Astronomers discovered that Rhea, the second biggest moon of Saturn, has a thin atmosphere of oxygen and carbon dioxide. Scientist believe Rhea's atmosphere is maintained by high-energy particles that constantly bombard the moon's icy surface.
It is incredibly thin, however. The density of O2, for example, is probably about five trillion times less dense than the oxygen that blankets Earth.
The presence of an exosphere, as it is more properly called, was confirmed by instruments on the Cassini probe which orbits the ringed planet and its moons.
The discovery is reported in the online version of Science magazine.
Oxygen exospheres have been seen at Jupiter's moons Europa and Ganymede, but this is the first time such a detection has been made in the Saturnian system.