Why is the Center of the Earth so Hot?

  • #1
I would think that after 4 billion years the thing would stabilize and cool off. I can't imagine that this is all due to continental drift and what not. Could there be an enormous nuclear fission furnace at the center of that iron core? Does solar wind drive an electric current in the center that inturn encounters resistance thereby produces heat?
 

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  • #2
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radiogenic heating
gravitational heating
latent heat
adiabatic heat

the effect of the solar wind heating the centre of the earth is negligible.
 
  • #3
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There is even one more

friction heating.
 
  • #4
baywax
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There is even one more

friction heating.

does "gravitational heating" mean pressure? Because pressure is a major cause of heat.
 
  • #5
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No, this one is about the interaction of torques, precession and nutation forces of the earth mantle and solid inner core.

see the two pdf links here:

http://www.me.ucsb.edu/dept_site/vanyo.htm [Broken]


This is maybe a more important reason for the second per day per million years decrease in spinning than the usual mentioned tidal friction.
 
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  • #6
BillJx
I'm not a scientist or engineer, merely a 2nd class power engineer (steam plant operator). However, I believe that fission of uranium contributes substantially. Note that we still have radioactive materials at the surface, and since they are so heavy, so we would expect to have a lot more in the core.

I think too, that tidal friction contributes. The earth is squeezed back and forth like a fistfull of window putty as it rotates.

Here's a related thought. If the Earth's internal heat was appreciably greater or less than it is, would a technological society be possible? If less, we wouldn't have the forces that bring iron, lead, gold etc to the surface. If greater, things would be too geologically unstable. Or so it seems to me.
 
  • #7
I'm not a scientist or engineer, merely a 2nd class power engineer (steam plant operator). However, I believe that fission of uranium contributes substantially. Note that we still have radioactive materials at the surface, and since they are so heavy, so we would expect to have a lot more in the core.

I think too, that tidal friction contributes. The earth is squeezed back and forth like a fistfull of window putty as it rotates.

Here's a related thought. If the Earth's internal heat was appreciably greater or less than it is, would a technological society be possible? If less, we wouldn't have the forces that bring iron, lead, gold etc to the surface. If greater, things would be too geologically unstable. Or so it seems to me.

Yeah I've read an article about this theory it's a little controversial but it would help to explain why the Earth's core is cooling so slowly, although there are other less controversial theories.

I seem to remember they proposed testing it by trying to determine where neutrinos produced by radioactive decay were coming from in the Earth, but I'm not sure if they got anywhere.
 
  • #8
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Why would we think that the core is cooling? With the friction going on, the temperature of the core could be stable until most rotational energy is transferred to heat.

Fission in the core is probably only highly incidentely as it is in regural natural uranium. There is no way that there is a natural design reactor with enriched uranium near critical mass and with moderating media to slow down the neutrons to sustain a chain reaction.

Neutrinos, I think, is yet another story, which is more related to the sun.
 
  • #9
BillJx
Why would we think that the core is cooling? With the friction going on, the temperature of the core could be stable until most rotational energy is transferred to heat.

Fission in the core is probably only highly incidentely as it is in regural natural uranium. There is no way that there is a natural design reactor with enriched uranium near critical mass and with moderating media to slow down the neutrons to sustain a chain reaction.

Neutrinos, I think, is yet another story, which is more related to the sun.

Andre, the conclusions of experts are at your fingertips.

http://en.wikipedia.org/wiki/Geothermal_(geology [Broken])
http://www.physorg.com/news62952904.html
http://www.physlink.com/News/121103PotassiumCore.cfm
http://www.newscientist.com/article.ns?id=mg18725103.700

Of course, there are minority opinions.
http://www.nov55.com/heat.html
 
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  • #10
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I have long learned that the conclusions of experts are something else than reality. The more consensus the less thinking out of the box.

Take Jim Vanyo for instance. Read this: http://www.me.ucsb.edu/dept_site/vanyo/computational.pdf [Broken]

Energy obtained by a ‘growing’ inner core is limited by the known small size of the inner core. Energy available from this model has a maximum near the minimum 1011 W needed for a geodynamo. Precessional energy is obtained from earth rotational kinetic energy and is limited by known estimates of secular deceleration by lunar and solar torques. Estimates of days/year from 850 Ma ago and 360 Ma ago of 435 (10,11) and 397 (12) , respectively, all compute to ~ 3.5x1012 W average continuous loss of rotational kinetic energy. Although this energy also powers other phenomena (lunar orbit changes, oceanic and solid earth tides), even 10% placed into core energy is three times the minimum of 1011 W needed.

