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.