It is one of nature’s upside-down towers that the deep interior of the Earth is as hot as the surface of the Sun. The iron sphere 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 is happening to iron at the center of the Earth when it is 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 has ever melted is likely to have an iron core, because the density of iron causes it to fall towards the center of gravity of a world. Astronomers believe that some iron asteroids are in fact the nuclei of planetesimals that have lost the rest of their mass due to collisions.
What happens to the iron when two planets collide? What happens to the iron in the heart of the Earth? In both scenarios, the iron is subjected to extreme heat and pressure. Most of what scientists know about iron in these extreme conditions comes from laboratory experiments involving lower temperatures and pressures. But researchers at the SLAC (Stanford Linear Accelerator Center) wanted to best recreate the extremes in the center of the Earth to test the behavior of iron.
The researchers, led by SÃ©bastien Merkel from the University of Lille, published an article reporting on their findings. The title of the article is “Femtosecond visualization of the strength and plasticity of hcp iron under shock compressionâAnd it is published in the newspaper Physical examination letters.
Under normal conditions on the Earth’s surface, iron is naturally arranged in a certain way. Atoms are arranged in nanoscopic cubes, with an iron atom in the center and one at each corner. When subjected to a sufficiently high pressure, the irons reorganize into hexagonal prisms. This configuration allows more iron to be compressed in the same space.
This is already known.
But what happens when the pressure increases even more, 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 iron in the laboratory to extreme temperatures and pressures. The second laser was the SLAC Linac coherent light source (LCLS) Free electron X-ray laser. The LCLS allowed the team to observe iron at an atomic level while under extreme conditions.
“We didn’t quite create the conditions for the inner core,” says co-author Arianna Gleason, a scientist in the High Energy Density Science (HEDS) division at SLAC. “But we have reached the conditions of the outer core of the planet, which is truly remarkable.”
Other materials like quartz, titanium, zircon and calcite have been tested in a similar way. But no one had ever observed iron under such extreme temperature and pressure.
âAs we keep pushing it, the iron doesn’t know what to do with that extra stress,â says Gleason. “And he has to relieve that stress, so he’s trying to find the most effective mechanism to do it.”
In response to all this stress, the iron does something called “twinning. “
Twinning occurs when atoms rearrange themselves so that they symmetrically share the points of the crystal lattice. Different materials exhibit different types of twinning, all governed by well-understood laws. In the case of iron, the hexagonal prisms rotate sideways almost 90 degrees. The point of attachment is called the twin plane or the composition surface.
When iron twins like this, it becomes extraordinarily strong. First. But over time, this force disappears.
âPairing allows the iron to be incredibly strong – stronger than we initially thought – before it begins to plastically flow over much longer timescales,â Gleason said.
This discovery revolved around an iron sample the size of a human hair. The iron was shocked by the optical laser in extreme heat and pressure. In a Press release, lead author SÃ©bastien Merkel described what it was like during the experiments. âThe control room is right above the experimentation room,â he said. âWhen you trigger the discharge, you hear a loud pop. “
Next, the LCLS observed the reaction of iron at the nanosecond scale to see how the atoms rearranged themselves. Prior to the experiment, the team did not know how fast the iron would react and whether they would be able to measure the changes. âWe were able to do a measurement in a billionth of a second,â said co-author Gleason. âFreezing atoms where they are in this nanosecond is really exciting. “
The team’s results were highlighted by an editor of Physical examination letters. In a comment, correspondent editor Merric Stephens said: âInitially, the shock wave changed the structure of the iron from a cubic-centered body to a compact hexagonal, which the team expected. let it happen. The hexagonal structure then elastically deformed for several nanoseconds before giving way, after which it adapted to the stress by rearranging itself into twin crystal pairs, a process that continued even after the stress dropped into below the yield strength.
Just being able to measure the changes that are happening so quickly, the researchers say, is a positive outcome in and of itself. âThe fact that the match is happening on a timescale that we can measure is an important outcome in and of itself,â Merkel said.
Prior to this experiment, much of our understanding of iron came from observing the element under less extreme conditions and then modeling it to higher extremes. But these results are an important step forward.
âNow we can give some physical models a boost for some really fundamental strain mechanisms,â Gleason said. âIt helps to strengthen some of the predictive capabilities we lack in modeling the reaction of materials under extreme conditions. “
Gleason says the recently upgraded LCLS made this experience a reality and will lead to more. âThe future is bright now that we’ve developed a way to do these measurements,â says Gleason. “Recent upgrades to the x-ray inverter as part of the LCLS-II project allow for higher x-ray energies, allowing studies on alloys and thicker materials that have lower symmetry and more complex x-ray prints. “
This experiment produced results that no one had seen before. But even with success, the team was unable to replicate the extreme conditions of the Earth’s inner core. They could only duplicate the outer core. But in the future, that will change.
ââ¦ We’re going to get more powerful optical lasers with approval to proceed with a new flagship petawatt laser facility, known as MEC-U,â said Gleason. âThis will make future work even more exciting, as we will be able to access the conditions of the Earth’s inner core without any problems. “
The new laser will be housed in an underground facility connected to SLAC’s existing LCLS. The petawatt laser will produce a trillion watts and will be able to study materials in the most extreme environments imaginable. The Matter Under Extreme Conditions (MEC-U) upgrade “… promises to dramatically improve our understanding of the conditions necessary to produce fusion energy and replicate a wide range of astrophysical phenomena here on Earth”, according to the Ministry of Energy.
There has been a lot of thinking and theories about the state of iron under extreme conditions in the heart of the Earth. Scientists speculated that a pairing would take place, as it does with other materials, but were not sure. Now there is experimental data to support some of these thoughts and to refute other conclusions.