The Borexino Collaboration announced the observation of geo-neutrinos at the underground Gran Sasso National Laboratory of Italian Institute for Nuclear Physics (INFN), Italy. The data reveal, for the first time, a definite anti-neutrino signal with the expected energy spectrum due to radioactive decays of U and Th in the Earth well above background.

The International Borexino Collaboration, with institutions from Italy, US, Germany, Russia, Poland and France, operates a 300-ton liquid-scintillator detector designed to observe and study low-energy solar neutrinos. The low background of the Borexino detector has been key to the detection of geo-neutrinos. Technologies developed by Borexino Collaborators have achieved very low background levels. The central core of the Borexino scintillator is now the lowest background detector available for these observations. The ultra-low background of Borexino was developed to make the first measurements of solar neutrinos below 1 MeV and has now produced this first, firm observation of geo-neutrinos.

Geo-neutrinos are anti-neutrinos produced in radioactive decays of naturally occurring Uranium, Thorium, Potassium, and Rubidium. Decays from these radioactive elements are believed to contribute a significant but unknown fraction of the heat generated inside our planet. The heat generates convective movements in the Earth's mantle that influence volcanic activity and tectonic plate movements inducing seismic activity, and the geo-dynamo that creates the Earth's magnetic field.

The importance of geo-neutrinos was pointed out by Eder and Marx in the 1960's, and a seminal study by Krauss, Glashow, and Schramm in 1994 laid the foundation for the field. In 2005 an excess of low-energy antineutrinos above background was reported by KamLAND, a Japan-US collaboration operating a similar detector in the Kamioka mine in Japan. Owing to a high background from internal radioactivity and antineutrinos emitted from nearby nuclear power plants, the KamLAND collaboration reported that the excess events were an "indication" of geo-neutrinos.

With 100 times lower background than KamLAND, the Borexino data reveal a clear low-background signal for anti-neutrinos that match the energy spectrum of Uranium and Thorium geo-neutrinos. The lower background is due to scintillator purification and radio-purity aware construction methods developed by the Borexino Collaboration, and to the absence of any nearby nuclear reactor plants.

The origin of the known 40 TW of power produced within the earth is one of the fundamental questions of geology. The definite detection of geo-neutrinos by Borexino confirms that radioactivity contributes a significant fraction, possibly most, of the power. Other sources of power are possible, the main one being cooling from the hot primordial condensation of the earth. A powerful natural geo-nuclear reactor at the center of the earth has been suggested, but is ruled out as a significant energy source by the absence of the high rate of geo-reactor anti-neutrinos that should have been observed in the Borexino data.

Although radioactivity can account for a significant part of the earth's internal heat, measurements with a global array of geo-neutrino detectors above continental and oceanic crust are needed for a detailed understanding. By exploiting the unique features of the geo-neutrino probe, future data from Borexino, KamLAND, and the upcoming SNO+ detector in Canada, will provide a more complete understanding the earth's interior and the source of its internal heat.