Neutrino Tomography of Earth: A New Look at the Planet's Interior
Using 10.7 years of muon neutrino data (500 GeV – 100 TeV) from the IceCube detector, including atmospheric and diffuse astrophysical fluxes, the radial density profile of the Earth has been reconstructed for the first time via the weak interaction. Neutrino absorption depends on the traversed columnar density and energy, so measuring the flux suppression as a function of zenith angle allows fitting a model of concentric homogeneous shells, accounting for fluxes, interaction cross sections, detector response, and glacial ice systematics. From the posterior density distributions, the Earth's mass and polar moment of inertia were computed — the most accurate values obtained by non-gravitational and non-seismic methods, consistent with PREM and independent gravitational measurements. The result demonstrates that neutrinos open a new way to probe planetary interiors, based on a different physical mechanism and complementing seismology and gravimetry.
Context
For nearly a century, Earth’s interior has been studied primarily through seismic waves, which are sensitive to the elastic properties of matter. These methods yielded the detailed PREM model, yet they rely on the macroscopic response of matter to gravitational and elastic forces. Neutrinos, predicted by Wolfgang Pauli almost a century ago, interact exclusively through the weak force, opening an additional window. Their ability to penetrate the planet’s bulk, partially absorbed depending on the density traversed, resembles the transit spectroscopy method used to study exoplanet atmospheres—except here, instead of a star, it’s the atmosphere, and instead of a planet, it’s Earth itself.
Methods
The analysis is based on data collected by the IceCube neutrino telescope at the South Pole. The detector registers Cherenkov radiation—flashes of light produced when charged particles travel through the ice faster than light in that medium. By measuring the flux attenuation as a function of energy, the researchers apply a kind of spectroscopy of Earth: different neutrino energies are sensitive to different depths. Out of 368,071 recorded tracks from neutrino interactions, events with energies from 500 GeV to 100 TeV were selected. For each event, the energy and zenith angle were reconstructed, the latter determining the path length through Earth. To link the observed flux attenuation to density, Earth was parameterized as a set of concentric shells of constant density. Then, using Monte Carlo methods and Bayesian analysis, by varying density parameters and numerous systematic uncertainties (including neutrino fluxes, interaction cross-section, and ice properties), a joint probability distribution was constructed.
Results
The reconstructed radial density profile, shown in Figure 2, agrees well with the standard seismic PREM model. The densities of the core, mantle, and crust, determined to within tens of percent, lie within expected ranges. From this profile, Earth’s mass was calculated: 7.25×1024 kg (68% confidence interval [6.31, 8.31]×1024 kg), which encompasses the gravitationally measured value of 5.97×1024 kg within the 95% interval. Similarly, the polar moment of inertia was found to be 1.05×1038 kg·m² ([9.04×1037, 1.24×1038] kg·m²), also consistent with the reference value of 8.01×1037 kg·m². This result is the most precise measurement to date of Earth’s internal structure using the weak interaction.
Implications
The success of neutrino tomography ushers in a new era in planetary science. We can now test models of Earth’s structure without relying solely on elastic waves and gravity. This method is especially valuable because it is based on an interaction described with high precision by the Standard Model of particle physics—the neutrino-nucleon cross-section is determined by the properties of partons inside protons and neutrons. Thus, geophysics gains a tool calibrated by the physics of the microcosm. Moreover, neutrino tomography could eventually be applied to other planets if sufficiently powerful detectors become available.
Future development
The near future of the method lies in increased statistics and reduced systematic uncertainties. The already planned IceCube upgrade and the under-construction Mediterranean telescope KM3NeT will boost the number of detected neutrinos, especially at high energies. Meanwhile, data on fluxes of atmospheric and astrophysical neutrinos, produced in objects like supernovae, neutron stars, and pulsars, as well as around black holes, are being refined. Improved models of nuclear effects in cross-sections at low Bjorken-x will help reduce uncertainties related to parton shadowing in nuclei.
Impact
Neutrino tomography will impact geophysics by providing an independent check on models of Earth’s interior; particle physics by stimulating more precise measurements of neutrino-nucleon cross-sections; and astrophysics, since it requires detailed knowledge of astrophysical neutrino fluxes, whose sources may include accreting black holes and exploding stars.
Next steps
Next steps include analyzing IceCube data with improved calibration of ice properties and incorporating information on neutrino oscillations (though negligible in this energy range). Joint fits with other experiments are also planned to reduce cross-section uncertainties.
Key open problems
Neutrino tomography is directly linked to the mystery of the composition and dynamics of Earth’s core, which generates the magnetic field. It may also help understand why Mars, a similar planet, lacks a global field. Moreover, the method bridges geophysics and particle physics, testing predictions of the Standard Model using an enormous nuclear target—Earth itself.
🎯 To detect a single neutrino, you would need a lead wall one light-year thick; yet Earth can attenuate a beam of 100 TeV neutrinos by almost half if they pass through the core.
Key numbers
- Years of data: 10.7 years
- Number of events: 368 071
- Energy range: 500 GeV – 100 TeV
- Measured mass of Earth: 7.25×10^24 kg
- Measured moment of inertia: 1.05×10^38 kg·m²