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Geoneutrinos are naturally produced electron antineutrinos from radioactive decays inside Earth. By detecting a tiny number of them, scientists can estimate how much heat uranium and thorium generate inside the planet—but the measurement is not a direct image, and separating the mantle signal from nearby crust requires geological models.
What are geoneutrinos?
Geoneutrinos are electron antineutrinos created by natural radioactive beta decays in Earth’s crust and mantle. The most useful detectable geoneutrinos come from decay chains of uranium-238 and thorium-232. Potassium-40 also produces antineutrinos and contributes to Earth’s radiogenic heat, but its antineutrinos have lower energies, which affects detector sensitivity.
They are not made by the detector. They arise from radioactive elements in Earth and can escape from deep underground because antineutrinos interact with matter only rarely. That same rarity makes them difficult to detect: experiments need large underground detectors and long observation periods.
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What can geoneutrinos tell us about Earth?
Geoneutrinos provide an independent way to study radioactive elements inside Earth and the heat those elements produce. The measured energy distribution and event rate help constrain the uranium-and-thorium contribution to Earth’s radiogenic power.
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That is one component of Earth’s heat loss, not the whole heat budget. Earth also releases primordial heat left from its formation and subsequent evolution. A geoneutrino measurement therefore informs the radiogenic part of the picture rather than directly measuring all heat flowing from the planet.
How do scientists measure them?
- Record rare interactions. Large underground detectors observe candidate antineutrino interactions over years. The underground setting helps reduce interference from cosmic-ray particles.
- Separate the signal from backgrounds. Researchers analyze event energies and rates, accounting for other antineutrinos, including those from nuclear reactors.
- Estimate the crust’s contribution. A detector receives geoneutrinos from nearby crust as well as from the mantle. Scientists use local geological information to estimate the lithosphere’s contribution.
- Infer what remains. After constraining the crustal contribution, researchers can estimate the mantle signal and use models to translate the observed signal into radioactive power.
The conversion from detected events to power is not a simple count-to-heat calculation. It depends on assumptions about the distribution and abundance of radioactive elements, the detector’s location, the uranium-to-thorium relationship, and how backgrounds and uncertainties are treated.
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What do the measurements say about Earth’s radiogenic heat?
Different published values describe different quantities and rely on different assumptions. A detected U/Th signal, an inferred mantle signal, and total radiogenic heat—including an assumed contribution from potassium-40—are not interchangeable.
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| Study or synthesis | Reported result | What the figure means |
|---|---|---|
| KamLAND Collaboration, 2005 | For an assumed thorium-to-uranium mass concentration ratio of 3.9, the 90% confidence interval for the total detected geoneutrino count was 4.5–54.2, compared with a model central value of 19; the paper set a 60 TW upper limit on U/Th radiogenic power. | An early measurement-era result, useful as historical context rather than a current central estimate. Nature report |
| KamLAND Collaboration, 2011 | 20.0 +8.8/−8.6 TW from U-238 and Th-232. | A reported U/Th radiogenic contribution to Earth’s heat flux. The paper also cited 44.2 ± 1.0 TW as the contemporary estimate of total heat flux to space available to that study; that is historical context, not a claim of a current global measurement. KamLAND paper |
| Borexino Collaboration, 2020 | 47.0 TNU, with +18.3/−17.2% total precision; 52.6 geoneutrinos within a 68% interval. | Based on 3,262.74 days of data collected from December 2007 to April 2019. The mantle estimate used knowledge of the local crust. Borexino analysis |
| Borexino Collaboration, 2020 | 38.2 +13.6/−12.7 TW total radiogenic heat. | A model-dependent total that assumes a mantle potassium-40 contribution and lithospheric heat contribution. It should not be read as a direct detector measurement or treated as interchangeable with a U/Th-only estimate. Borexino analysis |
| 2022 review | About 20 TW total present-day radiogenic power, or about 16 TW from U and Th alone. | A combined-analysis and model-dependent estimate, sensitive to lithosphere models—not an exact, model-free value. Earth and Planetary Science Letters review |
The spread is not a contradiction that can be resolved by choosing one number without context. Results differ according to what is being counted, the local crust beneath and around a detector, assumptions about uranium and thorium, whether potassium-40 is included, and how statistical and systematic uncertainty is handled. The 2020 Borexino total-heat estimate is notably conditional on its assumptions; the 2022 review’s preferred combined estimate is also model-dependent.
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What geoneutrinos cannot show
- They do not provide a detailed underground map. A detector measures rare events, not a picture of the interior.
- They do not isolate the mantle automatically. The observed signal includes nearby crust, so mantle conclusions depend on estimating that crustal contribution.
- Uranium and thorium do not account for every radioactive heat source. Potassium-40 contributes to radiogenic heat, but its lower-energy antineutrinos are harder to detect.
- An event rate is not a heat value by itself. Reporting power in terawatts requires model-based interpretation and assumptions.
Why detector location matters
Crustal composition varies by location, so detectors in different places receive different local contributions. A geographically distinct experiment can help test how much of a signal comes from the mantle rather than the lithosphere. SNO+ describes the goal of combining results from Canada’s detector with KamLAND and Borexino to infer uranium and thorium abundance in the deep mantle; that programme description explains the scientific rationale, not the latest status or a result. SNO+ science programme
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