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Earth's Core Slowly Gives Up Its Secrets
The northern lights hang along the planet's magnetic field, which is maintained by Earth's hot core.
Credit: NASA.

The behavior of Earth's core and the core's ingredients besides iron are major geological mysteries. Scientists can't exactly go take a sample. Yet understanding the core's exact makeup and conditions is a big deal for those who are trying to understand how our planet's complicated geophysical systems work together.

Not only is it likely the Earth's largely iron core plays a role in the movements of continentsover millions of years, it plays a major role in preserving life here: The roiling iron heart of our planet helps maintain the Earth's magnetic field, which helps shield life on the surface from damaging solar energy. In addition, it holds valuable clues about how the planet formed.

"Pinpointing the properties of iron is the gold standard — or, I guess, 'iron standard ' — for how the core behaves," Jennifer Jackson, assistant professor of mineral physics at Caltech, said in a statement. "That is where most discussions about the deep interior of the Earth begin. The temperature distribution, the formation of the planet — it all goes back to the core."

So how to study this inaccessible region lying roughly 1,860 miles (3,000 kilometers) below the planet's surface? Scientists at Caltech have used laboratory setups to put iron through the rigorous, high-pressure conditions inside the Earth to better understand its behavior there.

The researchers essentially sandwiched iron between small diamonds and squeezed until the pressure was 1.7 million times what we experience on the planet's surface. Then they put the compressed samples through tests to see how sound waves traveled through them, and compared the results with observations of how energy  waves produced by earthquakes travel through the planet.

The work helped shed light on iron's density and behavior in such high-pressure conditions, and helped the team get a better idea of iron's melting point at the boundary between the Earth's liquid outer core and solid inner core: around 5,800 degrees Kelvin, or nearly 10,000 degrees Fahrenheit.

Jackson said the new data will help narrow down which light elements are inside the core and help fuel convection there — the process that helps maintain Earth's magnetic field.

Recent research at Carnegie Institution's Geophysical Laboratory indicated oxygen may not be one of the core's ingredients, but the Caltech study authors suggest that is still a possibility.

"There are a few candidate light elements for the core that everyone is always talking about — sulfur, silicon, oxygen, carbon and hydrogen, for instance," Caitlin Murphy, co-author on the study, said in a statement. "Silicon and oxygen are a few of the more popular, but they have not been studied in this great of detail yet. So that's where we will begin to expand our study."

The study appears in the Dec. 20 issue of Geophysical Research Letters.

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