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seeriktus t1_j5oarvl wrote

Reply to comment by BoIshevik in What are the forces on Earth’s Inner Core that change its speed? by BayRunner

Heat retained from the formation of the earth, and compressive forces.

Insulation from the thick layers above.

Tidal heating - Continual squeezing and squashing of earths core by the moon orbiting around the earth. Though note that this doesn't occur with moons that are tidally locked.

Having a relatively warm surface from heating by the sun and thick atmosphere, though this is of lesser effect than say Venus. By having a warmer surface, the temperature differential is lessened, so less thermal energy moves across the gradient.

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CrustalTrudger t1_j5oemhv wrote

By far the two major sources of heat are primordial heat and radioactive decay. Changes in pressure can cause changes in temperature, but largely static pressure with depth does not generate heat. Tidal heating is not relevant for the Earth (though it's important for other planetary bodies in the solar system).

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Unlikely_Amount5932 t1_j5okrsl wrote

I'm confused. Are either of these processes related to the pressure of the mass above? I always thought that was what caused the heat. Two more questions. 1. Is radioactive decay triggered by something or does it just happen with those particular elements? 2. Is there something besides the insulating qualities of the crust that keep primordial heat from cooling quicker. Billions of years seems like a long time for something to cool down. I know it is a lot of mass but......

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CrustalTrudger t1_j5os2ad wrote

> Are either of these processes related to the pressure of the mass above? I always thought that was what caused the heat.

No. The mass above plays a role in the sense that it dictates the rates of heat transfer toward the surface from a given depth, but the fact that it's under pressure does not directly generate heat. There are various depth/pressure/temperature related processes that occur, specifically phase transitions at specific depth/pressure/temperature ranges that do change heat content, e.g., the 660 transition is characterized by an endothermic reaction and thus does contribute to the total heat budget.

> 1. Is radioactive decay triggered by something or does it just happen with those particular elements?

Radioactive decay, and its rate, is an intrinsic property of a given isotope. There is abundant literature and details on radioactive decay, but generally speaking, for a given isotope (e.g., ^(238)U, which is one that is relevant for the question), the "rate" of decay is actually a reflection that there is a fixed probability that any given 238 atom will decay, and given an arbitrarily large group of 238 atoms, we can consider this as a rate of decay.

> Is there something besides the insulating qualities of the crust that keep primordial heat from cooling quicker.

Insulating qualities of the entire planet, i.e., rock is not a great conductor, the relative inefficiency of radiation as a heat transfer mechanism (i.e., how heat is ultimately transferred out of the solid Earth), and the radioactive decay all play an important role. For the last bit, it's not just that radioactive decay is adding heat, but also in doing so, it's effectively slowing down the rate of heat loss. Broadly speaking, the rate of heat transfer is related to the gradient in temperature. Thus a reduced gradient (because the interior stays warmer) means that the rate of heat transfer is less. Things get more complicated as we have to think about convective heat transfer in the outer core and mantle, but broadly this point remains true.

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BoIshevik t1_j5obutc wrote

Earth and the moon are tidally locked aren't they? Interesting answers here and I actually had some keywords to use too lol thank you.

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seeriktus t1_j5oced4 wrote

The moon is tidally locked so it always faces the earth, but the earth doesn't always face the moon. It rotates unequally to how the moon orbits. There is also drift between the number of orbits and the ultimate position the moon ends up in a year. One month is roughly equal to a lunar month, but not exactly.

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