Using a telescope made of 1,000 tons of baby oil and benzene in a stainless steel tank, scientists have measured the total radioactivity of Earth for the first time, they are reporting today.
The telescope they used was designed to detect subatomic particles from nuclear reactors. The researchers simply pointed it downward, in effect, toward the center of Earth. Physicists and geologists said the measurement, which agrees roughly with geologists' calculations, was the start of a new era of being able to see inside Earth. Their findings, they said, would lead to a better understanding of what keeps the planet warm, volcanoes burbling, continents drifting, magnetic field churning - all things that contribute to enabling life.
Until now, scientists have had to rely on the reverberations from earthquakes to get a handle on what is going on down there.
The baby oil and benzene detector lies two-thirds of a mile below the Japanese island of Honshu in the Kamioka zinc mine. It recorded flashes caused by ghostly particles called neutrinos, which were produced by the radioactive decay of uranium and thorium deep in the heart of the Earth as they shot up through the ground and the detector.
According to the measurements of these "geoneutrinos," Earth's radioactivity generates about 19 billion kilowatts of heat, about half of the estimated 30 billion to 44 billion kilowatts that the planet produces. By comparison, all the world's nuclear power plants collectively generate about 1 billion kilowatts.
"In some sense this is the first real measurement of this quantity, so it's a very big deal," Giorgio Gratta, a physicist at Stanford University, wrote in an e-mail message.
Dr. Gratta and Atsuto Suzuki, from Tohoku University in Japan, led a team of 87 physicists from 4 nations who are reporting their results today in the journal Nature.
John Learned, a neutrino physicist at the University of Hawaii who was a member of the team, called the work "the start of something that should be much fun in the future."
In a commentary accompanying the Nature paper, William F. McDonough, a University of Maryland geologist who was not a member of the team, called the work "a landmark result," but added that the results "were not straightforward to obtain, and are not simple to interpret."
Neutrinos, which travel almost at the speed of light and can pass gracefully through miles of lead or the entire Earth without interacting with it, were first detected streaming from a nuclear reactor in 1956.
The detector, called Kamland - short for Kamioka Liquid-scintillator Anti-Neutrino Detector - was built in 1997 to detect antineutrinos, the antimatter opposites of neutrinos, emitted by Japanese nuclear reactors. Natural nuclear reactions, the decay of uranium and thorium in Earth, also make antineutrinos that Kamland can detect.
Scientists have estimated that about half of Earth's heat comes from radioactive decay, with the rest coming from other gravitational and chemical sources. But the proportions and the total energy outflow are uncertain.