Puny particles have given scientists a glimpse inside the Earth.
For the first time, physicists have measured the planet’s mass using neutrinos, minuscule subatomic particles that can pass straight through the entire planet. Researchers also used the particles to probe the Earth’s innards, studying how the planet’s density varies from crust to core.
Typically, scientists determine Earth’s mass and density by quantifying the planet’s gravitational pull and by studying seismic waves that penetrate the globe. Neutrinos provide a completely independent test of the planet’s properties. Made using data from the IceCube neutrino observatory at the South Pole, the new planetary profile agreed with traditional measurements, a trio of physicists reports November 5 in Nature Physics.
To make the measurement, the scientists studied high-energy neutrinos that were produced when protons and other energetic particles from space slammed into the Earth’s atmosphere. These neutrinos can zip clean through the entire Earth, but sometimes they smash into atomic nuclei and are absorbed instead. How often neutrinos get stopped in their tracks reveals the density of the stuff they’re traveling through.
Neutrinos that arrived at the IceCube detector from different angles probed different layers of the Earth. For example, a neutrino coming from the opposite side of the planet, at the North Pole, would pass through the Earth’s crust, mantle and core before reaching the South Pole. But one that skimmed in at an angle might pass through only the crust. By measuring how many neutrinos came from various angles, the team inferred the densities of different parts of the Earth and its total mass.
The technique doesn’t yet reveal anything new about the planet. But one day it might help scientists determine whether all of Earth’s mass comes from normal matter. Perhaps some of the mass is due to something that shuns neutrinos, such as a type of dark matter, a shadowy substance that scientists believe must exist to account for missing mass observed in measurements of other galaxies. Neutrinos could help physicists nail down whether the Earth harbors such dark matter within.