I got quite excited by the headline Saturn’s moon Rhea has thin atmosphere on the BBC. Even more so when I saw the atmosphere had oxygen. Rhea is the second largest moon orbiting Saturn, but it’s nothing like the size of Titan (which is bigger than Mercury), so finding an atmosphere is interesting. It’s much more interesting when you see that it’s partly made of oxygen.
Oxygen isn’t the sort of gas you’d expect to find in an atmosphere. That sounds daft, there a lot of it around Earth, but Earth is weird. Oxygen is a highly reactive gas. The reason the surface of Mars looks rusted is that Oxygen has reacted with chemicals in the rocks of Mars. To have Oxygen around long enough to spot it with astronomical instruments, you need a source to replenish what’s being lost. Life does that on Earth. On Rhea there’s ice, as a source of water, probably a rocky core — and we know of extremophiles that live deep within the rocks in Earth’s crust. If there’s a heat source, and planets can heat the cores of moons as they orbit by squishing them with tidal forces, then are we looking at bacteria on another world?
Rhea is in Saturn’s magnetic field. That means that lots of charged particles get accelerated and smash into the ice on Rhea’s surface. When they hit a water molecule with that kind of energy they break it H2O into its components and that’s how you get Oxygen. That then gets stripped away by the same forces, which is partly why Rhea hasn’t built atmosphere. It’s probably this lack of permanence that means exosphere is a better word for what’s around Rhea than atmosphere.
That’s slightly disappointing to me. I remember being told that Oxygen would be one of the best indicators of life on another world, and here it’s not the case. On the plus side, it’s another reminder to visit the Cassini site, where these results are coming from. There’s plenty of images as well as video.