Skip to comments.What keeps the Earth cooking?
Posted on 07/17/2011 3:00:13 PM PDT by decimon
Berkeley Lab scientists join their KamLAND colleagues to measure the radioactive sources of Earth's heat flow
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?
Radioactive decay of uranium, thorium, and potassium in Earth's crust and mantle is a principal source, and in 2005 scientists in the KamLAND collaboration, based in Japan, first showed that there was a way to measure the contribution directly. The trick was to catch what KamLAND dubbed geoneutrinos more precisely, geo-antineutrinos emitted when radioactive isotopes decay. (KamLAND stands for Kamioka Liquid-scintillator Antineutrino Detector.)
"As a detector of geoneutrinos, KamLAND has distinct advantages," says Stuart Freedman of the U.S. Department of Energy's Lawrence Berkeley National Laboratory (Berkeley Lab), which is a major contributor to KamLAND. Freedman, a member of Berkeley Lab's Nuclear Science Division and a professor in the Department of Physics at the University of California at Berkeley, leads U.S. participation. "KamLAND was specifically designed to study antineutrinos. We are able to discriminate them from background noise and detect them with very high sensitivity."
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.
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.
Hunting for neutrinos from deep in the Earth
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. In fact, reactor-produced antineutrinos were the first neutrinos to be directly detected (neutrinos and antineutrinos are distinguished from each other by the interactions in which they appear).
Because neutrinos interact only by way of the weak force and gravity, insignificant except on the scale of the cosmos they stream through the Earth as if it were transparent. This makes them hard to spot, but on the very rare occasions when an antineutrino collides with a proton inside the KamLAND detector a sphere filled with a thousand metric tons of scintillating mineral oil it produces an unmistakable double signal.
The first signal comes when the antineutrino converts the proton to a neutron plus a positron (an anti-electron), which quickly annihilates when it hits an ordinary electron a process called inverse beta decay. The faint flash of light from the ionizing positron and the annihilation process is picked up by the more than 1,800 photomultiplier tubes within the KamLAND vessel. A couple of hundred millionths of a second later the neutron from the decay is captured by a proton in the hydrogen-rich fluid and emits a gamma ray, the second signal. This "delayed coincidence" allows antineutrino interactions to be distinguished from background events such as hits from cosmic rays penetrating the kilometer of rock that overlies the detector.
Says Freedman, "It's like looking for a spy in a crowd of people on the street. You can't pick out one spy, but if there's a second spy following the first one around, the signal is still small but it's easy to spot."
KamLAND was originally designed to detect antineutrinos from more than 50 reactors in Japan, some close and some far away, in order to study the phenomenon of neutrino oscillation. Reactors produce electron neutrinos, but as they travel they oscillate into muon neutrinos and tau neutrinos; the three "flavors" are associated with the electron and its heavier cousins.
Being surrounded by nuclear reactors means KamLAND's background events from reactor antineutrinos must also be accounted for in identifying geoneutrino events. This is done by identifying the nuclear-plant antineutrinos by their characteristic energies and other factors, such as their varying rates of production versus the steady arrival of geoneutrinos. Reactor antineutrinos are calculated and subtracted from the total. What's left are the geoneutrinos.
Tracking the heat
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).
KamLAND's double-coincidence detection method is insensitive to the low-energy part of the geoneutrino signal from 238U and 232Th and completely insensitive to 40K antineutrinos. Other kinds of radioactive decay are also missed by the detector, but compared to uranium, thorium, and potassium are negligible contributors to Earth's heat.
Additional factors that have to be taken into account include how the radioactive elements are distributed (whether uniformly or concentrated in a "sunken layer" at the core-mantle boundary), variations due to radioactive elements in the local geology (in KamLAND's case, less than 10 percent of the expected flux), antineutrinos from fission products, and how neutrinos oscillate as they travel through the crust and mantle. Alternate theories were also considered, including the speculative idea that there may be a natural nuclear reactor somewhere deep inside the Earth, where fissile elements have accumulated and initiated a sustained fission reaction.
KamLAND detected 841 candidate antineutrino events between March of 2002 and November of 2009, of which about 730 were reactor events or other background. The rest, about 111, were from radioactive decays of uranium and thorium in the Earth. These results were combined with data from the Borexino experiment at Gran Sasso in Italy to calculate the contribution of uranium and thorium to Earth's heat production. The answer was about 20 terawatts; based on models, another three terawatts were estimated to come from other isotope decays.
This is more heat energy than the most popular BSE model suggests, but still far less than Earth's total. Says Freedman, "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."
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.
Says Freedman, "This is what's called an inverse problem, where you have a lot of information but also a lot of complicated inputs and variables. Sorting those out to arrive at the best explanation among many requires multiple sources of data."
"Partial radiogenic heat model for Earth revealed by geoneutrino measurements," by the KamLAND Collaboration, Itaru Shimizu of Tohoku University, Sendai, Japan, corresponding author, is published in Nature Geoscience and is available in advanced online publication at http://www.nature.com/ngeo/index.html.
