NASA’s Cassini spacecraft has discovered a tenuous atmosphere of molecular oxygen ions around Saturn’s moon Dione. The satellite’s atmosphere was initially detected in April 2010 by an instrument called the Cassini Plasma Spectrometer (CAPS). At the time, the Cassini probe was making its second close fly-by of the moon, passing just 503 kilometres above its surface. 

To an observer on the surface of Dione, the atmosphere would pretty much go unnoticed. It is so thin that there is only one oxygen ion per 11 cubic centimetres of space, which is as rarefied as Earth’s atmosphere at an altitude of 480 kilometres. Due to this low density, Dione’s atmosphere can also be thought of as an exosphere, similar to the very uppermost layers of a typical planetary atmosphere.

It is thought that Dione’s atmosphere is formed by a process called sputtering, which occurs when charged particles from Saturn’s magnetosphere collide with Dione’s surface. These collisions cause oxygen ions to be ejected from the moon’s surface, and the thin exosphere to be formed. Once liberated from the moon’s surface, Saturn’s magnetic field then strips the oxygen ions away from Dione altogether, preventing the atmosphere from getting thicker. Astronomers are also investigating the possibility that geological processes on the moon may be causing the release of oxygen ions.

While Dione’s exosphere is a significant discovery, the moon itself is not unique in this regard. Saturn’s second largest satellite, Rhea, has also recently been shown to have a thin atmosphere, comprised of oxygen and carbon dioxide. “We now know that Dione, in addition to Saturn’s rings and the moon Rhea, is a source of oxygen molecules,” says Robert Tokar, a Cassini team member and the lead author of the paper announcing the discovery. “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.”

With a diameter of 1,100 kilometres, Dione is the fourth-largest of Saturn’s moons, and was discovered in 1684 by Giovanni Cassini. It is a small, frozen world, comprised mostly of rock, covered in a thick layer of water ice, which makes it one of the most reflective bodies in the Solar System. Dione has a slightly chilly average surface temperature of -186 degrees Celsius, and lies within Saturn’s diffuse E-ring, where it orbits the planet once every 2.7 days.

The Cassini space probe has been in orbit around Saturn since 2004, where it has made many discoveries about the planet and its moons. The spacecraft has recently made an even closer fly-by of Dione, passing just 99 kilometres above its surface on 12 December 2011, which will likely be the closest that Cassini will ever get to this particular moon. Scientists are currently analysing the data recorded by the probe’s Ion and Neutral Mass Spectrometer (INMS) in order to discover if any other elements are present in the newly discovered atmosphere.