Best ever measurement of Earth's radioactivity.

Ghostly subatomic particles streaming from Earth's interior have enabled the most precise measurement yet of our planet's radioactivity.

These particles, called antineutrinos, suggest that about half of Earth's heat comes from the radioactive decay of uranium and thorium – and give clues to the location of geological stashes of these elements.

Heat is needed to drive the convection currents in Earth's outer core that create its magnetic field. But exactly how much of this heat comes from radioactive decay wasn't known until now.

In 2005, researchers from the international KamLAND collaboration used a detector buried in Japan to measure antineutrinos that are produced when elements decay, allowing a rough estimate.

Chemical window

Now they have enough data – 111 geological antineutrinos to be precise – to refine their measurement, suggesting that about 20 terawatts of heat come from radioactive decay. Earth's total heat production is about 40 terawatts.

The researchers also had enough neutrinos to confirm that some must be coming from places other than the crust, something that wasn't possible before. "The uncertainty is small enough that some contribution must be from the mantle," says Giorgio Gratta, a physicist at Stanford University in California who is part of the KamLAND collaboration.

The ability to determine the location of the radioactive elements could permit better models of the Earth's interior, says Gratta. Seismic waves tell us about elasticity of the crust and mantle: now we have a small window into their chemistry, which should allow their behaviour to be better modelled. The presence of radioactive elements in the mantle, for example, could affect its flow.

There's still some uncertainly in the new measurement, because detections of antineutrinos are so infrequent. Larger detectors would help improve the measurements and might even be used to monitor undeclared nuclear facilities from afar, says Gratta.