Geologists estimate that the Earth's core is a sweltering 5,700 K (5,427° C, 9,800° F), putting it about on par with the surface of the Sun – and yet the inner core is a solid ball of iron. Why it doesn't liquify is a bit of a mystery, but now a study from KTH Royal Institute of Technology puts forward a new theory, simulating how solid iron can remain atomically stable under such extreme conditions.
Here on the surface of the Earth, iron atoms arrange themselves into cubes, in what's known as a body-centered cubic (BCC) phase. Since this state is a product of room temperature and normal pressure, scientists have long believed that iron couldn't exist in this form in the broiling temperatures and intense pressure at the planet's center. Under those conditions, the crystal architecture of iron was expected to take on the shape of a hexagon, in a state called the hexagonal close-packed (HCP) phase.
Using the Swedish supercomputer Triolith, the new study from KTH crunched larger volumes of data than had previously been analyzed. The data indicated that the core was likely composed of 96 percent pure iron, with the remaining four percent made up of nickel and some light elements. But most importantly, the study found that BCC iron can indeed exist in the core, with its crystal structure remaining stable thanks to the very characteristics that were previously assumed to destabilize it.
"Under conditions in Earth's core, BCC iron exhibits a pattern of atomic diffusion never before observed," says Anatoly Belonoshko, one of the study's authors. "It appears that the experimental data confirming the stability of BCC iron in the core were in front of us – we just did not know what that really meant."
The crystal structures can be thought of as being divided into "planes" of atoms – that is, two-dimensional layers of atoms. So, iron atoms in a cubic phase are arranged in two planes of four atoms, making up the eight corners of a cube. These structures are normally fairly unstable, with planes sliding out of shape, but at extreme temperatures, the layers that slide off are reinserted into the mix, occurring reliably enough that it stabilizes the structure.
This diffusion normally destroys the crystal structure by liquifying it, but in this case, the iron manages to preserve its BCC structure. The researchers liken the planes to cards in a deck.
"The sliding of these planes is a bit like shuffling a deck of cards," says Belonoshko. "Even though the cards are put in different positions, the deck is still a deck. Likewise, the BCC iron retains its cubic structure. The BCC phase goes by the motto: 'What does not kill me makes me stronger.' The instability kills the BCC phase at low temperature, but makes the BCC phase stable at high temperature."
This finding also helps explain another inner-Earth mystery: why do seismic waves travel faster pole-to-pole than east-to-west, through the core? This phenomenon has been explained by the core being anisotropic, meaning it has a directional texture like the grain of wood. If that texture runs north-south, that difference would be expected, and the stable BCC phase iron could create this texture.
"The unique features of the Fe BCC phase, such as high-temperature self-diffusion even in a pure solid iron, might be responsible for the formation of large-scale anisotropic structures needed to explain the Earth inner core anisotropy," says Belonoshko. "The diffusion allows easy texturing of iron in response to any stress."
The research was published in the journal Nature Geosciences and the team explains the finding in the video below.