The Cassini mission has collected an impressive body of data from Saturn and its many moons and rings. Evidence collected in 2008 seemed to suggest that its second moon, Rhea (about half the size of our own moon) may have a tiny ring structure of its own. Follow-up observations showed that this discovery was mistaken and that these rings do not exist, but did reveal something just as interesting: An atmosphere rich in oxygen.
The chemistry is complex and scientists are still sifting through the mountain of information, but this is what Cassini team scientist Ben Teolis thinks is happening:
Based on its density, Rhea seems to be composed of three parts water to one part rock. Because of its cold location so far from the Sun, this water is frozen into ice. Now, solar radiation trapped by Saturn’s magnetic field gets whipped around and accelerated into Rhea, and this causes breaks the water molecules down into hydrogen and oxygen. The hydrogen dissipates easily into space but the heavier oxygen atoms are held back by Rhea’s gravity so that the atmosphere gradually becomes rich in oxygen. .
One other feature of Rhea’s atmosphere, a large amount of carbon dioxide, has not yet been explained. It is possible that this is from primeval material left over from the formation of the Solar System, similar to comets. If this is the case then there will be large amounts of organic compounds present on the little moon.
If these compounds are found in the theorised subsurface oceans (It is possible that tidal flexing from Saturn’s gravity could warm up the interior of Rhea enough to allow the existence of vast underground oceans), and if the atmospheric oxygen is being transported there as well, then we have a recipe for life!
In fact, such speculations have been made for years now about Jupiter’s moon Europa, so it seems that such scenarios could be common in our Solar System. And wouldn’t that be grand!