The early Earth was a molten ball covered in a global lava ocean, but over 4.5 billion years it has cooled enough for life to emerge. Its metallic core, still blazing nearly as hot as the surface of the Sun, constantly sends chemical messengers up through the planet's layers, including materials locked away since the Solar System's birth.
A team led by researchers at the University of Science and Technology of China set out to answer a longstanding question: Are the chemical differences observed in the deep mantle relics of the planet's crystallization from a magma ocean, or evidence that the core and mantle have exchanged chemicals ever since?
Using machine-learning-enhanced "path-integral simulations" to study the quantum behaviour of molecules and atoms, the researchers found that hydrogen drawn in from the primordial cloud of gas and dust that formed the Sun can escape the core and migrate into the mantle and crust. There, it combines with oxygen to form water, though trapped in minerals rather than as liquid.
The work adds to a growing body of evidence that the core is not a sealed vault. Previous studies have found elevated levels of helium-3 in volcanic rocks, indicating a primordial signature from deep within the planet. The new simulations suggest hydrogen, too, has been steadily leaking upwards over geological time.
If correct, the discovery could reshape understanding of Earth's water cycle, suggesting that a significant portion of the planet's water may have been generated internally rather than delivered solely by asteroid or comet impacts.