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Soft iron at the Earth’s core

Geologists study the interior of the Earth through the propagation of sound waves. The idea is fairly simple: sound waves travel by shifting matter back and forth (pressure waves) or side to side (shear waves). Both of these waves travel at different speeds and are reflected differently when they hit surfaces. In addition, liquids don't allow shear waves to pass through them, though at a liquid-solid interface, a shear wave may be partially converted to a pressure wave and vice versa, thus allowing a pressure wave in a liquid to excite a shear wave in a solid. By mapping out times and directions from many many seismic waves, researchers have built up a very detailed picture of the Earth's interior. But the inner core presented a challenge. 老域名购买

The problem is that shear waves move at an unexpectedly slow pace through the inner core. We know the core is iron or an alloy that is mainly iron. From lab experiments, we also know that shear waves in iron travel quite fast (4.4km.s), while in the core they proceed at a more leisurely 3.7km/s. This difference has been known for quite some time, but the explanations, such as liquid inclusions, seemed unlikely for a variety of reasons. In the case of liquid inclusions, they don't stay trapped in the inner core but are ejected into the liquid outer core.

The problem may now have been resolved1 by applying some computer modeling to examine the structure of iron in the inner core. It was found that the inner core forms very small crystals of iron that have many imperfections. These imperfections reduce the rigidity of the crystal, in turn slowing the speed of the shear waves. In addition, the surface layer of the crystals was much more mobile than expected—almost liquid-like. This allows the crystals to move relative to one another and effectively softens the iron even more, further slowing the shear waves.

To increase their confidence in the model they used to produce these results, the researchers tested their model under conditions where they could compare the results with laboratory results. These simulations showed iron crystals with shear and pressure wave velocities in agreement with those measured in the laboratory.

I always knew the Earth was an old softy.

1 Belonoshko, 316, pp 1603


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