It’s one of nature’s topsy-turvy tricks that the deep interior of the Earth is as hot as the Sun’s surface. The sphere of iron that resides there is also under extreme pressure: about 360 million times more pressure than we experience on the Earth’s surface. But how can scientists study what happens to the iron at the center of the Earth when it’s largely unobservable?

With a pair of lasers.

Earth is not the only body with an iron core. Mercury, Venus, and Mars have them, too. In fact, any world that was ever molten is likely to have an iron core, since iron’s density makes it fall toward the center of a world’s gravity. Astronomers think that some iron asteroids are actually cores from planetesimals that lost the rest of their mass due to collisions.

What happens to the iron when two planets collide? What happens to the iron at the Earth’s core? In both scenarios, the iron is subjected to extreme heat and pressure. Most of what scientists do know about iron in these extreme conditions comes from laboratory experiments involving lesser temperatures and pressures. But researchers at the DOE’s SLAC (Stanford Linear Accelerator Center) wanted to recreate the extremes at the Earth’s center as best they could to test iron’s behaviour.

The researchers, led by Sébastien Merkel of the Université de Lille, published a paper reporting their findings. The paper’s title is “Femtosecond Visualization of hcp-Iron Strength and Plasticity under Shock Compression” and it’s published in the journal Physical Review Letters.

Under normal conditions on the Earth’s surface, iron is arranged a certain way naturally. The atoms are arranged in nanoscopic cubes, with an iron atom in the center and one at each corner. When under sufficiently high pressure, the irons rearrange into hexagonal prisms. That configuration allows more iron to be compressed into the same space.

When under sufficient pressure iron forms hexagonal prisms. Image Credit: S. Merkel/University of Lille, France

This much is already known.

But what happens when the pressure is increased even further, to the same levels as the Earth’s outer core? To find out, the team of researchers used two lasers.

The first laser was an optical laser used to induce a shock wave that subjected the iron in the lab to extreme temperatures and pressures. The second laser was SLAC’s Linac Coherent Light Source (LCLS) X-ray free-electron laser. The LCLS allowed the team to observe the iron on an atomic level as it was subjected to extreme conditions.

“We didn’t quite make inner core conditions,” says co-author Arianna Gleason, a scientist in the High-Energy Density Science (HEDS) Division at SLAC. “But we achieved the conditions of the outer core of the planet, which is really remarkable.”

Other materials like quartz, titanium, zircon, and calcite have been tested in similar ways. But nobody had ever observed iron under such extreme temperature and pressure.

“As we continue to push it, the iron doesn’t know what to do with this extra stress,” says Gleason. “And it needs to relieve that stress, so it tries to find the most efficient mechanism to do that.”

In response to all that stress, the iron does something called “twinning.”

“We were able to make a measurement in a billionth of a second. Freezing the atoms where they are in that nanosecond is really exciting.”

Arianna Gleason, co-author, SLAC.

Twinning is when atoms rearrange themselves so that they share crystal lattice points symmetrically. Different materials exhibit different types of twinning, all governed by well-understood laws. In iron’s case, the hexagonal prisms rotate to the side nearly 90 degrees. The point of attachment is called the twin plane or the compositional surface.

When iron twins like this, it becomes extraordinarily strong. At first. But as time goes on, that strength disappears.

“Twinning allows iron to be incredibly strong — stronger than we first thought — before it starts to flow plastically on much longer time scales,” Gleason said.

This discovery revolved around a sample of iron the size of a strand of human hair. The iron was
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