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Geology from particle physics

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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.

[T]he 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. The heat continuously generated by all that radioactivity is about 20 terawatts, Shimizu says. Previous studies suggest that the radioactive decay of potassium-40, which can’t be measured by the Japanese sensors, provides another 4 terawatts. Altogether, the team estimates, this radiogenic heat accounts for about 54% of the heat flowing up through Earth’s surface.

Previous estimates of radiogenic heat are roughly the same as the new figure. But they were based on inferences of Earth’s chemical composition derived from analyses of meteorites, which presumably represent the overall proportions of elements in the cloud of dust and gas from which the solar system coalesced. So the team’s new estimate of Earth’s radiogenic heat is a significant result, says David Stevenson, a planetary physicist at the California Institute of Technology in Pasadena. “It’s nice to see this [estimate] emerging from an actual measurement.”

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Written by Arhopala Bazaloides

July 18, 2011 at 1:49 pm

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