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According to this, the Earth's core will be frozen 2.3 billion years from now.

Assuming humans still live on Earth, (a much smaller and inconceivably more technologically advanced population) how might we prevent this?

This is very broad, but not, in my humble opinion, too broad. What might we do to keep our magnetic field is a better question, and is, really, the underlying question in all of this, because we have no need for a molten core if we have another way to generate a magnetic field.

So, assuming humans live on Earth and want to maintain it as a monument as long as possible (we have likely spread through the stars by this point, assuming it is possible for us), how could we either maintain the magnetic field at its current strength or keep the core around to maintain it?

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  • $\begingroup$ Doesn't the sun become a red giant in 7 billion years that might solve your problem and pose a larger one?? $\endgroup$ – sdrawkcabdear Mar 14 '16 at 23:28
  • $\begingroup$ @sdrawkcabdear I think it is closer to 5 billion years. Our sun is about half way through its life cycle. Either way, 2.3 billion years from now != 5 or 7 billion, so Earth will likely still be around. $\endgroup$ – JDSweetBeat Mar 15 '16 at 13:59
  • $\begingroup$ @sdrawkcabdear I think it was originally my mistake; I originally miswrote the number as 23 million. $\endgroup$ – JDSweetBeat Mar 15 '16 at 16:56
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    $\begingroup$ Woah seriously? 2.3 billion seems way too low. It took 4 billion for the earth to go from molten surface to 10 km of crust. How is it only gonna take 2.3 billion years for the whole earth to freeze? $\endgroup$ – Ovi Mar 27 '17 at 1:34
  • $\begingroup$ I read with the slow expansion of the sun that within approx 1 billion years the earth temperature will reach an average of 120-130F. More and more sub atomic particles will be going through the earth and hopefully keeping the core molten. Surely with that much additional solar energy hitting the earth the core will remain molten, or at least not cold for much longer. Unless we humans use technology to drastically alter things the Earth will be UN-inhabitable longer before then. $\endgroup$ – cybernard Nov 28 '17 at 2:54
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sdrawkcabdear was spot on that we might have the problem solved if we wait long enough. The exact fate of Earth is up for debate, but it is generally accepted that the Sun will exit the main sequence in ~5-5.5 billion years and reach the tip of the red giant branch ~2 billion years beyond that (see Schroder & Smith (2008)). Its maximum radius will engulf much of the inner Solar System, possibly including Earth.

If the core can hold out for about 7.5 billion years, then the problem of reheating it will go away - as will the core, for that matter, along with the rest of Earth. Let us, however, assume that we see a best-case scenario, where the core cools off much sooner than this. We can then get back to the question at hand.

The Earth gets its internal heat from two places: The cooling core and radioactive decay of elements in the mantle (to a lesser extent). I would advise reading the answers to Why has Earth's core not become solid? for more information. Once the core finishes cooling to the level that a magnetic field is no longer produced, the residual heat in the core will be about exhausted. Radioactive decay will not help the magnetic field, though, as the elements are situated in the mantle and crust.

Obviously, reheating the core is going to be impossible. You would likely have to reform the Earth to do so, and that will cause many more problems than it will solve. The core will freeze. This then leaves us with only one other option to save the magnetic field: To create an external, artificial magnetic field. This answer notes that it is possible to induce a magnetosphere (not a magnetic field), as is the case with Venus and Mars (from the solar wind) and Titan (from Saturn). I see no reason why the solar wind would not induce a magnetosphere on Earth naturally.

That said, I think you’d be better off trying to replicate the effects of Saturn’s magnetic field “rubbing off” on Titan when the moon briefly passes through the planet’s magnetic field. Unfortunately, the effect only lasts for about three hours. However, perhaps a closer orbit involving more time spent inside the magnetosphere could increase this period. So, all you have to do to keep the magnetic field going is to bring Earth into an orbit around Saturn (or Jupiter, for that matter) and hope for the best. That said, perhaps you could merely copy the gas giants’s magnetic fields. All you have to do is build a massive artificial planet, create a core that will produce a magnetic field, and put Earth into orbit around it.

At this point, though, you’re probably better off just moving somewhere else, if you have the kind of technology to make this possible.

