4 April 2008—You probably haven’t noticed, but the length of a day is not what it used to be. Though only on the order of a few milliseconds’ difference and observable only over a period of decades, the time it takes the Earth to make one revolution varies. This anomaly has teased earth scientists for some time. They suspected that it is due to the exchange in angular momentum between the Earth’s fluid core and rocky mantle, but how that happens was still a question.
In today’s issue of Science , researchers at the Tokyo Institute of Technology provide evidence that a mineral phase in the Earth’s mantle is electrically conductive and could be doing the job by electromagnetically linking the core to the mantle. The Tokyo scientists say that the conductive mineral could point superconductor researchers in a new direction, too.
The conductive mineral, called post-perovskite, appears in the lowermost area of the Earth’s mantle, the rocky shell that surrounds the inner and outer cores of the Earth and is itself covered by the Earth’s crust. The most common component of the lower mantle is perovskite, which is rich in magnesium silicate (MgSiO 3 ).
Back in 2004, the Tokyo team reported that perovskite transitions into post-perovskite at depths below about 2600 kilometers because of the increasing pressure from the weight of the earth above. In making their discovery, the researchers had to recreate the temperatures and pressures at those depths in the lab. Other research groups in Japan and Switzerland independently made the same discovery at about the same time.
”Discovering the MgSiO 3 post-perovskite phase helped explain the differences in anomalous seismic-wave speeds propagating in the lowermost mantle,” says Kei Hirose, a professor of earth and planetary sciences at the Tokyo Institute of Technology and leader of the research group. ”And because the structure of the post-perovskite is very different from the structure of the perovskite, we expected their physical properties would also be different.”
To test this expectation, the group turned to the same lab techniques they employed to discover the mineral layer. They compressed samples of MgSiO 3 at room temperature using a diamond anvil and then heated the samples with a laser to synthesize both perovskite and post-perovskite. But this time, they also measured the materials’ conductivities.
In one particular set of experiments, perovskite was first synthesized at 104 gigapascals—about 1 million times standard atmospheric pressure. More pressure was exerted several times, reaching as high as 143 GPa, while the material was heated and reheated, with the highest temperature being 3000 Kelvin. This transformed the perovskite into post-perovskite, and its conductivity shot up by three orders of magnitude to 140 siemens per meter. That’s more than 20 times the conductivity of seawater at ordinary temperatures and pressures, but hundreds of thousands of times as resistant as copper.
Hirose theorizes that the conductivity change probably happens because the iron ions in the post-perovskite structure are closer together than they are in perovskite.
Such a conductive layer goes a long way toward explaining the variations in the day’s length. ”So if you have a layer of high electrical conductivity at the bottom of the mantle, it would create a strong magnetic coupling between the core and the mantle,” says Hirose. ”This would produce an exchange of angular momentum between them that could account for the change in the length of a day by several milliseconds on a time scale of decades.”
While the findings may be of more interest to researchers in the basic sciences than engineers working in their cubicles, Hirose says the work could have ”important implications” for researchers of room-temperature superconductivity.
”To date, all the materials being used in superconductivity research have a perovskite-type crystal structure,” he notes. ”But now we have post-perovskite, which has even higher conductivity, at least in the case of (Mg, Fe)SiO 3 post-perovskite. So this could be a lead to a new area of investigation.”
About the Author
JOHN BOYD writes about science and technology from Japan. He recently reported for IEEE Spectrum Online on an effort to mix MEMS and organic electronics in a flexible plastic substrate for low-power wireless communications and wireless power for portables.