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BoIshevik t1_j5no6bt wrote

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

What forces keep the core molten as it is? (I think)

Also is the heat dissipating? If so I'm sure the heat lost is close to the heat generated by whichever forces because the Earth has been hot & "hot as a mf" on the inside for so long.

Lol sorry & thanks, wouldn't bother a whole post, but seemed like you may know or be able to point me to a good set of terms to search to understand this better.

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mathologies t1_j5o8iud wrote

Core is molten for two reasons:

Initial formation of Earth generated a lot of thermal energy, because a lot of stuff smashed together

Continued decay of radioisotopes has contributed additional thermal energy-- without this, the core would have cooled by now

Thermal energy is lost from the core through heat transfer to the mantle, which brings it to the crust by way of convection; this is what ultimately causes plate movement, earthquakes, volcanism, subduction, etc. on Earth. Once the core is cool enough that mantle convection stops, tectonic forces will subside and uplift processes will cease. From that point, weathering and erosion will gradually erase all land above sea level, giving us an ocean planet. I think the Sun will be hot enough to boil our oceans well before that happens, though (solar brightening over geologic time).

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jabask t1_j5ogzt6 wrote

So if the oceans boil away, will the earth just become a smooth marble of rock with a bunch of fog over it?

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cklein0001 t1_j5ok4mp wrote

Think more "mars-scape, marscape?", as the core also gives us our magnetic field that protects our atmosphere from the solar winds. Once the core cools, the Earth's internal dynamo also slows, solar winds start buffeting our atmosphere out of Earth's gravitational field, and we start looking more like Mars.

Also scheduled to happen around the same geologic time as the Earth being swallowed / burned by the Sun.

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paul_wi11iams t1_j5pdu1z wrote

> swallowed

The version I heard was the expanding sun loses mass leaving Earth beyond its grasp.

> swallowed / burned

So its changed again, having become uncertain.

and even that is ignoring options for stellar engineering (assuming our descendants even care)

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cklein0001 t1_j5pep9v wrote

Yeah, several "what's gonna happen in unfathomable timescale" options to choose from.

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Rule_32 t1_j5ut7dr wrote

>sun loses mass leaving Earth beyond its grasp

This is incorrect. The Suns expansion does not equate to mass loss. Cosmologically its mass will have not decreased that much (which is what will determine where Earths orbit is) but it'll be fusing helium into carbon which is a hotter more energetic process and so the outer layers of the star will get pushed out and the star expands. Earths orbit won't change much however the Suns radius will expand to near or fully envelop Earth.

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cecex88 t1_j5nu9fn wrote

The core is molten because it's hot and under high pressure. Essentially, the Earth was very hot and it's cooling down.

In fact, the size of the solid part, i.e. the Inner Core, continuously increases.

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maineac t1_j5o8q5e wrote

Aren't there continuous nuclear reactions also that help keep the heat going? Likely caused by the pressure.

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cecex88 t1_j5oatsj wrote

Nuclear reactions happening in the Earth are all radioactive decays. These produce heat, and it's in fact the main contribution (I've written another comment here with a bit more detail) but most of it happens in the continental Crust and in the Mantle.

This nuclear process is not generally dependent on pressure, in fact it mainly happens in the external layers of our planet. The distribution of radiogenic heat depends on where the element that may undergo beta-decay are located. If you are curious, the main elements responsible for this on Earth are Uranium, Thorium and Potassium.

Despite there being large uncertainty, geochemical studies show that these element are not much soluable in the liquid core. Radioactive decay contributes to the heat production in the core for 0.2 TW, while the the total amount of heat produced by the core is around 10-15 TW.

EDIT: well, not all nuclear reactions in the earth are decays. A few occurrences of natural fission reactions have been found, where particular conditions, like more abundance of a certain uranium isotope and different amount of oxygen in the atmosphere, made the fission reactions possible. The only few occurrences known happened in the far geological past in Oklo (region of Gabon).

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alexm42 t1_j5o9pyn wrote

Not caused by the pressure (that's what stars do and the Earth isn't one) but rather the heat of radioactive decay. It's also believed that tidal forces from the Moon create some heat.