Inner core growth and precession together cannot supply the 4.2x1013 W of net earth heat loss requiring substantial radioactivity within the earth. Uranium 238 with half life of 4.5x109 yr and potassium 40 (1.3x109 yr) have been proposed. 238U is rare in the earth’s crust, K is abundant, but only 0.001% is 40K. Abundances in the lower mantle and core remain speculative, but radioactive sources in the earth’s core would augment geodynamo activity generated by buoyancy convection. Important aspects of precession include that it could generate 3x1016W but routinely has not and that its output is very sensitive to changes in precession rate, axis inclination, and especially magnetic core-mantle coupling
.
Emphasis original

Now Vanyo appears to encounter a surprise here, spin a raw egg and it will loose it rotational energy rather quickly due to the erratic movements inside, compared to a hard boiled egg. Earth compares to a raw egg, but the size of the solid inner core suggest that the precession torque did not do it's expected job. I think what Vanyo overlooks is that the size of the solid inner core is a function of available heat energy.

If the heat dissipates, the solid inner core grows under the pressure but the angular momentum of that solid core increases to the fifth power of the growing radius. A higher angular momentum means much more force to overcome the different precession rates of mantle and solid inner core which would tend to drift the spin axis of the inner core out of alignment with the mantle spin axis. This would put a bigger strain on the magnetic/mechanic coupling and as a consequence more friction. More friction is more heat, which would tend to reduce the size of the solid inner core again. In other words the size of the solid inner core is a function of the heat balance and overlooking this, could be the reason why Vanyo emphasized "routinely has not". I think it has, but the size of the core is at balance with dissipated spinning energy.

I think that the size of the inner core will be keeping balancing the heat, keeping it more or less constant, until it's sources are depleted, radioactivity prevailed at first perhaps in the young Earth but dissipation of Earth rotational energy takes over gradually as radioactivity declines.

What would happen if the inner core grows so big that the forces associated with precessing/drifting spin axis exceeds the mechanical/magnetical coupling and the inner core would spin completely out of alignment with the mantle?
 
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  • #11
BillJx
I'm not going to carry this on too far because 1) I don't think either of us is a physicist and 2) our approaches are so different that I doubt that we will come together.
Thinking out of the box leads to creative solutions, but it leads to many, many more wrong answers than right ones. In fact, logical analysis in general leads to many more wrong answers than right ones. That's why R&D is so expensive. Most ideas don't work, regardless of the intelligence behind them. It's also the reason that the ancient philosophers were unable to decipher the nature of the world. DaVinci had a beautiful explanation of the nature of image. So close to being right but he didn't know the physical nature of light and darkness. I appreciate your intelligence, but you seem to trust your own intellectual analysis over the consensus of experts whose conclusions are based on evidence, not to mention a much deeper theoretical underpinning than lay persons can attain.
Questioning is creative. Falling for your own conclusions is a dead end.

Also, some of your concepts seem a bit fuzzy. For example, "erratic movements" inside a raw egg will neither appreciably add to nor subtract from the rotational energy. i.e. they will reinforce the rotation as often as they oppose it. The reason the raw egg slows down faster is that the liquid core did not reach the same rotational speed as the shell. I suspect that if you spun both eggs at a steady speed for a long enough time before releasing them, that both would spin for about the same length of time. There's a little tabletop experiment that might be interesting to try!



Remember, everybody is wrong about everything, it's only a matter of degree.
 
  • #12
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i think you overestimate "experts". That's the autority fallacy. I did so too until I got to discuss with them a lot. Sure within the boundaries of their specialities you can learn a lot, but whenever you enlarge the thinking box, which means consulting many specialities simultaneously, then everybody is equal again. Moreover, the scientific method: observation, hypothesis, test does not depend on the status of the operator, only his skill to think of everything possible.

And the answer to:

What would happen if the inner core grows so big that the forces associated with precessing/drifting spin axis exceeds the mechanical/magnetical coupling and the inner core would spin completely out of alignment with the mantle?

is "Venus"
 
  • #13
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Andre, the conclusions of experts are at your fingertips.

http://en.wikipedia.org/wiki/Geothermal_(geology [Broken])
http://www.physorg.com/news62952904.html
http://www.physlink.com/News/121103PotassiumCore.cfm
http://www.newscientist.com/article.ns?id=mg18725103.700

Of course, there are minority opinions.
http://www.nov55.com/heat.html

With respect I really think you've misunderstood what the "experts" have said. I browsed through your references and I couldn't find anything that explicitly states that "fission of uranium contributes substantially" to the core's heating. Actually most of the Uranium is thought to be in the lower mantle and a fair bit is in the crust, of course it does do a lot of heating, but it doesn't heat the core, which was the original question!

