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?
closed as off-topic by called2voyage♦ Mar 10 '14 at 14:37
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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.