"The control room is just above the experimental room," Merkel says. The team fired both lasers at a tiny sample of iron about the width of a human hair, hitting the iron with a shock wave of heat and pressure. "It's been a door opener for other similar facilities in the world."
"At the time, LCLS was the only facility in the world where you could do that," says lead author Sébastien Merkel of the University of Lille in France. The second was SLAC's Linac Coherent Light Source (LCLS) X-ray free-electron laser, which allowed the researchers to observe the iron on an atomic level.
The first was an optical laser, which generated a shock wave that subjected the iron sample to extremely high temperatures and pressures. Reaching these extreme conditions required two types of lasers. "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. Twinning is a common pressure response in metals and minerals - quartz, calcite, titanium and zirconium all undergo twinning. The coping mechanism iron uses to deal with that extra stress is called "twinning." The arrangement of atoms shunts to the side, rotating all the hexagonal prisms by nearly 90 degrees. "And it needs to relieve that stress, so it tries to find the most efficient mechanism to do that." "As we continue to push it, the iron doesn't know what to do with this extra stress," says Gleason. No one had ever directly observed iron's response to stress under such high temperatures and pressures before, so the researchers didn't know how it would respond. "But we achieved the conditions of the outer core of the planet, which is really remarkable." "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. The group at SLAC wanted to see what would happen if you kept applying pressure to that hexagonal arrangement to mimic what happens to iron at the Earth's core or during atmospheric reentry from space. If you squeeze these cubes by applying extremely high pressures, they rearrange into hexagonal prisms, which allow the atoms to pack in more tightly. Most of the iron you encounter in your everyday life has its atoms arranged in nanoscopic cubes, with an iron atom at each corner and one in the center. The results appear in Physical Review Letters, where they have been highlighted as an Editor's Suggestion. But researchers at the Department of Energy's SLAC National Accelerator Laboratory have now observed for the first time how iron's atomic structure deforms to accommodate the stress from the pressures and temperatures that occur just outside of the inner core. As such, much of our understanding of planetary cores is based on experimental studies of metals at less extreme temperatures and pressures. Measuring what happens during the collision of celestial bodies or at the Earth's core is obviously not very practical. Some asteroids in our solar system are massive iron objects that scientists suspect are the remnants of planetary cores after catastrophic impacts. But in space, similar cores can collide with other objects, causing the crystalline materials of the core to deform rapidly.
Earth's planetary core is thankfully intact.