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LordMoriar t1_j5np35h wrote

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

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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