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Conflicts between cosmic rays and atmospheric nuclei create a continuous landless neutrinos shower. Tiny neutral particles rarely interact with other subjects, making it difficult to detect. But rarely it is never: When a neutrinos with energy of at least some TeV passes through something as big and dense as Earth, there is a small but unacceptable chance of being absorbed. The idea of analyzing this absorption to learn about the internal structure of the planet was first proposed more than 40 years ago. But until recently, the neutrinos detectors were not large enough to observe enough particles for such a study.
The IceCube Neutrino Observatory, which includes a cubic kilometer of ice near the South Pole, collected its first set of data in full operation in 2011-12. Sergio Palomares-Ruiz and Andrea Donini at the Institute of Cosmetic Physics (Spain's National Research Council and University of Valencia) and Jordi Salvado at the Cosmos Sciences Institute (University of Barcelona) have now used the publicly available data from the original execution to investigate the radial density of the Earth. They considered atmospheric neutrinos, whose number and distribution of energy were known, and examined how much neutrinos they did through the Earth on the detector. At each detection angle, neutrinos cross a different density landscape, as illustrated in the above figure. The researchers used the probability of absorbing a neutrinos, which depends both on the length of the path and on the density, in order to construct a one-dimensional radial density profile.
The number of atmospheric neutrinos with an energy exceeding 5 TeV that passed from the center of the Earth was about half that what researchers would expect if they were not absorbed. Attenuation decreased by reducing the energy and the length of the route until it became intolerable. Researchers used the neutrinos reduction to determine the average density of each layer in the shape. Although the neutrinos-based measurements are highly uncertain, the calculated Earth's mass value is in good agreement with the current acceptable value, as shown by the red dotted line in the left-hand graph below. The researchers found that most of the Earth's mass is at its core, as shown in the right graph, from 33% estimated by geophysical density models (red dashed line). However, the price previously accepted was 68% reliable, so the difference is not statistically significant.
Neutral tomography of the Earth's internal structure provides an independent measure of its makeup that can be used in parallel with seismic data. The method demonstrates the first measurement of Earth's mass using weak force instead of gravitational force. Neutrines also provide additional information about the Earth's core: Few seismic waves cross the core while longer neutrinos paths do. The measurements are expected to improve as more data will be available from IceCube and the future KM3NeT detector mileage across the Mediterranean floor. (A. Donini, S. Palomares-Ruiz, J. Salvado, Nat. Of course., 2018, two: 10.1038 / s41567-018-0139-1).
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