# Will the center of the earth ever cool?

If so, how long will it take? If not, why not?

Moderator’s Note: These questions should have answers. We’ll let them take a shot at it over in General Questions.

It will take a very long time though.

1. There is always heat being added to the core by radioactives decaying… But someday, this will run out.
2. It takes along time for all that heat to radiate away.
3. Of course there is the minor issue of the sun slowing this down.

But, eventually, given enough time, (Billions of years) it should happen.

Before the earth’s core cools, the sun, in it’s death throes, will swell and engulf the inner planets, burning them to a crisp.

I don’t believe that’s true. IIRC The Earths core is molten because of the tremundous pressure acting on it from the Earths own gravity. It’s like the old PV=nRT equation from physics and why ice melts when you apply pressure.

msmith537: The question was, “will the core cool”. Even if it did stay liquid under pressure, that would not prevent it from cooling. Regardless, I don’t believe the earth’s iron core would stay liquid without the heat.

Slip Mahoney: eek… this might be a problem. I believe you are right. The question I am not sure about now is, will the expanded outer layers of the sun put enough drag on the earth for its orbit to decay into the dying sun or not. If it stays in orbit after the sun dies, it will still cool off eventually.

Ice melts under pressure because water is a liquid at that pressure (that you apply) and temp. the act of pressuring the ice will cause some thermal energy to be imparted but once the ice/water reaches a steady pressure the temp will return to normal. Gravity can heat a body only if the gravity is causing some changing deformation (i.e. Jupiter and Europa).

The earths main source of heat is radioactive decay and that will take a very long time to ‘burn out’. the earth will most likely be burnt up, thrown out of orbit and freeze or dry out long before the core cools.

Just to clear up this quote.

Actually, the Earth’s Iron core is a solid because of the pressure caused by gravity. The Earth’s Mantle is what is liquid.

Sometimes everyday understandings of how physical laws work don’t equate to situations outside everyday experience. The pressures at or near the core are so great they overcome the kenetic energy of the heat energy. This doesn’t mean the core isn’t “hot” it just means the movement caused by the heat is confined within a solid matrix instead of a chaotic liquid. The Earth’s core is a solid crystal structure. A similar situation occurs on a vastly larger scale when a star becomes a nuetron star or black hole. However, the gravity is so strong at the Star’s core the force even overcomes the Strong Nuclear force, which is why we call them nuetron stars. In the case of the Black Hole, not even space/time can withstand the force of gravity focussed so narrowly and we get the infamous singularity.

And PV=nRT should only be used for gasses. Even then it is the “Ideal Gas Law” and there isn’t ever a real “Ideal” situation.

I believe Slip is right. The sun will destroy it before that happens.

However, if the sun (or ALL suns?) just “goes out” leaving leaving Earth hurtling through a void, I’m not sure.

I could can see where heat would be produced by gravitational pressure, BUT: Wouldn’t a steady-state earth producing heat through eternity violate the 2nd(?) law of thermodynamics? Wouldn’t this be essentially the same as a perpetual motion machine?

Ever is the vague part of the question. But if it does, who will be there to measure it? Will they care.

Actually, that explanation isn’t complete. Ice is less dense than water, which means that putting pressure on ice will make it go to the denser state of water. Even in a steady pressure situation, it will remain water, not because the pressure raised the temperature, but because it lowered the melting point. Rock is more dense as a solid, so higher pressure without higher temperature will tend to make it solid.

Yes and yes. And having something under high pressure does not make it hot. It is the process of going from low pressure to high pressure that increases the temperature. Once it reaches the high pressure no more work is being done and no more heat is being produced, so it starts to cool.

-----This is the same for the reverse process too, huh? Such as the case of volcanoes and lava, where the melted rocks flow as a liquid up the volcanoe as a direct result of the decreased pressure being applied.

Anyways…nothing lasts forever and that is why the earth’s core will cool at some time. Earth has two sources of energy, the core and the sun, and both will die someday.

Yes it would. If you take some substance and compress it, it will heat up. But keeping it compress does not keep it hot. In case of ideal gas, pv=nRT means that for a given pressure, you can have dense low-temperature gas or low-density but high temperature gas. (density = number of molecules per unit volume = n/v, so p=dRT.) Of course molten rock is not ideal gas but the principle is still the same - compress it and it gets hot and dense. Leave it like that and it will get cooler and even more dense.

Some things get very dense when you do this. Say heat production in a massive star stops because it runs out of fuel. The core of the star now has to support the same weight at lower temperatures, so it contracts and becomes more dense. This is how you end up with neutron stars and black holes, roughly speaking.

As I recall, the core of Mars has cooled and is now solid, which is why Mars doesn’t have a magnetic field.

According to a recent NASA press release, the Moon’s core is still somewhat mushy. Since Mars is bigger than the Moon, it’s likely that Mars is at least as mushy.

At least, that’s the expectation based purely on the size of the planets. Another factor is how much tidal flexing a body gets. Io, a moon of Jupiter, is smaller than the Moon, yet gets so much tidal flexing from the other Gallilean moons that it is the most volcanically active body in the Solar System. Since the Moon is pulled by both the Sun and the Earth, it probably gets some additional heating from that flexing. So it’s possible that its core is still mushy while Mars’ is not.

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Remember that current theory is that the moon was part of the earth and broken off when a object hit the earth. So the moon is younger then mars and made of lighter material (upper earth stuff) instead of the heavier stuff that would form in hte planets

I agree with this assessment. We’re about halfway through the life of the solar system, and the crust has only cooled to 30-50 miles. It might get another 50 miles thick by the time that the sun dies and engulfs the inner solar system.

Two questions:

1. How do we know that the earth has an iron core?

2. Is that core perfectly circular?

If the earth’s core simply started out with its current supply of heat, it would cool over a period of a few tens of millions of years. At the turn of the last century, this is one of the reasons why the physicists thought that the earth could only be about 50 million years old – that’s how long it would take to cool from a ball of molten rock to its present level of internal heat.

As we now know (and as several people already mentioned), the earth’s internal heat is replenished by radioactive decay. Since the isotopes that provide the energy are long-lasting (the main contributors are thorium-232 with a half-life of 14 billion years and uranium=238 with a half-life of 4.5 billion years), the heat supply will diminish only a little over the life of the solar system. However, if the earth is not incinerated by the sun’s red giant phase, the radioisotopes will eventually run out and the core will grow cold.