Seismic waves passing through Earth’s inner core have revealed much about our planet’s iron center: how it’s changing shape, reversing its spin, is weirdly textured, and contains an unusual state of matter.
Now, a new study seeking to explain anomalous data suggests Earth’s core may be layered like an onion.
Scientists in Germany wanted to specifically investigate the problem of seismic anisotropies – variations in the speed of seismic waves reverberating through Earth when they hit the inner core, depending on their direction of travel.
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“There have been several hypotheses for the origin of these anisotropies,” says mineralogist Carmen Sanchez-Valle, from the University of Münster.
“We set out to study the combined effect of silicon and carbon on the deformation behavior of iron.”
To figure out what’s happening, the researchers tested how these key elements of the inner core might interact under extreme pressures and temperatures as high as 820 °C (1508 °F).
Using X-ray diffraction, the researchers looked for a property called lattice-preferred orientation (LPO), which describes how crystals within solids align due to thermal patterns.
Previously, scientists lacked substantial data on how the LPO of iron might appear when mixed with silicon and carbon to form alloys.
LPO can affect the way that sound waves are transmitted through metals like iron, and it has been thought that this may explain seismic anisotropy. Here, it was tested on the tiniest of scales, with the alloys contained, squished, and heated in super small canisters.
“The diffraction patterns were analyzed after the experiment to derive plastic properties – specifically, yield strength and viscosity – of the iron-silicon-carbon alloys, which were further modeled through theory to extrapolate them to inner core conditions,” explains Sanchez-Valle.
The results showed that compared to pure iron, the addition of silicon and carbon did indeed change the crystal lattice arrangement of the iron alloy.
The resulting differences in seismic wave speed would match the anomalies observed in the outer part of the inner core.
It’s more evidence that Earth’s inner core actually has several layers – an impressive feat of science for a study of something that’s more than 5,000 kilometers (3,107 miles) beneath us, buried under rock and liquid metal.
The central part of the inner core may be low in silicon and carbon, resulting in strong seismic anisotropy, the researchers think, “while the increasing concentration of light alloying elements towards the outer layers of the inner core results in reduced anisotropy.”
Geologists are making steady progress in understanding the complexities of what lies beneath Earth’s surface, mainly by measuring how seismic waves travel and by recreating inner and outer core conditions in the lab.
The detailed work involves finding inconsistencies, thinking up possible explanations, and then putting those explanations to the test – something that the team behind this study was able to do successfully.
“The depth-dependent anisotropy pattern observed in the Earth’s inner core may result from chemical stratification of silicon and carbon following core crystallization,” conclude the researchers.
The research has been published in Nature Communications.