In what is being hailed as a new window on the inner workings of the Earth, scientists have gotten their first-ever look at our planet's rotating core, a glimpse they say should open up new vistas into realms previously accessible only in theory and science fiction.
"It curls your toes that they could do this," said UC Santa Cruz seismologist Thorne Lay.
The new view of the Earth's core could, among other things, help researchers understand why North and South magnetic poles of the Earth wander about on the surface, and completely change places every 100,000 years or so.
"We didn't think this was something we could ever hope to measure," said Paul Richards of Columbia University's Lamont-Doherty Earth Observatory, coauthor of the paper published today in the journal Nature.
By tracking the arrival times of waves from 38 earthquakes that rumbled through the Earth between 1967 and 1995, the researchers created what amounts to a sonogram of the inner Earth, similar to medical sonograms used to see inside the body. Because the speed of waves varies according to the properties of the material they traverse, the scientists were able to create an image of the core.
Richards and colleague Xiaodong Song discovered that the moon-sized core of the Earth spins slightly faster than the Earth's crust, but 100,000 times faster than the continents that drift about on its surface. "It's amazing that such a vast object could move so fast," Richards said.
Confirming these results, a paper soon to be published by UC Berkeley geophysicist Raymond Jeanloz and colleagues at Harvard University got almost exactly the same results using different methods.
"We were totally unaware that they were working on this, and vice versa, until a few months ago," Jeanloz said. "I think we're all amazed."
Song said he was "truly surprised" by the finding, because it contradicts the conventional geophysical mind-set about the inner dynamics of the planet. "We have tended to see the internal structure of the Earth as static," he said. Now it's clear that the inner Earth moves with astonishing swiftness.
Both groups are hoping to get more and better measurements that will tell them, among other things, if the spinning core speeds up or slows down over time, or even changes direction.
Such changes at the core can trigger significant consequences near and at the planet's surface. Just as the rotating Earth churns up hurricanes and storms in the atmosphere, currents of iron create similar weather patterns underneath.
"It's literally the analogue of circulation patterns [in the atmosphere]," said Jeanloz, who hopes to eventually track changes in terms of hours--in essence, producing hourly weather reports on the remote interior of the planet.
To the extent that these currents shape the planet's magnetic field, they influence the evolution of life on Earth, because the planet's magnetic umbrella shields us from dangerous solar winds and occasional high-powered storms.
Several theories could explain the core's rapid spin. The solid iron-crystal core could be getting spun around like a giant electric motor by the magnetic forces created by the molten iron that surrounds it.
Iron sinking toward the center could stir up the same kinds of forces that produce tornadoes on Earth, helping to "kick the inner core forward," Jeanloz said.
Finally, scientists have long known that the rotation of the entire planet gradually slows due to tidal drag from the moon. The core could simply be taking a longer time to lose energy and match the Earth's slowing rate.
All these mechanisms could well be intertwined, Jeanloz said. They all seem to confirm that the liquid part of the core has the consistency of water--even though it is made almost exclusively of iron.
The energy to drive this spinning motor, geophysicists think, comes from the gradual solidifying of the inner core from the liquid outer core. As the iron slowly crystallizes, the inner core grows about an inch in radius every 50 years.
It would take several billion years to freeze the core completely, Richards said. By that time, the Earth will have been incinerated anyway, a casualty of the sun's lethal expansion into what astronomers call a red giant.
Both groups were inspired by computer models of the inner Earth published last year by scientists at UCLA and Los Alamos National Laboratory. The models suggested that the core might spin at a different rate from the rest of the planet.
"It was only one sentence in the paper," said Jeanloz, "but the Lamont people picked up on it and came up with this clever way of doing it."
Up until now, he said, the way to measure the Earth's magnetic field was by detecting effects on the surface. Now, researchers will be able to get to the bottom of the Earth's inner turmoil.
"It's a very, very neat seismological observation," Lay said. "It's pretty astounding that sound waves traveling through the inner core can tell you how it's spinning."
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A New Spin