Our picture of the inner core is pretty simple—a lump of solid iron, made up of microcrystals with some defects and soft boundaries. However, recent measurements of seismic waves revealed that sound waves travel faster through the inner core along the Earth's axis. Looking even closer revealed that the speed up didn't begin immediately on entering the inner core, but began a few kilometers below the interface between the outer and inner cores.
To explain the anisotropic speed of sound in the inner core, some researchers had proposed that the inner core was, in fact, a single giant crystal with the hexagonal structure pictured to the far right. The hexagonal structure is justified because experiments and calculations have revealed it to be the stable structure adopted by iron under high pressure and temperature.
If this crystal was oriented along the Earth's axis and had lots of defects near the surface it might explain the observations. Considering that the Earth has had a few billion years to slowly cool, it is not so far-fetched to imagine a single large crystal in the center of the Earth.
The problem is that we can't easily replicate the conditions experienced by the iron in the inner core. We can manage the temperature or the pressure fairly easily—explaining why scientists proposed the hexagonal structure in the first place—but not both simultaneously, making it difficult to figure out if hexagonal iron has the right properties to explain the seismic observations. The single hexagonal theory was dealt a serious blow when calculations showed that although the hexagonal structure is stable at high pressure and temperature, it is also isotropic, meaning that sound has the same speed along every orientation, Science reports.
This is a lovely piece of research because it shows how an awkward, but sort-of works, solution can be replaced in one step. We started out needing to assume a single, oriented crystal in the Earth's core and replaced it with a morass of crystals, with only a minority of them aligned. In doing so, two unexplained observations were explained: the speed of sound, and its anisotropy in the Earth's inner core.
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