The interior of the Earth is hot. Drilling down a kilometer raises the temperature by about 25 °C. At the boundary between the crust and the mantle, it is several hundred degrees. At the core, it is hotter than the surface of the Sun.
Where does that heat come from? Two sources: the residual heat of formation, left over from when the planet coalesced four and a half billion years ago, and the ongoing radioactive decay of uranium, thorium, and potassium scattered throughout the mantle and crust. For decades, geophysicists could only estimate the relative contributions of these two sources. Now there is a third method: directly counting the antineutrinos that radioactive decay produces.
These are geo-neutrinos — the most peculiar messengers in modern geophysics.
Why the Earth glows in antineutrinos
When uranium-238, thorium-232, and potassium-40 decay inside the Earth, each decay emits an electron antineutrino (along with whatever charged particles, photons, and heat are produced). Because antineutrinos interact only weakly with matter, they stream out of the Earth’s interior essentially unimpeded, while the heat sits trapped in the mantle for tens of millions of years.
At any point on Earth’s surface, a flux of about a million antineutrinos per square centimeter per second is arriving from the planet’s interior. They have low energies — a few hundred keV to a few MeV — which makes them harder to detect than the higher-energy solar or atmospheric neutrinos, but possible.
If you can count those antineutrinos and measure their energies, you can directly infer how much uranium, thorium, and potassium is inside the Earth. That is a measurement no other technique can provide.
The detection
Two experiments have made geo-neutrino measurements: KamLAND in Japan and Borexino in Italy.
Both are large liquid scintillator detectors, with active masses of about one kiloton each. Both are buried deep underground — KamLAND in the Kamioka mine, Borexino in the Gran Sasso laboratory — to shield them from cosmic-ray backgrounds.
The detection technique is the same one Cowan and Reines used in 1956. An electron antineutrino interacts with a proton in the scintillator, producing a positron and a neutron. The positron annihilates immediately, giving a prompt flash of light. The neutron wanders for a few hundred microseconds before being captured, giving a delayed second flash. The double flash is a clean signature that almost no other process produces.
The challenge is that the same signature is also produced by reactor antineutrinos from nearby nuclear power plants — typically the dominant signal in both detectors. To extract the geo-neutrino contribution, the analysis carefully subtracts the known reactor flux, leaving the geological signal underneath.
KamLAND published its first geo-neutrino observation in 2005. Borexino reported its own measurement in 2010. Both experiments have continued accumulating events since.
What the data tells us
The combined picture from KamLAND and Borexino, as of the late 2020s, is that:
About half of the Earth’s interior heat comes from radioactive decay. The remainder comes from primordial heat and from minor sources like crystallization at the inner core boundary. This had been the prevailing geophysical assumption, but the antineutrino measurement is the first direct experimental confirmation.
Uranium and thorium are present in the mantle in roughly equal heating contributions. Each contributes about a third of the radiogenic heat. Potassium-40 contributes the remaining third.
The continental crust is enriched in radioactive elements compared to the deep mantle. This had also been geochemists’ best guess, but the geo-neutrino flux measured at different sites gives quantitative constraints on the difference.
These numbers feed back into models of mantle convection, the age of the Earth’s core, and the dynamics of plate tectonics. The Earth’s interior is famously inaccessible — the deepest borehole ever drilled reached just 12 kilometers, less than 0.2% of the planet’s radius. Geo-neutrinos provide a new probe of the remaining 99.8%.
What is still uncertain
The geo-neutrino measurements remain statistically limited. KamLAND and Borexino each see a few dozen geo-neutrino events per year. With combined data, the field has good measurements of the total radiogenic flux, but separating the contributions of the crust versus mantle, or of uranium versus thorium, requires much higher statistics.
The next major step is JUNO, the 20,000-ton scintillator detector that began full data-taking in 2025 in southern China. JUNO’s much larger mass should accumulate about as many geo-neutrino events in a year as KamLAND and Borexino collected in a decade.
Plans are also under way for dedicated geo-neutrino detectors sited in regions with minimal reactor background — for example in the deep ocean, far from any continental power plant. These would let physicists isolate the mantle contribution from the crust contribution, the most contested division in current models of the Earth’s heat budget.
