Observations from geology, geophysics, geochemistry and meteoritics allow for a range of non-unique solutions for the composition of the Earth. The relative proportion of Fe, O, Mg and Si in chondritic meteorites individually varies by ~15% each and reflects spatial and temporal differences in where these rocks formed in the early solar nebula. Likewise, refractory elements have 25% variation in their relative abundance, which translates into a factor of two in absolute concentration difference of these elements. Even greater enrichment factors of these elements occur when the volatile inventory (e.g., H 2 O, CO 2 , N 2 ) is mostly lost, as during terrestrial planet assembly. Finally, because the Earth’s core is taken to have negligible amounts of Th, U and K27,28,29, due to their limited solubility in core-forming metallic liquids, this becomes another 50% enrichment factor in the radiogenic elements in the silicate Earth. Consequently, compositional models predict between 10 and 30 ng/g U (and Th/U = 3.9, the chondritic ratio) for the silicate Earth. Given the planetary ratio of Th/U and K/U (1.4 ×104)30 and the absolute U content of the silicate Earth, its heat production for a 10 ng/g U model roughly corresponds to a surface heat flow of 10 TW and likewise 30 ng/g U to ~30 TW. Estimates of the Earth’s radiogenic heat production thus vary from low power models (10–15 TW of power from K, Th and U), through medium power models (17–22 TW) and to high power models (>25 TW)31. Accordingly, detecting the Earth’s flux of geoneutrinos can provide crucial data to test competing theories of the bulk Earth.

Two observatories, one in Japan (KamLAND) and one in Italy (Borexino), are making ongoing measurements of the surface flux of geoneutrinos at energies above the IBD threshold energy E ν ≥ 1.8 MeV. At Japan the flux measurement is (3.4 ± 0.8) × 106 cm−2 s−112, while at Italy the flux measurement is (4.3 ± 1.3) × 106 cm−2 s−113. Note that it is sometimes convenient to express geoneutrino flux as a rate of recorded interactions in a perfect detector with a given exposure using the Terrestrial Neutrino Unit (TNU)32, however in this work we focus on simple flux ( ) and luminosity ( ).

AGM models the Earth as a 3D point cloud consisting of roughly 1 million points. National Oceanic and Atmospheric Administration (NOAA) Earth TOPOgraphical 1 (ETOPO1) “ice” data33 is used to provide worldwide elevations with respect to the World Geodetic System 84 (WGS84) ellipsoid. Zero-tide ocean surface corrections to the WGS84 ellipsoid were obtained from the National Geospatial-Intelligence Agency (NGA) Earth Gravitational Model 200834 (EGM2008) for modeling the ocean surface elevations around the world. Underneath these surface elevations we model 8 separate crust layers using CRUST 1.035, shown in Fig. 4, as well as a 9th adjoining layer per Huang et al.36 which reaches down to the spherical mantle, creating a seamless earth model. Certain crust tiles which are too large (about 200 km across at the equator) to be adequately modeled as point sources are instead modeled as collections of smaller tiles using numerical integration, which recursively subdivides large tiles into progressively smaller sub-tiles until the contribution of each is less than 0.001 TNU.

Geoneutrino flux is produced from the decay of naturally occurring radioisotopes in the mantle and crust: 238U, 232Th, 235U, 40K, 87Rb, 113Cd, 115In, 138La, 176Lu and 187Re37. However, we only consider 238U and 232Th in our flux maps as all other elements’ energy spectrum is considerably below the IBD energy threshold of E ν ≥ 1.8 MeV. All abundances for the crust and mantle can be seen in Table 1. As shown in Table 2, K is the largest contributor to luminosity but its energy is below the IBD threshold. All elements other than U, Th and K have a negligible contribution to the Earth’s luminosity.

Table 1 AGM2015 distribution and properties of U, Th and K, which are the main emitters of electron antineutrinos. Full size table

Table 2 Contribution of geoneutrino luminosities L in AGM2015 for 238U, 232Th and 40K emitted by the Earth. Full size table

Successful detection of below 1.8 MeV remains elusive; if successful the incorporation of the remaining radioisotopes would be beneficial to future versions of AGM. The Earth’s core was assumed to have no significant contribution to the flux due to limiting evidence for a georeactor38 and no appreciable amount of 238U, 232Th, or 40K isotopes27. While certain core models support upper limits of K content at the ~100 ppm level28, which would be sufficient for up to ~1–2 TW of radiogenic heating in the present day, “constraints on K content are very weak”29 and in the absence of stronger evidence we’ve chosen to assume a K-free core.

Mantle abundances were derived from empirical geo-neutrino measurements at KamLAND12 and Borexino13. We deconstructed the reported geo-neutrino flux from each observation into separate contributions from U and Th according to a Th/U ratio of 3.9. From each of these, we subtracted the predicted crust flux contributions36 at each observatory, averaging the asymmetric non-gaussian errors, to arrive at estimates of the mantle contributions. We then combined the estimates of the mantle U flux and the mantle Th flux contributions from each observation in a weighted average. The resulting best estimates for the mantle U and Th flux contributions were finally converted to homogeneously distributed mantle abundances using the spherically symmetric density profile of the Preliminary Reference Earth Model (PREM)39 along with a corresponding correction to account for neutrino oscillations. Corresponding values for K were found by applying a K/U ratio of 13,800 ± 130030. The resulting AGM2015 U, Th and K mantle abundances are presented in Table 1. The main sources of uncertainty in these estimates are the observational errors in the flux measurements and limited knowledge of the subtracted crust fluxes. A detailed description of the methods and relevant conversion factors used here are presented in Dye31.

AGM2015 neutrino luminosities are for total numbers of neutrinos. Although almost all are originally emitted as electron antineutrinos, on average only ~0.55 of the total remain so due to neutrino oscillations. We calculate the total Earth luminosity to be s−1. A detailed breakdown of 238U, 232Th and 40K geoneutrino luminosity from the lithosphere and mantle can be seen in Table 2 (for all energies), as well as in Table 3 for E ν ≥ 1.8 MeV. Figure 5 shows the combined AGM2015 crust + mantle flux.

Table 3 Contribution of geoneutrino luminosities L in AGM2015 above the IBD threshold E ν ≥ 1.8 MeV for 238U, 232Th and 40K emitted by the Earth. Full size table