Different assumptions

Assuming the earth has a molten core is important, for theories from continental shift to the generation of the earth's magnetic field. There's a thought that with no molten core, the earth's magnetic field would "die", and the earth's atmosphere would be stripped by cosmic radiation, as it may have been on Mars.

For those theories relying on a molten core, the heat is a leftover, from the earth's formation, and cooling is taking place at about 100 degrees every billion years.

However, some vulcanologists believe that the molten core is self-perpetuating. For them, the source of the heat is radioactive decay.

This doesn't easily account for the coolness of Mars & the moon. They are smaller than the earth, and so they would cool more quickly. But their proportion of radioactive material is probably similar to the earth's.

This same criticism could be applied to any theory where volcanic heat is generated by some action of gravity against the earth's structure. Mars & the moon would have this process in proportion. And yet, they have no volcanic activity.

Geothermal energy & gravity

Aside from life-giving radiant heat from the sun, there is heat right under our feet. Geothermal energy moves continents, but we don't really agree on its cause.

What is the source of disagreement about the origins of geothermal energy? Like many problems, I believe it emerges from the "heroic age" of nuclear physics, when radioactivity explained nearly everything.

Basically, the current theory is that radioactive decay in rock provides this energy. The alternate theory (mostly discredited) relies upon leftover formation energy, from billions of years ago.

I believe a much more viable theory, and more testable, is that gravity, and structural resistance to gravity, is the cause of heat in the earth's crust. That would imply that nothing of much import goes on beneath the earth's crust. Which is pretty much what we experience.

Interestingly, the moon is used as a counter-argument to this theory ... why is there no geothermal activity there, if the answer is gravity? Or on Mars, where there was formally geothermal activity?

The same objections could be raised about the radioactive decay theory. But frankly, I don't think we know enough about martian & lunar geology in the context of this question, to use it as a logical counter.

I'm going to try to muddle through to a soultion here. It has caused me some consternation for years.