It’s a mystery that has puzzled generations of scientists: at the very center of our planet, within a liquid outer core, is a Pluto-sized orb of solid iron. That’s right, solid — even though it’s nearly the same temperature as the surface of the sun. How is that possible? Swedish scientists think they know.
I Am Iron Cube
The atoms in a solid block of iron are arranged in what’s known as a crystal structure. Those structures look different, depending on temperature and pressure. At the normal temperatures and atmospheric pressures we know, iron takes on what’s known as a body-centered cubic (BCC) phase—that classic cube shape with eight corner points and a center point. At extremely high pressures, though, iron’s structure morphs into what’s called a hexagonal close-packed (HCP) phase, with each point surrounded by 12 other points.
The pressure at Earth’s core, you might imagine, is extremely high—3.5 million times higher than the pressure you experience up here on the surface. You might expect, then, that iron crystals would take on a hexagonal formation there. Scientists did too: they believed that a cube structure simply couldn’t exist in those conditions. But for a study published in February 2017, scientists from KTH Royal Institute of Technology in Stockholm, Sweden crunched the numbers and came to a surprising conclusion.
Playing With A Full Deck
The researchers used a massive supercomputer to analyze a large amount of data collected three years previous at Livermore Lawrence National Laboratory in California. They found that the core is indeed in a cube structure, thanks to the very extremes that scientists thought made it impossible. At normal temperatures, that cube structure is unstable, and its atomic “planes” easily slide out of the structure into a liquid state. But in the extremes of the core, atoms are moving so quickly, so close together, that they don’t have anywhere to go. Like passengers on a packed subway car, they just switch positions, but maintain their original shape.
“The sliding of these planes is a bit like shuffling a deck of cards,” co-author Anatoly Belonoshko explains. “Even though the cards are put in different positions, the deck is still a deck. Likewise, the BCC iron retains its cubic structure.”
This explains more than why Earth’s core is solid. It also gives an explanation for why seismic waves (the kind that cause earthquakes) travel faster between the earth’s poles than through the equator. The way that atoms move among this cubic structure adds “texture” to the iron the way wood has a grain, giving it a “preferred” direction. Knowing that and other details about the way our planet is structured can help us make important predictions for what might happen to it in the future.
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