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What is the internal heat engine of the Earth?

Doing research, I found out that the external heat engine of the Earth is the Sun. However, when I did research on the internal heat engine of the Earth, I received multiple answers. Are there multiple answers to this question?

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For this question, Wikipedia is probably your best recourse.

To summarize, the heat in the earth's core comes from two places: it is left over from the formation of the earth and it is still being generated by radioactive decay of isotopes that were trapped in the earth as it formed.

The radiogenic heat is still being added to the earth through decay of long lasting isotopes. When the earth formed, there were probably many significant heat generating isotopes, but now after 4.5 billion years, it is pretty much just Thorium-232, Uranium-238, Potassium-40, and Uranium-235.

So there are multiple answers to this question in the sense that there are the two sources listed above, but otherwise, no, this is the answer.

This question has some more information related to heat transport to the surface that you may find relevant.

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  • $\begingroup$ Kingledion is correct. Almost all the heat generation is now from the radioactive decay of K, U and Th. Primordial heat, generated by gravitational collapse during planetary formation has largely been dissipated in the intervening 4.6 billion years. $\endgroup$ – Gordon Stanger Nov 2 '16 at 20:00
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    $\begingroup$ @GordonStanger -- Less than half of the internal heat escaping from the Earth is due to radioactive decay. The bulk of the remaining 50+% is due to primordial heat (secular cooling), the growth of the Earth's inner core, and gravitational energy from ongoing planetary differentiation. $\endgroup$ – David Hammen Nov 27 '16 at 0:46
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It's difficult for non specialists to understand the heat build-up in the Earth. Handle a specimen of uranium or thorium ore, which is just as radioactive as the Earth's interior, and you will not notice any heat from the rock. The rate of heat emission from radioactive decay is minute, but is real and continues for billions of years. The reason why the core of the Earth is as hot as the sun's surface is that the radiogenic heat has nowhere to go. It is effectively locked in by a thermal insulating blanket thousands of kilometres thick - i.e. the mantle.

Studies of neutrino sources indicate that 54% of the Earth's current internal heat comes from radioactive decay, the remaining 46% being primordial heat, presumably from initial gravitational condensation from the solar nebular.

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