Heat generated by the decay of radiogenic isotopes inside the planet contributes about half of total flux of heat from Earth into space, reports a paper published online this week in Nature Geoscience. The findings imply that the supply of residual primordial heat, left over after Earth formed, has not yet been exhausted.

Itaru Shimizu and colleagues, as part of the KamLAND Collaboration, measured the flux of geoneutrinos — electrically neutral particles that are emitted during radioactive decay of isotopes and can pass through the Earth virtually unaffected — using a detector in Japan. The researchers find that the radioactive decay of the elements uranium-238 and thorium-232 together contribute about 20 terawatts to the Earth’s outgoing heat flux. This value represents only about half of the total outgoing heat, implying that the remainder of the heat flux results from continued cooling of the Earth since its birth.