One of the most unusual things about the Earth is the large amount of oxygen in its atmosphere, which isn't stable and is only there because plants keep producing it.
The Cassini probe orbiting Saturn has discovered that Saturn's moon Rhea has an atmosphere made of mostly oxygen, and 30% carbon-dioxide. Unfortunately this doesn't mean that captain Kirk could beam down there and be comfortable in his polyester jump suit, and it doesn't indicate that there is a colony of plants down on the surface photosynthesising.
' alt='Saturn's moon Rhea' >Rhea's surface is mostly made of water ice at -180C, and it is getting irradiated by particles of solar wind trapped by Saturn's magnetic field. This breaks up the water releasing oxygen, which is forming the atmosphere. There is also some carbon dioxide being released, possibly from deeper within the moon. Whilst the atmosphere is 100km thick there are still only a few tonnes of oxygen spread out over the whole moon, which if it was on the earth would just fill a large building, so compared to earth's atmosphere it is immensely tenuous.
So this certainly isn't the big story it first sounds like, but it does show that when looking for life you have to be careful when interpreting what atmospheres are made of as there are lots of strange forms of chemistry that can go on without life being involved at all.
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