The paradox of the Earth's inner core formation has been solved: what caused the "heart" of our planet to freeze? (video)

The history of Earth spans billions of years, and it is no surprise that much about our planet remains unknown. Despite the extensive research and modeling conducted by scientists, the inner core of the planet is still shrouded in many mysteries that we have yet to uncover, as reported by IFLScience.

Studying the Earth's inner core is extraordinarily challenging for several reasons, including its depth of over 5,100 kilometers. The deepest hole ever drilled reached only 12,263 meters. However, scientists have found a solution: observing seismic waves traveling through the Earth and the lines of the planet's magnetic field, which are influenced by conditions in the core, can provide insights.

One of the puzzles that scientists have yet to solve is how the Earth's core "froze" — transitioning from a molten liquid state in the past to a solid state today. According to Alfred Wilson-Spencer, a research fellow in mineral physics at the University of Leeds and the lead author of a new study, the inner core of the planet was once liquid and has gradually solidified over time. As the planet cools, the inner core expands outward as the iron-rich liquid "freezes." Nevertheless, the heart of the planet remains extremely hot — around 4726.85°C.

Scientists believe that understanding this process in the future will lead to a better comprehension of Earth's magnetic fields. Given the role of the magnetosphere in protecting Earth from harmful solar radiation, this understanding could help us grasp the conditions necessary for life to thrive.

Researchers found that the freezing process releases elements like oxygen and carbon — both incompatible with being in a hot solid body. This process creates a hot, buoyant liquid at the bottom of the outer core. The liquid rises into the outer liquid core and mixes with it, generating electric currents that produce our magnetic field.

According to Wilson-Spencer, it remains challenging to understand how the Earth's core "froze," especially considering our location on the planet and the time frame, as cooling occurred over a billion years or more.

The traditional theory of this process suggests that the temperature at the planet's center decreased until it reached the melting point of the liquid iron alloy, after which the freezing of the inner core began. However, this picture is still incomplete, as it overlooks the physical requirement that all liquids must be significantly supercooled below the melting temperature before solids can form without remelting.

Previous models proposed that before the planet's heart froze approximately a billion years ago, liquid iron and other minerals must have been supercooled by about 700–1000 Kelvin. However, this presents some challenges.

If the core was indeed supercooled by 1000 K before freezing, the inner core should be somewhat larger than what is observed. Alternatively, if 1000 K is necessary for freezing but was never achieved, the inner core should not exist at all.

In the new study, scientists examined how the presence of other elements in the inner core could influence supercooling. The team modeled the interactions of iron and carbon atoms under high pressure using a supercomputer. The results suggest that in the presence of carbon, the core may cool and solidify with much less supercooling, possibly below 400 K, and within plausible timeframes.

It is worth noting that the findings of the scientists are still awaiting peer review and require further research, as the concept may be further complicated by the presence of other elements in the core, such as oxygen and silicon. Nevertheless, the researchers believe they have found a key to understanding how the Earth's inner core became solid.