Where is the methane coming from? Some reaction or mechanism must be sustaining it or it would have reacted away or bled off into space by now. Where's it coming from?

And where are the craters? Every other moon of Saturn is full of them, and given the proximity to the Kuiper belt there's some pretty damned big ones on Mimas, Iapetus and the rest. There don't seem to be any on Titan.

And why is it the only one with an atmosphere in a moon-field of rocks?

The questions go on...