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Neutrino Geoscience: Review, Survey, Future Prospects

William F. McDonough1,2,3 and Hiroko Watanabe1

1Research Center for Neutrino Science, Tohoku University, Sendai, Japan

2Department of Earth Sciences, Tohoku University, Sendai, Japan

3Department of Geology, University of Maryland, College Park, Maryland, USA

ABSTRACT

The Earth's surface heat flux is 46 ± 3 TW (terawatts, 1012 watts). Although many assume we know the Earth's abundance and distribution of radioactive HPEs (i.e., U, Th, and K), estimates for the mantle's heat production varying by an order of magnitude and recent particle physics findings challenge our dominant paradigm. Geologists predict the Earth's budget of radiogenic power at 20 ± 10 TW, whereas particle physics experiments predict TW (KamLAND, Japan) and TW (Borexino, Italy). We welcome this opportunity to highlight the fundamentally important resource offered by the physics community and call attention to the shortcomings associated with the characterization of the geology of the Earth. We review the findings from continent-based, physics experiments, the predictions from geology, and assess the degree of misfit between the physics measurements and predicted models of the continental lithosphere and underlying mantle. Because our knowledge of the continents is somewhat uncertain ( TW), models for the radiogenic power in the mantle (3.5-32 TW) and the bulk silicate Earth (BSE; crust plus mantle) continue to be uncertain by a factor of ~10 and ~4, respectively. Detection of a geoneutrino signal in the ocean, far from the influence of continents, offers the potential to resolve this tension. Neutrino geoscience is a powerful new tool to interrogate the composition of the continental crust and mantle and its structures.

1.1 INTRODUCTION

Core-mantle evolution involves understanding Earth's differentiation processes, which established the present-day distribution of the HPEs, and its dynamic consequences (i.e., the radiogenic heat left in the mantle powering mantle convection, plate tectonics, and the geodynamo). The energy to drive the Earth's engine comes from two different sources: primordial and radiogenic. Primordial energy represents the kinetic energy inherited during accretion and core formation. Radiogenic energy is the heat of reaction from nuclear decay. We do not have a constraint on the proportion of these different energy sources driving the present-day Earth's dynamics. In turn, this means that we do not have sufficient constraint on the thermal evolution of the planet, aside from first-order generalities. You might ask, is this important? We ask the question - how much energy (and time) is left to keep the Earth habitable?

We understand that the Earth started out hot due to abundant accretion energy, the gravitational energy of sinking metal into the center, a giant impact event for the formation of the Earth's Moon, and energy from short-lived (e.g., 26Al and 60Fe) and long-lived (K, Th, and U) radionuclides. From this hot start the planet should quickly lose some of its initial energy, although the amount and rate are unknowns. There are many significant unknowns regarding the thermal evolution of the Earth: (1) the nature and presence (or absence) and lifetime of an early atmosphere, which has a thermal blanketing effect; (2) the compositional model for the Earth, particularly the absolute abundances of the HPEs (K, Th, and U); (3) the cooling rate of the mantle (present-day estimates: 100 ± 50 K/Ga); and (4) the rate of crust formation and thus extraction of HPEs from the mantle.

The recent recognition (Krauss et al., 1984) and subsequent detection (Araki et al., 2005) of the planet's geoneutrino emission have opened up a new window into global scale geochemistry of the present-day Earth. The measurement of this flux presents Earth scientists with a transformative opportunity for new insights into the composition of the Earth and its energy budget. For the most part, solid Earth geophysics measures and parameterizes the present-day state of the planet. In contrast, solid Earth geochemistry measures and parameterizes its time-integrated state, mostly on a hand sample scale and then extrapolates these insights to larger scales. The advent of measuring the Earth's geoneutrino flux allows us, for the first time, to get a global measure of its present-day amount of Th and U.

