TEMPE, Ariz. — In a laboratory shrouded by heavy black curtains hung with danger signs, scientists are conjuring the scorching-hot, high-pressure conditions inside planets.
Welcome to the delicate, specialized and sometimes explosive world of high-pressure research.
Crushing and blasting samples in the lab helps scientists look inward, re-creating the conditions of otherwise unreachably deep portions of the planet. The work is key to probing some of the most profound questions about our existence: What makes Earth habitable? How did life emerge? And how do geologic processes that we still don’t fully understand sustain life today?
In recent years, the field’s focus has also swiveled outward, toward the thousands of planets known to orbit other stars. Astronomers have shown us that our galaxy is studded with a menagerie of water worlds, super-Earths, hot Jupiters and sub-Neptunes, some of which offer tantalizing hints that they might be able to support life. Since we can’t visit those planets, much less probe their deep interiors, the next best thing is to try to concoct them in the lab.
“So far in the earth science community, our primary goal has been explaining our existence,” said Dan Shim, a mineralogist at Arizona State University whose lab works to create planetary innards in miniature. “But now, with the [discovery of] exoplanets, we have new questions being asked.”
To help fuel these explorations, the United States is about to get its first really “big press” — a two-story-high crusher that scientists have been dreaming about for a decade, powered by a hydraulic ram that can exert 6,000 tons of force. The instrument, nicknamed Ichiban, will allow scientists to compress significantly larger samples, improving our understanding of what happens to materials under extreme conditions.
It’s part of a $13.7 million, nationally funded facility, called FORCE, that will house a suite of instruments with new squishing, squeezing and twisting capabilities.
“We’re at an exciting time. Our ignorance is still so profound that we’re asking really basic questions,” said Joseph O’Rourke, a planetary scientist at ASU.
O’Rourke doesn’t run high-pressure experiments, but he takes the findings and integrates them into models to better understand how planets formed and evolved. Sometimes, those models yield extraordinary possibilities. He recalled a recent seminar where a colleague presented a model of an exotic planetary interior made of miles and miles of diamonds.
“You can make all these models, and these things are technically conceivable, according to physics and chemistry,” O’Rourke said. “We’ve got a huge amount of work to do to collect the data to know if our stories are science fiction or science fact.”
‘Cook and look’
The inner life of planets appears tidy and simple in textbook illustrations. Earth’s layers — solid inner core, molten outer core, mantle, crust — look concentric, color-coded and symmetrical, like a gobstopper candy.
But in reality, things get far more complicated.
By measuring how seismic waves from earthquakes move through the planet, geologists have been mapping the temperature, pressure and properties of the rock or fluid inside, allowing them to decipher structural features in the planet’s innards, like a doctor reading an ultrasound. Increasingly, these maps show that the planet’s nested layers are irregular, occasionally mountainous and highly dynamic.
“At the base of the mantle, where the mantle and the core come together, there’s a hint of something really intriguing happening down there,” said David Lambert, a program officer at the National Science Foundation.
Understanding the anatomy of Earth’s interior is a critical step to figuring out why the exterior is habitable. And a lot of it comes down to basic chemistry.
Earth’s solid inner core grows as molten material solidifies around it, releasing lighter elements in the process that fuel turbulent convection in the outer core. That, in turn, generates a magnetic field that shields life from harmful cosmic rays and prevents the sun from blowing away our atmosphere. Heat also moves from the liquid outer core into the mantle, helping drive tectonic activity that gives us earthquakes and volcanoes, which release gases and dust that can alter the climate.
“The engine that keeps our magnetic field going may be dependent on the chemical behavior of oxygen or sulfur embedded within Earth’s roiling outer iron core,” said Quentin Williams, a planetary scientist at the University of California at Santa Cruz.
A recent drilling expedition thrilled scientists when it pulled up mantle rocks that are unusually close to the ocean floor. But the distance from the surface to the core is nearly as long as the distance from Washington, D.C., to Phoenix — far too deep to drill.
So to gain insight into what’s going on at depth, scientists “cook and look” — they squeeze and heat different combinations of chemicals in the lab, and then see what’s produced.
Diamond anvils
Scientists know which chemical elements dominate Earth’s layers from multiple streams of evidence, including measurements of the density of the planet, the composition of meteorites, which are leftover planetary building blocks, and bits of Earth’s innards that make their way to the surface through volcanic eruptions.
For decades, scientists have subjected these ingredients to immense heat and pressure by turning the screws of a simple, palm-size device called a diamond anvil cell, one of Shim’s specialties. They were invented in the late 1950s, when the General Services Administration made diamonds confiscated from smugglers available to scientists for free.
Today, diamond anvil cell research has become such an established niche it’s possible for scientists to buy diamonds from a research supplier instead of traveling to jewelers in Manhattan, as Shim did early in his career.
