Here’s what you’ll learn when you read this story:
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Researchers in England believe carbon played a key role in freezing Earth’s core.
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As it contributes to the cooling and freezing of the molten outer core, the solid, iron-rich center of Earth continues to grow.
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Researchers used atomic-scale computer simulations to discover the importance of carbon in forming a solid core.
The literal center of the Earth has been a mystery ever since humans grew curious about our planet. How did Earth’s core form? Researchers in England now believe they’ve solved a hidden chemistry riddle that helps explain the solid, iron-rich mass that serves as Earth’s inner core. And carbon plays a starring role.
We know that today, our solid, iron-rich inner core slowly and continuously grows as it cools, freezes, and hardens the molten outer core surrounding it. How this freezing ever started in the first place, though, has been a question without an answer for as long as we’ve known it happens. Cooling the core to the point of freezing isn’t just about temperature—it also requires maintaining the precise chemical composition to create crystallization (like water droplets in clouds), which cools the outer core before it can freeze without producing a slew of adverse side effects.
If this supercooling of the core wasn’t so precise, we’d have seen all sorts of ramifications on Earth, such as a much larger inner core than we have or even the failure of Earth’s magnetic field. Because those things haven’t happened, scientists believe the core originally cooled at no more than about 250°C below its melting point, as opposed to rapidly supercooling at more than 800°C below that point.
A new study—published in Nature Communications and lead by scientists from the University of Oxford, the University of Leeds, and University College London—aimed to ascertain how the inner core exists without past supercooling. Getting the answer required computer simulations of the freezing process and an understanding of how elements like silicon, sulfur, oxygen, and carbon might impact the freezing process.
“Each of these elements exist in the overlying mantle and could therefore have been dissolved into the core during Earth’s history,” Andrew Walker, co-author and associate professor of Earth sciences at Oxford, said in a statement. “As a result, these could explain why we have a solid inner core with relatively little supercooling at this depth. The presence of one or more of these elements could also rationalize why the core is less dense than pure iron, a key observation from seismology.”
The team ran atomic-scale computer simulations of around 100,000 atoms at supercooled temperatures and pressures equivalent to those in the inner core. The simulations allowed the team to track the way that often small, crystal-like clusters of atoms formed from a liquid. It’s these “nucleation” events that provide the first steps toward freezing.
The team was surprised to find that silicon and sulfur, two elements often associated the inner core, would slow down the freezing process—the first hint that what we’ve previously believed about the core is wrong. Carbon, on the other hand, turned out to be a key accelerator in Earth’s freezing, making it likely to be more abundant in Earth’s core than we ever thought.
The researchers ran simulations until they found that if 3.8 percent of the core’s mass is carbon, then supercooling could occur at 266 °C, the “only known composition that could explain both the nucleation and observed size of the inner core.”
The results show not only way more carbon in the core than we had previously thought, but also that, without carbon, we wouldn’t even have a solid portion of the inner-most part of Earth at all.
“The experiments also show,” the team wrote, “that the inner core freezing was possible with just the right chemistry, and unlike water when it forms hail, it did so without nucleation seeds, tiny particles which help to initiate freezing. This is vital, because when tested in previous simulations, all of the candidates for nucleation seeds in the core have melted or dissolved.”
Carbon will now, hopefully, help provide even more answers to outstanding questions about Earth’s core.
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