The earth is too large to fit into an X-ray scanner or MRI, so a group of physicists in Spain have discovered a fascinating new way of looking inside – they used subatomic particles that constantly flow through the planet.
The team examined the interior of the Earth using neutrinos – and even used them to carry out a completely new measurement of the mass of our planet.
Neutrins are weird little things. They are one of the most abundant molecules in the universe, and yet they are difficult to detect. They are similar to electrons, but they do not have an electric charge, and their mass is almost zero, so they interact very little with normal matter when they flow through the universe at a speed close to the speed of light.
Billions of neutrinos are now combing your body. You can see why they are called "ghost particles".
While historically our detection efforts were weak, the IceCube neutrino detector in Antarctica opened a new, brave world of neutrino science.
Using the IceCube's annual data, physicists at the Institute of Particle Physics (IFIC) in Valencia, Spain, could even study the guts of the Earth.
It worked almost like X-rays. When you are x-rayed, rays are transmitted through your body. They usually pass through soft tissues such as muscles and organs. However, a denser material, such as a bone, absorbs the beams at a higher rate, so fewer X-rays hit the detector on the other side, creating an image of your skeleton.
Instead of X-rays, the team used atmospheric neutrinos whose currents arise when energy particles from space collide with the Earth's atmosphere. They then pass through the Earth – but they can be absorbed by the atomic nuclei of the material they pass through.
The denser the material, the higher the rate of absorption; which in turn allowed the research team to study the density of the planet.
"The use of atmospheric neutrinos allows us to have neutrinos from all directions, with a wide range of energy and a known stream with sufficient precision," explains IFIC physicist Sergio Palomares-Ruiz.
"The amount of absorption of the atmospheric neutrino stream depends on the amount of material digested, as well as on the neutrino energy, so by studying the variation in the magnitude of absorption in different directions for neutrinos with different energy, we can determine the Earth's density distribution."
Different angles are an important factor here. Some of the neutrinos traveled through the entire core of the Earth, while others traveled at an oblique angle that completely bypassed the nucleus.
Analysis of concentration and angle gave the team a tool to calculate the density of the planet at different depths.
Traditionally, the density of the Earth is extrapolated on the basis of the propagation of seismic waves produced by earthquakes. However, seismic waves can not penetrate into the internal core.
"On the other hand, neutrinos go through all of this, offering valuable information about the unknown core of the Earth, where the magnetism of the planet is generated," said IFIC physicist Andrea Donini.
The team did not learn anything new about our planet. The density map of the Earth's interior, the mass of the planet and the moment of inertia were calculated on the basis of neutrino data – all of them were consistent with previous measurements made with other methods and less detailed.
But the purpose of the research was not to learn anything new about Earth – it was about learning something new about what we can learn from neutrinos. And in this case the results are really angry.
"Our results demonstrate the feasibility of this approach to the study of the internal structure of the Earth, which is complementary to traditional methods of geophysics," the researchers wrote in their work.
At present, however, neutrino data is still relatively rare; scientists hope that their research will encourage the publication of more recent data from IceCube, as well as further research on neutrinos in the coming years.
The article was published in Physics of nature.