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Iron in the Earth’s core weakens before melting.

11 October 2013

The pressures and temperatures in the Earth’s core are extreme – the main component, iron, is squeezed to half its surface volume, while the temperatures are similar to those on the surface of the sun. 

While seismic waves give us a clue as to what is happening, seismologists tell us that the behaviour of the materials in the inner core is very different to that expected from experiments and computer simulations. Specifically, the proposed mineral models for the inner core have always shown a faster wave speed than that observed, especially in the shear waves (VS); this mismatch has given rise to several complex theories about the state and evolution of the Earth’s core.

Science -October 2013

Figure. Computed wave velocities of iron at 360 GPa as a function of T/Tm where Tm is the melting temperature. Seismic data (PREM) are given as dashed lines.

In our recent Science paper, we propose a more straightforward explanation for this discrepancy, based on the idea that materials may become extremely weak just before melting. We used quantum mechanical calculations to evaluate the wave velocities of solid iron at inner-core pressure up to melting. Our work shows that these velocities decrease linearly to about 95% of the melting temperature (see figure), but thereafter they drop dramatically. At about 99% of the melting temperature, the calculated velocities agree with seismic data for the Earth’s inner core. Since, other geophysical results also suggest that the inner core is likely to be at 99-100% of its melting temperature, these results give a compelling explanation as to why the seismic wave velocities are lower than those predicted previously.

This is, however, not the end of the story as other factors need to be taken into account before a definitive core model can be made. As well as iron, the core contains nickel and light elements, such as silicon and sulphur. Their effect on the properties of the core is not currently known, although we are working on this problem. Despite this, the strong pre-melting effects shown in this Science paper are an exciting new development in understanding the Earth’s inner core.

Full reference: Strong pre-melting effect in the elastic properties of hcp-Fe under inner-core conditions.  Authors: Benjamí Martorell, Lidunka Vočadlo, John Brodholt, Ian G. Wood; Science DOI: 10.1126/science.1243651

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