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July 28, 2005 Print   E-mail story   Most E-mailed
THE NATION
Earth Core's Particles Analyzed
  • Antineutrinos detected in a 'revolutionary' observatory provide data about the planet's mysterious interior.

  • Physicists at an underground observatory in Japan have for the first time detected and analyzed antineutrinos — elusive particles 500,000 times lighter than an electron — created by radioactive fires deep within the Earth.

    The observation sheds new light on the complicated processes that generate heat below the Earth's crust and cause the movement of tectonic plates.

    The findings are "a landmark result [that] … will allow better estimation of the abundances and distributions of radioactive elements in the Earth," University of Maryland geologist William G. McDonough wrote in an editorial accompanying the report in today's issue of the journal Nature.

    An 87-member team led by physicists Giorgio Gratta at Stanford University and Atsuto Suzuki at Japan's Tohoku University snared the tiny subatomic particles with the help of the Kamland observatory, which is essentially a giant vat of baby oil, benzene and fluorescent material that flashes when an antineutrino passes through it.

    The detector is buried in a cavern under a mountain in Kamioka, Japan, to reduce "pollution" from nuclear reactors, which can also generate antineutrinos. Physicists were able to determine which antineutrinos were from the Earth's interior because antineutrinos from nuclear reactors had a different energy spectrum.

    "What may be most revolutionary is the alternative [that the Kamland detector] provides to traditional probing methods, which simply bore down from the surface — a very costly technique that can trigger earthquakes," Stanford geophysics professor Norman Sleep said. The deepest borehole to date is about 7.6 miles deep, or 0.2% of the way to the Earth's center.

    For more than a century, seismologists and plate tectonics experts have had one main source of information about the center of the Earth — the vibrations produced by earthquakes. The Kamland results could tell them about the exact chemical nature of matter in the Earth's core, mantle and crust.

    The technique "promises to give geologists and seismologists better data to predict volcanoes, earthquakes and other volatile Earth dynamics," Sleep said. "The Kamland data could, for instance, help a scientist trying to predict how quickly a new arc of volcanic islands will rise and then cool in a convergent plate in the Pacific."

    Gratta said the Kamland results, particularly the ratios of thorium to uranium, supported "earlier theories about the nature of matter in the Earth's crust, core and lower mantle."

    The team concluded that about 16.2 million antineutrinos per square centimeter per second were streaming out of the interior. They calculated that radioactive decay producing that level of radiation was probably generating about 24 terawatts of heat continuously.

    That is about the amount of heat generated by chemical reactions and phase changes, such as crystallization of liquids, and heat left over from the Earth's formation.

    Gratta cautioned, however, against "reading too much into our simple chemical analyses."

    "It's a bit of a misconception to say we 'knew' anything about the inner Earth, since current theories are only wild guesses based on what little information we've been able to gather from lavas that erupted billions of years ago, lavas that erupted more recently and meteorites," Gratta said. "Essentially, antineutrinos reveal just some basic chemistry about the inner Earth. But still, when you know nothing, knowing a little bit can make a big difference."





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