[Hetty_Fauxvert]

Ignorant question alert: would it be (remotely) possible to reinvigorate Mars’ core by starting some sort of nuclear reaction there?

[jmacusa]

I've wondered that myself. The diameter of Mars is around 4,220 miles, slightly larger than the Moon. I'm not sure how much of that comprises the core. An iron/nickel core is much like a blast furnace, the heat is generated by the atoms in the iron and nickel colliding with each other and compressing into a molten state. Once that process ceases you're left with a large mass of metal. Re-igniting it in a massive nuclear type blast would destroy most of the planet and send chunks flying all over the solar system, quite possibly into us. A slow, sustained and controlled reheating? Theoretically speaking it's possible. A similar idea was proposed in the form of a kind of drill to pierce the frozen surface of Europa, one of Jupiter's moons to a supposed liquid water ocean underneath but on a small scale, the logistics are beyond the current capability. In the case of Mars they'd be impossible.