Earth's crust studied anew

Non-intrusive probe finds tiny particles that could give clues to quakes, volcanoes

Science

July 29, 2005|By Alex Raskin | By Alex Raskin,Los Angeles Times

Physicists at an underground observatory in Japan have for the first time detected and analyzed anti-neutrinos -- elusive particles 500,000 times smaller 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 generate 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," wrote University of Maryland geologist William G. McDonough in an editorial accompanying the report in yesterday's issue of the journal Nature.

An 85-member team led by physicists Giorgio Gratta at Stanford 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 anti-neutrino passes through it.

The detector is buried in a cavern under a mountain in Kamioka, Japan, to reduce "pollution" from nuclear reactors, which also can generate anti-neutrinos. Physicists were able to distinguish between the two tiny particles because

anti-neutrinos from nuclear reactors have a different energy spectrum than those from the Earth's interior.

The Kamland "results were not straightforward to obtain, and are not simple to interpret. Various 'pollutants' must be removed from the energy spectrum of the anti-neutrinos to achieve a pure signal," McDonough observed in his editorial. He noted that more than 50 percent of the total signal came from nearby nuclear power reactors.

"The detector is situated in the center of the largest Japanese island, Honshu, in a mine one kilometer below the summit of Mount Ikenoyama, to reduce the effects of cosmic rays formed from particles other than anti-neutrinos," McDonough wrote.

He said observations are expected to continue there and at a detector under the Gran Sasso mountain in central Italy, which begins operations next year. The method "will generate more data and provide greater sensitivity in testing the nature and sources" of the particles, he said.

"What may be most revolutionary is the alternative it provides to traditional probing methods, which simply bore down from the surface -- a very costly technique that can trigger earthquakes," said Stanford professor Norman Sleep, a geophysicist. The deepest borehole drilled to date is about 7.4 miles deep, reaching only to 1 / 500th of the Earth's radius.

For more than a century, seismologists and plate tectonics experts have had only one tool with which to peer into the center of the Earth -- the vibrations produced by earthquakes. The Kamland results promise to give them something more -- data 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 it discovered, "support earlier theories about the nature of matter in the Earth's crust, core and lower mantle."

The team concluded that about 16.2 million anti-neutrinos per square centimeter per second are streaming out of the interior. They calculated that radioactive decay producing that level of radiation is likely generating about 24 terawatts of heat continuously.

That is about the same amount of heat as is 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."

"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," Gratta said.

Later this year, particle physicists and Earth scientists will meet in Hawaii to discuss the research.

The Los Angeles Times is a Tribune Publishing newspaper. A Sun staff writer contributed to this article.

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