Oxygen in space

I saw this report from the Cassini spacecraft that two of Saturn’s moons, Rhea and Dione have an extremely thin atmosphere with very small amounts of oxygen.

At the Dione surface, this atmosphere would only be as dense as Earth’s atmosphere 300 miles (480 kilometers) above the surface…The density of oxygen at the surfaces of Dione and Rhea is around 5 trillion times less dense than that at Earth’s surface… scientists suspected molecular oxygen would exist at Dione because NASA’s Hubble Space Telescope detected ozone. But they didn’t know for sure until Cassini was able to measure ionized molecular oxygen on its second flyby of Dione.

Now I find this fascinating because I’ve worked on a fair number of spacecraft orbiting Earth around that 480-km altitude. Between 100 km and roughly 1000 km, ultraviolet radiation from the Sun breaks down the molecular oxygen in our atmosphere into atomic oxygen. The exact amount varies with solar activity. Think of it as rusting on steroids, except it doesn’t just react with metal. Any polymer or plastic that contains hydrogen, carbon, sulfur, or nitrogen will react with atomic oxygen and gradually erode away.

Now you see ’em.

Now you don’t.

That’s four years’ exposure, but I know some of the samples on this experiment were badly damaged after just one year. We do a lot of testing because if you’re going to launch a $500M satellite, you want it to last as long as possible.

I wonder if the oxygen at Rhea and Dione is molecular or atomic. Another pass by Cassini, and maybe we’ll find out.