As I recall, the main cause of the high temperatures inside e.g. the Earth is thermal heating due to the decay of radioisotopes. The thinking is that:
- Although small, the heat has nowhere to go, so it piles up.
- Similarly, thermal flux from solar heating at the surface has a hard time propagating down.
- Residual leftover heating from the solar system's formation would be expected to be only a minor component by now.
I was thinking about this, and realized that it implies a somewhat interesting conclusion: if the rocky planets/bodies are of similar composition, then their core temperatures should be functions of their radius, with their distance to the sun being basically irrelevant.
For example, I might expect that Ganymede (radius 2634km) has a warmer core temperature than Mercury (radius 2440km), even though Mercury is 13.4 times closer to the sun.
My question: about how accurate is this conclusion? Certainly, composition varies somewhat, but as a heuristic in general, is it fair to say that larger rocky bodies have warmer interiors?