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Comment by BurningFrog | Hacker News Reader

The major force keeping our core hot is nuclear fission.

https://blogs.scientificamerican.com/observations/nuclear-fi...

You've got to be careful here. This article refers to the heat budget of the _entire_ earth, not the Earth's core. The Earth's core has relatively little in the way of radiogenic elements now.

Most of the heat originating in the core comes from

- The formation of the earth (called primordial heat) - Latent heat (released when iron freezes onto the solid core) - Differentiation (e.g. settling of heavy stuff to the bottom)

The mantle is chock full of radiogenic elements though.

Well, yes, but it basically means the core is wrapped in a heat-producing blanket (the rest of the Earth), so we can say that the fissile materials on the outer parts of the Earth does keep the core stay warm.

That's what my physics instincts tell me. It might take centuries for heat to conduct across the planet interior, but it has nowhere else to go.

I'm open to being told my instructions are wrong in this case though.

I wouldn't say that the best way of thinking about it is the blanket analogy because conduction is _not_ the primary way that the earth sheds heat, convection in the mantle is.

Though I haven't run the numbers, I would strongly bet that the core would be _hotter_ than it is right now if the mantle did not have internal heating.

The reason being that, the rate at which you remove heat from the core (e.g. the rate at which the core cools) is entirely determined by the rate at which mantle convection removes heat. Radiogenic heat is a _strong_ driver of mantle convection in the earth, without radiogenic isotopes you would get much more sluggish mantle convection and a much lower rate of heat removal from the core.

The reason for this is twofold:

- Internal heating leads to higher temperatures which leads to lower viscosity and more vigorous mantle convection - Internal heating will locally heat cold "blobs" and make the buoyant

I would also (much less strongly) bet that plate tectonics would not occur without internal heating. We only see plate tectonics in a narrow slice or parameter space in mantle convection models and once you get plate tectonics you get _much_ more heat removal and _much_ faster cooling.

Err, that SciAm author misunderstood that research... what they wrote is exactly backwards. The neutrino observations they're talking about disproved the natural nuclear reactor hypothesis. There is no antineutrino signal from the Earth of the type a nuclear fission process would emit.

https://arxiv.org/abs/1909.02257 ("Comprehensive geoneutrino analysis with Borexino")

What Borexino found was about 25 terawatts of radioactive decay occurring inside the Earth—alpha decay of 238U and 232Th. But, no nuclear fission. If there are fissioning critical masses inside the Earth, the total amount, constrained by neutrino counts, is no more than 2.4 terawatts (and consistent with 0).

Interesting read. Though it says the nuclear fission happens in the crust and mantle, not the core.

Here's a survey paper with a deeper treatment of the topic, for Earth and other solar system bodies:

https://link.springer.com/article/10.1007/s11214-020-00656-z

"On the Distribution and Variation of Radioactive Heat Producing Elements Within Meteorites, the Earth, and Planets"

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