This chapter is organized as follows: we provide the rationale for the field of neutrino geoscience and define some terms (section 1.2). We review the existing and developing detectors, the present-day detection methods, and future technologies (section 1.3). We discuss the latest results from the physics experiments (section 1.4). We present the range of compositional models proposed for the Earth (section 1.5) followed by a discussion of the geological prediction of the geoneutrino fluxes at various detectors (section 1.6). We finish with a discussion on determining the radioactive power in the mantle (section 1.7) and future prospects (section 1.8).

1.2 NEUTRINO GEOSCIENCE

The field of neutrino geoscience focuses on constraining the Earth's abundances of Th and U and with these data we can determine: (1) the absolute concentration of refractory elements in the Earth and from that determine the BSE's composition (crust plus mantle), and (2) the amount of radiogenic power in the Earth driving the planet's major dynamic processes (e.g., mantle convection, plate tectonics, magmatism, and the geodynamo). These two constraints set limits on the permissible models for the composition of the Earth and its thermal evolutionary history.

First, the refractory elements are in constant relative abundances in all chondrites. There are 36 of these elements (e.g., Al, Ca, Sr, Zr, REE, Th, and U) and by establishing the absolute abundance of one defines all abundances, since refractory elements exist in constant ratios to each other (McDonough & Sun, 1995). Most of these elements are concentrated in the bulk silicate Earth, but not all (e.g., Mo, W, Ir, Os, Re, etc.) and these latter ones are mostly concentrated in the metallic core. Knowing the Earth's abundance of Ca and Al, two of the eight most abundant elements (i.e., O, Fe, Mg, Si, Ca, Al, Ni, and S) that make up terrestrial planets (i.e., 99%, mass and atomic proportions) define and restrict the range of accepted compositional models of the bulk Earth and BSE.

Second, the decay of 40K, 232Th, 238U, and 235U (i.e., HPE) provides the Earth's radiogenic power, accounts for 99.5% of its total radiogenic power, and is estimated to be TW (1 TW = 1012 watts). This estimate, however, assumes a specific BSE model composition (McDonough & Sun, 1995; Palme & O'Neill, 2014). It must be noted that there is no consensus on the composition of the BSE, and so predictions from competing compositional models span from about 10-38 TW (Agostini et al., 2020; Javoy et al., 2010). This uncertainty in our present state of knowledge means that the field of neutrino geoscience plays a crucial role in resolving fundamental questions in Earth sciences.

1.2.1 Background Terms

The field of neutrino geoscience spans the disciplines of particle physics and geoscience, including geochemistry and geophysics. The following list of terms are offered to support this interdisciplinary research field.

Alpha () decay: a radioactive decay process that reduces the original nuclide () by four atomic mass units by the emission of a He nucleus and reaction energy (). Commonly, the particle is emitted with between 4 and 9 MeV (1 MeV = 106 eV) of discrete kinetic energy. The basic form of decay is as follows:

Beta decay : a radioactive decay process that transforms the original nuclide () into an isobar (same mass ) with the next lower proton number () during either electron capture () or decays or, alternatively, the next higher proton number () during decay. During each decay, there is an exchange of two energetic leptons (i.e., beta particles) and reaction energy (). Basic forms are as follows:

Beta particles : first-generation energetic leptons, either matter leptons (electrons and neutrinos: and ) or antimatter leptons (positrons and antineutrinos: and ).

Chondrite: an undifferentiated stony meteorite containing chondrules (flash-melted spheres, sub-mm to several mm across), matrix [fine grained (micron scale) aggregate of dust and crystals], and sometimes Ca-Al-inclusions and other refractory phases. They are typically mixtures of silicates and varying amounts of Fe-Ni alloys and classified into groups based on their mineralogy, texture, and redox state. Three dominant groups are the carbonaceous, ordinary, and enstatite type chondrites, from most oxidized to reduced, respectively. Isotopic observations are also used to create a twofold classification of chondrites and related meteorites (i.e., the NC and CC groups). The NC (non carbonaceous) group includes enstatite and ordinary chondrites and is believed to have formed in the inner solar system inside of Jupiter. The CC (carbonaceous) group includes carbonaceous chondrites and is believed to have formed in the outer solar...