Researchers need a steady supply because under enough pressure even diamonds can break, in what scientists call a blowout. It sounds like a spoon clinking on a champagne glass, but the real impact is the emotional one for the scientists, who have to start their experiments again from scratch. Shim has two boxes of busted diamonds in the back of his lab, some of them mementos of especially important experiments.
Research with diamond anvil cells has so far yielded a number of exciting revelations, helping lay bare the colorful layers of deep Earth. The upper mantle is dominated by green-tinged olivine, but under pressure and higher temperatures, it transforms to a bluish mineral called wadsleyite, then an even deeper blue ringwoodite. A form of silicate perovskite, recently named bridgmanite, dominates the lower mantle.
Two decades ago, diamond anvil cell experiments revealed a new mineral phase at the enigmatic boundary between the core and mantle. That layer fascinates geoscientists because it is instrumental in how heat moves between the core and mantle and helps drive plate tectonics and volcanoes.
The experiments can offer insights into the guts of planets beyond Earth, too. On a recent afternoon in Shim’s lab, postdoctoral scientist Sibo Chen was exploring the question of why Mars is a dry, barren wasteland.
Features and minerals found by a fleet of rovers suggest that water must have been abundant on the Red Planet billions of years ago. One possible explanation for why Mars is so dry today is that water deep inside the planet killed the planet’s protective magnetic field.
Chen squeezed specks of ringwoodite, a Mars mantle mineral that can hold water, next to iron, which dominates its core and can alloy with hydrogen under pressure. The experiment basically sets up a contest to see where water — or its major ingredient, hydrogen — ends up under pressure and heat. Precise twists of Allen wrenches brought the sample to the pressure at Mars’s core-mantle boundary, about 200,000 times Earth’s atmospheric pressure.
“We’re ready to roll,” Chen said, clicking the laser on and watching on a screen as a glowing dot came into focus where the beam was heating the sample. As the temperature increased, the dot began to flicker.
Days later, Chen packed the diamond anvil cell into a special padded case and traveled with it to an X-ray beam in California to examine the structure of the mineral. He is still analyzing the data, but if the experiment shows that hydrogen is drawn into the iron core, it could help unravel why Mars’s magnetic field petered out about 4 billion years ago. If a light element like hydrogen migrated into the liquid iron core, it could float to form a stable layer at the top, interrupting the convective churn that powers the magnetic field.
Gas guns and a big press
As revelatory as these experiments have been, diamond anvil cells have their limits. For one, the samples must be microscopic, thinner than a human hair. To squeeze larger samples, scientists turn to another device, called the multi-anvil press.
Kurt Leinenweber, one of the leaders of the new FORCE facility at ASU, recalls a trip he took to Japan in 1989, when he first got to use a multi-anvil press that could create in a lab some of the mineral changes that occur inside Earth.
“I was so excited, I couldn’t sleep at night,” he recalled. “Just lying there, I couldn’t stop thinking of the things you could do.”
Today, multi-anvil presses are used in labs across the country, but scientists will be able to use the supersize Ichiban to test samples the size of a pencil eraser and take measurements that are difficult at smaller scales.
“You can do measurements of their physical properties — sound velocities, electrical conductivity, thermal conductivity — because you have something in your hand,” not a speck under a microscope, said Robert C. Liebermann, a research professor in the Department of Geosciences at Stony Brook University. He has worked in the field for more than four decades and is thrilled to see the United States finally get a big press, catching up to Japan, Germany and China.
But to reliably reach the extreme pressures at Earth’s core — or inside even larger planets — scientists need new tools.
“The behavior of the core — that’s where experimental techniques are pretty primitive,” said Alexandra Navrotsky, director of the Navrotsky Eyring Center for Materials of the Universe at ASU.
Scientists seeking to generate the most extreme pressures shoot projectiles out of a gas gun or use powerful laser pulses. Researchers may get only a millionth or billionth of a second of data, but they can get their samples to pressures akin to those at Earth’s core — or higher. That’s especially exciting for exoplanet studies, since some of the alien worlds found so far are more extreme than anything in our solar system.
Super-Earths are one category of exoplanet that has raised a habitability eyebrow. A key question is whether those planets contain a churning liquid core inside that generates a protective magnetic field, making their surfaces potentially habitable.
Sally June Tracy, at the Carnegie Institution for Science in Washington, performs these split-second experiments. She said that one of the things she finds most exciting about the entire field is that the experiments span wildly different scales of time and size, probing the miniature to fill in planet-size pictures.
“We’re doing ultrafast experiments to think about the history of the solar system, and working with very small samples … to understand large-scale processes that affect the evolution of planetary bodies,” Tracy said.
“That just blows my mind.”
