Antineutrinos reveal a primordial source of Earth’s radiated heat

Every second, Earth expels 44 terajoules of energy into space. Much of that energy arises from the decay of radioactive isotopes, principally uranium-238 and thorium-232. The remainder flows from an energy reservoir created billions of years ago, as our planet formed and gravitational potential energy was converted into heat. (Subsequent radioactive decay also contributed to that reservoir.) Now the Kamioka Liquid-Scintillator Anti-Neutrino Detector (KamLAND) collaboration reports that 24 ± 9 TW of the radiated power has a radiogenic origin; the remaining heat flux—about half—is primordial. In deriving that conclusion, the researchers combined seven-plus years of their own observations of antineutrinos formed in Earth’s interior and measurements obtained from the Borexino experiment in Italy. KamLAND’s mineral-oil-filled detector (see the figure; the person at the top gives the scale) observed 841 antineutrino events at energies consistent with ²³⁸U or ²³²Th decay. The great majority of those were background events that were generated by reactions at nearby nuclear facilities and that yielded much of the collaboration’s statistical error. Ironically, the experiment benefited from a natural catastrophe: Background flux was significantly reduced beginning in July 2007, when the Kashiwazaki–Kariwa nuclear power station was completely shut down for 21 months following a magnitude 6.6 earthquake. (A. Gando et al., KamLAND collaboration, Nat. Geosci., in press, doi: 10.1038/ngeo1205.)—Steven K. Blau