One article mentions that there could be K in the core, which is a radiogenic element, this might well be true and I have found a (slightly more reputable) source which backs this up:

The fact that the core is largely composed of Fe was
firmly established as a result of Birch’s (1952) analysis
of mass-density/sound-wave velocity systematics. Today
we believe that the outer core is about 6–10% less
dense than pure liquid Fe, while the solid inner-core
is a few percent less dense than crystalline Fe (e.g.
Poirier, 1994a). From cosmochemical and other considerations,
it has been suggested (e.g. Poirier, 1994b;
Allègre et al., 1995; McDonough and Sun, 1995) that
the alloying elements in the core might include S, O,
Si, H and C. It is also probable that the core contains
minor amounts of other elements, such as Ni and K.
(Vocadlo et al. 2003)

The newscientist article you referenced makes the distinction that the antineutrinos are coming from within the core, however I fail to see how they can be sure that these antineutrinos are not coming from the lower mantle - or even the continental crust! I suspect this is a sloppy article and without seeing a more robust source I will consider it to be incorrect.

Anyway, the fact is the experts do not think that there is a significant U source in the core. If there were, how would you explain that the average density of the core is observed to be ~3% less than that of iron (at the appropriate pressures)? Also, how would the inner core be cooling if there were U constantly heating it up? These are big problems for your theory which - as I have stated - is a theory with little backing from the experts.
 
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  • #14
BillJx
Thanks Billiards. I find that very surprising; obviously I'm guilty of letting preconceptions get in the way of understanding. I did do a bit more online reading and learned a bit more, so thank you for that. Another skewed perception ever-so-slightly straightened!

Andre, I don't think that assigning credibility to credible sources is what the Authority Fallacy refers to. Apparently though, understanding those sources might be something else again . . .

I'll confess that I wrote a rather ignorant reply the other day. Apparently I closed out without sending it. The guardian angel of fools at work I guess.
 
  • #15
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ANDRE said:
"There is no way that there is a natural design reactor with enriched uranium near critical mass and with moderating media to slow down the neutrons to sustain a chain reaction."

And don't forget that one of the so-called experts said that gravity at the centre of the earth sorted the heavy elements from the lighter ones at the core.There is little or no gravity at the centre of the earth..
 
  • #16
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ANDRE said:
"There is no way that there is a natural design reactor with enriched uranium near critical mass and with moderating media to slow down the neutrons to sustain a chain reaction."

And don't forget that one of the so-called experts said that gravity at the centre of the earth sorted the heavy elements from the lighter ones at the core.There is little or no gravity at the centre of the earth..

I don't know who that so-called expert was but the fact remains that light elements are sorted from the heavier ones at the inner core boundary (ICB), although this is merely a chemical process whereby the newly crystallising Fe rejects the lighter elements. Kind of like when sea ice forms it rejects the brine.

I don't see why people are so determined that the inner core has a ball of U at its centre, the theory is riddled with problems, of which I've thought of two already (see earlier post), here's a third: Why is there no seismic reflection from the huge thermal and chemical boundary that the U ball would create?

Besides, if you wanted to get serious with your theory you would need to use ab initio methods whereby you solve schrodingers equation for a crystal of U (or whatever you think the crystals chemistry might be), you need to find a way that your crystal would be stable and exhibit the elastic properties that have been observed by seismology, you also need to explain why the density is so low. Hmmmm, big problems there.....
 
  • #17
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Well there is always the odd crackpot with an exploding Earth due to the nuclear reactor going nuts caused by global warmng.

Warning, don't believe a word of it:

http://sci-e-research.com/geophysics.html

Just notice how convincing boulderdash can be sold if it appeals to our weaknesses.
 
  • #18
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I was just lurking at old threads and came upon this one.
ANDRE said:
"There is no way that there is a natural design reactor with enriched uranium near critical mass and with moderating media to slow down the neutrons to sustain a chain reaction."

There have actually been sites found where exactly this has taken place. Apparently, water was the moderator which flooded the pile and evaporated cyclically causing intermittent power generation for thousands of years. Not to suggest that this kind of thing was enough to contribute significantly (it looks like a pretty small reactor,) but it is interesting that it can happen.

http://en.wikipedia.org/wiki/Natural_nuclear_fission_reactor
 
  • #19
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The Earth's rotation has a wobble as most everyone knows. The huge mass of the Earth is being shaken as a result of the wobble which causes molecular fristion which produces large amounts of heat. The wobble also causes shifting of the Earth's crust. The symptoms of the shifting crust are earth quakes and volcanoes.
 
