Purple Monkey Dishwasher

Will the centre of the Earth ever cool? If so, how long will it take? If not, what's keeping it so fricken hot?

Corwin

Kind of a tough one....

What's keeping it hot? Pressure. All that rock weighs a LOT... the more you pressurize something, the more energy it picks up... and the hotter it gets. What gets me is where is all this energy coming from? I suppose it's gravity that's the root cause... but where does the energy for that come from? (Somehow I don't think anybody's going to have an answer... mostly because the answer to this question is sort of the holy grail of modern physics... grand unified theory and/or theory of quantum gravitation... whoever figures it out is going to get a Nobel...)

tronvillain

I have no idea what you are talking about Corwin.

Corwin

Pressure creates heat. The center of the earth? Has several billion tons of rock on top of it.

This produces a fair amount of pressure.... similar to the way that water pressure on the ocean floor is enough to crush a human being like an eggshell... well... rock is heavier and there's a lot more of it... and the center of the earth is where all that weight is centered on.

Corwin

Let me rephrase the beginning of that....

Pressure is kinetic energy. When it can't move, as in pressing against an immovable object, that energy has to go somewhere. If the immovable object is also unbreakable.... the energy begins to dissipate. Heat is sort of the 'lowest common denominator' of energy... if energy is going to degrade, it will usually do so in the form of heat. (Such as from electrical resistance or blocking light.)

Bill

When asked about why the center of the Earth is so hot, NASA has this to say <a href="http://helios.gsfc.nasa.gov/qa_earth.html#hot" target="_blank">HERE</a>: We should also not lose sight of the effects of the Earth's magnetic field, nor of the Earth's spin. Both of these contribute to "stirring the pot" (so to speak) deep down in the core (which is, after all, mostly liquid iron).

About 4.7 (or so) billion years ago, the Earth was nothing but a liquid metal ball spinning around the Sun. After a while, the spinning did what any good centrifuge does: it separated the components into layers. The iron stayed mostly in the core, and the crustal materials (mostly) rose up to the surface. It was actually the lighter materials which rose to the top because the centrifuge effect had less strength than the force of gravity. So, while the spin did stir things up, we still ended up with the bulk of the heaviest parts in the core and the bulk of the lightest parts on the surface. This is really lucky for us humans because nobody knows how to get life out of "heavy metal."

Anyway, the core of the Earth is gradually cooling, and is probably still mostly liquid (with a solidified middle), as opposed to the core of the Moon, which has probably cooled to the point of being solid all the way through (as per NASA's observations, above).

Here are some more articles for you to read:

• <a href="http://volcano.und.nodak.edu/vwdocs/vwlessons/plate_tectonics/part1.html" target="_blank">Introduction to Plate Tectonics</a>
• <a href="http://www.sigmaxi.org/Amsci/articles/95articles/Wysession-full.html" target="_blank">The Inner Workings of the Earth</a>
• <a href="http://www.maths.gla.ac.uk/~drf/compconv.htm" target="_blank">Compositional Convection in the Earth's Core</a>
• <a href="http://spike.geophys.washington.edu/SEDI/dialog/dial10.htm#fifth" target="_blank">Studies of the Earth's Deep Interior : Phases and Properties of Iron and the Core</a>
• <a href="http://phoenix.liu.edu/~divenere/gly_511/interior.htm" target="_blank">Seismic Structure of Earth’s Interior</a>, which has some interesting answers to the question of "How do we know" some of these facts.

== Bill

Bill

The center of the Earth is continuously cooling. I suppose that you have some target temperature in mind? If so, you need to specify that in your question. At present, the center of the core has cooled to the point where it is solid, but that center is still surrounded by a liquified zone.

Given that the Earth is almost as old as the Sun, and that we anticipate that the Sun will "flame out" (become a red giant) in about 5 billion years, it could easily happen that the core of the Earth might still be liquified to some degree when the Earth is burnt to a crisp by the expanding Sun. But all of this is clearly speculative. It would seem that it will take billions of additional years for the core of the Earth to become completely solid.

Of course, the NASA article answered your last question: radiation keeps the core heat up, along with (probably) several other factors discussed above.

== Bill

Baloo

(quick physics lecture)
Pressure does not create heat. Pressure is simply a measure of how much force is "pressing" against an area, and is most commonly measured in pounds per square inch (atleast in the good ole US of A). I weigh 240 pounds. If I stand on a 10"x10" rectangular board, the overall pressure I exert is 2.4 pounds per square inch. Similarly, hundreds of miles of atmosphere, pulled by the force of gravity, presses the air against the ground at about 15 pounds per square inch. Only changes in pressure can create heat, and the amount of heat created is equal to the amount of work necessary to change the pressure (both heat and work are measures of energy, which is conserved). Since the mass of the earth is not significantly changing, it's graviational pull is not significantly changing, and therefore the pressure due to the gravitational pull of the earth is not changing. IOW, whatever is going on at the earth's core, if it is producing heat (i.e., it is not something that was very hot and is slowly cooling off), it is not producing it due to pressure.

Why is the core actually hot? The leading theories seem to be:
1) The core was hot from the time of formation, but is well insulated, and therefore has been cooling very slowly since, reeleasing heat to the surface in the process.
2) Gravity and existing heat are sufficient to drive a scaled-down nuclear reactor (scaled down from what most stars are capable of, at least).
3) A combination of 1 and 2.

Synaesthesia

Ah, I thought there was something a little strange about the notion that pressure created heat. If that were the case, it seems a violation of the 2nd law of thermodynamics would be possible. (Since heat would be radiated as long as there are massive bodies in the universe!)

KeithHarwood

Yup.
Nope, not by many orders of magnitude. Even Jupiter isn't within one order of magnitude of doing this.

However, there is some nuclear heating. Small quantities of radioactive elements are distributed thoughout the rock the Earth is made of. When an atom decays it released a small amount of energy. That energy can't go anywhere fast so it stays around and heats up the rock.

So, you have three sources of heat, the original gravitational infall, gravitational energy from fractionating (heavy materials moving inwards and light materials moving outwards) and radioactive decay; the whole topped off by a couple of thousand miles of insulation that pretty well ensures that the heat stays there.

Yes, the Earth will cool down, after all the radioactives have decayed and all that heat has finally leaked out. But it's going to take a lloonngg time. So, yes, you can start that long novel you were going to read and maybe have time for War and Peace afterwards.