Washington, July 18 (IANS) Some 44 trillion watts of heat continually flow from Earth's interior into space. Where does this come from?

One trillion is 1,000 billion. So 44 terawatts works out to 44,000 billion watts. And how did geologists come by the staggering figure? They relied on temperature measurements from more than 20,000 boreholes around the world.

Radioactive decay of uranium, thorium, and potassium in earth's crust and mantle is a principal source of this heat, reports the journal Nature Geoscience.

In 2005, scientists in the Japan-based KamLAND (Kamioka Liquid-scintillator Antineutrino Detector) collaboration first showed that there was a way to measure the contribution directly.

A neutrino, more similar to an electron, is an elementary particle that travels close to the speed of light, but unlike electrons, doesn't carry an electric charge, according to a statement by Berkeley Lab, which is a major contributor to KamLAND.

The trick was to catch what KamLAND dubbed geoneutrinos -- more precisely, geo-antineutrinos -- emitted when radioactive isotopes (same chemical element with different masses) decay.

"As a detector of geoneutrinos, KamLAND has distinct advantages," says Stuart Freedman, member of US Department of Energy's Berkeley Lab.

Freedman, also professor in physics at the University of California, Berkeley, said: "KamLAND was specifically designed to study antineutrinos. We are able to discriminate them from background noise and detect them with very high sensitivity."

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.

Antineutrinos are produced not only in the decay of uranium, thorium, and potassium isotopes but in a variety of others, including fission products in nuclear power reactors.