Randyll writes "During its Saturn flyby in March, the Cassini space probe detected an oxygen-rich atmosphere on Rhea, Saturn's second-largest moon. While 100 times thinner than the atmospheres of Europa or Ganymede, Rhea's atmosphere contains a surprising amount of carbon dioxide. There is an explanation for the oxygen — the decomposition of surface ice — while the origin of the carbon dioxide is a mystery. A few of the possible explanations are that Rhea has carbon-rich organic molecules or that the gas is seeping from Rhea's interior. However, researchers have been unable to determine the exact source for the gas."
While "richness" is relative — the study's abstract
refers to Rhea's atmosphere as "tenuous," and oxygen concentrations are trillions of times lower there than they are on Earth — the finding still puts Rhea in rare company among the planets and moons of the solar system.