PERSPECTIVES

GEOPHYSICS Neutrinos created by nuclear decay may allow geoscientists to measure the distribution of Mapping the Earth’s Engine radioactive elements in the Earth. William F. McDonough

article physicists and geophysicists optimal location for measuring the rarely meet to compare notes, but ear- Continental crust (>50%) distribution of heat-producing ele- lier this year researchers from these two Proposed Mantle (<50%) ments in the ancient core of a P geoneutrino disciplines gathered to discuss antineutrinos continent. Here, the antineutrino detectors (the antiparticle of the neutrino) (1). These signal will be dominated by the fundamental particles are a by-product of crustal component at about the reactions occurring in nuclear reactors and 80% level. This experiment will pass easily through Earth, but they are also SNO+ provide data on the bulk composi- generated deep inside Earth by the natural tion of the continents and place radioactive decay of uranium, thorium, and Core (~0%) limits on competing models of the potassium (in which case they are called continental crust’s composition. geoneutrinos). Particle physicists have re- The Boron Solar Neutrino experi- cently shown that it is possible to detect Hanohano ment (Borexino) detector, situated geoneutrinos and thus establish limits on the in central Italy (and hence some- amount of radioactive energy produced in the what removed from the regions of interior of our planet (2). This year’s joint France with many reactors), has meeting was aimed at enhancing communica- begun counting (5). This detector tion between the two disciplines in order to will accumulate a geoneutrino sig- on August 31, 2007 better constrain the distribution of Earth’s nal from a younger continental radioactive elements. Inside story. One model of the distribution of radioactive ele- region and surrounding Medi- Researchers from the Kamioka Liquid ments potassium, thorium, and uranium in Earth. New antineu- terranean ocean basin, thus receiv- scintillator Anti-Neutrino Detector (Kam- trino detectors in Canada (SNO+) and Hawaii (HANOHANO) ing a greater proportion of its sig- LAND) in Japan reported results that are con- should allow more precise determination of these distributions. nal from the mantle. sistent with the power output produced from Particle physicists from Hawaii the decay of thorium and uranium (16 TW), the reactor signal overwhelmed the geoneu- and their colleagues from elsewhere in the and the abundances of these elements in trino signal. New detectors are being devel- United States, Japan, and Europe are propos- Earth, as estimated by geoscientists (3). oped, deployed, and positioned in locations ing a 10,000-ton, portable geoneutrino detec- www.sciencemag.org (Potassium geoneutrinos cannot be detected at that have substantially smaller contributions tor that is deployable on the sea floor. This present due to the high background in this from nuclear reactors, and thus will provide detector, called Hawaii Antineutrino Ob- region of the spectrum.) The initial measure- more precise measurements of neutrinos and servatory (HANOHANO, which is also Ha- ment is also broadly consistent with the Th/U antineutrinos to both the Earth science and waiian for “magnificent”), would allow the ratio for Earth being equal to that of chondritic astrophysical communities. measurement of the geoneutrino signal com- meteorites, which is a fundamental assump- In addition to detecting geoneutrinos, these ing almost exclusively from deep within tion used by geochemists to model planetary facilities are designed to detect neutrinos from Earth, far removed from the continents and Downloaded from compositions. However, the upper power limit supernovae and determine their oscillation nuclear reactors (6). Thanks to the capability determined by the experiment (60 TW at the properties (like antineutrinos, neutrinos can of multiple deployments, this detector would 3σ limit) exceeds Earth’s surface heat flow by oscillate among their three different states). As provide the exciting possibility of obtaining a factor of 1.5 and is thus not very useful as a particle physicists continue to count geoneutri- signals from different positions on the globe. constraint for the models. nos, the signal-to-noise ratio will improve Ultimately, these different detectors will Nevertheless, there is great excitement and, with more counts, the uncertainty in the allow Earth scientists to test various models within the two communities, as advances in radioactive energy budget of Earth will shrink for the vertical and lateral distribution of tho- antineutrino detection are anticipated. The and the measured Th/U ratio of the planet will rium and uranium in Earth and will yield KamLAND detector was intentionally sited be determined to a greater precision. Mea- unparalleled constraints on the composition of near nuclear reactors in order to characterize surement uncertainties of 10% or better the continents and the deeper Earth. Insights antineutrino oscillation parameters (the reac- are possible with the new detectors, and are from geoneutrinos will also allow us to decide tor produces so-called electron antineutrinos, achievable with only 4 years of counting. among competing models of Earth’s interior. and antineutrinos can oscillate between the What does this mean for the Earth sci- Decades of research on the state of mantle three different “flavors”—the electron, muon, ences? Geoneutrino detectors will be sited on convection have assumed wide-ranging and tau antineutrinos)—and sense fluctua- continental crust of different ages, including values of the Urey ratio, the proportion of tions in reactor power output. Consequently, ancient cratons, the oldest pieces of continents radioactive energy output to the total energy (see the figure). One proposal is to convert the output of the planet. Geochemists have Sudbury Neutrino Observatory (SNO) to deduced a Urey ratio of ~0.4, whereas geo- The author is in the Department of Geology, University of Maryland, College Park, MD 20742, USA. E-mail: “SNO+” (4). This 1000-ton detector is sited in physicists prefer constructing mantle convec- [email protected] a mine in Ontario, Canada, and represents an tion models assuming higher Urey ratios that

