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Scientists have for the first time observed Earth’s internal structure using ghostly particles — neutrinos.
Abstract
Scientists "weighed" the Earth using neutrinos — almost elusive particles that pass through the planet. Like X-rays, they shine through the interior, and by the attenuation of the flux, the density of layers can be determined. The results matched known values, but now geology has a fundamentally new method. Who would have thought that ghostly particles could weigh the Earth?
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Almost a hundred years ago, physicist Wolfgang Pauli predicted the existence of neutrinos — lightweight particles that pass through matter like a knife through butter. Only much more so: to stop even a single neutrino, you’d need a lead wall a light-year thick. Nevertheless, if a neutrino is very energetic, Earth isn’t completely transparent to it — especially the dense iron core. That’s exactly what the scientists used: just as the shadow of an object reveals its shape, so the attenuation of the neutrino flux revealed the interior of our planet.
Imagine you’re shining a bright flashlight through fogged-up glass. By how dim the beam gets in different spots, you can guess where the glass is thicker or dirtier. For neutrinos, Earth is just such fogged-up glass, only the beam is a shower of particles from space.
For over ten years, the giant IceCube detector, frozen into Antarctic ice, caught flashes from neutrinos that had passed straight through the Earth. The detector spotted more than three hundred thousand such events. When a neutrino collided with an atom in the ice, a charged particle was born, which sped faster than the local speed of light and caused a bluish glow. From the brightness and direction of this flash, they determined the neutrino’s energy and how much it had weakened while struggling through the depths. In this way, a kind of planetary spectral analysis was carried out: neutrinos of different energies faded differently, revealing layers of different density — the crust, mantle, and core.
The highest-energy neutrinos come to us from the most violent events in the Universe: from supernova explosions, frantically spinning pulsars, collisions near black holes, and from the surfaces of neutron stars. In essence, distant cosmic catastrophes are X-raying the Earth’s core.
As a result, scientists built a density map of Earth from the surface to the very center. The obtained mass and moment of inertia matched those we know from other measurements, but now they are derived using the weak interaction, which is precisely described by the Standard Model of particle physics. It’s like another set of scales — not spring-based, but, say, laser-based: the principle is different, but the number is the same. This transit method, previously used for the atmospheres of distant planets, was applied for the first time to peer into our own planet. This means that in the future we can test our notions about the Earth’s interior in an independent way and, possibly, figure out why Earth has a magnetic field while Mars doesn’t.
🎯 To stop even a single neutrino, you’d need a lead wall a light-year thick, yet high-energy neutrinos are absorbed by half when passing through Earth’s core.
