Geo-neutrino: status and prospect

Geological models are at odds over the radioactive power of the earth, predicting anywhere from 16 to 42 TW. The actual value constrains the thermal evolution and indicates the meteoritic origin of the planet. An estimated 20% of this radioactive power escapes to space in the form of geo-neutrinos. The remaining portion heats the planet with significant geo-dynamical consequences. The amount of radiogenic heating, conveniently expressed as a fraction of the 47-TW surface heat flow, discriminates earth models and characterizes the rate of planetary temperature change. A fraction greater than one means the earth is heating up; a fraction less than one means the earth is cooling down. Radiogenic heating in the planet primarily springs from unstable nuclides of uranium, thorium, and potassium. Closely associated with radiogenic heating is the production of geo-neutrinos. Large sub-surface detectors efficiently record the infrequent interactions of the highest energy geo-neutrinos, which originate from uranium and thorium. The measured energy spectrum of the interactions estimates the relative amounts of these heat-producing elements, while the intensity assesses planetary radiogenic power. Recent geo-neutrino observations suggest radioactivity accounts for less than all of the surface heat flow. This assessment, implying the earth is cooling down, approaches sensitivity to model predictions. Future observations at selected locations have the potential to constrain the thermal evolution of the planet and to resolve earth models. This contribution reviews the science and status of geo-neutrino observations and the prospects for measuring the radioactive power of the planet.