Antineutrinos created inside our planet reveal how it stays warm.
An international team of researchers has used particles called antineutrinos that are produced in the bowels of the Earth to estimate how much of the core's heat comes from natural radioactivity.
This is the first time these elusive particles have been used to study geology. Previous work on neutrinos and their antimatter counterparts, antineutrinos, has been concerned with fundamental physics and astrophysics.
The results rely on measurements of antineutrinos made at the Kamioka liquid scintillator antineutrino detector (KamLAND), which is housed in a mine underneath a mountain in central Japan. They establish neutrino science as a way to look at the deep Earth. They may also help to solve some long-standing mysteries about how hot our planet is at its core, and how long it will take to cool.
“It's a revolution, but these are very difficult measurements. Giorgio Gratta , Stanford University, California”
"For the first time we can say that neutrinos have a practical interest for other fields of science," says Atsuto Suzuki, director of the Research Center for Neutrino Science at Tohoku University in Japan and a member of the team that carried out the work.
Fire down below
The 87-strong team, from Japan, the United States, China and France, say the results, reported in Nature1, confirm that about half of the heat inside the Earth comes from the decay of radioactive elements that produces antineutrinos. The rest comes from processes in the planet's iron-rich core and from heat left over from the Earth's fiery birth 4.5 billion years ago.
This heat plays a central role in the behaviour of the Earth. It causes convection (the slow churning of sluggish rock in the planet's mantle), which in turn drives the movements of tectonic plates at the Earth's surface. This drives everything from mountain formation to earthquakes and volcanism.
Much of the interior of the Earth is still a mystery to scientists. "Essentially, we only know the crust of our planet," says physicist Giorgio Gratta of Stanford University in California, a member of the KamLAND team. "We can drill holes a few kilometres deep, but beyond that, you simply don't have access."
Most of what is known comes from studying seismic tremors passing through the planet. These can reveal boundaries between different kinds of rock. But antineutrinos produced by radioactivity in the mantle can tell researchers something about the chemistry down below.
Antineutrinos have hardly any mass, and barely interact with other matter at all, so they tend to pass straight through the Earth. But they can be detected by KamLAND, where flashes of light are triggered by occasional collisions between antineutrinos and other particles in a 13-metre-wide tank filled with oil and benzene.
Despite counting antineutrinos for more than two years, the researchers haven't yet spotted enough of them to make a very accurate measurement of the heat produced by radioactivity. But they hope that this will improve as they gather more data and combine them with those obtained from a similar detector called Borexino in Italy, which is scheduled to begin operations in 2006.
Norm Sleep, a geophysicist at Stanford, says that better data could ultimately revolutionize some areas of geoscience. "I'd like to see a more precise result," he says, "but this shows it can be done."
With several antineutrino detectors in operation, Sleep adds, it should be possible to conduct a kind of chemical tomography of the Earth, measuring how the radioactive elements are distributed and therefore how uniform the mantle is.
"It's a revolution," Gratta agrees, "but these are very difficult measurements and the detectors are very expensive." He thinks that it will be 10 or 20 years before there are enough neutrino detectors to obtain firm answers to geological questions.
ArakiT., et al. Nature, 436. 499 - 503 (2005).
Stanford University, California