Abstract: A quarter of a century ago, particle physicists began detecting the Earth's geoneutrino flux, opening up a new way to investigate our planet's internal composition. This approach now provides an unparalleled view into Earth's deep interior. Above all, it provides a unique way to quantify the total amounts of uranium and thorium inside the planet—something that had previously eluded direct measurement—and significantly tightens the range of acceptable models for Earth's bulk composition. It also yields an independent determination of the chondritic Th/U ratio, a key parameter underpinning the widespread assumption that the rocky planets in our solar system possess chondritic proportions of refractory elements. In addition, by establishing how much radiogenic fuel remains within Earth, we can assess how much primordial heat the planet still preserves relative to its residual radioactive energy sources. Besides refining our understanding of Earth's thermal evolution, this information illuminates the energy budgets that drive plate tectonics, volcanism, mantle convection, and the geodynamo. The geodynamo, in turn, produces Earth's magnetic field, which serves as an essential shield for life by deflecting damaging cosmic radiation.
These pioneering efforts were neither simple nor free from obstacles and errors. However, once more, we have seen that we can advance further when we work together and collaboratively. We now possess U & Th results from four detectors located in the northern hemisphere. In addition, we have compelling plans for the next generation of detectors, including instruments capable of measuring potassium geoneutrinos and a mobile, seafloor-based detector designed to obtain direct observations of "mantle-only" geoneutrino flux with directional sensitivity.
The opportunity to detect ⁴⁰K geoneutrinos constitutes discovery-level science and has the potential to significantly transform our understanding of Earth's budget of moderately volatile elements. Currently, this budget is the subject of active debate and is only loosely constrained. Estimates of the potassium content of the bulk silicate Earth (BSE) vary by about a factor of two. Precisely establishing the planet's potassium abundance would essentially finalize the inventory of the principal heat-producing elements (HPE), since K, Th, and U together account for 99.5% of the radiogenic power fueling Earth's internal heat. In addition, it would yield crucial insight into the total quantities of water and CO₂ sequestered within the planet.
Determining the distribution of HPE inside the Earth is a grand challenge in Earth sciences. Seismologists have discovered Large Low-Velocity Provinces (LLVPs)—two continent-sized regions (up to 1000 km tall) in the lowermost mantle that exhibit unusually low seismic velocities. Establishing whether these regions are enriched in HPEs is crucial for understanding their nature and origin. However, current methods cannot resolve the spatial pattern of the geoneutrino signal. Directional geoneutrino detection can overcome the intrinsic non-uniqueness of flux-only measurements, providing a new observational approach to probe compositional variations in the deep Earth.
Quantifying the HPE stored in Earth allows us to constrain the cooling history of the mantle more tightly. However, due to current uncertainties (approaching the 100% level), models of the thermal evolution of the Earth still produce a wide range of estimates for the remaining primordial and radiogenic heat in the interior. Upcoming geoneutrino observations are expected to substantially narrow these uncertainties (to ≤ 15%) in both the magnitude and spatial distribution of radiogenic heat, thus making the heat flux from the core (10 ± 5 TW) the dominant unknown.
Moreover, neutrino oscillation measurements in the 2–8 GeV energy range provide a means to determine the electron density in the Earth's core and mantle, which can then be used to infer the hydrogen content of these regions. In addition, neutrinos with energies exceeding a few TeV are absorbed by the Earth and are mainly employed to constrain density contrasts across internal boundary layers.
Consequently, the emergence of Neutrino Geophysics has ushered in a new and transformative era in interdisciplinary geoscience, offering insights into our planet that are inaccessible through any other observational technique.