It’s not quite breathable, but Saturn’s moon Dione still has a bit of molecular oxygen within its very thin atmosphere. The oxygen atoms are very sparse, only about one molecule of oxygen per 11cubic centimeters (cc) of space. The fact was reported by NASA’s Cassini Spacecraft.
But how did it get there?
The interesting bit about the oxygen is how it comes to be! The oxygen is not made by biological organisms, unlike here on Earth, but by physical processes like dissociation of molecules in the atmosphere due to the bombardment by highly energetic photons. They can also come from geological activity. How the tiny Dione holds on to this thin layer of gases is, however, not understood. Saturn’s biggest moon Titan, possibly the biggest in the Solar System, has a thick atmosphere, but then it’s much bigger than Dione. Dione’s atmospheric problem compounds the problem astronomers have of explaining the atmosphere around Rhea, another small satellite of Saturn.
Not that big a surprise
The detection of molecular oxygen was done using ion and neutral mass spectrometers. Earlier, Hubble had picked up the signature of ozone, so molecular oxygen was always on the cards, since ozone is just the oxygen molecule with one more oxygen atom.
It is not clear whether there are rarer gases in Dione’s atmosphere and, if present, what their composition is.
The conclusion comes from analysis of the data taken on the Cassini flyby on Dec 12, 2011.