www.sciencemag.org SCIENCE VOL 317 31 AUGUST 2007 1177 Published by AAAS PERSPECTIVES

range up to 1.0 (7). In addition, geoneutrino composed of high-density metal, has a References 1. Deep Ocean Anti-Neutrino Observatory Workshop, data, coupled with local heat-flow data, will markedly higher electron density than the sili- Honolulu, Hawaii, 23 to 25 March 2007, be used to evaluate models of bulk continental cate shells of Earth. Likewise, there is a www.phys.hawaii.edu/~sdye/hano.html. crustal composition. Competing models differ marked contrast in electron density for the 2. T. Araki et al., Nature 436, 499 (2005). 3. W. F. McDonough, in The Mantle and Core, R. W. Carlson, by almost a factor of 2 in their concentrations inner and outer core. Measurement of neu- Ed. (Elsevier-Pergamon, Oxford, 2003), vol. 2, pp. of potassium, thorium, and uranium, with trino dispersion in these layers would substan- 547–568. some models critically dependent on heat- tially improve our knowledge of the absolute 4. M. J. Chen, Earth Moon Planets 99, 221 (2006). 5. M. G. Giammarchi, L. Miramonti, Earth Moon Planets 99, flow data (8). radius of the core and hence the precision of 207 (2006). Beyond determining the amount and dis- global seismological models. Such beam 6. J. G. Learned, S. T. Dye, S. Pakvasa, in Proceedings of the tribution of heat-producing elements in Earth, studies could also place limits on the amount XII International Workshop on Neutrino Telescopes, M. Baldo-Ceolin, Ed. (Istituto Veneto di Scienze, Lettere ed particle physicists at the workshop described of hydrogen in the core. Arti Padora, 2007), pp. 235–269. future experiments, only a decade or so away The range of novel experiments underway 7. T. Lyubetskaya, J. Korenaga J. Geophys. Res. 112, from implementation, that would allow more and those just over the horizon will directly B03212 (2007). 8. R. L. Rudnick, S. Gao, in The Crust, R. L. Rudnick, Ed. precise determination of Earth’s structure. interrogate the interior of Earth in exciting and (Elsevier-Pergamon, Oxford, 2003), vol. 3, pp. 1–64. Dispersion of neutrino beams penetrating unparalleled ways (9); these tools will essen- 9. N. H. Sleep, Earth Moon Planets 99, 343 (2006). Earth are a function of the electron density of tially provide new ways of “journeying” to the different layers of the planet. The Earth’s core, center of the Earth. 10.1126/science.1144405

ENGINEERING New technologies are being developed to protect the privacy of individuals in today’s Privacy By Design information society.

