Inner Workings: Diamond anvils probe the origins of Earth’s magnetic field

Why is Earth covered by oceans and teeming with life, when Mars remains arid and apparently lifeless? The reasons are many, ranging from atmospheric characteristics to the planet’s distance from the Sun. But one key condition may involve the ability of the Earth’s core to generate a magnetic field.

Diamond anvils squeeze samples to extreme pressures while probing them with lasers directed through the diamonds. Geophysicists use the anvils to simulate the extreme pressures of the Earth’s interior. Image courtesy of Sergey Lobanov (Geophysical Laboratory, Carnegie Institution of Washington, Washington, DC).

And yet the origins of the Earth’s magnetic field remain a mystery for a variety of reasons. For one, there’s the small matter of accessing that chiefly iron core, which is buried under 1,800 miles of rock. This has left scientists resorting to lasers and diamond anvils in the laboratory to heat and squeeze iron to recreate the kind of temperatures and pressures found in the deep Earth. Thus far, these experiments have offered conflicting results, raising more questions than answers (1, 2).

Controversial Conductivity

Earth’s magnetic field is generated by its dynamo: the electrically conducting liquid metals in the planet’s outer core that churn or convect because of heat, like boiling water roiling in a pot. The strength of this convection depends on how much heat flows from the outer core to Earth’s mantle, which in turn depends on the thermal conductivity of iron and its alloys.

The planet, including its core, is cooling. If condensed-matter scientists can determine the present-day temperature of the Earth’s core and the thermal conductivity of iron, “we can estimate how the core’s temperature and Earth’s magnetic field have evolved over time,” says James Badro, a geophysicist at the Institute of Earth Physics of Paris.

But researchers still don’t know iron’s thermal conductivity at very high pressures and temperatures. Early predictions (3) suggested that iron in Earth’s outer core had low thermal conductivity. This theory implied that the outer core cooled slowly over time, and that Earth’s dynamo likely operated since the beginning of the planet’s history. It’s a model supported by evidence that Earth’s magnetic field might be at least 4 billion years old (4).

But recent calculations have ignited controversy (5, 6). Those contentious results suggest that iron possesses high thermal conductivity, which indicates that Earth’s magnetic field “existed over only the past 1 billion years or so,” says Zuzana Konôpková, a geophysicist at the European X-Ray Free-Electron Laser facility in Hamburg, Germany.

Discovering whether iron’s thermal conductivity is high or low can lead to dramatically different models of the history of Earth’s magnetic field, and in turn to different models of terrestrial evolution. “Earth’s magnetic field protects the hydrosphere and atmosphere from solar bombardment that would break down water; without a global magnetic field, you end up with a dry planet, as we think happened with Mars,” Badro says. “You need more than just a magnetic field to sustain life, but it seems that you can’t sustain life without a magnetic field. Understanding the history of Earth’s magnetic field addresses the question of why Earth is a habitable planet.”

The alternative possibility, that iron’s thermal conductivity is high at the Earth’s core, sparked “a whole new area of research into other ways to produce an ancient magnetic field for Earth other than by thermal evolution,” Badro says. For example, Badro and others have suggested that as Earth’s inner core solidified, it expelled lighter elements, stirring up the outer core and potentially helping to drive Earth’s dynamo (7, 8).

Studying the Earth’s Core in the Laboratory

To conduct experiments that shed light on the Earth’s dynamo, geophysical scientists place samples under extreme pressure. Squeezed between diamond anvils a few millimeters thick, they’re then heated to extreme temperatures by shining lasers through the diamonds. “Building the set-up is challenging, and keeping it all aligned and operational is also challenging,” says Alexander Goncharov, a mineral physicist at the Carnegie Institution of Washington.

But measuring iron’s thermal conductivity at the extreme temperatures and pressures found in the Earth’s outer core has proven to be a very difficult task. “The samples in diamond-anvil cells are really tiny, just a couple of microns large, so they’re hard to measure,” Konôpková says.

In addition, researchers often attempt to make sure that samples are detached from any sensors measuring their properties, or else they can suck heat from the samples and keep them from reaching the desired temperatures, Goncharov says. Also, the samples should be centered and flat relative to the anvils to ensure uniform heating, and the sample chamber should be free of water and other impurities, he says. Laser pulses that are too long or too strong “will destroy the sample through chemical reactivity and diffusion,” he explains.

“There's a fundamental difference of a factor of three between these groups' results.”

—James Badro

The accuracy and reliability of laser-heated diamond-anvil cell research has improved dramatically in the past decade or so, Badro says. “Ten years back, the community was notorious for contradictory papers: for different groups conducting the same experiment with opposite results,” he notes. “In the last few years, we’ve seen much less of that, I think due mostly to the technological innovations with the lasers.”

About 20 years ago, researchers used powerful lasers to generate high temperatures, but they were clunky and unstable. Smaller lasers had stability but could not produce the high temperatures. “It was a really bad situation,” Badro says. He lauds the arrival, in the last 10 years, of advances in fiber lasers as the telecommunications industry moved from copper wires to fiber optics. Fiber lasers are stable, use little energy, and efficiently convert most of that energy to laser power.

Contradictions Continue

Two groups recently used laser-heated diamond-anvil cells to analyze iron samples smaller than a grain of sand at temperatures more than 4,000 kelvin and pressures greater than 1 million atmospheres. “There are probably a dozen or so labs around the world that can operate at this level,” Badro says.

Using different methods to measure iron’s thermal conductivity, the two groups achieved dramatically contradictory results. Kenji Ohta at the Tokyo Institute of Technology and his colleagues measured the electrical resistivity of iron wires, a feature closely linked with thermal conductivity. Their results suggest a relatively high thermal conductivity of about 90 watts per meter per kelvin, in line with recent numerical simulations.

Konôpková, Goncharov, and their colleagues measured how long it took for heat pulses to propagate through bits of iron foil by measuring changes in the brightness and wavelength of the light from the samples. Their results suggest a relatively low thermal conductivity of about 18–44 watts per meter per kelvin, in accord with older estimates. “There’s a fundamental difference of a factor of three between these groups’ results,” Badro says.

Why the discrepancy? It may be that electrical resistivity and thermal conductivity are not linked at high temperatures and pressures in the way assumed, Konôpková says. Plus, using wires to measure electrical resistivity is very sensitive to sample irregularities, or the wires only probe activity of the surface of samples as opposed to their bulk property. On the other hand, Badro notes that the findings of Konôpková, Goncharov, and their colleagues might have problems rooted in the geometry of their diamond-anvil cell, or in their interpretation of what light from the samples indicated about the heating process.

Future research needs to replicate both sets of very high-pressure experiments at least once, “ideally by different groups,” Badro says. Konôpková notes that further work should also measure the thermal conductivities of other metals, which could help researchers develop better models of thermal conduction under extreme conditions. “With more experiments performed by different groups and perhaps new independent techniques, [this discrepancy] will be solved soon, perhaps in 1 to 2 years,” Goncharov says.

“The most exciting thing … about these findings is that it relaunches the whole debate about the Earth’s dynamo,” Badro says. “It’s aggravating that we still aren’t certain what iron’s thermal conductivity is, but at least the fact that that drives discussion is stimulating.”