Looking at the earlier suggestion to put a superconducting ring around Mars -- would that not gradually heat the core through hysteresis, for instance? Granted, the time scale would be ridiculous. But maybe multiple lines of attack? Superconductiong ring providing inductive heating, plus H-bombs dropped down a deep shaft, plus microwave energy being beamed to the poles and used in some kind of either heating or further induction... I'm just spitballing, and I freely admit I haven't a clue.