George Duncan on August 31, 2007

nformation privacy used to come by advocates strengthening the powers of the default, mainly because of the high costs U.K. Information Commissioner to include Iimposed on any snooper. Yet today, tech- substantial penalties for misuse of data. nology has lowered the costs of gathering There are many important reasons to use information about individuals, linking per- personal information. For example, under sonal details, storing the information, and Megan’s Law, Web sites permit the public to broadcasting the results. Inexpensive net- locate and identify convicted sex offenders worked surveillance cameras capture our digi- in the United States. Depersonalized data on www.sciencemag.org tal image across time and place. Terabyte patient drug use can be mined to better target RAID (redundant array of independent disks) marketing efforts for pharmaceuticals; this drives provide cheap storage. Real-time data approach is used, for example, by Verispan (6). integration software turns fragmented per- Web-based social networks like Jaiku or Twitter sonal data into composite pictures of individu- facilitate peer-to-peer exchange of personal als (1). Communication that is universal, details. Road tolls can be debited electronically

instantaneous, unlimited in capacity, and free from a driver’s personal account while monitor- Downloaded from for all (2) is becoming ever more plausible. establishment of a Federal Privacy Com- ing every vehicle’s speed and recording With cost barriers lowered for data cap- missioner or Privacy Commission, greater fed- safety violations. ture, storage, integration, and dissemination, eral regulation of businesses that use personal But in the wrong hands, this personal our privacy is no longer implicitly protected information, and more government action to information can be used to exploit or harm (3). Instead, those charged with protecting protect individual information privacy. individuals; for example, released sex offend- information privacy must now give it explicit The report from the U.K. Royal Academy ers may be subject to harassment, employers attention. This is the purpose of two thought- of Engineering emphasizes that, because of may discriminate against those with certain provoking reports released this year (4, 5). human rights law, organizations maintaining medical conditions, children on social net- In its report, the U.S. National Research systems that use personal information should works may be targeted by those with evil Council recommends that fair information be accountable for designing them to provide intent, and car owners may be held account- practices be adopted by businesses in the use privacy (5). The report recommends less able for what thieves may do with their cars. of personal information and that mechanisms intrusive data use (such as preferring client To help balance privacy concerns and the be developed to give individuals more control authentication—“are they valid users?”— need for personal data, a new paradigm is over the use of their information (4). Perhaps over identification—“who are they?”), emerging, in which system designers conduct the most controversial recommendations research on how camera surveillance can privacy risk assessments and incorporate pri- involve increased privacy regulation: the ignore law-abiding activities, developing clar- vacy as a fundamental design parameter. As ity about privacy expectations, formation of Alan Greenspan has remarked (7), “The most trusted third-party organizations as guardians effective means to counter technology’s ero- The author is at the H. John Heinz III School of Public Policy and Management, Carnegie Mellon University, Pittsburgh, of personal data, and making data collection sion of privacy is technology itself.” To illus-

PA 15213, USA. E-mail: [email protected] and use transparent to the data subject. It trate how privacy-enhancing technologies PETER HOEY CREDIT:

1178 31 AUGUST 2007 VOL 317 SCIENCE www.sciencemag.org Published by AAAS 16

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Courtesy of Lawrence Berkeley National Laboratory geoneutrinos For a big view of inner Earth, catch a few ... ingly opaque telescope. The Earth, though, is frustrat- www.sciencenews.org W feature “ electrons. to partner antimatter the are positrons as just counterpart, ter antimat- neutrinos’ are which trinos, Park.sity ofMarylandinCollege McDonough, a geochemist at the Univer William saystime,” real in Earth the of possibility of measuring the composition produced withintheplanet itself. by detecting “geoneutrinos” Earth’sthe interior illuminate to down, looking and telescope neutrino a taking are scientists earth Now cosmos. the in nos originating in the sun and elsewhere neutrino neutri- study to decades for telescopes used have Astrophysicists neutrinos. as known particles atomic sub- curious the using are they physics, astro from page a Borrowing innards. Earth’sthe probing for tool new a oped rocks, meteorites andthesun. near-surface in found elements the on rely chemistry internal Earth’s about Inferences image. ultrasound of kind a from studying seismic waves, which give planet’scomes the structure of internal Electrons and positrons have opposite have positrons and Electrons Earth,’ the inside from coming ‘antineutrinos than say to natural radioactivity insideEarth. ted todetectantineutrinosproduced by couldbe retrofit- Neutrino Observatory detectoratthe Sudbury the four-story ticles fromspacecalledneutrinos, Originally usedtodetectelusive par-

