Turning the world inside out: Research gives new insight into formation of materials in Earth's core

By combining high-power lasers, diamonds and the brilliant X-rays produced by Argonne's Advanced Photon source, scientists from around the world can investigate materials at temperatures and pressures equal to those in the Earth's core. The knowledge they gain could illuminate Earth's distant past while revolutionizing the creation of many future materials from semiconductors to ceramics.

The astronomer Edmund Halley thought that it was hollow. The mathematician Leonhard Euler thought it contained a star. The author Jules Verne wrote that it was home to dinosaurs and mastodons. But when it comes to determining the composition of the core of the Earth, geoscientist Mark Rivers has a few thoughts of his own.

The core of the matter: the matter of the core

"We know there's iron there for sure," said Rivers, the associate director of the GeoSoilEnviroCARS (GSECARS) beamline at Argonne's Advanced Photon Source (APS). "But until now we've really had no way of knowing exactly what state the iron is in, or what other elements or compounds might be down there."

At GSECARS – run by the University of Chicago for the U.S. Department of Energy – and at several other APS beamlines, geoscientists from around the world use tools that can dig deeper than any drill and descend farther than any mineshaft. By using powerful lasers as well as presses that range from pocket-size to several tons, these scientists can expose materials to the blazing temperatures and crushing pressures found in the Earth's outer core.

The smaller device, called a diamond anvil cell, consists of two tiny diamonds that act as a vise, squeezing together a small sample. Because pressure is the measure of a force divided by the area over which it is applied, Rivers and his colleagues need to use only a small amount of force to create an enormous amount of pressure. "With a tiny sample like this, you can get to the pressure of the center of the Earth with just a small Allen wrench," Rivers said.

The pressures in the Earth's outer core are roughly two million times that of our atmosphere, strong enough to crush any known living organism and even alter the fundamental atomic arrangement of most substances. Temperatures in the outer core soar to more than 7,000 degrees Fahrenheit, but scientists do not really have a firm idea of exactly how hot it really is.

"If you pick a point inside the Earth and tell me how deep it is, we can pretty easily figure out about the pressure there. But although temperature generally increases with depth, there's never been a good way to know the exact value," said Yanbin Wang, a GSECARS researcher. "We can't exactly dig our way there."

Because the researchers could calculate the pressure at the core's boundary, they were able to use the diamond anvil cells to more accurately ascertain the temperature of the outer core. First, Argonne geophysicist Guoyin Shen squeezed a small iron sample inside the diamond anvil cell at the appropriate pressure, and then heated it with a laser. Because the outer core primarily contains liquid iron, Shen, who now runs the beamline of the High-Pressure Collaborative Access Team, could determine the minimum temperature of that iron by heating the sample until it melted.

The generation of the tremendous pressures and temperatures at the APS represents only the first step in the investigation of materials under these conditions. The high-energy X-rays produced by the APS hold the key to unlocking never-before-seen chemical properties.

Making an impact: the birth of the Earth

By gaining the ability to see how materials respond to extreme stresses, the geoscientists using the APS can open a window not only into the Earth's center but also into its past. "One of the biggest mysteries in earth science asks how the Earth originally formed," Rivers said. "In the early days of Earth's formation, the planet was likely entirely molten and then cooled over time. The APS hard X-ray facility at Argonne provides us with a unique capability of understanding the effects of extreme temperatures at both the macro and molecular scale."

The extreme temperatures and pressures produced in the Earth's early history and found inside it today can radically change the arrangement of atoms in a material. Even though all the atoms in a material stay the same, different formations of these atoms yield exceptionally different materials. For instance, at the Earth's surface, carbon atoms frequently arrange themselves in parallel planes in the form of soft, flaky graphite. However, deep inside the Earth, the intense pressure forces these atoms to rearrange themselves into strong pyramids, creating a diamond.

In these crystalline materials, the bonds between atoms act like small springs. As the initial pressure is applied to them, these "springs" compress. However, if the material experiences extreme pressure or temperature, the atoms suddenly contort themselves into a new, denser configuration.

The high-energy X-rays produced by the APS collide with the atoms in the crystal structure, which causes them to scatter. A detector on the other side of the sample captures the scattered X-rays, and the pattern that they create enables the scientists to calculate the position of individual atoms.

As the Earth cooled, its relatively uniform composition began to separate into a number of layers as different compounds solidified at different rates. Without a way to directly observe the action of those distant millennia, Wang and his GSECARS colleagues turned to a technique known for its use in diagnosing kidney stones, not looking inside rocks.

Computed axial tomography, better known as a CT or CAT scan, can separate structures of different densities inside of a given object. The scientists at GSECARS designed and built a special device, which, while being compressed in a hydraulic press, can spin the sample so that CT images can be collected at extreme pressure and temperature. Taken together, the images produced by scans of materials at different temperatures and pressures give geoscientists a glimpse into the development of the young Earth.

In order to enrich the account of the Earth's early days, Rivers and his colleagues at GSECARS used X-rays produced by the APS to determine the composition of meteorite fragments. The hope, Rivers said, is to find analogies between the formation of the meteorite and the development of the Earth.

According to Rivers, geoscientists believe that as the Earth and solar system formed between four and five billion years ago, thousands upon thousands of large meteorites collided with the surface of our infant planet. These collisions determined the composition of our planet, and scientists believe one extremely large and violent collision created Earth's moon.

"There's pretty much a consensus that the core contains not only iron and nickel but also light elements like oxygen or sulfur," he said. "These meteorites contain many of the same elements and were solidifying at the same time as the Earth cooled, so they could give us clues as to what things look like thousands of kilometers down or billions of years ago."

Under the volcano: sizing up seismic activity

Through their attempts to glean more information about the Earth's interior and its geologic history, the researchers at the APS slake their scientific curiosity while providing key data that could help officials predict and prepare for natural disasters.

In addition to the diamond anvil cell, the GSECARS scientists use a device called a large volume press – which essentially combines many different anvils – to investigate the viscosity of a variety of melted minerals and compounds. The viscosity of a material measures how much it resists flow.

The effects of high pressure and temperature on viscosity determine the structure of volcanoes and how the seismic waves of earthquakes propagate through the Earth's interior, Wang said. "Volcanoes form when the liquid magma is less dense than the surrounding rock, but since liquids are more compressible than solid rock the density and viscosity of the magma typically increases the lower you go. The style of the eruption depends primarily on the viscosity."

Although scientists have already mapped out the major fault lines along which earthquakes occur, that knowledge provides them with only a limited ability to predict and understand the mechanisms that produce potentially catastrophic earthquakes. The viscosity research "gives us a three-dimensional view of an earthquake's path," Rivers said. "The research we're doing on these tiny samples of iron and rock could end up eventually saving lives."

By exposing materials to extreme conditions, scientists do not merely seek to appease their geological curiosity. Under these stresses, materials show novel and unexpected properties that help scientists and engineers to understand why certain substances act in the way that they do. The creation of new generations of electronics, textiles, ceramics, catalysts and fuels requires an understanding of the entire spectrum of material properties. — by Jared Sagoff

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For more information, please contact Jared Sagoff (630/252-5549 or jsagoff@anl.gov) at Argonne.