Why is the Earth's core still hot and will it ever freeze?

Why is the Earth's core still hot and will it ever freeze?

Our planet has existed for 4 billion 600 million years. That's a long time, and yet, for some reason, the Earth's core has not cooled down and continues to surprise with activity.

How long can a stone cool down? Even if it is pretty large - say, the size of a planet - millions, not to mention billions of years, should more than suffice for its complete cooling and solidification. Our intuition, supported by the invincible second law of thermodynamics, tells us this. We all know that every body gives up their heat to the environment, and every bonfire must go out at some point. Yet, against all common sense, deep beneath the surface of the Earth's crust, there seems to be an eternal heat.

Let's have a look at the very core of our planet. A nickel-iron sphere with a diameter of 7 thousand kilometers, which concentrates in itself almost 1/3 of the mass of the whole globe, remains constantly heated up to over 5.500 °C. After 4.6 billion years, our planet's interior still generates thick terawatts of energy and steams not much weaker than the surface of the Sun.

And so that there is no doubt, the heat from the mantle and the nucleus is leaking, if only in the process of convection. The molten matter under our feet indefatigably travels upwards, giving up a part of its temperature, then it densifies and again begins to descend to the centre. (This, however, does not apply to the inner core itself. Despite the enormous temperatures, sufficient to melt any metal without any problems, the crushing pressure keeps it as a solid). It would seem that typically such a process should have cooled our world long ago and led to its geological death.

Is it possible that planets just slowly lose the energy they gained in the process of their turbulent birth? It turns out yes. However, it would not be possible without the help of their independent power source in the form of nuclear reactions. To avoid misunderstandings: it is not about thermonuclear processes, that is, the fusion of atomic nuclei typical for the interiors of stars. The planets do not have enough mass (nor sufficient fuel, in our case) to provide the conditions necessary to sustain the fusion. However, we have admixtures of heavy radioactive isotopes at our disposal, which willingly undergo spontaneous decays, accompanied by the release of large portions of energy.

Curious readers may wonder from where we know about nuclear reactions happening entirely out of our sight. Indeed, this is quite unusual since a large part of current geological models has been built using neutrino detectors, or more precisely, electron antineutrinos. We often associate these tiny, all-penetrating particles with cosmic sources (e.g. solar neutrinos), but their emission accompanies many physical phenomena, like individual nuclear decays. In 2005, the team operating the Japanese KamLand detector started to catch these geoneutrinos, from which they made a thorough assessment of the phenomena occurring inside the Earth. According to the current model, nuclear decays generate up to 20 terawatts of energy, with about 40% of this coming from the decay of uranium-238, another 40% from the decay of thorium-232 and 20% from the decay of potassium-40.

Two more facts should be noted. First, our Earth's heat balance theories are not complete and still leave room for debate. Radioactivity is a powerful force, but it probably does not account for all the energy generated. Second, isotope decays occur in the mantle of our planet but not in the core. According to physicists and geologists, uranium, thorium, and potassium are practically absent from the Earth's inner core, so all the radiogenic heat must be generated somewhat higher up.

So what is the correct answer to the title question? It seems that the nucleus is burning with primordial heat, being a relic after the planet's birth. However, it has not cooled down since it remains wrapped in a thick layer of molten rocks, continually heated by nuclear decays. Thus, the mantle can be perceived here not even as a quilt but as an electric blanket with its power supply.

Does all this mean that the Earth will never freeze? Of course not, but the process of cooling its core is incredibly slow. Taking into account the rate of heat release and all the rest, it will take between 55 and 90 billion years for the core to solidify completely. Fortunately for us, because high temperatures and convective movements of trillions of tons of molten iron are a condition for the existence of the Earth's magnetosphere.

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