‘Geoneutrinos’ is just an easier word word easier an just is ‘Geoneutrinos’ Geoneutrinos are actually antineu- actually are Geoneutrinos the have we time, first the for “Now, devel- have geoscientists Recently, down to its outer core with a with core outer its to down kilometers 2,900 gaze might ere the Earth a crystal ball, you | —

geoneutrinos to light. Most knowledge ” McDonough says. McDonough ” — neutrinos neutrinos - - dynamics of geology that we see we that geology of dynamics the of all essentially for responsible from 30billion to 44billionkilowatts. power, the Earth puts out; estimates range troversy over how much heat, in terms of playersisn’tof known. There’s evencon- number a of one is or action heating the dominates decay radioactive whether But Manoa. at Hawaii of University the says John Learned, a particle physicist at spreading,” seafloor and continents ing Powering Earth tion intheEarth’s mantle. convec- driving turn, in and, Earth the up heating in play may elements these decayof the role crucial the about more mantle. Earth scientists are keen to learn and crust Earth’s the in potassium and thorium uranium, of decay radioactive one oftheproposedexperiments. Sudbury,in tember Canada particle physicists at a conference in Sep- with developments new discussed tists ican Geophysical Union, and earth scien- Eos, the weekly newspaper of the Amer in October in experiments the of view over an gave Hawaii of University the ocean detector. deep- submersible mobile, a to Europe, and States United the Canada, in tors istry. These range from deep-mine detec better glimpse of the Earth’s inner chem- even an get to poised are experiments new proposed of array an Now 2005. in Japan in mine a inside deep detector a or may not bethesame particle. nos and antineutrinos, confusingly, may antineutrinos have and no charge. So neutri- neutrinos but charges, electrical Te ovcin n h mnl is mantle the in convection “The enurns rgnt fo the from originate Geoneutrinos McDonoughfrom colleagues two and in observed first were Geoneutrinos — the site of site the — mov - - - - the moltenouter core. stretches 2,900 kilometers from crust to vast,viscous, slowly layerchurning that mantle the in elements these of Earth’s dynamics is knowing the amount understanding to key But rock. of so or kilometers 30 top the crust, the in dant These elements are probably most abun- 40 to 60 percent of Earth’s interior heat. also from potassium, accounts for at least but thorium and uranium from mainly for theradioactive contribution. detectors could pin down better numbers sources. But the new suite of geoneutrino heat might have come from this or other conditions.” sipated it, depending on the atmospheric amount of kinetic energy and rapidly dis- large a with started have could we “or McDonough, saysit,” dissipated slowly large amount of kinetic energy, and we’ve radiated thisheatintospace. thermal energy. Over time, the Earth has became other,energy each kinetic their into slammed meteorites the As Earth. become to mass enough accreted tually even - blocks building planetary These meteorites. colliding by et’sformation plan- the from over left heat is sources sources drivingtheEarth’s engine?’ energy the are ‘What is, today question The sources. other from portion some coal, but some portion from nuclear and from electricity our of all get don’t “We It’s like the energy mix in homes, he says. sources.”energy of subset a or fuel tive radioac- entirely either is that for fuel the “and McDonough, says ride, nents conti the which upon plates geologic siae ae ht radioactivity, that are Estimates much how measure to difficult It’s a with out started have could “We energy possible other the Among the of movement the drives Energy January 17,January 2009

