Why hasn't the earth cooled down by now?

[Dropship]

I mean, mother earth is supposed to be billions of years old, but it's still full of that hot molten stuff!

[swansont]

10 minutes ago, Dropship said:

I mean, mother earth is supposed to be billions of years old, but it's still full of that hot molten stuff!

2 reasons. One is various radioactive materials inside the earth, with billion year half-lives, and the other is a yellow orb sending us thermal radiation from a 6000K surface.

[Dropship]

9 minutes ago, swansont said:

2 reasons. One is various radioactive materials inside the earth, with billion year half-lives, and the other is a yellow orb sending us thermal radiation from a 6000K surface.

Exactly what type of radioactive stuff is in there? i thought radiation was dangerous?

[joigus]

Google: Search string: "half life of uranium 238"

Quote

The half-life of uranium-238 is about 4.5 billion years, uranium-235 about 700 million years, and uranium-234 about 25 thousand years.

Google: Search string: "how abundant is uranium on earth"

Quote

Uranium is a naturally occurring element with an average concentration of 2.8 parts per million in the Earth's crust. Traces of it occur almost everywhere. It is more abundant than gold, silver or mercury, about the same as tin and slightly less abundant than cobalt, lead or molybdenum.

Google: Search string: "most abundant radioactive materials on earth"

Quote

Most common are potassium-40, uranium-238, and thorium-232, which all have fairly long half-lives. Additionally, there are small quantities of shorter-lived materials, such as radium-226, which is a decay product of U-238, and radon-222, which is a product of Ra-226.

 

Wikipedia: https://en.wikipedia.org/wiki/Uranium

Iso­tope	Abun­dance	Half-life (t1/2)	Decay mode	Pro­duct
232U	syn	68.9 y	SF	–
			α	228Th
233U	trace	1.592×105 y	SF	–
			α	229Th
234U	0.005%	2.455×105 y	SF	–
			α	230Th
235U	0.720%	7.04×108 y	SF	–
			α	231Th
236U	trace	2.342×107 y	SF	–
			α	232Th
238U	99.274%	4.468×109 y	α	234Th
			SF	–
			β−β−	238Pu

 

Radiation is dangerous, but how dangerous it is depends on how exposed you are to it, as well as the radiactive material.

Radioactive materials are present in many rocks, as eg. granite, but not so concentrated that they will give you cancer in any noticeable time.

Edited March 11, 2022 by joigus
minor correction

[StringJunky]

Also, the space around Earth is like a giant vacuum flask, so heat loss by convection (molecule to molecule energy transfer) is minimal. 

If I may add a relevant question to the more knowledgeable: Does gravitational compression on the core by the surrounding mass contribute heat to it in a gradient from the surface to the centre? Also, geological friction from internal siesmic activity? I would have thought these mechanical stresses never reach equilibrium, since Earth's geological makeup is continually changing over time..

Edited March 11, 2022 by StringJunky

[StringJunky]

Had a search and found this Phys.org article: https://phys.org/news/2006-03-probing-earth-core.html

[swansont]

11 hours ago, Dropship said:

Exactly what type of radioactive stuff is in there? 

Potassium-40 (1.25 billion year half-life), Uranium (predominantly U-238), Thorium-232 (17 billion years)

 

[Dropship]

2 hours ago, swansont said:

Potassium-40 (1.25 billion year half-life), Uranium (predominantly U-238), Thorium-232 (17 billion years)

 

Thanks, so if it's bubbling away down there and isn't harmful, you'd think scientists would drop boreholes to suck it up to use as an energy source to heat our homes.

In fact there are hot springs all over the place like Old Faithfull so the stuff is often close to the surface anyway.  

[Phi for All]

14 minutes ago, Dropship said:

Thanks, so if it's bubbling away down there and isn't harmful, you'd think scientists would drop boreholes to suck it up to use as an energy source to heat our homes.

