a device called a plasma spectrometer while flying within 503 kilometres of the icy moon's surface in April 2010.
The results were published in the journal Geophysical Research Letters.
Dione has a diameter of just 1,123 kilometres — less than a third the diameter of Earth's moon.
Prior to this study, researchers weren't sure Dione was big enough to hold onto even a very thin atmosphere, said Amanda Hendrix, a scientist at NASA's Jet Propulsion Laboratory in Pasadena, Calif., who was not directly involved in the study.
Nevertheless, they had suspected molecular oxygen, which is made up of two oxygen atoms, might exist on Dione because the Hubble Space Telescope had detected ozone there. Ozone is made up of three oxygen atoms and is usually created from molecular oxygen.