| By DianaSteele science news science —

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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. For all their promise, these geoneu- provided the first glimpse of geoneutrinos sphere surrounded by electronic eyes trino detectors won’t be able to unearth and a first approximation of uranium and that scan the fluid for flashes of light char- Mantle signature the whole picture of Earth’s interior. thorium’s contribution to the Earth’s heat. acteristic of geoneutrinos’ presence. SNO+ is the furthest along of the wouldn’t begin for at least several years. das in astrophysics and particle physics Stevenson points out that all the Unfortunately, because the number of Chen, a particle astrophysicist at up-and-coming geoneutrino experi- These detectors would be, for the most as well as earth science. geoneutrino detectors proposed share geoneutrinos was so small, the estimated Queens University in Kingston, Canada, ments. Other detectors under discus- part, counting geoneutrinos that origi- With funding, Hanohano could be built a shortcoming: They can’t detect the heat contribution, in terms of power, had hopes SNO+ will start up in late 2010. sion include one in the Homestake mine nate in the Earth’s crust, where thorium within about two years, McDonough geoneutrinos coming from radioactive a range of 19 billion to 60 billion kilowatts; It should catch about 50 geoneutrinos in South Dakota and a large detector to and uranium are concentrated. Looking estimates. Preliminary design stud- potassium-40. These geoneutrinos have consistent with but not more precise than a year, considerably more than either be built in Europe, possibly in Finland. close to the crust is like having your eye ies for Hanohano are underway, and too little energy to trigger the detector. previous estimates. KamLAND or Borexino. The longer Startup and operating costs are esti- close to a bright flashlight. McDonough is hoping for another $5 mil- “So there is one part of the expected Since then, the geoneutrino detector SNO+ runs, the better the picture it will mated to be on the order of hundreds To get a better idea of what’s going lion of seed money to continue design. heat production that we cannot measure, Borexino, located in an Italian mine, has get of the inner Earth. of millions of dollars, and construction on in the mantle — a dim and more dis- But to construct and deploy Hanohano and haven’t figured out how to measure,” come online. And new experiments are tant flashlight — requires a geoneutrino and to keep it running for 10 years could he says. And although only a tiny fraction on the horizon, though it may be at least detector situated in a place where the cost around $200 million, he estimates. of potassium-40 is actually radioactive, two years before the first of the next gen- crust is only a few kilometers thick, like That’s expensive, but it’s about a fac- and most of it has already decayed over eration of detectors starts up. at the bottom of the ocean. tor of 10 less expensive than sending a Earth’s 4.6-billion-year history, potas- The SNO+, or Sudbury Neutrino Obser- McDonough and his colleagues are spacecraft to another planet, points out sium still contribute up to 20 percent of

vatory Plus, would sit two kilometers lat; detector photo proposing a 10,000-ton submersible David Stevenson, a planetary physicist at the radioactive heat, Stevenson says. b underground in the Creighton nickel detector they have named “Hanohano” the California Institute of Technology in “But if you go back in time, potas-

mine near Sudbury, Ontario. SNO+ would . koren (Hawaiian for “magnificent”), for the Pasadena, who is not directly involved in sium-40 becomes increasingly impor- j y piggyback on and use the same detector as b Hawaii Anti-Neutrino Observatory. the geoneutrino experiments. And “it’s tant,” he adds. “And that’s why, if you a highly successful solar neutrino proj- Hanohano would be about 10 times inconceivable to me that we could get the want to reconstruct the history of the ect called the Sudbury Neutrino Obser- the size of the SNO+ detector and filled same information with the accuracy we Earth, you would like to know how much vatory, which played an important role with the same scintillator fluid. It would desire by any other method,” he says. potassium you had.” s in solving a long-standing conundrum. be towed out to sea on a barge and sunk, He also hopes for the unexpected. oration; top: graphic

Researchers had detected fewer neutri- b anchored about 4,000 meters deep and “I’ve learned as a planetary scientist, Diana Steele is a freelance science writer nos coming from the sun than expected. about 90 m from the bottom. After catch- that when you go to a planet, you actu- based in Ohio.

SNO showed that neutrinos have a tiny + colla ing neutrinos for a year or two, it could ally discover things, you are surprised,” bit of mass and are shape-shifters, turn- SNO be serviced and redeployed elsewhere. he says. “And in the case of the neutri- Explore more age: P ing from a detectable form into another, A physicist boats in water surrounding Canada’s SNO detector, which is filled with Like SNO+, Hanohano will be a multi- nos, you may be surprised. You may be ss Visit the SNO+ website at

undetectable one. heavy water to detect neutrinos. A new fluid will be used to detect geoneutrinos. LEFT m ISTOCKPHOTO / A lesVeluscek fro courtesy of b erkeley la ; E arth i m age faceted experiment, with research agen- surprised, for example, to discover that snoplus.phy.queensu.ca/

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