EARTH'S natural radioactivity has been measured for the first time. The measurement will help geologists find out to what extent nuclear decay is responsible for the immense quantity of heat generated by Earth. Our planet's heat output drives the convection currents that churn liquid iron in the outer core, giving rise to Earth's magnetic field. Just where this heat comes from is a big question. Measurements of the temperature gradients across rocks in mines and boreholes have led geologists to estimate that the planet is internally generating between 30 and 44 terawatts of heat. Some of this heat comes from...