  • #20
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What the heck is gravitational heating? Where does the energy come from?
 
  • #21
baywax
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The Earth's rotation has a wobble as most everyone knows. The huge mass of the Earth is being shaken as a result of the wobble which causes molecular fristion which produces large amounts of heat. The wobble also causes shifting of the Earth's crust. The symptoms of the shifting crust are earth quakes and volcanoes.

Hi oliason, can we get some references to back your claims?
 
  • #23
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Given that the temperature of the surface is almost all due to sun heat, and almost none due to earth heat, I suspect that the insulation of the crust is a big player in the temperature of the core.

It just hasn't cooled down much yet.
 
  • #24
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Tapping into the gravitational energy reservoir.

http://www.as.utexas.edu/geeifc/talks/utgal08_birnboim.pdf

"Where does the energy of gravity come from?"

Physicsforums Thread...

https://www.physicsforums.com/showthread.php?t=164152

As far as I know gravity is a static force and represents only potential energy.

Hmm, I didn't follow that. If "the gravitational energy reservoir" means gravitational potential energy, then does that mean something has to be "falling" as this heating occurs? Would that mean the Earth gets gradually smaller and denser with time? Like some parts of things just fall in somehow? Hard to picture.
 
  • #25
baywax
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Hmm, I didn't follow that. If "the gravitational energy reservoir" means gravitational potential energy, then does that mean something has to be "falling" as this heating occurs? Would that mean the Earth gets gradually smaller and denser with time? Like some parts of things just fall in somehow? Hard to picture.

We need an astrophysicist for this. But, when you think about it, you need mass to create gravity. The gravity then acts on the mass and the surrounding space and light. I think that as the density of the mass increases, gravity does as well and in this case there is a huge amount of energy taking place during the compression of the mass. In the extreme case this compressed mass can form a black hole and the potential energy of its gravity is actualized by the surrounding light/matter being effected by it.
 
  • #26
D H
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As far as I know gravity is a static force and represents only potential energy.
Changes in gravitational potential, however, manifest as kinetic energy; e.g., increased temperature. Gravitational potential energy led to the heating of the Earth in several ways.

1. Accretional heating. As the proto-Earth became ever larger, its gravity made incoming bolides hit ever harder. The proto-Earth's gravitational field gave those incoming bolides a boost in kinetic energy before they hit the proto-Earth in purely inelastic collisions. Energy is still conserved in such collisions. The kinetic energy of the incoming bolides was turned into heat.

2. Compressional heating. As proto-Earth grew in bulk it started to compress itself gravitationally. There is a considerable difference in self-induced gravitational potential energy between a planetesimal-sized loose collection of rocks and a smaller, but equally massive planetesimal-sized solid body. The resultant change in gravitational potential energy manifests itself in the form of heat.

3. Differentiation heating. The Earth is highly differentiated, with a dense iron core, a considerably less dense mantle, and an even less dense crust. This differentiation represents a *huge* change in gravitational potential energy, which once again manifests itself in the form of heat.
 
  • #27
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However when there is strong evidence of stable liquid water some 4.3 billion years ago or ~250 million years after start counting, then we may have to scratch our heads about how hot the protoplanet really was.
 
  • #28
D H
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A couple of points.

1. 250 million years is a long time.
Ignoring heating from below, a spherical shell of rock the size Earth will cool from an arbitrarily high temperature to 300 K in a mere 30,000 years. (Aside: This is what led Lord Kelvin to erroneously estimate the age of the Earth to be no more than 100 million years.) This Kelvin cooling time scale is orders of magnitude smaller than cited figure of ~250 million years. The Earth had plenty of time to begin forming a significant temperature gradient in that time span.


2. The Earth did not have to cool to ~300 K.
It didn't even have to cool to 373 K (boiling point at 1 atmosphere). The primordial atmosphere was much denser than today's atmosphere. Some posit pressures as high as 150 atm based on the amount of CO2 sequestered in limestone and other mineral deposits. The boiling point at 150 atm is 344 C, or 617 K. This reduces the Kelvin cooling time even further.
 
  • #29
baywax
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Changes in gravitational potential, however, manifest as kinetic energy; e.g., increased temperature. Gravitational potential energy led to the heating of the Earth in several ways.

