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It's believed the main reason that the interior of the Earth hasn't cooled down in 4 billion years is because of the ongoing decay of radioactive elements that keeps adding more heat than is lost to the various cooling processes. Is this a proven fact or are there other possibilities?

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closed as off-topic by called2voyage Mar 10 at 14:37

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Radioactive decay heat is a fact. But there is also primordial heat. See also Wikipedia: en.wikipedia.org/wiki/Earth%27s_internal_heat_budget –  Gerald Mar 9 at 19:14

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There are really only six ways you could heat the interior of a planet. Gravitational collapse and compression, fission, fusion, crystallization, a very large impactor, and tidal flexing.

Fusion is out because Earth would be a rock-star. (Been waiting to use that one.)

Of the remaining possibilities, nuclear fission explains the majority of Earth's current temperature gradient.

http://en.wikipedia.org/wiki/Geothermal_gradient

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What makes you say the majority of the Earth's current temperature gradient is explained by nuclear fission? According to the Wikipedia article on the Earth's internal heat budget "the relative contribution of the two main sources of Earth's heat, radiogenic and primordial heat, are highly uncertain because their direct measurement is difficult." –  Steve d'Apollonia Mar 10 at 0:29
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To be perfectly honest, I'm recalling the results of a problem from Math 340C (Partial Differential Equations and advanced Engineering Mathematics) some 32 years ago where we solved the heat equation for a spherical Earth beginning at a uniform temperature losing heat through radiation at the surface. The result was a much colder interior than we actually see. I'll admit advances in science may have passed me by. –  Marc Mar 10 at 0:59

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