And now from NASA:
NASA’s unmanned Cassini spacecraft has detected molecular oxygen ions in the atmosphere of Dione, one of Saturn’s moons. This discovery confirms the presence of a very thin atmosphere. In fact, scientists reveal that the oxygen ions around Saturn’s moon are extremely exiguous (one for every 11 cubic centimeters of space or approximately 2,550 per cubic foot). This fact highlights the thinness of Dione’s atmosphere. For a comparison that might make more sense to the average reader, scientists believe that the atmosphere at Dione’s surface would only be as dense as Earth’s atmosphere 300 miles above the surface.
Scientists believe that Dione’s oxygen comes from either solar photons or energetic particles from space assaulting the moon’s water ice surface and freeing oxygen molecules. Despite this reasoning, scientists will continue to examine data from the Cassini spacecraft to see if the presence of oxygen around Saturn’s moon can be explained by geological processes or other events.
While a number of solid solar system bodies, like Earth, Venus, Mars and Saturn’s moon Titan, have atmospheres, they are usually a lot denser than the tenuous atmosphere detected around Dione. Back in 2010, however, Cassini scientists found a thin exosphere around Saturn’s moon Rhea, one that was very similar to what has recently been discovered around Dione. In fact, the density of the oxygen at the surfaces of Dione and Rhea is approximately 5 trillion times less than the density of the atmosphere at Earth’s surface.
Scientists reckon more moons around Saturn and Jupiter could also have oxygen in their atmospheres. They also think some of the moons have liquid oceans deep below their surfaces. With the possibility of both water and oxygen, experts hope to explore the moons further to see if there are any signs of life or if humans could survive there.