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For a Big View of Inner Earth, Catch a Few

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For a Big View of Inner Earth, Catch a Few FEATURE | GEONEUTRINOS For a big view of inner Earth, catch a few ... By Diana Steele geoere the Earth a crystal ball,neutrinos you electrical charges, but neutrinos and Energy drives the movement of the might gaze 2,900 kilometers antineutrinos have no charge. So neutri- geologic plates upon which the conti- W down to its outer core with a nos and antineutrinos, confusingly, may nents ride, says McDonough, “and the telescope. The Earth, though, is frustrat- or may not be the same particle. fuel for that is either entirely radioac- ingly opaque — to light. Most knowledge Geoneutrinos were first observed in tive fuel or a subset of energy sources.” of the planet’s internal structure comes a detector deep inside a mine in Japan It’s like the energy mix in homes, he says. from studying seismic waves, which give in 2005. Now an array of proposed new “We don’t get all of our electricity from a kind of ultrasound image. Inferences experiments are poised to get an even coal, but some portion from nuclear and about Earth’s internal chemistry rely better glimpse of the Earth’s inner chem- some portion from other sources. The on the elements found in near-surface istry. These range from deep-mine detec- question today is, ‘What are the energy rocks, meteorites and the sun. tors in Canada, the United States and sources driving the Earth’s engine?’ ” Recently, geoscientists have devel- Europe, to a mobile, submersible deep- Among the other possible energy oped a new tool for probing the Earth’s ocean detector. sources is heat left over from the plan- innards. Borrowing a page from astro- McDonough and two colleagues from et’s formation by colliding meteorites. physics, they are using the curious sub- the University of Hawaii gave an over- These planetary building blocks even- atomic particles known as neutrinos. view of the experiments in October in tually accreted enough mass to become Astrophysicists have used neutrino Eos, the weekly newspaper of the Amer- Earth. As the meteorites slammed into telescopes for decades to study neutri- ican Geophysical Union, and earth scien- each other, their kinetic energy became nos originating in the sun and elsewhere tists discussed new developments with thermal energy. Over time, the Earth has in the cosmos. Now earth scientists are particle physicists at a conference in Sep- radiated this heat into space. taking a neutrino telescope and looking tember in Sudbury, Canada — the site of “We could have started out with a down, to illuminate the Earth’s interior one of the proposed experiments. large amount of kinetic energy, and we’ve by detecting “geoneutrinos” — neutrinos Geoneutrinos originate from the slowly dissipated it,” says McDonough, produced within the planet itself. radioactive decay of uranium, thorium “or we could have started with a large “Now, for the first time, we have the and potassium in the Earth’s crust and amount of kinetic energy and rapidly dis- possibility of measuring the composition mantle. Earth scientists are keen to learn sipated it, depending on the atmospheric of the Earth in real time,” says William more about the crucial role the decay of conditions.” McDonough, a geochemist at the Univer- these elements may play in heating up It’s difficult to measure how much sity of Maryland in College Park. the Earth and, in turn, driving convec- heat might have come from this or other Geoneutrinos are actually antineu- tion in the Earth’s mantle. sources. But the new suite of geoneutrino trinos, which are neutrinos’ antimat- detectors could pin down better numbers ter counterpart, just as positrons are Powering Earth for the radioactive contribution. the antimatter partner to electrons. “The convection in the mantle is Estimates are that radioactivity, ORATORY B “ ‘Geoneutrinos’ is just an easier word responsible for essentially all of the mainly from uranium and thorium but A L to say than ‘antineutrinos coming from dynamics of geology that we see — mov- also from potassium, accounts for at least inside the Earth,’ ” McDonough says. ing continents and seafloor spreading,” 40 to 60 percent of Earth’s interior heat. ATIONAL ATIONAL N Electrons and positrons have opposite says John Learned, a particle physicist at These elements are probably most abun- the University of Hawaii at Manoa. But dant in the crust, the top 30 kilometers Originally used to detect elusive par- whether radioactive decay dominates or so of rock. But key to understanding ticles from space called neutrinos, the heating action or is one of a number Earth’s dynamics is knowing the amount AWRENCE BERKELEY AWRENCE L the four-story detector at the Sudbury of players isn’t known. There’s even con- of these elements in the mantle — the Neutrino Observatory could be retrofit- troversy over how much heat, in terms of vast, viscous, slowly churning