You'd think: https://www.energy.gov/eere/geothermal/geothermal

Quote

Geothermal energy is heat derived below the earth’s surface which can be harnessed to generate clean, renewable energy. This vital, clean energy resource supplies renewable power around the clock and emits little or no greenhouse gases -- all while requiring a small environmental footprint to develop.

 

[exchemist]

12 hours ago, StringJunky said:

Also, the space around Earth is like a giant vacuum flask, so heat loss by convection (molecule to molecule energy transfer) is minimal. 

If I may add a relevant question to the more knowledgeable: Does gravitational compression on the core by the surrounding mass contribute heat to it in a gradient from the surface to the centre? Also, geological friction from internal siesmic activity? I would have thought these mechanical stresses never reach equilibrium, since Earth's geological makeup is continually changing over time..

I suppose there may be some further compression occurring, due to the slight reduction of volume in the core as it cools and progressively solidifies. From that link you provided, it seems the latent heat of fusion released as more of the core becomes solid will slow the rate of cooling. If the solid is also denser than the liquid, then the gradual growth of the solid core at the expense of the liquid core will reduce the volume of the core and the rest of the earth will move downward to fill the space and thereby become further compressed, releasing more heat.

Regarding tectonic movement, my understanding is that this does indeed release heat due to friction (e.g. in descending slabs at subduction zones, it contributes to the partial melting that takes place which is responsible for island arc volcanoes). But since the source of the motion is thermal convection in the first place, I should have thought that would be a redistribution of heat from core to crust/upper mantle, rather than an additional source of it.

But this isn't my field so I speak only as an interested amateur.  

[swansont]

1 hour ago, Dropship said:

Thanks, so if it's bubbling away down there and isn't harmful, you'd think scientists would drop boreholes to suck it up to use as an energy source to heat our homes.

In fact there are hot springs all over the place like Old Faithfull so the stuff is often close to the surface anyway.  

The deepest we've ever drilled is about 12 km, and you need to go about 2900 km to get to the outer core. As you and Phi point out, you don't have to do that to tap into geothermal energy.

[StringJunky]

1 hour ago, exchemist said:

I suppose there may be some further compression occurring, due to the slight reduction of volume in the core as it cools and progressively solidifies. From that link you provided, it seems the latent heat of fusion released as more of the core becomes solid will slow the rate of cooling. If the solid is also denser than the liquid, then the gradual growth of the solid core at the expense of the liquid core will reduce the volume of the core and the rest of the earth will move downward to fill the space and thereby become further compressed, releasing more heat.

Regarding tectonic movement, my understanding is that this does indeed release heat due to friction (e.g. in descending slabs at subduction zones, it contributes to the partial melting that takes place which is responsible for island arc volcanoes). But since the source of the motion is thermal convection in the first place, I should have thought that would be a redistribution of heat from core to crust/upper mantle, rather than an additional source of it.

But this isn't my field so I speak only as an interested amateur.  

Cheers

[beecee]

3 hours ago, swansont said:

The deepest we've ever drilled is about 12 km, and you need to go about 2900 km to get to the outer core. As you and Phi point out, you don't have to do that to tap into geothermal energy.

https://www.sciencealert.com/confidence-grows-in-mit-spin-off-aiming-to-make-the-deepest-hole-for-limitless-energy

 

 

Hmmm, sounds a bit airy fairy to me. 

[Bufofrog]

20 hours ago, Dropship said:

i thought radiation was dangerous?

Radiation is a rather broad area.  Radiation from the sun heats the earth.  Radiation from the sun help humans to synthesize vitamin D.  Ionizing radiation on the other hand can be quite dangerous and deadly at high levels of exposure.

