Seeing Inside the Earth express
Deep beneath our feet, within the planet's bulk, radioactive elements are slowly decaying. This process spawns tiny particles — geoneutrinos. They pass through the crust, mantle, and core, barely interacting with anything. But if we could catch even a few of these particles, we could learn exactly what the Earth's interior is made of — how much potassium, uranium, or thorium is down there. It's like listening to a whisper from the depths.
Conventional detectors catch geoneutrinos but don't know where they came from. The new approach uses a liquid that emits a faint flash of light when a particle strikes it. The shape of that flash reveals the particle's direction of travel. This turns the detector into a kind of camera pointed at the Earth's interior. The main nuisance is solar neutrinos, which outnumber geoneutrinos by thousands to one. They are filtered out by discarding events coming from the Sun's direction.
If you want to go further and build a map, to understand how heat sources are distributed inside the Earth, you'll need even more patience. To distinguish a uniform glow from a patchy picture requires more than 27,000 ton-years. But such a map would tell us how the planet has cooled over billions of years and why it remains warm to this day.
Geoneutrinos are mostly produced by the decay of potassium-40, uranium-238, and thorium-232—the very elements that have been heating our planet from within for 4.5 billion years.
In Arthur C. Clarke's novel Rendezvous with Rama, an alien spacecraft is explored from the inside; here, humanity is learning to 'see through' its own planet without cutting it open.