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A high concentration of carbon within Earth鈥檚 inner core could explain a long-standing mystery about how the deepest part of our planet froze solid 鈥� a process that kick-started the magnetic field protecting life on the surface.

Earth鈥檚 inner core presents a paradox for geophysicists: it first formed as a massive liquid ball of mostly iron, then began to solidify within the last billion years. In order for that freezing process to start in a pure iron object, it would have had to cool by at least 700 kelvin in that time period. But such a large and relatively fast drop in temperature is impossible given how big the inner core is. 鈥淭he mineral physics has to be wrong,鈥� says at the University of Leeds in the UK.

Wilson and his colleagues tackled this 鈥渋nner core nucleation paradox鈥� by simulating how the cooling of the inner core would change using a more realistic composition of elements than pure iron alone. The core mostly consists of iron and nickel, but around 10 per cent is made up of an unknown composition of light elements like silicon, sulphur, oxygen and carbon.

Using a supercomputer, the researchers simulated the interactions of several hundred iron and carbon atoms under the extreme pressure and temperature conditions of the inner core, then scaled up to tens of thousands of atoms and started lowering the temperature below the melting point of the material.

With carbon atoms making up about 15 per cent of the mix, solid clusters of atoms started to form with as little as 250 K of cooling, a plausible temperature change for the inner core to undergo in roughly a billion years, says Wilson. This potential resolution to the paradox also hints at what makes up the unknown portion of Earth鈥檚 inner core: lots of carbon.

鈥淚 think this is a really promising approach and that maybe this could be a solution, but it鈥檚 too early to tell whether carbon really is going to work,鈥� says at Case Western Reserve University in Ohio. He says the high carbon concentration and the magnitude of cooling still required is 鈥渁t the edge of what could possibly be a solution鈥�.

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Wilson says the cooling afforded by carbon is only 鈥渃lose enough鈥�. But he adds that a more complex simulation 鈥� including interactions between iron, carbon and oxygen 鈥� might reveal a scenario where the inner core could freeze with even less cooling required, building a more detailed view of the core鈥檚 make-up. That knowledge could, in turn, underpin 鈥渙ur overall understanding of how the planet has thermochemically evolved since its formation鈥�, he says.