Why does Rhea have a thin oxygen atmophere?

[Dr. Kerbal]

Why does Rhea have a thin oxygen atmosphere? How did this come to be? Why? How? Why hasn't this oxygen frozen or liquefy yet? How!

[SOXBLOX]

As far as I can tell, it forms because of radiation splitting water molecules in ice on the surface. I guess it survives because there's not much condensation going on. "Frozenness" isn't a property of individual atoms or molecules, only groups, so the thin atmosphere can't properly "freeze".

This answer is probably at least partially wrong.

[Piscator]

I assume the atmosphere as a whole survives because it is constantly replenished, rather than because  of the individual molecules staying around for any serious amount of time. There might be a gas torus effect though, in which escaping gas molecules remain in orbit around Saturn and occasionally return to the moon.

[UmbralRaptor]

 

17 hours ago, SOXBLOX said:

As far as I can tell, it forms because of radiation splitting water molecules in ice on the surface. I guess it survives because there's not much condensation going on. "Frozenness" isn't a property of individual atoms or molecules, only groups, so the thin atmosphere can't properly "freeze".

This answer is probably at least partially wrong.

Well, that's what the paper says.  It's entirely possible that this "atmosphere" is a surface-boundary exosphere, given the low particle density.