NASA's Cassini spacecraft has detected a very tenuous atmosphere
known as an exosphere, infused with oxygen and carbon dioxide
around Saturn's icy moon Rhea. This is the first time a
spacecraft has directly captured molecules of an oxygen
atmosphere - albeit a very thin one - at a world other than
Earth. The oxygen appears to arise when Saturn's magnetic field
rotates over Rhea. Energetic particles trapped in the planet's
magnetic field pepper the moon's water-ice surface. They cause
chemical reactions that decompose the surface and release
oxygen. The source of the carbon dioxide is less certain. Oxygen
at Rhea's surface is estimated to be about 5 trillion times less
dense than what we have at Earth. But the new results show that
surface decomposition could contribute abundant molecules of
oxygen, leading to surface densities roughly 100 times greater
than the exospheres of either Earth's moon or Mercury. The
formation of oxygen and carbon dioxide could possibly drive
complex chemistry on the surfaces of man