Measuring the Neutrino Tomography of Earth

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In November of 2018, an international team of physicists managed to measure Earth’s mass by using neutrino tomography and the data captured by the South Pole’s IceCube Neutrino Observatory. They published their report in the Nature Physics journal.

A neutrino is a tiny subatomic particle that lacks an electric charge and is produced in high-energy collisions. Because of their properties, they interact only with the weakest natural forces and show extremely weak gravitational interaction. They are massless, unaffected by magnetic fields, and pass through normal matter unobstructed and undetected – unless you make an effort to detect them. And that is exactly what scientists do, for various reasons. Because of their unique features, neutrinos can convey valuable and rare astronomical information –  including those about extreme cosmic events such as exploding stars, gamma-ray bursts, and phenomena involving black holes.

Besides catastrophic space events, neutrinos are also produced when energetic particles such as protons collide with our atmosphere. As previously insinuated, those neutrinos can just pass right through the entire planet. However, sometimes they will hit an atomic nucleus and get absorbed instead. These neutrino “setbacks” reveal the density of the matter they are traveling through, and it is exactly what scientists used for their calculation. The neutrinos that the IceCube detector had “caught” came from different angles because they probed different layers of Earth. By measuring how many neutrinos came from different angles, the scientific team was able to measure the densities of various Earth’s segments, and from that – the total mass of Earth.

For this occasion, the Antarctica-based IceCube Neutrino Observatory gathered data from the interaction of cosmic rays with the Earth’s atmosphere. The neutrino tomography confirmed the traditional measurements on Earth’s mass, based on gravitational calculations and seismological data. That is especially interesting because neutrino tomography is a conceptually different and independent method – it doesn’t rely on gravity, but purely on weak interactions and the nucleon masses.

IceCube's Antarctic lab accompanied by a cartoon depicting long strands of detectors frozen into the crystal clear ice below

IceCube’s Antarctic lab accompanied by a cartoon depicting long strands of detectors frozen into the crystal clear ice below

While nothing new was discovered, the fact that a new technique has proven the old calculation right is exciting in its own terms. Next time you want to dismiss a scientific theory based on the “It’s just calculations” argument, think again. In the near future, neutrino tomography will have more uses in geoscience. First of all, the data can be used to provide a clearer picture of Earth’s density profile. It could help solve a number of long-standing problems in geophysics: the dynamics of the Earth’s core and mantle, the bulk composition of Earth, and the geomagnetic dynamo mechanisms.

Perhaps most intriguingly, neutrinos could help determine if all of Earth’s mass comes from “normal” matter, or a portion of it is made up of the elusive dark matter. Dark matter is still a hypothesis, believed to be composed of some undiscovered subatomic particles. It should account for the missing mass in astrophysical measurements throughout the universe. Dark matter should repel neutrinos. In theory, we could use neutrino measurements to determine if the Earth itself contains this mysterious entity.

IceCube Neutrino Observatory is primarily funded by the The National Science Foundation (NSF) and led by University of Wisconsin–Madison, with assistance from funding agencies and scientific institutions from around the world.

Resources

The Study

Donini A. et al. 2018. Neutrino tomography of Earth. Nature Physics Vol. 15, p. 37–40 (2019). https://www.nature.com/articles/s41567-018-0319-1

Articles

Conover, E. 2018. “Physicists measured Earth’s mass using neutrinos for the first time”. Science News Magazine Vol. 194, No. 11, Dec 8, 2018, p. 14. https://www.sciencenews.org/article/physicists-measured-earth-mass-using-neutrinos-first-time

“IceCube Explained”. IceCube South Pole Neutrino Observatory. https://icecube.wisc.edu/about/overview

“All About Neutrinos”. IceCube South Pole Neutrino Observatory. https://icecube.wisc.edu/info/neutrinos

“How can the weight of Earth be determined?”. Scientific American. https://www.scientificamerican.com/article/how-can-the-weight-of-ear/

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