Anyway, in conclusion, you won’t be able to reheat the Earth’s core without essentially recreating the birth of the planet, but with a little a lot of luck, time, money, and energy, you can maybe induce a new magnetic field.

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  • $\begingroup$ The Earth loses heat at 47 TW. The Sun radiates the Earth with 89300 TW. It is easily possible to reheat the earth with just the energy from the sun, (with poor efficiency: since the surface of the sun is about the same temperature as the earth's core, it can't be done directly). But a far-future civilization should manage. We're actually getting fairly close to having enough energy even wo harnessing the sun: world energy production is almost 4 TW, about 8% of what is required. $\endgroup$ – lirtosiast Mar 15 '16 at 4:24
  • $\begingroup$ @lirtosiast Gathering the energy isn't the problem; using it to heat up the core is the problem. You can gather those 89300 TW, but you're not going to have an easy time transferring that heat to the core, especially as the temperature gradient helps heat move to the surface. $\endgroup$ – HDE 226868 Mar 15 '16 at 21:40
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One of the reasons the Earth's core is still "hot" stems from the fact that it contains significant quantities of heavy, radioactive isotopes (including Uranium, Thorium, Potassium, and others). The heat produced from this radioactive decay helps maintain the mantle in a hot liquid state.

Presumably, injecting additional quantities of radioisotopes would help active lifetime of the inner mantle, allowing it to remain liquid and sustain a magnetic field. Over geological timescales (such as what you're talking about) additional radioactive material could be gradually introduced at subduction zones, where its density relative to other crustal material would allow it to sink toward the inner mantle.

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Since much of the heat in the Earth's core is produced from the decay of radioactive elements, it may seem trivial to simply assume that you can inject more radioactive elements into the core to make up the deficit.

The real problem is that the Earth's core is in the centre of the Earth, and the static pressure will collapse any conceivable tunnel or bore hole long before you reached the core (probably before you even penetrate the crust and reach the mantle)

Getting energy into the core might be achieved in several ways:

Firstly, drop a slug of anti-matter condensed to neutronium density into the core. Neutronium will pass through most ordinary matter like it isn't there, and with some clever calculations, it can be launched in a trajectory which allows it to come to a stop at the centre of the Earth's core, where internal gravitational effects are essentially zero (gravity is cancelled out because all the matter attracting you surrounds you in a sphere. I have not been able to find the total heat energy contained in the Earth's core, but the Atomic Rocket's "Boom Table" suggests that the Earth has a heat flux of 4.4X10^13 J/sec. The upper limit is the binding energy of the Earth itself; exceed that amount and you have a glowing cloud of gravel rapidly expanding through the solar system (2.9X10^32J). (Boom table is found here: http://www.projectrho.com/public_html/rocket/usefultables.php

By controlling the amount of antimatter, the reaction will convert a small fraction of the core into energy, which will radiate outwards and melt the rest of the core. Since one milligram of antimatter + one gram of matter releases 1.8X10^11J of energy (about 4X more than a MOAB), you can see that this is a delicate balance.

The second semi plausible suggestion is an immensely powerful neutrino beam, as suggested in an article in New Scientist: https://www.newscientist.com/article/dn3734-neutrino-beam-could-neutralise-nuclear-bombs/.

The beam would not only deliver energy to knock nucleons from the densely packed atoms in the Earth's core, but also release a vast amount of alpha and neutron radiation as well. To maximize the effect. having three beams intersecting at right angles at the core would provide a "hot spot" capable of melting the core and releasing heat over a period of centuries. This is much slower than using antimatter, but probably a bit easier to calibrate. Since flinging chunks of antimatter through the solar system might be taken the wrong way by polities orbiting other planets, this might be more acceptable to whoever or whatever is inhabiting the solar system at this time.

Finally, in about one billion years, the increasing luminosity of the Sun will have baked away most of the Earth's oceans and atmosphere. If anything is "alive" on Earth at this time, it may be some form of machine intelligence or something incomprehensible to us, which isn't bothered by the lack of a magnetosphere, oceans or atmosphere anyway.

