Deep Earth
Earth’s core is solid, after all

Seismic waves passing through Earth’s center have long puzzled researchers, as some waves travel fast enough to indicate that Earth’s inner core is solid iron-nickel crystals, but they do not travel quite as quickly as scientists would expect, based on studies of stiff iron alloys. New data now suggest an answer to this conundrum: Under the intense temperature and pressure conditions within the core, these solid crystals may be behaving somewhat fluid-like.

Past explanations have suggested that because the outer core is molten, the inner core may also contain some small amounts of melt, which slows down certain types of seismic waves. Liquid inclusions in the inner core, however, are unlikely because of the core’s high heat and pressure, which would squeeze the liquid out. Another possibility is that a structure composed of multiple iron-nickel crystals may behave in unexpected ways when subjected to high heat and pressure, says Anatoly Belonoshko, a materials physicist at the Royal Institute of Technology in Stockholm, Sweden, and lead author of a study published June 15 in Science.

To better understand what might occur under these conditions, Belonoshko and his team constructed cube-shaped crystals from melts of iron and nickel, and subjected the polycrystalline structures to intense shear stresses, stresses which slide along parallel to the surface of a structure, stretching and bending its molecular bonds. The samples showed “shear relaxation” over time, in which atoms were able to diffuse more easily within the grains and across the crystal grain boundaries, Belonoshko says, as a result of both tiny defects within the grains and alterations to the boundaries between the crystals under the simulated core conditions.

At the high pressures and temperatures in the inner core, the boundaries separating different crystal grains become “liquid-like,” Belonoshko says. “Imagine a structure, parts of which are nailed to each other,” he says. Such a structure is strong and impossible to shear. If the nails are replaced with rubber bands, however, the structure becomes shakier and the boundaries between its elements behave more like a fluid, he says. “We showed that the grains in Earth’s core are not nailed to each other, but rather tied together with rubber bands.” As a result of that lowered resistance, seismic shear waves traveling through the inner core would also slow down, even without any liquid inclusions present — and that could finally explain the long-standing mystery, he says.

Although Belonoshko’s modeling study is “top-notch” in its methods, it leaves open some questions about its real-world application, says David Price, a mineral physicist at University College London in the United Kingdom. For one thing, he says, the crystals in the inner core are likely to be much larger than the crystals Belonoshko modeled. That would mean many fewer grain boundaries, greatly reducing the impact of their liquid-like effect on shear waves. Another question, he says, is whether the short timescale of this study is long enough to accurately simulate how shear waves behave within the inner core. “It’s a very elegant study,” Price says. “The question is, is it relevant to Earth?”

Carolyn Gramling

Links:
"Mercury's gooey center," Geotimes, July 2007

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