Why this is unusual physics
Geo-neutrino science sits at a strange intersection. The detectors were built primarily for particle physics — to study neutrino oscillation, antineutrino spectra, and reactor monitoring. The geo-neutrino results were a side benefit, almost a bonus.
But the geological community now treats them seriously. Geo-neutrino measurements have been cited in textbooks on geophysics, in mantle convection models, and in discussions of how much primordial heat the Earth still contains.
It is one of those interdisciplinary moments that happens occasionally in physics: a tool built to answer one question turns out to be the only tool that can answer a quite different one.
The Earth, it turns out, has been glowing with antineutrinos for four and a half billion years. Only in the last twenty have we been able to actually see the light.
For the detectors involved, see KamLAND and Borexino on the detector atlas. For the basics of antineutrino detection, see Cowan and Reines, 1956.
Frequently asked
What are geo-neutrinos?
Electron antineutrinos produced by the radioactive decay of long-lived isotopes — primarily uranium-238 and thorium-232, plus smaller contributions from potassium-40 — inside the Earth. The flux is small (about 10⁶ per cm² per second at the surface) but measurable with large underground detectors.
Why measure them?
Because geo-neutrinos directly probe how much of Earth's internal heat (~47 TW total) comes from radioactive decay versus primordial sources left over from the planet's formation. Roughly half of Earth's heat is now attributed to radioactivity, but the exact fraction depends on the abundances of uranium and thorium in the mantle and crust — quantities that are otherwise hard to measure.
Who first detected them?
KamLAND in Japan reported the first significant geo-neutrino detection in 2005, after several years of accumulated data. Borexino at Gran Sasso added an independent confirmation. The combined results from these two experiments are now precise enough to constrain the radioactive content of Earth's interior.
How are they distinguished from reactor antineutrinos?
By energy. Geo-neutrinos from U-238 and Th-232 chains have endpoints around 3.3 MeV and 2.3 MeV respectively. Reactor antineutrinos extend to higher energies. By analyzing the antineutrino spectrum below 3 MeV, detectors separate the geological signal from the reactor background. Reactor-free periods (like Japan's post-Fukushima shutdown) made the geo-neutrino measurement at KamLAND cleaner.
What do geo-neutrinos tell us about plate tectonics?
They constrain the heat budget that drives mantle convection and plate motion. If radioactive heating accounts for ~half of Earth's heat flow, the remaining heat must come from primordial cooling. The split affects models of Earth's thermal evolution, the rate at which the core is cooling, and ultimately the timing of mantle dynamics over geological time.
Cite this article 5 formats
APA
Neutrino Times Editorial Team. (2025, September 2). Geo-neutrinos: listening to Earth's radioactive heart. Neutrino Times. https://neutrino-times.com/articles/geo-neutrinos-listening-to-earths-radioactive-heart/
Chicago
Neutrino Times Editorial Team. "Geo-neutrinos: listening to Earth's radioactive heart." Neutrino Times, September 2, 2025. https://neutrino-times.com/articles/geo-neutrinos-listening-to-earths-radioactive-heart/.
MLA
Neutrino Times Editorial Team. "Geo-neutrinos: listening to Earth's radioactive heart." Neutrino Times, 2 Sep. 2025, https://neutrino-times.com/articles/geo-neutrinos-listening-to-earths-radioactive-heart/.
BibTeX
@misc{neutrino-times-geo-neutrinos-listening-to-earths-radioactive-heart,
author = {Neutrino Times Editorial Team},
title = {Geo-neutrinos: listening to Earth's radioactive heart},
howpublished = {Neutrino Times},
year = {2025},
month = {sep},
url = {https://neutrino-times.com/articles/geo-neutrinos-listening-to-earths-radioactive-heart/},
note = {Accessed: 2025-09-02}
} RIS
TY - GEN TI - Geo-neutrinos: listening to Earth's radioactive heart AU - Neutrino Times Editorial Team PY - 2025 DA - 2025-09-02 PB - Neutrino Times UR - https://neutrino-times.com/articles/geo-neutrinos-listening-to-earths-radioactive-heart/ ER -