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seeriktus t1_j5oarvl wrote

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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LordMoriar t1_j5np35h wrote

Have you ever felt a spray bottle get colder as you release the pressurised gas/content?

In Earth's core it's the other way around. High pressure makes the core hot. The high pressure itself is from the mass of the kilometers of dirt and stone and water etc.

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GeneralBacteria t1_j5nqvvp wrote

releasing pressure makes something get colder. increasing pressure makes it get hotter.

so where is the increase in pressure coming from that keeps the core hot?

what actually keeps the core hot is radioactive decay on unstable isotopes

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theredhype t1_j5ntbvi wrote

Gravity causes the pressure. It’s the weight of everything above pressing down, which naturally increases the deeper you go.

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PlotRatio t1_j5nx2iy wrote

But its a static pressure isn't it?

Otherwise something highly compressed would radiate heat indefinitely which ain't going to happen.

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Unlimited_Emmo t1_j5nxb27 wrote

Yes, somewhat, there are fluctuations but mainly the earth is hot, it was heated by the pressure, and is now cooling down.

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silent_cat t1_j5o5oeb wrote

> Otherwise something highly compressed would radiate heat indefinitely which ain't going to happen.

Sure, the earth is cooling down. The mantle however is a reasonably good insulating layer though (mostly because it's so damn thick). The heat loss is is estimated at 47±2 TW (or about 3 times to total energy usage by humans). Still, the Earth will be destroyed by the Sun before it cools down.

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PlotRatio t1_j5o8h0c wrote

Sure, I agree with all of that.

>Gravity causes the pressure. It’s the weight of everything above pressing down, which naturally increases the deeper you go which really isn't the case as no work is being done.

I just read the above as suggesting that a static pressure will result in an increase in temp.

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rivalarrival t1_j5nyfgo wrote

The relationship between temperature and pressure does not work the way you are describing. A substance is not at a given temperature just because it is at a given pressure. Pressurize one cylinder of nitrogen to 30PSI, and another cylinder of nitrogen to 3000PSI. Leave them alone in a room for awhile, and they will both become room temperature.

The temperature of a given mass is not dependent on its static pressure, but on changes to its pressure.

You are (effectively) arguing that adiabatic heating is responsible for the heat of the earth's core. To make this argument, you will have to show that the earth's volume is shrinking, or otherwise demonstrate that the pressure at the core is not just high, but increasing.

Without a pressure change, we need to look at the heat entering or exiting the system. The simple fact is that relative to the total amount of heat within them, very little heat actually leaves the core and mantle. At the current rate of dissipation, it will take billions of years to remove a significant amount of that heat.

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Aethyx_ t1_j5nzzpz wrote

Following the analogy... Isn't it so that the earth was pressurised to a tremendous amount of psi, and is now in that process of cooling down to ambient temperature?

Of course many other processes play a role, but the pressurised can analogy kind of works if you scale it up?

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rivalarrival t1_j5o6oem wrote

I don't know if adiabatic processes are responsible for the temperatures in the core, but if it is, it would be more accurate to describe this in the past tense, rather than the present tense that the other commenter used:

>"High pressure makes made the core hot"

You made the same distinction:

>the earth was pressurised

That being said, I doubt adiabatic heating plays a significant role. Adiabatic processes operate through compression, not pressurization.

Suppose I have a sealed tank of water. I put a balloon inside it. Then I pressurize the water to double the pressure in the tank. The volume of the balloon shrinks.

Here's the important part: Even though the balloon is half the size now, it still has the same amount of heat: none has entered or exited yet. The same amount of heat in a smaller volume means the temperature has risen. That's adiabatic heating.

Replace the balloon with an iron or nickel ball. When you double the pressure, the volume of the ball doesn't change. Increase the pressure a hundred times, a thousand times, it doesn't matter: the volume of the ball stays the same. The heat within the ball is not concentrated. There is no adiabatic process involved.

With the core of the earth being primarily comprised of non-compressible materials, I don't think adiabatic heating explains the temperature of the core.

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Implausibilibuddy t1_j5o6ohb wrote

> the pressurised can analogy kind of works if you scale it up?

