It seems oxygen is far more abundant than we ever suspected, particularly on moons that seem to be completely frozen solid. We recently found evidence of oxygen on Jupiter's moons Europa and Ganymede, and now this finding on Europa. In fact, because the region of space surrounding Saturn's rings has an oxygen atmosphere, it's thought even more of the icy moons within the gas giant's magnetosphere likely have little atmospheres of their own.
According to new data from the Cassini probe, the moon's thin atmosphere is kept up by the constant chemical decomposition of ice water on the surface of Rhea. It's likely that Saturn's fierce magnetosphere is continually irradiating this ice water, which is what helps to maintain the atmosphere. Researchers suspect a lot of Rhea's oxygen isn't actually free right now, but is instead trapped inside Rhea's frozen oceans.
While the presence oxygen is relatively easy to understand, the carbon dioxide is actually even more intriguing. The gas is likely created by reactions between organic molecules and oxidants down on the moon's surface. That seems rather shockingly Earth-like, or at least like the Earth of a few billion years ago. This is just further proof that the building blocks and basic prerequisites of life exist all throughout the solar system, even if it was apparently only on Earth where conditions were good enough for it to actually lead very far.