Real Earth Cooking
What spreads the sea floors and moves the continents? What melts iron in the outer core and enables the Earth’s magnetic field? Heat. Geologists have used temperature measurements from more than 20,000 boreholes around the world to estimate that some 44 terawatts (44 trillion watts) of heat continually flow from Earth’s interior into space. Where does it come from? Initially the earth heated up using energy released buy gravitational collapse, and while this energy completely melted the planet, this heat would have all been lost by now as the Earth is 4.6 billion years old. However, the earth is still hot in its core as we can see from all the volcanic activity on our planet. The energy which keeps the core hot and the volcanoes active is produced by radioactive decay. Heavy, radioactive elements such as uranium sank to the Earth's core along with the Iron and Nickel early in Earth's history (when it was all molten) and these radioactive elements have been heating the core (rather like a nuclear power station) ever since.
In 2005 scientists in the KamLAND collaboration, based in Japan, first showed that there was a way to measure the heat contribution directly. The trick was to catch what KamLAND dubbed geoneutrinos — more precisely, geo-antineutrinos — emitted when radioactive isotopes decay.
Anti-neutrinos are made in the beta-decay of neutron-rich daughter fragments in the fission process. A neutrino meaning "small neutral one", is an elementary particle that usually travels close to the speed of light, is electrically neutral, and is able to pass through ordinary matter almost unaffected. This makes neutrinos extremely difficult to detect. Neutrinos have a very small, but nonzero mass. Neutrinos are similar to the more familiar electron, with one crucial difference: neutrinos do not carry electric charge.
KamLAND scientists have now published new figures for heat energy from radioactive decay in the journal Nature Geoscience. Based on the improved sensitivity of the KamLAND detector, plus several years’ worth of additional data, the new estimate is not merely consistent with the predictions of accepted geophysical models but is precise enough to aid in refining those models.
All models of the inner Earth depend on indirect evidence. Leading models of the kind known as bulk silicate Earth (BSE) assume that the mantle and crust contain only lithophiles (rock-loving elements) and the core contains only siderophiles (elements that like to be with iron). Thus all the heat from radioactive decay comes from the crust and mantle — about eight terawatts from uranium 238 (238U), another eight terawatts from thorium 232 (232Th), and four terawatts from potassium 40 (40K).
This is more heat energy than the most popular earth core model suggests, but still far less than the Earth’s total. Says Freedman (One of the authors), "One thing we can say with near certainty is that radioactive decay alone is not enough to account for Earth’s heat energy. Whether the rest is primordial heat or comes from some other source is an unanswered question." Other sources — primordial heat left over from the planet’s formation, and possibly others as well — must account for the rest.
Better models are likely to result when many more geoneutrino detectors are located in different places around the globe, including midocean islands where the crust is thin and local concentrations of radioactivity (not to mention nuclear reactors) are at a minimum.
The study is published in Nature Geoscience and is available in advanced online publication at http://www.nature.com/ngeo/journal/vaop/ncurrents/abs/ngeo1205.html.
For further information: http://newscenter.lbl.gov/news-releases/2011/07/17/kamland-geoneutrinos/