What spreads the sea floors and moves the continents? What melts iron in the outer core and enables the Earth’s magnetic field? Heat. Geologists have used temperature measurements from more than 20,000 boreholes around the world to estimate that some 44 terawatts (44 trillion watts) of heat continually flow from Earth’s interior into space. Where does it come from?
Radioactive decay of uranium, thorium, and potassium in Earth’s crust and mantle is a principal source, and in 2005 scientists in the KamLAND collaboration, based in Japan, first showed that there was a way to measure the contribution directly. The trick was to catch what Kamioka Liquid-scintillator Antineutrino Detector (KamLAND) dubbed geoneutrinos – more precisely, geo-antineutrinos – emitted when radioactive isotopes decay.
“As a detector of geoneutrinos, KamLAND has distinct advantages,” says Stuart Freedman of the U.S. Department of Energy “KamLAND was specifically designed to study antineutrinos. We are able to discriminate them from background noise and detect them with very high sensitivity.”
KamLAND scientists have now published new figures for heat energy from radioactive decay in the journal Nature Geoscience. Based on the improved sensitivity of the KamLAND detector, plus several years’ worth of additional data, the new estimate is not merely “consistent” with the predictions of accepted geophysical models but is precise enough to aid in refining those models.
One thing that’s at least 97-percent certain is that radioactive decay supplies only about half the Earth’s heat. Other sources – primordial heat left over from the planet’s formation, and possibly others as well – must account for the rest.