Rhea is one of 60 moons that orbit Saturn and as Cassini flew above its surface it found an extremely thin atmosphere of oxygen and carbon dioxide.
Rhea’s isn’t the only other atmosphere in the universe, but it is so thin that Cassini had to fly through it just to confirm that it was there at all (other atmosphere’s have been detected and studied from afar by tools like the Hubble Space Telescope). According to Cassini’s onboard science instruments, Rhea’s atmosphere contains something like 50 billion oxygen molecules per cubic meter, matched by 20 billion carbon dioxide molecules. Rhea’s atmosphere is far too thin – and the moon is far too distant and cold – to have any near-term implications for space missions beyond the usual unmanned variety. But the discovery is significant in that it proves that chemistry involving oxygen occurs naturally elsewhere in our own solar system, and could be quite common outside of it as well.via popsci