A new study by researchers at the University of Oxford, University of Leeds, and University College London has identified a new constraint on the chemistry of Earth's core, by showing how it was able to crystallise millions of years ago. The study has been published today (4 September) in Nature Communications.
The researchers showed that the core would need to be made of 3.8% carbon for it to have begun crystallising. This result indicates that carbon may be more abundant in Earth's core than previously thought, and that this element could have played a key role in how it froze, offering a rare glimpse into the processes occurring at the heart of our planet.
Earth's inner core, the solid iron-rich mass at the centre of our planet, is slowly growing as the surrounding molten outer core cools and freezes. But this process has been a source of debate amongst scientists for decades.
Inner core formation is not just a matter of determining when the core cooled to its freezing point, but instead involves the process of crystallisation which depends on its exact chemical composition. Like water droplets in clouds, which can cool to -30 °C before forming hail, molten iron must be supercooled (cooled to below its melting point) before it can freeze.
Previous calculations have suggested that 800-1000 °C of supercooling would be needed to initiate freezing of the core if it were made of pure iron.
However, if the core is supercooled to this degree, researchers have shown that the inner core would grow massively, and the Earth's magnetic field would fail. But neither of these outcomes have occurred during our planet's history. Instead, scientists believe that in the past, the core could have cooled to no more than about 250 °C below its melting point.
This new research aimed to understand how the inner core exists as observed today with such limited supercooling in the past. Without direct access to the Earth's deep interior, the research team needed to rely on computer simulations of the freezing process.
They looked at the presence of other elements, specifically silicon, sulphur, oxygen, and carbon, and how these might affect 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," explained co-author Associate Professor Andrew Walker (Department of Earth Sciences, University of Oxford). "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."
Using atomic-scale computer simulations of around 100,000 atoms at supercooled temperatures and pressures equivalent to those in the inner core, the research team tracked how often small crystal-like clusters of atoms formed from a liquid. These "nucleation" events are the first steps toward freezing.
What they found was surprising: silicon and sulphur, elements often assumed to be present in the core, actually slow down the freezing process. In other words, more supercooling would be needed to start forming the inner core if these elements were abundant in that part of the Earth.
On the other hand, they found that carbon helped to accelerate freezing in the simulation.
In the study, the researchers tested how much supercooling would be required to freeze the inner core if 2.4% of the core's mass were made of carbon. The result: about 420 °C, still too high, but the closest result to viability yet.
But when they extrapolated their results to a case where 3.8% of the core's mass is carbon, the required supercooling dropped to 266 °C. This is the only known composition that could explain both the nucleation and observed size of the inner core.
This result indicates that carbon may be more abundant in Earth's core than previously thought, and that without this element, the formation of a solid inner core may never have happened.
The experiments also show that 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.
Lead author Dr Alfred Wilson (School of Earth and Environment, University of Leeds) said: "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."
Scientists have debated when the inner core began to solidify for decades, with some arguing for an ancient inner core (with freezing beginning more than two billion years ago) and others suggesting a much younger age (less than half a billion years). With this new information about the carbon content of the core, we are one step closer to constraining its chemistry and physical properties, and therefore how it evolved.
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