Earth generates heat. The deeper you go, the higher the temperature. At 25km down, temperatures rise as high as 750°C; at the core, it is said to be 4,000°C. Humans have been making use of hot springs as far back as antiquity, and today we use geothermal technology to heat our apartments. Volcanic eruptions, geysers and earthquakes are all signs of the Earth's internal powerhouse. //

The average heat flow from the earth's surface is 87mW/m2 – that is, 1/10,000th of the energy received from the sun, meaning the earth emits a total of 47 terawatts, the equivalent of several thousand nuclear power plants. The source of the earth's heat has long remained a mystery, but we now know that most of it is the result of radioactivity. //

The decay of one uranium-238 nucleus, for example, releases an average of 6 neutrinos, and 52 megaelectronvolts of energy carried by the released particles that then lodge in matter and deposit heat. Each neutrino carries around two megaelectronvolts of energy. According to standardized measures, one megaelectronvolt is equivalent to 1.6 10-13 joules, so it would take around 1025 decays per second to reach the earth's total heat. The question is, can these neutrinos be detected? //

Two recent experiments have added to the research: KamLAND, a detector weighing 1,000 metric tons underneath a Japanese mountain, and Borexino, which is located in a tunnel under the Gran Sasso mountain in Italy and weighs 280 metric tons. Both use "liquid scintillators." To detect neutrinos from the earth or the cosmos, you need a detection method that is effective at low energies; this means exciting atoms in a scintillating liquid. Neutrinos interact with protons, and the resulting particles emitted produce observable light.

KamLAND has announced more than 100 events and Borexino around 20 that could be attributed to geoneutrinos, with an uncertainty factor of 20-30%. We cannot pinpoint their source, but this overall measurement—while fairly rough—is in line with the predictions of the simulations, within the limits of the low statistics obtained.

Therefore, the traditional hypothesis of a kind of nuclear reactor at the center of the earth, consisting of a ball of fissioning uranium like those in nuclear power plants, has now been excluded. Fission is not a spontaneous radioactivity but is stimulated by slow neutrons in a chain reaction.