The earth is a heat engine. It remains geologically and biologically active, and evolves, because there are two great sources of energy. One source of energy is from the earth’s molten core (that drives the geology), and the second is from the sun (that drives life and the atmosphere.)However, considering the fact that the solar system began as a cloud of gas and dust that was near absolute zero we might wonder where the earth’s internal heat came from to drive the plate tectonics. The problem is more perplexing when we realize that virtually every other planetary body in the solar system (including some moons that are larger than some planets) is geologically dead (they have no internal heat of their own). Similarly, the earth is the only planetary body we know at present that is biologically alive too.
Earth may have formed more than 4.5 billion years ago, but it’s still cooling. A new study reveals that only about half of our planet’s internal heat stems from natural radioactivity. The rest is primordial heat left over from when Earth first coalesced from a hot ball of gas, dust, and other material.The new finding comes from experiments carried out deep inside a Japanese mountain. Itaru Shimizu, a particle physicist at Tohoku University in Sendai, Japan, and his colleagues used geoneutrinos—particles produced in a variety of ways, particularly during certain types of radioactive decay—to more directly estimate the amount of radiogenic heat produced inside Earth.
44 terawatts of heat!
We all know that the Earth runs on massive amounts of heat – enough to melt iron in the outer core, create magnetic field, spread the sea floors and move the continents. However, where all this heat comes from was a mystery until now. According to a new research, only about half of our planet’s internal heat stems from natural radioactivity. The rest is primordial heat left over from the planet’s formation, and possibly others. Geologists estimate that some 44 terawatts (44 trillion watts) of heat is constantly flowing out of the Earth’s interior into space.
Where does it come from?
Geologists relied on temperature measurements from more than 20,000 boreholes around the world. Radioactive decay of uranium, thorium, and potassium in Earth’s crust and mantle is a principal source, and in 2005 scientists in the KamLAND collaboration, based in Japan, first showed that there was a way to measure the contribution directly. Using the Kamioka Liquid-scintillator Antineutrino Detector (KamLAND) located under a mountain in Japan, they analysed geoneutrinos – emitted by decaying radioactive materials within the Earth.
The antineutrinos theory
KamLAND scientists have now published new figures for heat energy from radioactive decay. Based on the improved sensitivity of the KamLAND detector, plus several years’ worth of additional data, the new estimate is not merely “consistent” with the predictions of accepted geophysical models but is precise enough to aid in refining those models. Antineutrinos are produced not only in the decay of uranium, thorium, and potassium isotopes but in a variety of others, including fission products in nuclear power reactors.
All models of the inner Earth depend on indirect evidence. Leading models of the kind known as bulk silicate Earth (BSE) assume that the mantle and crust contain only lithophiles (“rock-loving” elements) and the core contains only siderophiles (elements that “like to be with iron”). Thus all the heat from radioactive decay comes from the crust and mantle – about eight terawatts from uranium 238 (238U), another eight terawatts from thorium 232 (232Th), and four terawatts from potassium 40 (40K).
Primordial or some other heat?
Additional factors that have to be taken into account include how the radioactive elements are distributed (uniformly or concentrated), variations due to radioactive elements in the local geology, antineutrinos from fission products, and how neutrinos oscillate as they travel through the crust and mantle. Alternate theories were also considered, including the speculative idea that there may be a natural nuclear reactor somewhere deep inside the Earth. This is more heat energy than the most popular BSE model suggests, but still far less than Earth’s total.
Geothermal gradient is the rate of increasing temperature with respect to increasing depth in the Earth’s interior. Away from tectonic plate boundaries, it is 25–30°C per km of depth in most of the world. Strictly speaking, geo-thermal necessarily refers to the Earth but the concept may be applied to other planets. The Earth’s internal heat comes from a combination of residual heat from planetary accretion (about 20%) and heat produced through radioactive decay (80%). The major heat-producing isotopes in the Earth are potassium-40, uranium-238, uranium-235, and thorium-232. At the center of the planet, the temperature may be up to 7,000 K and the pressure could reach 360 GPa. Because much of the heat is provided by radioactive decay, scientists believe that early in Earth history, before isotopes with short half-lives had been depleted, Earth’s heat production would have been much higher. This extra heat production, which was twice that of present-day at approximately 3 billion years ago, would have increased temperature gradients within the Earth, increasing the rates of mantle convection and plate tectonics, and allowing the production of igneous rocks such as komatiites that are not formed today.
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