Saturn's rings are separate from the planet they circle, and then even have an atmosphere of their own. During several flybys, Cassini has been able to detect very small amounts of molecular oxygen floating around the rings. Molecules of water are broken apart by ultraviolet light from the Sun; the hydrogen and some of the oxygen is lost into space, and some of the oxygen is frozen back into the rings. But there's enough of a cloud of these atoms around the rings that this process must be ongoing and kept in a continual balance.