URANIUM PROSPECTING WITH NEUTRINOS. Up to 40% of the 40 terawatts of energy leaving the Earth's surface is believed to come ultimately from the radioactive decay of uranium-238 and thorium-232. Simple geological models predict where these isotopes are to be found: half in the crust beneath continents and half in the mantle. But geophysicists would like to map the deposits more accurately, especially since radiogenic heat has had a large role in determining the dynamics of our planet's interior. Now, scientists at Bell Labs, the Technical University of Munich, and Tohoku University (Japan) are proposing a scheme in which neutrinos would survey the Earth just as positrons reveal the presence of metabolic activity (parts of the brain lighting up, say) through the process of positron emission tomography. In this case radioactive deposits would announce themselves by the neutrinos they cast off. These neutrinos, which effortlessly plow through the Earth's bulk, would register in three surface detectors: Borexino, to come online in 1999 in Italy; Kamland in Japan, around the year 2001; and the proposed Geomanda detector at the South Pole. The numbers and energies of the neutrinos can be used not only to chart the density of the U and Th hoards, but also to differentiate between the two nuclides, thus providing a sort of global analytical chemistry of the Earth. (R.S. Raghavan et al., Physical Review Letters, 19 January 1998; contact Raju Raghavan, 908-582-4351, raju@lucent.com; picture at Physics News Graphics website.)