EARTH鈥橲 natural radioactivity has been measured for the first time. The measurement will help geologists find out to what extent nuclear decay is responsible for the immense quantity of heat generated by Earth.
Our planet鈥檚 heat output drives the convection currents that churn liquid iron in the outer core, giving rise to Earth鈥檚 magnetic field. Just where this heat comes from is a big question. Measurements of the temperature gradients across rocks in mines and boreholes have led geologists to estimate that the planet is internally generating between 30 and 44 terawatts of heat.
Some of this heat comes from the decay of radioactive elements. Based on studies of primitive meteorites known as carbonaceous chondrites, geologists have estimated Earth鈥檚 uranium and thorium content and calculated that about 19 terawatts can be attributed to radioactivity. But until now there has been nothing definitive about exactly how much uranium there is in the planet, says geologist Bill McDonough of the University of Maryland in College Park. 鈥淭here are fundamental uncertainties.鈥�
Advertisement
There is one way to lessen this uncertainty, and that is to look for antineutrinos. These particles are the antimatter equivalent of the uncharged, almost massless particles called neutrinos and are released when uranium and thorium decay to form lead. If antineutrinos are being created deep within the planet they should be detectable, because they can pass through almost all matter.
Now, the KamLAND antineutrino detector in Kamioka, Japan, has counted such antineutrinos. An international team of scientists analysed the data and found about 16.2 million antineutrinos per square centimetre per second streaming out from Earth鈥檚 core. They calculate that the nuclear reactions creating these particles could be generating as much as 60 terawatts, but are most likely putting out about 24 terawatts (Nature, vol 436, p 499). 鈥淲e have made the first measurements of the radioactivity of the whole of Earth,鈥� says John Learned, who heads the KamLAND group at the University of Hawaii in Manoa. The KamLAND group鈥檚 finding is like unwrapping a birthday present, says McDonough.
With time, as more antineutrinos are detected, KamLAND may be able to determine once and for all whether radioactivity is entirely responsible for heating Earth or whether other sources, such as the crystallisation of liquid iron and nickel in the outer core, also play a significant role. 鈥淸Detecting anti-neutrinos] is the way of the future in terms of hard numbers about the system,鈥� says McDonough.
鈥淭he number of antineutrinos show that nuclear reactions are probably generating about 24 terawatts of the Earth鈥檚 heat鈥�
Antineutrinos could also reveal the radioactive composition of the crust and mantle, which will give geologists clues as to when and how they formed. But to do that, they will have to be able to pin down exactly where the antineutrinos are coming from, and this will require a whole network of detectors. 鈥淲e are heading towards doing neutrino tomography of the whole Earth,鈥� says Learned. 鈥淭his is just the first step.鈥�
