Neutrinos reveal Earth’s inner structure

Collisions between cosmic rays and atmospheric nuclei create a constant terrestrial shower of neutrinos. The tiny neutral particles rarely interact with other matter, which makes detecting them challenging. But rarely is not never: When a neutrino with an energy of at least a few TeV passes through something as large and dense as Earth, there is a small but nonnegligible probability that it will be absorbed. The idea of analyzing that absorption to learn about the planet’s inner structure was first proposed more than 40 years ago. But until recently, neutrino detectors were not big enough to observe a sufficient number of particles for such a study.

The IceCube Neutrino Observatory, which encompasses a cubic kilometer of ice near the South Pole, collected its first data set at full operation in 2011–12. Sergio Palomares-Ruiz and Andrea Donini at the Institute of Corpuscular Physics (Spanish National Research Council and the University of Valencia) and Jordi Salvado at the Institute of Cosmos Sciences (University of Barcelona) have now used the publicly available data from that initial run to investigate Earth’s radial density. They considered atmospheric neutrinos, whose number and energy distributions are known, and looked at how many neutrinos made it through Earth to the detector. At each detection angle, the neutrinos traversed a different density landscape, as depicted in the figure above. The researchers used the probability of a neutrino being absorbed, which depends on both path length and density, to construct a one-dimensional radial density profile.

The number of atmospheric neutrinos with energies above 5 TeV that passed through the center of Earth was about half of what researchers would expect if none were absorbed. The attenuation diminished with decreasing energy and path length until it became unmeasurable. The researchers used the neutrino reduction to determine the average density of each layer in the figure. Though the neutrino-based measurements have a large uncertainty, the calculated value of Earth’s mass is in good agreement with the currently accepted value, as shown by the red dashed line in the left graph below. The researchers also found a larger fraction of Earth’s mass to be in its core, shown in the right graph, than the 33% estimated by geophysical density models (red dashed line). However, the previously accepted value falls within the 68% credible interval, so the difference is not statistically significant.

Neutrino tomography of Earth’s inner structure provides an independent measurement of its makeup that can be used alongside seismological data. The method demonstrates the first measurement of Earth’s mass using the weak force instead of the gravitational force. Neutrinos also provide additional information about Earth’s core: Few seismic waves cross the core, whereas the longest neutrino paths do. The measurements are expected to improve as more data become available from IceCube and the future kilometer-scale KM3NeT detector on the floor of the Mediterranean Sea. (A. Donini, S. Palomares-Ruiz, J. Salvado, Nat. Phys., 2018, doi:10.1038/s41567-018-0319-1 .)