Putting a New Spin on Earth's Core
By William J. Cromie
The metallic core of our planet is spinning faster than the rest of it, according to evidence unearthed by Harvard geologists. And this hellishly hot core, almost as big as the moon, apparently is growing in size.
"It's like a planet within a planet," says Adam Dziewonski, Baird Professor of Science.
It's the first major finding about Earth since the 1960s, when geologists confirmed that continents and ocean bottoms drift across the planet's surface at a rate of less than an inch to about four inches a year. "You very seldom make planetary-scale discoveries like these," Dziewonski notes.
The whole Earth spins completely around once a day, while the inner core rotates an extra three degrees or so each year. In approximately 120 years, the planet within completes an extra lap (360 degrees).
If you could stand on the surface of the inner core and look up at the bottoms of continents, you would see them falling behind at a languid rate of about a half-mile a day.
The solid inner is surrounded by a liquid outer core. They, in turn, sit inside a 1,800-mile deep mantle of rock like a partly cooked yolk enveloped by egg white. Atop the white lies a 20-mile-thick cracked "shell" representing the moving continents and oceans.
The inner core itself was only discovered in 1936. Dziewonski and Freeman Gilbert of the University of California, San Diego, proved it was solid, rather than liquid, a scant 25 years ago.
In 1986, Andrea Morelli, John Woodhouse, and Dziewonski, working at Harvard, found a strange unevenness, or anisotropy, in the inner core. Shock waves from earthquakes travel through it in a north-south direction faster than in other directions. Geologists attribute this to the crystalline structure that iron, its major ingredient, assumes under the intense pressure near Earth's center, more than a million pounds on every square inch.
Two years ago, Dziewonski and research associate Wei-jia Su showed that the axis of symmetry of the iron tilts about 12 degrees from the north-south axis of its rotation. Dziewonski and Su located the asymmetry axis when they analyzed records from 15,722 earthquakes that sent shock waves though the inner core.
Earth scientists began to discuss whether the inner "planet" might rotate at a different speed than the rest of Earth. While visiting the University of California, Berkeley, last year, Dziewonski and geophysicist Raymond Jeanloz talked about how they might solve that riddle.
"We were riding around in a car sightseeing and discussing how we could measure such rotation," Dziewonski recalled. "We came up with the idea of using the axis of symmetry as a marker. The many years of data we had collected could tell us if and how much its position changed with time. If it moved around, then the inner core must rotate at a different speed."
Su divided 30 years of earthquake records into three 10-year periods, but he couldn't come up with a clean result. Some months later, Dziewonski suggested using six five-year periods. When that was done, the change showed up clearly. They found that path along which shock waves travel fastest, the axis of symmetry, moved from west to east about 3 degrees a year.
Talk of Torque
Meanwhile, seismologists at Columbia University analyzed the travel paths of quakes that had rattled the ocean floor south of South America. These temblors sent shock waves through the center of Earth and out the other side at Fairbanks, Alaska. They, too, conclude that the inner core spins independently but at a slower speed -- one degree a year. At that rate, it would take the solid iron core 360 years to lap the rest of Earth.
The extra rotation apparently comes from a twisting force generated by the interaction between the magnetic fields of the inner and outer cores. The inner core, more than 3,000 miles below our feet and roasting at a temperature of about 7,000 degrees Fahrenheit, steadily releases its heat to the liquid outer core. This heat stimulates convective motion in the latter, causing molten iron to move like air over a radiator. Hot fluid moves upward, cools, then slips downward.
The highly conductive iron moving in a magnetic field generates electricity, creating the equivalent of a huge generator, or dynamo, at the planet's center. This electricity, in turn, has its own magnetic field which is responsible for compasses pointing north, northern and southern lights, and other effects at the planet's surface.
Magnetic fields in the core reach strengths of 200 or more times greater than at the surface. The intense field at the bottom of the outer core penetrates into the inner core, coupling the two together.
"We believe this coupling provides enough twisting motion, or torque, to power for extra rotation of the inner core," Dziewonski says. The mechanism works somewhat like that of a motor wherein a rapidly changing electromagnetic force causes a rotor to rotate. In this case, the motor rotor is the size of the moon.
This neat explanation leaves one frustrating question: where did the magnetic field come from that originally started the geodynamo? "Once you get things going, the electric current generated by the dynamo can reinforce it," Dziewonski notes. "But we don't know how things got started in the first place."
Sometime after Earth formed, nearly 5 billion years ago, it must have been completely molten. During that time, heavy metals like iron sank toward the center.
"At first, there was no inner core," Dziewonski says. "Now it is about 1,500 miles wide and 4,700 miles in circumference. We believe it grows by the freezing-out of iron as Earth cools. The heat that is released provides the energy to roil the outer core and drive the geodynamo."
When the inner core began to solidify, and why it developed its anisotropy, remain unknown.
It's also possible that the inner core changes its axis of symmetry over time, or that the overlying mantle is slowing down compared to the inner core rather than the other way around. However, Dziewonski says, "the best explanation of what we see is that the anisotropy is frozen so its position changes correspond to movement of the inner core."
Discovery Within Discovery
During their analysis of earthquake records, Su and Dziewonski found something they call "every bit as exciting" as discovering the independent spin. Around 1971, the axis of anisotropy, and presumably the inner core, shifted ahead some 50 degrees. That's as much motion as takes place in about 17 years, or 50 years if you take the Columbia researchers' slower rotation rate.
"We were surprised and stunned when we saw it," Dziewonski remarked.
At nearly the same time, the magnetic field at the surface underwent an abrupt change known as a "magnetic jerk."
"This instability probably came from a change in the outer core, where the field originates," Dziewonski guesses. "The change may also have produced a temporary torque that jerked the inner core forward. The motion of the outer-core fluid is complex and poorly understood, however, so this may be just a hand-waving explanation."
To answer questions raised by finding a planet within a planet, and a discovery within a discovery, "we need to constantly monitor the inner core," Dziewonski says.
That's not easy. Earthquakes occur frequently only in certain parts of the world, and many of them don't produce shock waves that pass through the metal core. To do a better job, Dziewonski wants to add more recording stations to the worldwide net of some 100 stations already in existence. He also sees a need for more powerful computers to find out exactly how the geodynamo works and how it interacts with the inner core.
"The expense would not be unreasonable," he maintains. "If we made similar discoveries about Mars or our moon, there would be a clamor to send a space mission there."
Copyright 1998 President and Fellows of Harvard College