Berkeley Lab and UC Berkeley members of the KamLAND Collaboration include Thomas Banks, Thomas Bloxham, Jason Detwiler, Stuart Freedman, Brian Fujikawa, Ke Han, Richard Kadel, Hitoshi Murayama, Thomas O'Donnell, and Herbert Steiner. Besides Tohoku University, Lawrence Berkeley National Laboratory, and the University of California at Berkeley, member institutions of the KamLAND Collaboration are the Institute for the Physics and Mathematics of the Universe, Tokyo University, Kashiwa (of which Hitoshi Murayama is also the director); the University of Alabama, Tuscaloosa; the California Institute of Technology, Pasadena; Colorado State University, Fort Collins; Drexel University, Philadelphia; the University of Hawaii at Manoa; Kansas State University, Manhattan; Stanford University, Palo Alto, California; the University of Tennessee, Knoxville; Triangle Universities Nuclear Laboratory, Durham, and Duke University, North Carolina Central University, and the University of North Carolina at Chapel Hill; the University of Wisconsin at Madison, and NIKHEF (National Institute for Subatomic Physics), Amsterdam.
KamLAND is supported by the Japanese Ministry of Education, Culture, Sports, Science and Technology and the U.S. Department of Energy's Office of Science.
Lawrence Berkeley National Laboratory addresses the world's most urgent scientific challenges by advancing sustainable energy, protecting human health, creating new materials, and revealing the origin and fate of the universe. Founded in 1931, Berkeley Lab's scientific expertise has been recognized with 12 Nobel prizes. The University of California manages Berkeley Lab for the U.S. Department of Energy's Office of Science. For more, visit www.lbl.gov.
Credit: Lawrence Berkeley National Laboratory
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Watts happening ping.
He keeps the earth cooking.
And who hung a nuclear device 93 million miles out in space from schools and hospitals fer cryin’ out loud??? Didja ever see the size of that thing????
Don’t forget, that thing is not shielded!
Colossians 1:17And He is before all things and in Him all things hold together.
My guess is HOT AIR from DC is funneled into the core.
What about all the kinetic energy involved when a mass the size of Earth rotates around its axis and simultaneously is rotating around both the Sun, our galaxy and whatever galactic super-cluster we are a part of?.
Calling Mr. Hoagland. Hyperdemsional physics anybody?.
Yea, that too. Where the heck is the EPA?? Why haven’t they shut that thing down???
Wow! Heavy stuff.
Does Algore’s Nobel prize enable him to understand this stuff, or is he more baffled than I am??
It’s still cooking because HE Said it WILL!
Science is OK, but one has to blend it with the teachings of The Giver of All Science.
That huge hole in the picture seems to be centered at California. That’s about the right size of a hole for the state budget....
The greenies won't let us use it, then.
Neither Earth’s rotation nor the periodic bending/warping of Earth’s crust by the Moon is not even close enough to raise Earth’s core to any more than 20 degrees Celcius.
The story of the discovery of radioactivity starts with - of all things - Darwin’s book: On the origin of Species (published in 1859). Darwin understood that evolution takes a very long time and he needed to support the evolution theory by showing that Earth is old enough to bring about life and evolution. Darwin assumed an erosion rate of an inch a century, and then extrapolated that some 300 million years were apparently necessary to explain the total amount of erosion that had occurred.
In 1862, Lord Kelvin published his analysis in a paper titled On the Secular Cooling of the Earth. He arrived at a best estimate for the age of the Earth of 100 million years. Essentially, Lord Kelvin stated that if Earth is more than 100 million years old, there would be no volcanic activity (tidal forces would be unsufficiant) and the planet would be totally solid to the core.
The Christian Fundamentalists were in an uproar against Darwin and held up Lord Kelvin’s study to show that Darwin is incorrect. However, the sedimentation record of the Grand Canyon implies that Earth is at least 2 billion years old. Other studies concluded that Earth is much younger than 1 billion years old. There was much acrimony in the scientific world. It came to the point that Lord Kelvin stated that if one can discover an unknown - at that time - source of energy then he will acknowledge that his models were wrong.
Then in 1896, Henri Becquerel accidentally discovered radioactivity.
I learn the above story by reading 20 years ago a science book published before WW2. Today, all science textbooks start the radioactivity story with Becquerel.
You can read more about the radioactivity story at www.lewrockwell.com/orig9/deming4.1.1.html
Depends on who is doing the implying and how/why they interpreted facts and also whether they interpreted their "facts" based upon other mere implications thought to be facts but based upon other implications often based upon outdated science.
I know you are trying to be witty but you have failed miserably. At least 3 Nobel prizes (chemistry, physics, medicine) were award for work on sedimentation. The relevant researcher is The Svedberg, 1926 Chemistry Nobel prize.
However, The did not do the Grand Canyon work but his predecessors in the field of sedimentation.
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