It did 4 billion years ago when the debris in our Sun's accretion disk coalesced to form our planet, and again when whatever planet sized object hit us to form our Moon, but since then we've been cooling off like a pot of old coffee. Fortunately there's a lot of mass left to cool off, and it's stored in the best Thermos ever created...

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HerraTohtori t1_j5ooerf wrote

The increase of pressure occurred billions of years ago when the Earth was originally formed from the cloud of gas and dust accumulated in a disc around the early Sun.

The collisions between particles created a lump that started attracting more and more particles and dust and larger debris pieces. As the mass increased, gravity also increased and each layer started to push on the layer under it with increasing weight. Eventually the force of gravity was strong enough to slowly deform the core of the object, until the proto-Earth reached a state called hydrostatic equilibrium.

In English this means the object was now big enough that it formed into a sphere under its own weight. This shape change caused a lot of heat through friction, and of course the Earth was not yet done growing.

As the planet grew and the pressure within increased, the core started to melt, which meant that heavy elements sunk into the core. However, eventually the planet grew so big that the pressure at the core actually started to solidify the iron and nickel there.

This is how Earth ended up with a solid iron-nickel inner core, surrounded by a thick layer of outer core which is also mostly iron and nickel, but in liquid phase.

So, the pressure increase that caused the Earth's core to heat up originally occurred billions of years ago when the planet was forming.

Currently, it's believed that some of Earth's core heat is still residual or "primordial" heat from the formation of the planet, and the rest is from tidal forces generated by the Moon, or heat from radioactive decay of active isotopes.

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LeifRoberts t1_j5nqbla wrote

The core's main heat source comes from radioactive decay of elements leftover from planetary formation, not from pressure.

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cecex88 t1_j5nv1rz wrote

In reality, geochemical studies suggest that radiogenic heat plays a small role in the energy balance of the core. Cooling and phase transition are the main processes.

The part where radioactive decay is the main heat source is the mantle.

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Aurora_Fatalis t1_j5nxaxf wrote

To what extent is it kept warm simply by heat retention from primordial times and the formation of the earth? As in, how does the thermal energy generated by the core/mantle over the past few billion years compare to the thermal energy lost and the thermal energy it started out with when the planet was a mostly molten blob?

I guess to simplify the question, I'm curious whether, in the absence of radioactive decay in the mantle, we'd be another Mars right now.

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cecex88 t1_j5nzkcf wrote

The main heat sources in the core are secular cooling (i.e. losing primordial heat), latent heat due to the ongoing solidification of the inner region, compositional energy (essentially gravitational energy, the lighter elements in the core do not solidify and some fraction of the core solidify these elements rise up to the liquid part) and radioactive decay.

The estimates in Earth's Core (by Cormier et al., nice book) are around 0.3 TW for radioactive decay and a few TW (2 to 6) each for the others.

As an order of magnitude estimation, compositional changes, phase transition and original heat loss contributes equally, while radiogenic heat is only a minor contribution.

The heat balance we measure at the surface has obviously much more than this. We have to take into account the secular cooling of the mantle itself (16 TW), plus the heat production of continents (8TW) and again the mantle (11 TW), which are mainly radiogenic (data here from the Encyclopedia of Solid Earth Geophysics).
Note that every estimate, despite being in line with scientific consensus, is subject to high uncertainties, due to the very difficult nature of these kind of measures/models.

To close going slightly OT, this combination of heat production and heat loss is the driving force of hot spots and plate tectonics. Which means that the cooling dynamics of the earth is responsible for essentially the entirity of what we observe in the solid earth. Earthquakes, volcanic eruptions, tectonic movement, but also interactions with surface geomorphology are all byproducts of a ball of molten rock cooling.

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RickSchwifty t1_j5o0byk wrote

As far as I understand earth will inevitably become similar to mars: as the earths core cools down and solidifies our magnetic field begins to disintegrate, seismic and volcanic activities will disappear which ultimately will thin out our atmosphere to a point where life will be impossible. We talking billions of years ofc.

According to science only half of earths internal heat stems from radioactivity, the rest being primordial heat. This in turn means our planet is obviously much cooler than it used to be.

https://www.science.org/content/article/earth-still-retains-much-its-original-heat

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