Around half the 44 trillion watts of heat that flows from the interior of the Earth is the remaining heat from the planet's creation.
y capturing what have been called geoneutrinos – more precisely, geo-antineutrinos – emitted when radioactive isotopes decay - scientists at the KamLAND collaboration in Japan have precisely calculated the amount of heat generated by radioactive decay.
They've found that radioactive decay supplies only about half the Earth's heat, with the rest coming from primordial heat left over from the planet's formation.
The current most popular model of the inner Earth, known as bulk silicate Earth (BSE) assumes that all the heat from radioactive decay comes from the crust and mantle – about eight terawatts from uranium 238, another eight from thorium 232, and four from potassium 40.
KamLAND detected 841 candidate antineutrino events between March 2002 and November 2009, of which about 730 were reactor events or other background. The rest, though, were from radioactive decays of uranium and thorium in the Earth.
These results were combined with data from the Borexino experiment at Gran Sasso in Italy to calculate the contribution of uranium and thorium to Earth's heat production.
The answer was about 20 terawatts; based on models, another three terawatts were estimated to come from other isotope decays. This is more heat energy than the most popular BSE model suggests, but still far less than Earth's total.
"One thing we can say with near certainty is that radioactive decay alone is not enough to account for Earth's heat energy," says Stuart Freedman of Kamland contributor Berkeley Lab. "Whether the rest is primordial heat or comes from some other source is an unanswered question."
One - rather speculative - idea is that there may be a natural nuclear reactor somewhere deep inside the Earth, where fissile elements have accumulated and initiated a sustained fission reaction.