Radiogenic heating is a key component of the energy balance and thermal evolution of the Earth. Geo-‐neutrino observations are beginning to measure the present radiogenic power of our planet. The precision of this estimate, which assumes negligible radioactivity in the core and relatively well-‐defined radiogenic heating in the crust, is ultimately limited by uncertainty in the quantity and distribution of heat-‐producing elements in the mantle. This project explores the geological consequences of the precision of the mantle radiogenic heating measurement and investigates the geological uncertainties limiting these measurements, such as the radiogenic power of MORB-‐source mantle and the nature of seismic discontinuities in the deep mantle.
Geoscientists have made important progress understanding the energy budget of the Earth. They have measured the heat flow from the surface of the planet, totaling 47 ± 3 TW (Davies and Davies, 2010), with impressive precision. The main sources of this flow, comprising heat passing into the mantle from the core, mantle cooling, and radiogenic heating in the crust and mantle, have been identified. Whereas radiogenic heating in the crust has been robustly assessed at 7 ± 1 TW (Huang et al., 2013), the corresponding value for the mantle has proved difficult to define. Reasoning on evidence from geochemistry (McDonough and Sun, 1995; Boyet and Carlson, 2006), geodynamics (Turcotte and Schubert, 2002), and cosmochemistry (O’Neill and Palme, 2008; Javoy et al., 2010), geoscientists have calculated disparate model estimates of radiogenic heating in the mantle ranging from 3 to 30 TW.
In 2002 particle physicists began recording geo-‐neutrino interactions using 1000 metric tons of scintillating liquid beneath Mt. Ikenoyama in Japan. They published the first observation of geo-‐neutrinos three years later as a cover article in Nature (Araki et al., 2005). Since this pioneering observation, they have continued to operate and refine the detector in Japan and have deployed a second observatory, employing 300 metric tons of scintillating liquid, beneath Gran Sasso in Italy. With the joint exposure of these two detectors, amounting to about 6100 metric ton years, particle physicists have recorded 130 ± 30 geo-‐neutrino interactions. Analyses of the geo-‐neutrino data (Gando et al., 2013; Bellini et al., 2013), coupled with independent estimates of the signal expected from the crust (Enomoto et al., 2007; Coltorti et al., 2011), have directly assessed 2 to 24 TW of radiogenic heating in the mantle (Ludhova and Zavatarelli, 2013).
The measurement of mantle radiogenic heating by physicists using geo-‐neutrinos neatly overlies the range of model estimates calculated by geoscientists. Although the error on the present measurement is too large to rule out any of the models, the potential of geo-‐neutrino measurements to inform geology is clearly demonstrated. Indeed, if one combines the mantle radiogenic heat production measurement expressed as 13 ± 11 TW with the 7 ± 1 TW of radiogenic heating in the crust and the recently estimated 10 ± 5 TW of heat entering the mantle from the core (Lay, Hernlund, and Buffett, 2008), a constraint on mantle cooling of 17 ± 13 TW results. It is evident that the error on the geo-‐neutrino measurement of mantle radiogenic heating not only impedes resolution of geological models but also dominates the error on mantle cooling.
The relatively poor precision of mantle radiogenic heat production originates from both geo-‐neutrino measurement errors and geological uncertainties. Improving the precision requires more exposure to geo-‐neutrinos, strategically placed detectors, and reduction of geological uncertainties. Assuming standard observation methodologies, a preliminary analysis indicates the following precision can be achieved (Dye 2013): ± 5—7 TW with a combined exposure of ~25,000 metric ton years at four continental observatories; and ± 3—5 TW with an exposure of ~35,000 metric ton years at a single oceanic observatory. Smaller uncertainty corresponds to lower radiogenic heating. The most effective observational strategy depends on the required precision.
What are the levels of precision in the assessment of radiogenic heat production that are needed to address significant issues in geology? What observational strategies most effectively produce the required levels of precision? The answers to these questions define the roadmap for neutrino geosciences.
We plan to gather leaders in geochemistry, geodynamics, seismology, and mineral physics together with leaders in neutrino and nuclear physics to evaluate the major questions that can be addressed by increasingly more precise assessments of radiogenic heat production and the role of uncertainties in these assessments. Following organizational meetings of the conveners by teleconferencing, we propose holding at KITP during CIDER several meetings of the working group of geoscientists and particle physicists to discuss relevant issues, define project deliverables, describe project activities, and assign tasks. We anticipate producing a white paper and a journal article, which document the results of the project.