 

[Ken Fabian]

A lot of the health issues around radioactive materials are actually chemical in nature, by being chemically toxic and biochemically carcinogenic. Eg, Strontium 90 when ingested will be used in place of calcium in cells and tissues, but then the Strontium undergoes radioactive decay, breaking apart the molecules they were part of (which may become toxic in turn) and becoming Yttrium 90 and then Zirconium 90 whilst releasing alpha, beta and gamma radiation, which induces further unwanted and potentially harmful biochemical reactions in surrounding tissue. 

Re the Thermal History of Earth there is still a lot of uncertainty but the fundamental processes appear understood. Like heat from compression being ultimately attributable to the formation of the Earth - the energy came with the materials that coalesced to make Earth. And whilst we may not perceive rock as being a good insulator it is in fact a very good one when it is hundreds of kilometres thick, so much of that heat has still not managed to get out.

[Dropship]

On 3/11/2022 at 2:59 AM, StringJunky said:

Also, the space around Earth is like a giant vacuum flask, so heat loss by convection (molecule to molecule energy transfer) is minimal. 

If I may add a relevant question to the more knowledgeable: Does gravitational compression on the core by the surrounding mass contribute heat to it in a gradient from the surface to the centre? Also, geological friction from internal siesmic activity? I would have thought these mechanical stresses never reach equilibrium, since Earth's geological makeup is continually changing over time..

 

It's not my field but I'd say the moon has something to do with keeping the earth's innards hot by making the molten stuff slosh around causing friction.

[iNow]

13 minutes ago, Dropship said:

It's not my field but

You should’ve stopped there 

[Dropship]

3 minutes ago, iNow said:

You should’ve stopped there 

 

17 minutes ago, Dropship said:

 

It's not my field but I'd say the moon has something to do with keeping the earth's innards hot by making the molten stuff slosh around causing friction.

 

If you expect me to pretend something is my field when it's not, you're gonna have to get yerself another boy..:)

None of us are experts in every scientific field, for example as a single man I know nothing about women but am hoping to learn from the experienced men of the world in this forum.

I remember as a kid asking my mum where babies come from and she replied "You buy them from hospitals for £15 each", but as I've grown older I'm beginning to suspect otherwise.

[Bufofrog]

36 minutes ago, Dropship said:

as a single man I know nothing about women

Huh?  That seems rather strange...

[exchemist]

1 hour ago, Dropship said:

 

It's not my field but I'd say the moon has something to do with keeping the earth's innards hot by making the molten stuff slosh around causing friction.

You mean tidal heating? No, it is not significant for the Earth, though it is for some of the Jovian satellites, I understand. 

[Dropship]

4 minutes ago, Bufofrog said:

Huh?  That seems rather strange...

It's true mate, it's true because I was a shy kid and got off to a slow start..:)

For example as a nerdy young teenager I plucked up the courage to ask a girl for a date but she replied "**** off and kill yerself you ugly b*****d".

My mate consoled me by explaining- "Don't worry, it means she really likes you, just wait a bit and she'll be asking you for a date"

50-odd years later i'm still waiting..

7 minutes ago, exchemist said:

You mean tidal heating? No, it is not significant for the Earth, though it is for some of the Jovian satellites, I understand. 

Yeah that's another thing, I hear some distant planets and moons have got molten innards and volcanoes even though they're zillions of miles from the sun, therefore the suns heat can't be contributing much to keeping them hot inside.

[mistermack]

The heating effect of tidal forces will affected by how close together the two bodies are. The heating effect might be negligible now, but the Moon was once much closer to the Earth, so maybe some of the heat stored inside the Earth might be due to tidal effects from the past. 

Also, the greenhouse effect of our atmosphere must surely slow down the rate of conduction. 

The average surface temperature on Earth is 14 deg C, and it's been warmer for most of the past. That will slow down the flow of heat from below.

Edited April 4, 2022 by mistermack

[Moontanman]

OMG! There is a nuclear reactor inside the Earth! We're all gonna die! 

Spoiler

A completely factual statement!

