THE iron lump at the centre of the Earth seems to have a split personality.
Geologists studying how seismic waves ripple through the Earth say the results
suggest the inner core has two distinct layers.
The inner core, which is 1220 kilometres across, is thought to be a chunk of
solid iron. For more than a decade, however, geologists have noticed that it
does not act like a perfectly smooth glob of iron.
鈥淧eople found that there was something strange about it,鈥� says Xiaodong Song,
a geologist at the Lamont-Doherty Earth Observatory in New York. It turns out
that when sound waves from earthquakes pass through the inner core, they don鈥檛
travel at the same speed in every direction. 鈥淲aves travel faster from north to
south than they do from east to west,鈥� says Song.
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Song and his colleague Don Helmberger have now calculated that the theory
that best explains the varying speeds is that the core has two distinct layers.
鈥淭here seems to be a boundary which hasn鈥檛 been observed before,鈥� says Song.
He believes that the outer layer, about 200 kilometres thick, is isotropic,
so sound waves from earthquakes travel through it at the same speed in all
directions. But he thinks that the rest of the inner core is anisotropic, which
means that sound waves travel faster from north to south than they do from east
to west. The results appear in last week鈥檚 Science (vol 282, p
924).
The difference might be because the iron crystals in the outer layer orient
themselves randomly, while the iron crystals in the central part tend to align
themselves from north to south. Nobody yet knows why. 鈥淚t鈥檚 still under debate,鈥�
says Song. 鈥淚t might be magnetic, but it might have to do with heat flows.鈥� He
hopes that more data from earthquakes will explain the mystery.