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  • $\begingroup$ People can survive 47 degrees Celsius with the advanced technology that we would presumably have by this point in time. Being exposed to this environment might not be pleasant, but definitely survivable (at least at the poles where it will be much cooler than that). Sending 'solar shades' into space would be trivial if we are still a technological species. The same type of shade used in terraforming a place like Venus could be used to keep Earth at a decent temperature. $\endgroup$ – JDSweetBeat Mar 15 '16 at 15:44
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I'll avoid trivial answers. I shall assume the Earth can be protected from the growing heat of the sun and the goal of this project is to keep the core of the earth warm. Also, this is not a science based tag so there will be no equations.

Let's begin:

The Magnetic Feild

The Earth's Magnetic Feild is thought to be produced by a Geodynmao and is the result of the interactions between convection in the core and Earth's rotation. The outer core is still liquid due to the primordial heat and radioactive decay (there is even evidence for a fission reactor in the core), while the inner core is solid due to the immense pressure.

Inner and Outer Core

As has been pointed out they are hot and hard to reach. You can read up on the specifics of the inner core here and outer core here. For the purposes of this project, there are only two things we need care about: the inner core needs to stay solid and the outer core needs to stay liquid.

Save the core, save the magnetic field

All we need to do is keep the outer core liquid. How? By delivering an enormous (by our standards at least) amount of energy to it. Firstly it could be done without remelting the whole of the earth. Humanity of the far future should be able to harness that kind of energy.

I don't see a chunk of neutronium as being able to work because neutron matter when not in a neutron star decays rapidly, plus there is no real good way to keep it together to the core. Making it out of antimatter won't help if it can't reach the core either.

A beam or beams of neutrinos could very well deliver that heat to the core but the overwhelming majority of the energy in the beam would pass through the earth and be wasted.

So how do we keep the core of the Earth molten is a relatively practical way? Here me out: An Ultra High Energy Neutron Beam (UHENB) look closely not neutrino. Now this beam of neutrons won't be able to penetrate all the way to the core of the earth so it will need to be installed deep within the earth. By this time the mantle will have cooled greatly, and to be fair it's not really even molten. There will need to be a shaft a couple thousand miles deep into the earth for this project. This is doable, firstly a cooler mantel means the digging will be easier. The harder part will be preventing the shaft from collapsing under the pressure. I won't hand wave and say some super strong material. It could be supported by many layers of walls nested together with the spaces between the layers pressurized. The same principal was employed in old vacuum chambers. There are plenty of materials that can tolerate the heat and throw in a giant cooling system for good measure.

Why a UHENB? Because the neutrons have a half-life of about ten minutes and when they decay you get a good amount of energy. Those neutrons will also release energy as their kinetic energy is dumped during collisions or breaking apart atoms they encounter which is a bonus because those decay products add more heat. Your run of the mill neutron beam has pretty good penetration, amp it up several orders of magnitude and the beam could penetrate hundreds or thousands of miles in the core. But here is the key, at such high energies the beam will initially pass completely through the matter in encounters only after slowing down will it really begin to heat up the material it is passing through.

The Set Up

A thousand mile long particle accelerator that's business end is deep in the earth, protected from the heat and pressure by pressurized nested layers of walls. It's powered by the sun since the sun radiates more than enough heat energy to warm the earth. The whole system would be built at the poles with the neutrons directed into the outer core on a path tangent to the inner core and pointed slightly in the direction of earth's rotation. There could even be several of them. So now you are dumping energy (heat) into the outer core, adding a little energy to its rotation and increasing the radioactivity (heat). The outer core stays molten and rotating, the inner core stays solid and all the energy in the beam is captured by the earth.

Edit:

Turns out neutrons don't actually behave quite the way I described. According to this there are better are better forms of particle radiation for the task. An Ultra High Energy Particle Accelerator is what you will need, and a beam of whatever particles will do the trick.

Also, should add this will take a long time and the beams of particles should be swept around to improve heating and reduce hot spots because super volcanos are bad.

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  • $\begingroup$ See edit before commenting on neutron radiation. Dammit, Jim, I'm an engineer, not a nuclear physicist. $\endgroup$ – Joe Kissling Mar 27 '17 at 5:20

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