New research has identified a new restriction on the chemistry involved in the Earth’s core, explaining how it was able to crystallize millions of years ago. According to this work, the core would have needed to be made of 3.8 percent carbon for crystallization to begin, which means carbon is likely far more abundant in the core than previously believed. This is a rare insight into the processes that formed the heart of our planet.
The core is the iron-rich mass at the center of the planet that plays an important role in determining the structure and dynamics of the Earth’s deep interior. The core is not a static mass; it’s actually gradually growing as the molten outer layers cool and then freeze. But while scientists have known about this for a long time, there is still debate over the processes involved.
Inner core formation is not just a matter of determining when the core cooled to its freezing point, which could be understood as a mechanical process, but rather how it becomes crystallized, which is a chemical process involving a knowledge of its composition.
Molten iron, like water droplets in clouds, must be supercooled before it can freeze. This means its temperature must fall below its melting point. Previous calculations have indicated that 800-1,000°C (1,440-1,800°F) of supercooling would be needed to start freezing the core if it were made of pure iron. But if the core is supercooled to this degree, so other research has suggested, the inner core would grow significantly, and the Earth’s magnetic field would stop. Of course, neither of these outcomes have happened, so scientists believe that, in the past, the core can’t have cooled to more than about 250°C (450°F) below its melting point.
To understand how the inner core can exist with such limited supercooling in the past, researchers from the University of Oxford, University of Leeds, and University College London created computer simulations of the freezing process. This is because it is not possible for them to have direct access to the Earth’s deep interior.
They examined the presence of different elements in the core, including silicon, sulfur, oxygen, and carbon, and explored how they could influence 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,” Associate Professor Andrew Walker, Department of Earth Sciences, University of Oxford, explained 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 rationalise why the core is less dense than pure iron, a key observation from seismology.”
The team used atomic-scale computer simulations of around 100,000 atoms at supercooled temperatures and at pressures believed to be close to those of the inner core. This allowed them to track how often small crystal-like clusters, called “nucleation events”, formed from the liquid. These events are regarded as the first steps towards freezing.
To their surprise, the researchers found that silicon and sulfur, two elements that are often assumed to be present in the core, actually slowed down the freezing process. If these elements were abundant in that part of the core, the team argue, then it would need more supercooling to start the freezing process.
Instead, the researchers found that carbon helped accelerate freezing in the simulation. In the study, they tested how much supercooling would be needed to freeze a core if 2.4 percent of its mass was made of carbon. They found that about the required supercooling was about 420°C (756°F), which is still too high. But when they extrapolated their results to an example where 3.8 percent of the core’s mass is carbon, the required supercooling dropped to 266°C (478.8°F).
This is the only known composition that could account for both nucleation and the observed size of the inner core.
It seems the Earth’s core contains more carbon than previously thought and without it, the formation of the inner core may never have happened. It also seems that core freezing is possible with the right chemistry; unlike water when it forms hail, the core did not require “nucleation seeds” – tiny particles that help initiate the freezing. This may seem like a small point, but it is extremely important as when tested in a previous simulation, all the candidates for nucleation seeds melted or dissolved.
“It is exciting to see how atomic scale processes control the fundamental structure and dynamics of our planet. By studying how Earth’s inner core formed, we are not just learning about our planet’s past. We’re getting a rare glimpse into the chemistry of a region we can never hope to reach directly and learning about how it could change in the future,” Dr Alfred Wilson, School of Earth and Environment, University of Leeds, added.
For decades, scientists have debated when the core began to solidify. Some have argued for an ancient inner core – with freezing beginning more than 2 billion years ago – with others suggesting a much younger age – less than half a billion years. This new research offers a big step towards understanding how its chemical and physical properties work, and therefore how it evolved.
The study is published in Nature Communications.
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