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Scanning Earth With Neutrinos

Experiments being built in Italy and Japan to study the sun could deliver a bonus for earth scientists. Meant to detect neutrinos, fleeting particles from the sun, these detectors should also register the passage of neutrinos expelled from radioactive elements inside Earth, physicists will report next week in Physical Review Letters. The huge, oil-filled detectors thus may open a unique window on processes that have heated Earth since it formed.

Earth's surface emits about 40 trillion watts of heat, and perhaps 40% of this comes from the slow decay of the unstable isotopes uranium-238 and thorium-232. The decay heat helps drive processes such as plate tectonics and volcanism, but geophysicists have never had a direct way to determine how these elements are distributed in Earth's interior. The Italian and Japanese detectors may change that by scanning Earth in much the same way as a positron emission tomography scan illuminates active areas in the brain.

The trick is to spot the neutrinos that zip through Earth when uranium and thorium decay. These ghostly particles generally slip effortlessly through matter. But one in every billion or so should spark flickers of ultraviolet light in the detectors, says physicist Raju Raghavan of Lucent Technologies Bell Laboratories in Murray Hill, New Jersey. The new detectors, unlike existing experiments, should be able to tell these neutrinos apart from ones given off by the sun by their energies. And because neutrinos from uranium and thorium also have subtly different energies, "this technique could produce the first global transuranic chemical analysis of Earth," says Raghavan. The method may, for example, reveal the ratio of these elements in continental versus oceanic crust--a constraint for models of how Earth's surface segregated from the rest of the molten planet.

Physicist Lawrence Krauss of Case Western Reserve University in Cleveland, Ohio, who proposed the neutrino scheme in a 1984 Nature paper with Nobel laureate Sheldon Glashow and the late cosmologist David Schramm, is pleased to see that the technique may soon be feasible. "It is striking to me how much we can learn about geophysics from solar neutrino detectors," Krauss says. He cautions, however, that sources of interference, such as neutrinos from nearby nuclear reactors, will make the scientific analysis challenging.