Salad dressing explains the Earth’s magnetic fields

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Researchers may have found a new factor to help explain the ebb and flow of Earth’s magnetic field—and it’s something familiar to anyone who has made a vinaigrette for their salad.

Earth’s magnetic field, produced near the center of the planet, has long acted as a buffer from the harmful radiation of solar winds emanating from the sun. Without that protection, life on Earth would not have had the opportunity to flourish. Yet our knowledge of Earth’s magnetic field and its evolution is incomplete.

In a new study in the Proceedings of the National Academy of Sciences, researchers found that molten iron alloys containing silicon and oxygen form two distinct liquids under conditions similar to those in the Earth’s core. It is a process called immiscibility.

Researchers used a laser-heated diamond anvil cell to simulate the pressure and temperature conditions of Earth’s core. Inset shows a scanning electron microscope image of a quenched melt spot with immiscible liquids. (Credit: Sarah M. Arveson)

“We observe liquid immiscibility often in everyday life, like when oil and vinegar separate in salad dressing. It is surprising that liquid phase separation can occur when atoms are being forced very close together under the immense pressures of Earth’s core,” says lead author Sarah Arveson, a graduate student at Yale University.

Immiscibility in complex molten alloys is common at atmospheric pressure and has been well documented by metallurgists and materials scientists. But studies of immiscible alloys at higher pressures have been limited to pressures found in Earth’s upper mantle, located between Earth’s crust and its core.

Even deeper, 2,900 kilometers (just under 1802 miles) beneath the surface, is the outer core—a more than 2,000-kilometer (1243-mile) thick layer of molten iron. It is the source of the planet’s magnetic field. Although this hot liquid roils vigorously as it convects, making the outer core mostly well-mixed, it has a distinct liquid layer at the top. Seismic waves moving through the outer core travel slower in this top layer than they do in the rest of the outer core.

Scientists have offered several theories to explain this slow liquid layer, including the idea that immiscible iron alloys form layers in the core. But there has been no experimental or theoretical evidence to prove it until now.

Using laser-heated, diamond-anvil cell experiments to generate high pressures, combined with computer simulations, the researchers reproduced conditions in the outer core. They demonstrated two distinct, molten liquid layers: an oxygen-poor, iron-silicon liquid and an iron-silicon-oxygen liquid. Because the iron-silicon-oxygen layer is less dense, it rises to the top, forming an oxygen-rich layer of liquid.

“Our study presents the first observation of immiscible molten metal alloys at such extreme conditions, hinting that immiscibility in metallic melts may be prevalent at high pressures,” says Kanani KM Lee, an associate professor in the geology and geophysics department.

The researchers say the findings add a new variable for understanding conditions of the early Earth, as well as how scientists interpret changes in Earth’s magnetic field throughout history.

Additional coauthors are from Louisiana State University. The National Science Foundation and the Connecticut Space Grant Consortium funded the research.

Source: Yale University