A new experiment has quantified the fraction of the heat that stems from radioactive decay by analysis of anti-neutrinos originating in decays in the Earth.
Using the Kamioka Liquid-scintillator Antineutrino Detector (KamLAND) located under a mountain in Japan, they analyzed geoneutrinos — ones emitted by decaying radioactive materials within the Earth — over the course of more than seven years.
The specific amount of energy an antineutrino packs on the rare occasions one does collide with normal matter can tell scientists about what material emitted it in the first place — for instance, radioactive material from within the Earth, as opposed to in nuclear reactors. If one also knows how rarely such an antineutrino interacts with normal matter, one can then estimate how many antineutrinos are being emitted and how much energy they are carrying in total.
The researchers found the decay of radioactive isotopes uranium-238 and thorium-232 together contributed 20 trillion watts to the amount of heat Earth radiates into space, about six times as much power as the United States consumes. U.S. power consumption in 2005 averaged about 3.34 trillion watts.
A very important trick, I assume, is distinguishing these from the Sun's very copious flow of neutrinos produced in fusion.