Oxygen found around Rhea but alas no life

Nov 26, 2010 Alun Science 2 Comments Astrobiology, Astronomy, Saturn

Rhea orbit­ing Sat­urn. Photo NASA/JPL/Space Sci­ence Institute.

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?

Sadly, no.

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.