@ Kullat Nunu
Maybe the oxygen on Callisto and Ganymede is from microbes
But i see the point of impacts from meteors, guess there is no life on the icy surface of Europe and if the amount of oxygen is close to the same as Europe i guess that explains it, but it could be life - ok, i don't know much about those two moons molecules so maybe i'm entirely wrong.
@ Beowolf
That makes sence, but then i have to add i don't see the point that Saturn does not have a core greater than any of its moons. I've always heard of Saturn to be pure (or almost pure) gasses, but with greater gravity to pull the astroids towards it, it should have solid ground (not to mention the pressure that transforms many gasses into solid.
What about atmospheres around the moons of Uranus and Neptune? Did you mention those too (as said i don't know much about the moons out there except that i know there are many of them).