About half of the heat that the generates itself is due to radioactive decay, scientists have concluded. While a recent study published in Nature Geoscience by the Japan-based KamLAND collaboration (which runs an important radiation detector) has shed light on processes deep within the bowels of the planet, it still leaves open the question of what's generating the other half?

"One thing we can say with near certainty is that radioactive decay alone is not enough to account for Earth's heat energy," Stuart Freedman of the U.S. Department of Energy's Berkeley Lab said in a statement. "Whether the rest is primordial heat or comes from some other source is an unanswered question. This is what's called an inverse problem, where you have a lot of information but also a lot of complicated inputs and variables."

The earth continuously generates about 44 terawatts of heat, according to measurements from approximately 20,000 boreholes all over the world. It's this heat, and how it affects substances in the core and mantle, that leads to the gradual motion of continents and the creation of the earth's magnetic field. Exactly how this heat is generated has been difficult to measure, since the inner layers of the planet can't be probed directly.

Scientists have long believed that the heat is probably from two major sources: "primordial" heat that's left over from the earth's fiery formation, and radioactive decay. Now, thanks to the study, they can put an amount on the latter source, with radioactive decay accounting for about 23 terawatts. The models suggest that all of this radioactive heat comes from the crust and mantle, since the heart of the planet is thought to contain only substances that are friendly to the nickel-iron core.

The researchers based their conclusion on data that measured the number of antineutrinos emanating from the earth. When a substance like uranium decays, one of the byproducts is this nearly weightless particle, which can pass through the earth's crust like it was transparent. After filtering out any antineutrinos that came from other sources (like nuclear reactors), the scientists were able to conclude not only the amount of heat coming from radioactive decay within the earth, but also exactly which substances were responsible. Eight terawatts were from uranium, eight are from thorium, and four are from potassium. Another three are from decays of various isotopes of those elements.

Now that we know how much heat comes from radioactive decay, scientists are left with the task of accounting for the other half. Primordial heat is the main candidate, though there could be other sources still unknown. One thing's for sure: mole people aren't likely to be involved.