Scientists have used the principles of quantum physics to answer a long-standing puzzle: How can seismic waves travel at different speeds in different directions through Earth’s inner core?
The velocities of seismic waves going north-south through the center of Earth are slightly faster than those traversing the core in an east-west direction. On average, however, the travel times of these waves indicate that the planet’s inner core is made of iron that has a density of about 13 grams per cubic centimeter.
That’s about double iron’s density at the less hellish temperatures and pressures of Earth’s surface.
In Earth’s core, iron atoms pack together and make crystals in the shape of hexagonal prisms, says Gerd Steinle-Neumann, a geophysicist at the University of Michigan in Ann Arbor.
Thermodynamic equations that incorporate the quantum behavior of iron atoms under high pressures show that vibrations travel at a different speed across the hexagonal crystals’ cross sections than along their axes, he notes.
Most geophysicists have assumed that the crystals in the core have random orientations. The observed mismatch between seismic waves traveling north and south and those traveling east and west, however, can be explained if the cross sections of about one-third of the hexagonal iron crystals in the core are aligned with
Earth’s rotational axis. Such an alignment could result from external forces such as Earth’s magnetic field, Steinle-Neumann says. Bilham and his colleagues report their finding in the Sept. 6 Nature.