The deepest hole that has ever been dug is about 12 km deep. Geochemists analyze samples from the Earth's crust and from the top of the mantle. Seismology can reconstruct the density profile throughout all Earth, but not its composition. In this respect, our planet is mainly unexplored. Geo-neutrinos, the antineutrinos from the progenies of U, Th and K40 decays in the Earth, bring to the surface information from the whole planet, concerning its content of natural radioactive elements. Their detection can shed light on the sources of the terrestrial heat flow, on the present composition, and on the origins of the Earth. Geo-neutrinos represent a new probe of our planet, which can be exploited as a consequence of two fundamental advances that occurred in the last few years: the development of extremely low background neutrino detectors and the progress on understanding neutrino propagation. We review the status and the prospects of the field.
The deepest hole that has ever been dug is about 12 km deep, a mere dent in planetary terms. Geochemists analyze samples from the Earth's crust and from the top of the mantle. Seismology can reconstruct the density profile throughout all Earth, but not its composition. In this respect, our planet is mainly unexplored.
Geo-neutrinos, antineutrinos from the progenies of U, Th, and K decays in the Earth, bring to Earth’s surface information coming from the whole planet. Differently form other emissions of the planet (e.g., heat, noble gases), they are unique in that they can escape freely and instantaneously from Earth’s interior.
Detection of geo-neutrinos is becoming practical as a consequence of two fundamental advances that occurred in the last few years: a) development of extremely low background neutrino detectors and b) progress on understanding neutrino propagation. In fact, KamLAND has reported in 2005 (Araki et al., 2005a) evidence of a signal originating from geo-neutrinos, showing that the technique for geo-neutrino detection is now available.
Geo-neutrinos look thus a promising new probe for the study of global properties of Earth and one has to examine their potential. Let us enumerate a few items which, at least in principle, can be addressed by means of geo-neutrinos1 .
There are large uncertainties on Earth’s energetics, both on the value of the heat flow (estimated between 30 and 45 TW) and on the separate contributions to Earth’s energy supply (radiogenic, gravitational, chemical . . . ). Estimates of radioactivity in the Earth’s crust, based on observational data, account for at least some 8 TW. The canonical Bulk Silicate Earth (BSE) model provides about 20 TW of radiogenic heat. However, on the grounds of available geochemical and/or geophysical data, one cannot exclude that radioactivity in the present Earth is enough to account for even the highest estimate of terrestrial heat flow.
An unambiguous and observationally based determination of the radiogenic heat production would provide an important contribution for understanding Earth’s energetics. It requires determining how much uranium, thorium and potassium are inside the Earth, quantities which are strictly related to the anti-neutrino luminosities from these elements.
The BSE model presents a chemical composition of the Earth similar to that of CI chondritic meteorites see, e.g., (McDonough, 2003;Palme and O’Neill, 2003). The consistency between their composition and that of the solar photosphere points towards considering CI representatives of the material available in the pre-solar nebula and the basic material from which our planet has been formed. Some authors, however, have argued for a genetic relationship of our planet with other chondrites, such as enstatite chondrites, which are richer in long lived radioactive elements (Javoy, 1995).
We remind that BSE is a basic geochemical paradigm consistent with most observational data, which however regard mostly the crust and an undetermined portion of the mantle. The global abundance of no element in the Earth can be estimated on the basis of observational data only. Geo-neutrinos could provide the first direct test of BSE (and/or its variants) by measuring the global abundances of natural heat radiogenic elements.
Heat generating elements in the crust: a test of the estimated abundances.
The amount of radioactivity in the Earth’s crust is reasonably well constrained by observational data, with the exception of the lowest portion. Most of the uncertainty on the amount of radioactivity in the crust arises from the different estimates about the lower crust. In this respect, a detector located well in the middle of a continent, being most sensitive to geo-neutrinos from the crust, might provide a significant check of the estimates on the crustal content of heat generating elements.
A measurement of heat generating elements in the mantle.
The estimated content in the mantle is based on cosmochemical arguments and implies that abundances in deep layers have to be much larger than those measured in samples originating from the uppermost layer (Jochum et al., 1983;Zartman and Haines, 1988). Uncertainties on the heat generating elements content of the Earth essentially reflect the lack of observational data on the bulk of the mantle. A geo-neutrino detector located far from continents would be mainly sensitive to heat radiogenic elements in the whole mantle, as the oceanic crust is thin and poor in these elements.
What can be said about the core? Geochemical arguments are against the presence of radioactive elements in the core, although alternative hypothesis have been advanced see, e.g., (Herndon, 1996;Rama Murthy et al., 2003).
Present non directional detectors can say little about the core; however some extreme hypothesis can already be tested. If a natural fission reactor were present in the Earth’s core, as advocated by Herndon in a series of paper (Herndon, 1998(Herndon,
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