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:

  1. Although small, the heat has nowhere to go, so it piles up.
  2. Similarly, thermal flux from solar heating at the surface has a hard time propagating down.
  3. 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?

  • $\begingroup$ "..the heat has nowhere to go" ..you've never heard of 'volcanoes'? $\endgroup$ – Andrew Thompson Jul 18 '15 at 2:19
  • $\begingroup$ @AndrewThompson Earth is more than a trillion cubic kilometers, and all the volcanoes are on the outside. $\endgroup$ – imallett Jul 18 '15 at 5:42
  • $\begingroup$ Don't forget it will depend on the exact composition of the core. Different bodies will have different radio-isotopes (or ratios of them). Now the inner planets might have formed from very similar compositions, but going out to Ganymede you'll probably see significantly different composition. Also, don't forget pressure. Cores are under enormous pressure from gravity. Pressure and heat are highly related via ideal gas law PV = nRT. So a small increase in radius may lead to a large increase in heat even if the composition is the same. $\endgroup$ – DrZ214 Sep 10 '16 at 21:05

You idea is supported by this source, which states:

Of the terrestrial planets in our solar system, Earth has the most internal heating because it is the most massive.

It is speculated that Venus also has a high internal temperature due to volcanism. An interesting fact that supports this concept is that Venus is very similar in size to Earth, being slightly smaller. This source lists the objects in the solar system by size.

As you mention, composition is also very important. A rocky mantel and crust are better able to withstand the temperature of a hot core than water ice. Additionally, rock acts as a good insulator of heat and the thickness of the mantel and crust increases the insulating affect. The other critical part of composition and heat generation is the presence of sufficient quantities of longer lived radioactive isotopes such as "potassium-40, thorium-232, and uranium-235 and uranium-238".


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