Why Earth’s inner core doesn’t melt

An energy distribution cycle keeps the core solid despite it being hotter than the surface of the Sun

Scientists have discovered why the crystallised iron core of the Earth remains solid, despite being hotter than the surface of the Sun.

Researchers at the KTH Royal Institute of Technology in Sweden found that on the edge of the inner core, pieces of crystals’ structure continuously melt and diffuse only to be reinserted due to high pressure like “shuffling deck of cards.”

This energy distribution cycle keeps the crystal stable and the core solid.

Spinning within Earth’s molten core is a crystal ball — actually a mass formation of almost pure crystallised iron — nearly the size of the moon.

Understanding this strange, unobservable feature of our planet depends on knowing the atomic structure of these crystals — something scientists have been trying to do for years.

As with all metals, the atomic-scale crystal structures of iron change depending on the temperature and pressure the metal is exposed to.

Atoms are packed into variations of cubic, as well as hexagonal formations. At room temperatures and normal atmospheric pressure, iron is in what is known as a body-centred cubic (BCC) phase, which is a crystal architecture with eight corner points and a centre point.

However at extremely high pressure, the crystalline structures transform into 12-point hexagonal forms, or a close packed (HCP) phase.

At Earth’s core, where pressure is 3.5 million times higher than surface pressure — and temperatures are some 6,000 degrees higher — scientists have proposed that the atomic architecture of iron must be hexagonal.

Anatoly Belonoshko from KTH said data showed that pure iron likely accounts for 96% of the inner core’s composition, along with nickel and possibly light elements.

Temperature impact

At low temperature, BCC is unstable and crystalline planes slide out of the ideal BCC structure. But at high temperatures, the stabilisation of these structures begins much like a card game — with the shuffling of a “deck.”

Mr. Belonoshko said in the extreme heat of the core, atoms no longer belonged to planes because of the high amplitude of atomic motion.

“The sliding of these planes is a bit like shuffling a deck of cards. Even though the cards are put in different positions, the deck is still a deck. Likewise, the BCC iron retains its cubic structure,” he said.

Such a shuffling leads to an enormous increase in the distribution of molecules and energy — which leads to increasing entropy, or the distribution of energy states.

That, in turn, makes the BCC stable, he said.