layer that ted to detect antineutrinos produced by power, the Earth puts out; estimates range stretches 2,900 kilometers from crust to OURTESY OF CLOCKWISE, TOP LEFT: CREDIT ONE, SHARED; CREDIT TWO; CREDIT THREE SHARED; CREDIT TWO; TOP LEFT: CREDIT ONE, CLOCKWISE, C natural radioactivity inside Earth. from 30 billion to 44 billion kilowatts. the molten outer core. 16 | SCIENCE NEWS | Month X, XXXX www.sciencenews.org www.sciencenews.org January 17, 2009 | SCIENCE NEWS | 17 FEATURE | GEONEUTRINOS Like the better-known solar neutrinos, Converting SNO into SNO+ — which Ontario’s nuclear power plants are They came from Earth there’s a major source of radioactivity geoneutrinos can pass through thou- would detect the lower-energy geoneutri- far enough away to not overwhelm the Like its cousin the neutrino, a geoneutrino in a layer just above the core” — an idea (an antineutrino produced in Earth) can pass sands of miles of solid rock without being nos — means changing out the fluid that geoneutrino signal. “Certain problem- through Earth unimpeded and, researchers proposed early last year by Dutch and stopped or even deflected. That makes filled the detector. SNO operated from atic backgrounds from cosmic rays are hope, into detectors built to catch it. South African scientists writing in the them ideal for studying deep Earth — but 1999 to 2006 using heavy water — water even further reduced because we just South African Journal of Science. At the proposed SNO+ detector in Canada, also makes them very difficult to catch. with atoms of deuterium, heavy hydro- happen to be deeper underground than passing geoneutrinos would collide with Another scenario, which Stevenson gen — to snag solar neutrinos. Pending other, similar detectors,” Chen says. protons in fluid inside the detector (bottom ProtoP onn thinks is extremely unlikely but Learned Catching geoneutrinos final approval of funding, the detector With SNO+, he says, it will be possible right). Such a collision gives off a positron — acknowledges “would be quite cool,” is producing a flash of light — and changes the Neutronn One surefire way to catch some is to will be filled with a hydrocarbon-based to do some interesting things “with less proton into a neutron (right). When the neutron that enough uranium exists in the core build a detector near a concentrated scintillation fluid, which, when a geoneu- background and improved precision.” approaches another proton, the two bond to that there is essentially a nuclear reactor source of antineutrinos. Conveniently, trino is caught, will luminesce and trig- SNO+ has an ambitious scientific create deuterium (a heavy version of hydro- Positront humming away down there. gen), producing a second flash of light. “You’ve the uranium and other radioactive ele- ger the detector. agenda that includes better understand- got a flash associated with the positron pro- San Diego–based independent scien- ments used in a nuclear reactor provide The fluid is a common, mass-produced ing the fundamental nature of neutrinos. duction, and 200 microseconds later you have tist J. Marvin Herndon first proposed Geoneutrino a flood of these ghostly particles. petrochemical called linear alkylben- One goal is to pin down the mass of the another flash,” says University of Maryland, such a core reactor in 1993. Although College Park geochemist William McDonough. Proton That’s why the first geoneutrino detec- zene, or LAB, used to make clear deter- neutrino — a quantity even more elu- “And you say, ‘Eureka! I’ve got an antineutrino not widely believed, his hypothesis tor was built near a cluster of reactors in gents, like liquid hand soap. It’s less toxic sive than the neutrino itself. Another that’s come in.’ ” would explain some puzzling observa- Japan, with an aim to further character- than most chemical liquid scintillators. is to determine whether neutrinos and tions, such as an excess of an isotope of ize antineutrinos. Consequently, parti- “It produces a lot of light, and it’s very antineutrinos are the same particle — the INNER EARTH SNO+ helium emitted from volcanoes, Learned cles from the reactors swamped those transparent, but it’s a safer scintillator,” lack of charge makes it difficult to tell. points out. produced by naturally occurring ura- says SNO+ director Mark Chen. “It’s That question “connects to our under- Hanohano would be able to tell fairly Inner Core nium in the crust and mantle. much easier to use it, especially in a set- standing of the early universe and might Outer Core quickly whether such a reactor exists at Nonetheless, in 2005, this experiment, ting where we are taking a thousand tons inform us about why … we see matter in Mantle all, Learned reported last May at a neu- called KamLAND (short for Kamioka Liq- of it into an active mine.” the universe but much less antimatter,” Crust trino symposium in New Zealand. uid Scintillator Anti-Neutrino Detector), The detector is a four-story acrylic Chen says.
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