 

Edited April 4, 2022 by Moontanman

[beecee]

10 hours ago, mistermack said:

The heating effect of tidal forces will affected by how close together the two bodies are. The heating effect might be negligible now, but the Moon was once much closer to the Earth, so maybe some of the heat stored inside the Earth might be due to tidal effects from the past. 

Spot on imo...the Moon causes the high tide closest to it by attracting the water, and then attracting Earth itself so that high tides also occur on the opposite side. Some effect, but minimal at this time. And of course just as the Moon was much closer to Earth in the past, so to will it be much further away in the future, moving at a rate of a couple of cms a year. It (the Moon) is also via the friction of the tides, causing the Earth's revolution about its axis to slow. This will continue until a day on Earth is equal to a lunar month. and Earth will have one side permantly facing the Moon, just as the Moon has one side facing Earth. None of this is due though, for at least a couple of billion years. 

Edited April 5, 2022 by beecee

[sethoflagos]

On 3/11/2022 at 5:06 PM, exchemist said:

Regarding tectonic movement, my understanding is that this does indeed release heat due to friction (e.g. in descending slabs at subduction zones, it contributes to the partial melting that takes place which is responsible for island arc volcanoes). But since the source of the motion is thermal convection in the first place, I should have thought that would be a redistribution of heat from core to crust/upper mantle, rather than an additional source of it.

 

A bit after the lord mayor's show,. However ...

While thermal expansion must play some part in mantle convection, it seems that the primary driver is phase change (ref: https://authors.library.caltech.edu/25038/145/Chapter 5. The eclogite engine.pdf). This implies that most of the transported energy is locked up in the enthalpies of various sequences of crystal structural readjustments with depth (eg the olivine-wadsleyite-ringwoodite-perovskite sequence) rather than as thermal energy. 

Hence there is within the mantle a huge reservoir of energy bound up in high pressure mineral polymorphs, that may be released as heat at specific lower pressure locations (mid-ocean ridges. volcanic island arcs etc) moreorless independently of the overall thermal gradient. This localised heat release drives the processes of crustal magmatic fractionation, adding low density granitic material to the continents, and consequently increasing the density of the returning subduction slab.

The excess of gravitational potential energy of this continental scale mass of dense material over and above the surrounding asthenosphere represents a second huge reserve of energy that is partially returned as heat back to the base of the mantle, but is mainly consumed in restoring the high pressure polymorphs of its constituent mineralogy.

Overall, the process of continent building via magmatic convection and fractionation might be approximated as raising 3 billion km3 of granite (SG ~2.5) through 1,000 km of mantle (SG ~ 4) which looks like 4.5 x 10^28 Joules over a period of 4.5 billion years resulting in a heat output of 0.3 TW purely on isostatic considerations.

Contrast this with the estimated total crustal heat flow of 47+/-2 TW (based on 38,000 measurements).

If we were to consider the approach toward gravitational equilibrium of all structures within the earth (not just the surface ones), then the process of planetary differentiation (ref: https://en.wikipedia.org/wiki/Planetary_differentiation) would seem to amount to an appreciable percentage of the total. And that's based only on the isostatic aspect. We are as yet nowhere near full planetary chemical equilibrium either, and that must also factor into the balance sheet.  

I've seen figures of ~20 TW given for the heat produced by radioactive decay, and can only add the comment that it seems credible.

More critically some authors have ascribed the balance to 'primordial heat'. I don't quite know what they mean by that. Or rather, I do. It's the heat generated during the initial accretionary growth of the planet, the sum total of initial gravitational potential energy of all its constituent parts and released as heat on impact. But isn't this just the initial phase of planetary differentiation? The phase where space was gravitationally displaced by matter? And has it been sat around doing nothing since? I think not. Rather its being doing what heat does - driving convection currents and fuelling endothermic reactions for the last 4.5 billion years. 

Seen in this light, I'm tending to lean towards there being an approximate balance between radiogenic heating and the nett release of gravitational and chemical potential energy arising from planetary differentiation.