Well, there's always radioactive isotopes.
They are always taken into consideration when judging the solidity of a planet or moon's core.
Also, it is under a glacier, right? Energy from the motion of the ice must cause some heat. Maybe there's a special mechanism that's causing more heat than one would suspect.
Just brainstorming. I don't give either thought much credence.
-edit- fixed my html. Dang! I can't seem to post around here without having to fix my errors.