Scientists have to use some roundabout methods to weigh the Earth and measure what’s inside it—typically, they’ve used sound waves and the strength of gravity to make their calculations. But one team has weighed the Earth in a whole new way: by measuring mysterious cosmic particles that pass through it.
A team of Spanish researchers used neutrinos passing through the Earth to measure our planet’s mass and density. It’s super early days for this method, but one day perhaps it might become a useful tool for Earth scientists. And the number they got was pretty close to the figure calculated by current state-of-the-art methods.
“Our results demonstrate the feasibility of this approach to study the Earth’s internal structure, which is complementary to traditional geophysics methods,” the authors write in the study published today in Nature Physics.
Particle physics is the study of particles interacting through forces. The particles you deal with most often are the protons, neutrons, and electrons that make up atoms, which interact through gravity, electromagnetism, the strong nuclear force (which binds the components of atomic nuclei together), and the weak nuclear force (which also occurs over the tiny length scales inside the atomic nucleus and is partially responsible for some aspects of radioactive decay).
The neutrino is one of the universe’s most abundant particles (second only to the photon, the particle that makes up light). But neutrinos are difficult to detect because they interact only with the weak nuclear force. Neutrinos are constantly slamming the Earth, reaching us from the Sun as well as from outer space. They’re so weakly interacting that the detectors built to study them face down toward the Earth, picking up neutrinos that have traveled through the entire planet.
Meanwhile, there are geophysicists hoping to study the inside of the Earth, including the behavior of the core and mantle and the planet’s magnetic fields. Typically, they do that through indirect measurements, like observing how sound waves change as they pass through the planet or detecting local changes to the force of gravity.
The physicists behind the new study thought, well, we already have these neutrino detectors and neutrinos are passing through the Earth, so why don’t we try measuring the Earth with the neutrinos?
And so they did. The researchers used data from a neutrino-observing telescope at the South Pole called IceCube, which detects neutrinos that create photons after interacting with the ice through the weak force. They measured the number of neutrinos per angle that they struck the detector, where neutrinos traveling directly upward through the detector would pass through more of the Earth than the ones hitting at an angle. They compared this data to the number of neutrinos they’d expect if the Earth weren’t below the detector.
Using this data, they were able to measure the mass of the Earth, albeit very imprecisely, and got a number similar to the presently accepted value found using gravity. They also measured the mass of the core (their number was a little bigger than the accepted number). They also found that the core is denser than the Earth. Yes, we already knew this, but it’s fun to learn it again using particles passing through the Earth from space.
The researchers are very upfront about the fact that they’re not working with a lot of data—perhaps with 10 more years of observation from IceCube and other detectors under construction, the method could become useful. One researcher not involved in the study, Véronique Van Elewyck from Paris Diderot University, noted in a Nature commentary that researchers still don’t know how many neutrinos hit the Earth, and need to learn more about the optical properties of the ice to decrease the experiment’s uncertainty. Additionally, there’s still a lot of experimental uncertainty around how neutrinos interact with matter more generally, she wrote.
But with more data and more experiments (and maybe a decade or two’s time), neutrinos really might be a useful way to measure the inside of Earth. And using neutrinos to measure the inside of Earth is just about as fun as physics gets.