July 18, 2011

Half Of Earth’s Internal Heat Comes From Radioactive Decay

The radioactive decay of uranium, thorium, and potassium contained in the Earth's crust provides nearly half of the planet's internal heat, claim the authors of a new study that has been published in Nature Geoscience and currently available at the journal's website.

"The Earth has cooled since its formation, yet the decay of radiogenic isotopes...in the planet's interior provides a continuing heat source," the international team of American, Japanese, and Dutch researchers behind the project wrote, adding that the current total heat flux from the Earth to space is approximately 44.2 terawatts "but the relative contributions from residual primordial heat and radiogenic decay remain uncertain."

"However, radiogenic decay can be estimated from the flux of geoneutrinos, electrically neutral particles that are emitted during radioactive decay and can pass through the Earth virtually unaffected," they continued.

The scientists collected data from antineutrino detectors in Japan and Italy, and through their observations, they were able to determine "that heat from radioactive decay contributes about half of Earth's total heat flux."

Those findings lead the experts to determine that the planet's primordial heat supply "has not yet been exhausted," thought as David Stevenson, a planetary physicist at the California Institute of Technology, told Science Now reporter Sid Perkins, that will eventually change.

According to Perkins, Stevenson believes that the planet's internal radioactivity and primordial heat levels are both expected to diminish in the years to come. In fact, the Science Now writer reports that the Earth is currently cooling at a rate of approximately 100 degrees Celsius each 1 billion years.

In their research, the international team--which was spearheaded by Itaru Shimizu, a particle physicist at Tohoku University in Sendai, Japan, and included contributors from Tokyo University, the University of California at Berkley, Stanford University, the University of Alabama, the University of Wisconsin, the University of Tennessee, Drexel University, and Colorado State University, among others--used sensors to study neutron release inside Japan's Mount Ikenoyama from March 2002 through November 2009.

"About 485 of those neutrinos were produced by nuclear power plants and other reactors and by nuclear waste, the team estimates. Another 245 were probably generated by sources such as cosmic rays striking gas molecules in the atmosphere. So only 111 of the neutrinos were associated with natural radioactivity within Earth, the researchers report online today in Nature Geoscience. Using a different analytical technique, they trimmed that tally to 106," Perkins said.

"Despite the small number, the team estimates that about 4.3 million of the particles generated by the radioactive decay of uranium-238 and thorium-232 pass through each square centimeter of Earth's surface each second," he said, adding that team estimates that radiogenic heat "accounts for about 54% of the heat flowing up through Earth's surface."

That heat, according to a July 17 Department of Energy/Lawrence Berkeley National Laboratory press release, is responsible for spreading the sea floor, moving continents, melting iron in the planet's outer core, and enabling the planet's magnetic field. That press release also noted that geologists recorded temperatures at over 20,000 locations to determine how much flows into outer space at one time.


Image 1: 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. Credit: Lawrence Berkeley National Laboratory

Image 2: 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. Credit: KamLAND Collaboration


On the Net: