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Geoneutrinos reveal that radioactive uranium and thorium inside Earth are producing heat, and their measured energy and rate can constrain how much of those elements is present. But today’s detectors do not make a layer-by-layer map: the signal combines contributions from different depths, and estimating the mantle’s share depends on subtracting a geologically modeled crust contribution.
What geoneutrinos are and how detectors find them
Geoneutrinos are electron antineutrinos released in radioactive decays inside Earth. Decay chains involving uranium-238 and thorium-232 produce both antineutrinos and heat. Potassium-40 also contributes to Earth’s radiogenic heat and produces antineutrinos, but current detectors are not sensitive to potassium-40 geoneutrinos.
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Antineutrinos interact so weakly that many pass through Earth without interacting. A large detector deep underground can nevertheless register some of them. The detector records interaction events and their energies; it does not directly measure an isotope concentration or a quantity of heat. Scientists interpret the event rate and energy spectrum using neutrino physics, background estimates, and models of where radioactive elements are distributed inside Earth.
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What the event rate and energy spectrum tell scientists
- Event rate: constrains the overall U/Th geoneutrino signal and, through modeling, the amount of uranium and thorium contributing to it.
- Energy spectrum: carries information that can help distinguish uranium from thorium contributions. The precision of that separation depends on the number and quality of recorded events.
- Radiogenic heat: is inferred from the radioactive-element content and decay physics. It is a model-derived estimate, not a heat measurement made by the detector.
The distinction matters: measuring antineutrino events is a direct observation of a particle signal from Earth, while assigning that signal to particular layers and converting it into heat require interpretation.
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What geoneutrinos can reveal about Earth
Radioactive heat inside the planet
Uranium and thorium geoneutrinos provide a particle-based constraint on the radiogenic part of Earth’s heat budget. The results help scientists test models of the bulk silicate Earth—the silicate portion represented by crust and mantle—and evaluate how much heat radioactive decay could supply. They do not, by themselves, account for all sources of Earth’s internal heat.
Constraints on uranium and thorium
The combined event rate constrains the U/Th signal, and spectral information can help constrain the relative contributions of the two elements. Those constraints are not a unique solution for Earth’s composition: interpretations may depend on an assumed thorium-to-uranium ratio and on whether the mantle is treated as homogeneous or as having layers.
A test of mantle models, if the crust can be accounted for
Crustal rocks contain uranium and thorium, so a detector on land sees a substantial contribution from the crust as well as any mantle signal. To estimate the mantle component, scientists model the local crust and subtract its expected contribution from the measured signal. The result therefore depends on local geology and on the crust model, not just on the detector’s event count.
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Borexino’s 2020 analysis used detailed knowledge of the local crust to extract a mantle signal and excluded a zero-mantle-signal hypothesis at 99.0% confidence. That is evidence for a mantle contribution within that analysis; it is not a direct image or a model-free measurement of mantle composition.
What published measurements show—and what their numbers mean
The results below are not interchangeable estimates of one directly measured quantity. Some are fitted event signals, while the heat figures are interpretations made under specified assumptions.
| Analysis | Reported result | What it represents |
|---|---|---|
| Borexino Collaboration, 2020 | 47.0 +8.4/−7.7 (statistical) +2.4/−1.9 (systematic) TNU | U/Th geoneutrino signal fitted from 3,262.74 days of data collected from December 2007 through April 2019. The collaboration reported total precision of +18.3/−17.2%. |
| Borexino Collaboration, 2020 | 21.2 +9.5/−9.0 (statistical) +1.1/−0.9 (systematic) TNU | Extracted mantle signal, using detailed local-crust knowledge; the zero-mantle-signal hypothesis was excluded at 99.0% confidence in that analysis. |
| Borexino Collaboration, 2020 | 38.2 +13.6/−12.7 TW | Total Earth radiogenic-heat estimate under the analysis’s assumptions, including an 18% mantle potassium-40 contribution and lithosphere radiogenic heat of 8.1 +1.9/−1.4 TW. This is a model-dependent heat estimate, not a detector-only measurement. |
| KamLAND Collaboration, 2022 | 174 +31/−29 U/Th geoneutrinos | Fitted combined count when uranium and thorium contributions were allowed to vary. Fixing the chondritic thorium-to-uranium mass ratio at 3.9 gave 183 +29/−28. |
| KamLAND Collaboration, 2022 | High-heat model prediction disfavored at 99.76% confidence for a homogeneous mantle, and 97.9% for a sunken-layer mantle | Model comparisons under those stated mantle assumptions; the confidence levels do not make the result independent of the assumed interior model. |
| KamLAND Collaboration, 2005 | 60 TW upper limit | Historical result from the first search. The paper characterized its statistical power as limited, so this figure should not be treated as a current precision estimate. |
TNU means terrestrial neutrino unit, a way of expressing an expected interaction rate normalized to the number of target protons in the detector. The different Borexino and KamLAND results above reflect different analyses and reported quantities; they should not be collapsed into one exact global value.
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What geoneutrinos cannot currently tell us
They do not provide a detailed interior map
Large subsurface detectors can record the energies of these infrequent events, but not their directions with useful precision. As Stephen T. Dye explains in his 2012 review, “Large subsurface detectors efficiently record the energy but not the direction of the infrequent interactions of the highest-energy geoneutrinos, originating only from uranium and thorium.” Without useful event direction, the observations are integrated constraints rather than a layer-by-layer or location-by-location image.
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A measured U/Th signal does not, on its own, reveal how much came from nearby crust versus deeper mantle. Mantle estimates rely on crust models, while assumptions about the thorium-to-uranium ratio and mantle structure affect how the observations translate into composition and heat. Different model choices can therefore produce different inferred heat budgets from the measured signal.
They do not measure potassium-40 geoneutrinos with current methods
Although potassium-40 is an important potential contributor to radiogenic heat, the 2022 KamLAND analysis states that existing detectors are insensitive to its geoneutrinos. A heat estimate that includes potassium therefore relies on assumptions or other information rather than a current direct potassium-geoneutrino measurement.
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They do not establish detailed core chemistry
The measurements discussed here constrain uranium and thorium decay signals and the associated radiogenic power. They do not establish the detailed chemical composition of Earth’s core.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Why results depend on detector location and background control
Continental detectors face a strong crust contribution, and local differences in crustal geology affect how confidently that contribution can be estimated. Reactor antineutrinos and other backgrounds must also be modeled. More exposure and improved event selection have helped later analyses, but event statistics and background treatment remain part of the uncertainty.
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An oceanic detector is a prospective way to improve mantle separation: with less crustal contribution, its signal could offer tighter constraints than a continental measurement. Dye’s review presents this as a potential advantage, not as a result from an existing oceanic geoneutrino observation.
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What future measurements may add
More events and better energy-spectrum precision could improve separation of uranium and thorium contributions and strengthen tests of interior models. A 2026 JUNO sensitivity study projects that, in less than a year, the detector could collect a geoneutrino sample comparable in size to the entire existing world dataset. That is a forecast, not a reported JUNO detection. The study’s sensitivity estimate accounts for the site, reactor backgrounds, detector uncertainties, and TAO constraints.
Two capabilities remain especially important for extending what the signal can reveal: techniques sensitive to potassium-40 geoneutrinos and methods that determine event direction well enough to add spatial information. Until such methods are available, geoneutrinos are best understood as a powerful but integrated probe of radioactive heat and U/Th abundance—not a direct scan of Earth’s interior.
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