July 18, 2011 | 32
Nuclear fission powers the movement of Earth’s continents and crust, a consortium of physicists and other scientists is now reporting, confirming long-standing thinking on this topic. Using neutrino detectors in Japan and Italy—the Kamioka Liquid-Scintillator Antineutrino Detector (KamLAND) and the Borexino Detector—the scientists arrived at their conclusion by measuring the flow of the antithesis of these neutral particles as they emanate from our planet. Their results are detailed July 17 in Nature Geoscience. (Scientific American is part of the Nature Publishing Group.)
Neutrinos and antineutrinos, which travel through mass and space freely due to their lack of charge and other properties, are released by radioactive materials as they decay. And Earth is chock full of such radioactive elements—primarily uranium, thorium and potassium. Over the billions of years of Earth’s existence, the radioactive isotopes have been splitting, releasing energy as well as these antineutrinos—just like in a man-made nuclear reactor. That energy heats the surrounding rock and keeps the elemental forces of plate tectonics in motion. By measuring the antineutrino emissions, scientists can determine how much of Earth’s heat results from this radioactive decay.
How much heat? Roughly 20 terawatts of heat—or nearly twice as much energy as used by all of humanity at present—judging by the number of such antineutrino particles emanating from the planet, dubbed geoneutrinos by the scientists. Combined with the 4 terawatts from decaying potassium, it’s enough energy to move mountains, or at least cause the collisions that create them.
The precision of the new measurements made by the KamLAND team was made possible by an extended shutdown of the Kashiwazaki-Kariwa nuclear reactor in Japan, following an earthquake there back in 2007. Particles released by the nearby plant would otherwise mix with naturally released geoneutrinos and confuse measurements; the closure of the plant allowed the two to be distinguished. The detector hides from cosmic rays—broadly similar to the neutrinos and antineutrinos it is designed to register—under Mount Ikenoyama nearby. The detector itself is a 13-meter-diameter balloon of transparent film filled with a mix of special liquid hydrocarbons, itself suspended in a bath of mineral oil contained in a 18-meter-diameter stainless steel sphere, covered on the inside with detector tubes. All that to capture the telltale mark of some 90 geoneutrinos over the course of seven years of measurements.
The new measurements suggest radioactive decay provides more than half of Earth’s total heat, estimated at roughly 44 terawatts based on temperatures found at the bottom of deep boreholes into the planet’s crust. The rest is leftover from Earth’s formation or other causes yet unknown, according to the scientists involved. Some of that heat may have been trapped in Earth’s molten iron core since the planet’s formation, while the nuclear decay happens primarily in the crust and mantle. But with fission still pumping out so much heat, Earth is unlikely to cool—and thereby halt the collisions of continents—for hundreds of millions of years thanks to the long half-lives of some of these elements. And that means there’s a lot of geothermal energy—or natural nuclear energy—to be harvested.
Image: Courtesy of USGS