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Detectors zero in on Earth’s heat: Geoneutrinos paint picture of deep-mantle processes.

A window on the deep Earth opened unexpectedly in 2011, when Japan’s nuclear reactors were shut down after the Fukushima disaster. Before the closure, an underground particle detector called KamLAND based in Kamioka, Japan, was monitoring a torrent of neutrinos streaming from dozens of nearby nuclear reactors, seeking clues to the nature of these hard-to-catch subatomic particles. After those plants fell silent, KamLAND scientists could see more clearly a signal that had largely been obscured: a faint trickle of neutrinos produced inside the planet.

Neutrinos are generated in stars, reactors, and deep in Earth’s crust and mantle by the radio­active decay of elements such as uranium and thorium. KamLAND reported the first tentative detections of these ‘geoneutrinos’ in 2005 (ref.1). But last month at a conference in Takayama, Japan, KamLAND scientists reported seeing them in meaningful quantities — as did a team at the Borexino neutrino detector at the Gran Sasso National Laboratory near L’Aquila, Italy.

These detections are not just curiosities. Geoneutrinos offer the only way to measure one of Earth’s internal heat sources. The total heat flow, measured with sensors in deep mines and amounting to 47 terawatts (TW) of power, drives everything from plate tectonics to Earth’s magnetic field. Some of it comes from the decay of radioactive elements, the rest is primordial heat left over from when Earth was formed by the violent collision of planetary building blocks.

But no one knows the proportions. Geologists assume that Earth contains the same amount of radioactive elements as certain primitive meteorites, but they aren’t sure. “We’re after trying to understand how Earth was built,” says William McDonough, a geologist at the University of Maryland in College Park.

Ultimately, geoneutrino researchers would like multiple detectors spaced around Earth, so that they could perform a sort of tomography on the mantle. That could help scientists to discern between models that favour the uranium and thorium being spread throughout the mantle, versus those in which the elements are concentrated near the core–mantle boundary. Such a difference could help to determine where and how long heat will continue to flow to drive geological processes such as plate tectonics — and how long it will take Earth to cool.

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