Neutrino Geoscience: Geoneutrinos and Heat Production in the Earth

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What powers plate tectonics, mantle convection, and the Earth's geodynamo? The Earth radiates 46 terawatts (46 million millon watts) to space, and this power emission reflects contributions from primordial and radiogenic sources, with the former constituting planetary accretion and core-formation sources. Inside the Earth, the decay of 3 radioactive elements (potassium, thorium, and uranium) produces 99% of the Earth?s nuclear power. Existing measurements of the Earth's flux of geoneutrinos, electron antineutrinos from terrestrial natural radioactivity, reveal the amount of uranium and thorium in the Earth, but these data come with considerable uncertainty. Given the understanding of the amount and distribution of these elements in the continental crust, it is known that they contribute about 7 TW of radiogenic power to the continental heat flux, and this component plus an underlying mantle flux accounts for about 1/3 of the total power lost from the Earth. The remaining 2/3 of the Earth's surface heat flux comes up beneath the oceans, but it is not known how much of this mantle flux is primordial versus radiogenic contributions. Compositional models of the Earth collectively allow for up to a factor of 30 in estimates of the mantle's radiogenic power. The understanding of the Earth's thermal evolutionary history is intimately linked to knowing the total radiogenic power of the mantle. Consequently, this project seeks to understand the rate and magnitude by which the planet is cooling. Thus, by determining the amount of radioactive energy that powers the Earth's engine, one can make a 'fuel gauge' that identifies the proportion of primordial to radioactive fuel left in the planet. In addition, there will be collaboration with particle physicists and members of the U.S. intelligence community in the detection of electron antineutrinos (from nuclear reactors and the Earth) for nuclear nonproliferation purposes. Reactor antineutrinos are the background for the analyses of geoneutrino research and geoneutrinos are the background for reactor monitoring.Despite best efforts there remains an order of magnitude uncertainty in the amount of radiogenic power driving mantle dynamics, given the competing models of the Earth's composition. Direct measurements of the abundance of radiogenic, heat-producing elements (K, Th and U) present in the mantle and much of the deep continental crust do not exist. Importantly, this picture is rapidly changing because of new, larger and more sensitive geoneutrino detectors that are coming on line in the coming years. In the next 8 years, a suite of 5 experiments will define the mantle's radiogenic contribution to the surface heat loss and when tested to a reference model these data can define the radiogenic heat from the mantle. These results will fix limits on the composition of the silicate Earth and will set bounds on permissible values for models defining the mode of mantle convection.The abundance and distribution of the heat-producing elements in the Earth will be studied, and the major tasks include: 1) Improve predictions and reduce systematic errors in defining the regional geoneutrino signal to SNO+ detector (Ontario, Canada), 2) Model geological, geochemical, and geophysical data of the regional lithologies surrounding the KamLAND, JUNO (Guangzhou, China) and Jinping (Sichuan, China) detectors to improve geological predictions, 3) Develop and improve the global reference model at the 1x1 degree scale, making it a community resource that goes beyond applications in geoneutrino studies, 4) Test existing and future data from all detectors against estimates of the regional and global contribution, assuming all detectors see approximately the same mantle signal (within +/-10%), and 5) Use above data to test models of the bulk silicate Earth. Data from current and planned detectors can bring resolution to several major issues in Earth sciences, such as 1) what are the building blocks used to make the planet; 2) what is the present-day proportion of radiogenic heat relative to the residual heat of accretion, core formation and extinct nuclides; 3) what is the present-day fraction of radiogenic heat in the continental crust relative to that in the mantle; and 4) what is the composition of the silicate Earth, upper mantle, and lower mantle? Answers to these questions will, in turn, define the power that is driving plate tectonics, mantle convection and the geodynamo, as well as the structure of mantle convection. Neutrino geoscience offers a great potential to address these broad interdisciplinary issues.