1. Accretional heating. As the proto-Earth became ever larger, its gravity made incoming bolides hit ever harder. The proto-Earth's gravitational field gave those incoming bolides a boost in kinetic energy before they hit the proto-Earth in purely inelastic collisions. Energy is still conserved in such collisions. The kinetic energy of the incoming bolides was turned into heat.

2. Compressional heating. As proto-Earth grew in bulk it started to compress itself gravitationally. There is a considerable difference in self-induced gravitational potential energy between a planetesimal-sized loose collection of rocks and a smaller, but equally massive planetesimal-sized solid body. The resultant change in gravitational potential energy manifests itself in the form of heat.

3. Differentiation heating. The Earth is highly differentiated, with a dense iron core, a considerably less dense mantle, and an even less dense crust. This differentiation represents a *huge* change in gravitational potential energy, which once again manifests itself in the form of heat.

Thanks D H. This helps... does the difference between a loosely packed planetesimal sized group of rocks and the smaller, solid planetesimal sized body amount to differences in "density"?
 
  • #30
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Changes in gravitational potential, however, manifest as kinetic energy; e.g., increased temperature. Gravitational potential energy led to the heating of the Earth in several ways.

1. Accretional heating. As the proto-Earth became ever larger, its gravity made incoming bolides hit ever harder. The proto-Earth's gravitational field gave those incoming bolides a boost in kinetic energy before they hit the proto-Earth in purely inelastic collisions. Energy is still conserved in such collisions. The kinetic energy of the incoming bolides was turned into heat.

2. Compressional heating. As proto-Earth grew in bulk it started to compress itself gravitationally. There is a considerable difference in self-induced gravitational potential energy between a planetesimal-sized loose collection of rocks and a smaller, but equally massive planetesimal-sized solid body. The resultant change in gravitational potential energy manifests itself in the form of heat.

3. Differentiation heating. The Earth is highly differentiated, with a dense iron core, a considerably less dense mantle, and an even less dense crust. This differentiation represents a *huge* change in gravitational potential energy, which once again manifests itself in the form of heat.

Thanks! That's helpful, but it isn't obvious how much that effect still matters today. I'd guess the differentiation is still ongoing, so #3 is probably still in force, at least. How much heat does that contribute these days?
 
  • #31
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does "gravitational heating" mean pressure? Because pressure is a major cause of heat.

Pressure is not a cause of heat. When a gas is compressed, the total heat energy contained in the gas is squeezed down to a smaller volume. This heat goes up because the volume is smaller. The total energy is about the same as compressing it does add energy, but mostly is the smaller volume.

Put a chunk of iron in a vice and squeeze it as hard ad you can. Apply all the pressue you you can, it will not get hotter. (Not any hotter than you and I can mearusre with our figer tips.)
 
  • #32
D H
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Pressure is not a cause of heat. When a gas is compressed, the total heat energy contained in the gas is squeezed down to a smaller volume. This heat goes up because the volume is smaller.
You have a mistaken concept of heat. Objects do not contain heat. You also have misconception regarding pressure and temperature. Compressing a gas adiabatically (no heat flow) *does* cause the gas to get hotter. Diesel engines would not work if compressive heating did not exist.
 
  • #33
Xnn
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Objects above absolute zero, do contain internal heat energy.

Anyhow, up to 90% of the Earth core heat is from radioactive decay.
About 5 to 10% is from friction caused by gravity.
About 5 to 10% is residual left over from the original formation of earth.
A small amount is latent heat released when molten materials turn into solids.

As I understand it, tidal push and pulls are part of the 5 to 10% gravity friction portion.

Here is a link to an article on the subject: http://www.physorg.com/news62952904.html
 
  • #34
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The core of Jupiter is 24,000K (4 times that of the surface of the Sun).

Compression from gravity creates the heat.

You can also think of it in terms of stars. Stars need to reach about 10 million K in their cores (mainly caused by gravitational compression) before they can start to initiate fusion. The earliest stars had no radioactive elements. [Note there is some fission heat produced by hydrogen and helium isotope burning before fusion starts, but the process is inevitable without this].
 
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  • #35
D H
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Objects above absolute zero, do contain internal heat energy.
Objects above absolute zero contain internal energy. Calling this internal energy "internal heat energy" is a misnomer. This phraseology recalls the long-falsified phlogiston and caloric theories of heat. Objects do not contain "heat".

Heat transfer is a process variable rather than a state variable. Internal energy is a state variable. How that internal energy changes due to heat transfer and work depends on path taken through state space. That the change in energy is path dependent is why it is invalid to say that objects contain heat.
 

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