Discuss how radioactivity is heating the earth. Be sure to include the radioacti

Question

Discuss how radioactivity is heating the earth. Be sure to include the radioactive isotopes that are the largest contributors to the heat produced in the interior of the earth.

“What Keeps the Earth Cooking?” (July 11, 2011) http://earthsky.org/earth/what-keeps-the-earth-cooking

Geologists have used temperature measurements from more than 20,000 boreholes around the world to estimate that some 44 trillion watts (44 terawatts) of heat continually flows from Earth’s interior into space.

Where does that heat come from? While primordial heat left over from the planet’s formation accounts for some heat, the radioactive decay of uranium, thorium, and potassium in Earth’s crust and mantle likely provides half of Earth’s heat, according to scientists participating in the KamLAND (Kamioka Liquid-scintillator Antineutrino Detector) Collaboration. Their new measurements appear in the July 17, 2011 online issue of Nature Geoscience.

A main source of the 44 trillion watts of heat that flows from the interior of the Earth is the decay of radioactive isotopes in the mantle and crust. Scientists using the KamLAND neutrino detector in Japan have measured how much heat is generated this way by capturing geoneutrinos released during radioactive decay. Image Credit: Lawrence Berkeley National Laboratory

In 2005, scientists based in Japan first showed there was a way to measure the radioactive decay in Earth’s crust and mantle by catching geoneutrinos (geologically produced antineutrinos) – more precisely, geoantineutrinos – emitted when radioactive isotopes decay.

Antineutrinos (the antiparticles to neutrinos) are produced not only in the decay of uranium, thorium, and potassium isotopes but in the fission products of nuclear power reactors. In fact, reactor-produced antineutrinos were the first neutrinos directly detected (neutrinos and antineutrinos are distinguished from each other by the interactions in which they appear).

Neutrinos stream through the Earth as if it were transparent. This makes them hard to spot, but on the very rare occasions when an antineutrino collides with a proton inside the KamLAND detector – a sphere filled with a thousand metric tons of scintillating mineral oil – it produces an unmistakable double signal.

Being surrounded by nuclear reactors in Japan means KamLAND’s background events from reactor antineutrinos must also be accounted for in identifying geoneutrino events. This is done by identifying the nuclear-plant antineutrinos by their characteristic energies and other factors, such as their varying rates of production versus the steady arrival of geoneutrinos. Reactor antineutrinos are calculated and subtracted from the total. What’s left are the geoneutrinos.

The KamLAND anti-neutrino detector is a vessel filled with scintillating mineral oil and lined with photomultiplier tubes (inset), the largest scintillation detector ever constructed, buried deep underground near Toyama, Japan. Image Credit: KamLAND Collaboration

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 (elements that “love rocks”) and the core contains only siderophiles (elements that “like to be with iron”). Thus all the heat from radioactive decay in these types of models would come 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).

KamLAND detector schematic. Via Wikipedia

KamLAND detected 841 candidate antineutrino events between March of 2002 and November of 2009, of which about 730 were reactor events or other background. The rest, about 111, were from radioactive decay of uranium and thorium in the Earth. Using data from another experiment in Italy, the researchers were able to calculate the contribution of uranium and thorium to Earth’s heat production. The answer was about 20 terawatts, plus an estimated three terawatts coming from other isotope decays.

This is more heat energy than the most popular BSE model suggests, but still far less than Earth’s total. Stuart Freedman of the U.S. Department of Energy’s Lawrence Berkeley National Laboratory, explained:

One thing we can say with near certainty is that radioactive decay alone is not enough to account for Earth’s heat energy. Whether the rest is primordial heat or comes from some other source is an unanswered question.

Better models are likely to result when many more geoneutrino detectors are located in different places around the globe, including mid-ocean islands where the crust is thin and local concentrations of radioactivity (not to mention nuclear reactors) are at a minimum.

Freeman said:

This is what’s called an inverse problem, where you have a lot of information but also a lot of complicated inputs and variables. Sorting those out to arrive at the best explanation among many requires multiple sources of data.

Bottom line: The radioactive decay of uranium, thorium and potassium in Earth’s crust and mantle likely provides half of Earth’s heat, according to scientists participating in the KamLAND (Kamioka Liquid-scintillator Antineutrino Detector) Collaboration. Their new measurements appear in the July 17, 2011 online issue of Nature Geoscience.

Explanation / Answer

The decay radioactive isotopes in earth’s crust and mantle heating the earth.

The radioactive isotopes 238U and 232Th, and 40K are the main contributors to heat earth. 238U is producing 8.0 TW, 232Th is producing about 8.3 TW and 40K is producing around 3.0 TW heat. The total radioactive source from these three isotopes is around 19.3 TW.

The heat measurements from >20,000 boreholes estimated about 44 trillion watts (TW) of continuously flowing from earth interior to space.

But from radioactive isotopes decay of Uranium, Thorium and Potassium contributing around 20 TW heat flow from earth.

The remaing heat flow of 24 TW (44-20=24 TW) may be contributed by other radioactive isotopes available in the earth’s crust and mantle.

Though core of the earth is of iron magma but its may also contributing in heating the earth.

The radioactive materials in volcanic lave may also contributing in heating the earth.

Similarly pressure and temperature involved in plate tectonic movements, orogeny movements may also contributing in heating the earth. The heat generated during continuous period of foldings and faultings may be one of the reason in contributing heating of the earth.

If we put all these activities together it may come to 44TW heat that escaping to the space from earth.

But studies like Kam LAND should be carried out all over the world to prove accurately the contribution from each and conclude final figure of 44 TW heat

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