The Sciences

Unlocking The Secrets of Earth’s Other Moon

Is what's inside Earth simply the other half of our only satellite?

By Daniel WoodOct 24, 2005 12:00 AM


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Far below your feet—about 3,219 miles down—is Earth’s other moon. Rotating independently of the planet, turning at a different speed within a fluid outer core, this solid, satellite-size sphere holds clues to understanding Earth’s earliest history and perhaps even life on the planet. For 300 years, little was added to the postulation that Earth had at its center a homogeneous ball of pure iron. But in the past decade or so, geophysicists, using new data and laboratory simulations, have started to map and analyze it. Every year, more features are discerned from seismic evidence. Scientists don’t yet agree on what all the information means, but one thing is becoming clear: Earth’s center is far more varied and unusual than anyone had previously thought.

“If the inner core were a featureless ball, it doesn’t take you far,” says geophysicist Ken Creager of the University of Washington. “But the more you look, the more details you see. Complexity gives clues to origins and to evolution. Now we see that the core is enormously complex. It’s telling us how Earth works.”

The composition of the inner core is affected by extraordinary pressures: about 52 million pounds per square inch, or 3.5 million times Earth’s atmospheric pressure. The pressure keeps the inner core solid, despite temperatures as high as 11,000 degrees Fahrenheit. Seismic data show that sound waves, descending from earthquakes in the mantle and crust above, slow as they pass through the molten outer core. Some waves then carom off the surface of a deeper inner core; others pass rapidly right through the inner core.

By precisely measuring the arrival times of these waves at distant seismometers, geophysicists have gleaned unexpected information. For example, Miaki Ishii, of the Scripps Institution of Oceanography at the University of California at San Diego, speculates that the inner core contains anomalies, including a small “seed” at its very center. Scientists have also detected zones of unique crystallization elsewhere in the core, and—quite possibly—an ever-growing outer surface that may contain shallow hills and valleys. These surface features may result from the iron crystallization process and may grow outward, much in the manner that snowflakes form.

The concept of an expanding, layered inner core could go a long way toward clarifying the history of the early Earth. It would help explain how the inner core first came to be and would help account for the incredible amounts of heat that lie deep within the planet. Geologists believe that Earth was struck by a Mars-size asteroid about 4.5 billion years ago. The impact created sufficient heat to melt most of the planet’s rocks; the heavier iron in the rock sank toward the center, which has been shedding heat ever since. That’s the theory, anyway. Just as paleontologists deduce ancient events by examining fossils and strata, so geophysicists believe the inner core contains solid, “fossilized” evidence—areas of varying crystallization and chemistry—that shed light on how the planet developed.

A better understanding of the structure of Earth’s inner core would clarify its role in locking the planet’s magnetic field in its north-south position, which has the effect of insulating terrestrial life from a lethal bombardment of cosmic rays. Putting all the puzzling pieces together won’t be easy. “Because we don’t have direct access to the core, we have an imperfect ability to model what’s happening there,” says planetary scientist David Stevenson of Caltech. “The problem is similar to modeling Earth’s atmosphere or oceans: It’s too complicated. We know that every planet is unique; we know Venus, Mars, and the moon have no magnetic field. So what’s different about Earth? How did they get that way and we get this way?”

Core geophysicists admit they’re like the 16th-century explorers who could map the outer shape of the continents but had only the slightest inklings of internal terrain. As Creager says, “The core is a place of plausibility, with few certainties.”

Nonetheless, Creager and his colleagues believe science can unravel many of the core’s mysteries within the next decade. Researchers have positioned thousands of seismometers worldwide, and many more are on the way. By listening in on seismic waves, the instruments will provide ever more precise measurements of the core. For example, the National Science Foundation is investing more than $100 million to establish the USArray, a nationwide network of 400 linked seismometers that will be used, in part, to create a more detailed map of the inner core. Meanwhile, laboratory simulations of deep-core physics are improving daily. Before long, perhaps, our inner moon will be as familiar to us as the outer one, and its role in Earth’s cosmological trajectory will see full light.

The Moon

Age: About 4.5 billion years Diameter: 2,160 miles Rotation rate: Once every 27.3 days Density: 208.5 lbs. per cubic foot Mass: 81 x 1018 tons Features: 43-mile-thick outer crust; 788-mile-thick mantle; 248-mile-wide iron-rich core Activity: Tectonically dead. Without an active liquid core, the moon has no mountain building, no volcanoes, and no magnetic field

Earth’s center

Age: About 4.5 billion years Diameter: 1,510 miles Rotation rate: Once every 23.89 hours—or 0.2 percent faster than Earth. Every 1,800 years, the inner core turns one more time than the planet Density: 811.6 lbs.

per cubic foot

Mass: 1.07 x 1020 tons Features: Numerous 3- to 6-mile-wide blobs of crystallized material and evidence of layers hundreds of miles thick and perhaps a 360-mile-wide innermost inner core of iron Activity: Convective currents in the outer core give rise to a dynamo process—a natural generator. This geodynamic action creates Earth’s protective magnetic field.

Bruce Buffett, a University of Chicago geophysicist, studies the inner Earth.

What would an asteroid strike 4.5 billion years ago explain about Earth’s core?

B: It fits what we know. There were a lot of Mars-size bodies floating around then. The chance of things colliding was very likely. The whole planet would have been molten; the iron would have sunk, the silica would have floated. It would explain, at least in part, the core’s heat. It would explain the basic structure of the planet.

What has changed recently about

our understanding of the core?

B: We used to think the core was isotropic—uniform throughout. Yet there’s now evidence of large-scale structures and very-small-scale structures frozen inside it. It’s not homogeneous. It’s complex. It’s anisotropic. These structures may be “fossils” from earlier times. There’s some process that aligns the crystals there that we don’t understand. There may be different processes going on, at macro and micro levels. Understanding the core’s anisotropic structures may tell us how it’s growing—and if the growing will end.

And if it is growing?

B: Albert Einstein said that one of the biggest unsolved mysteries in physics is a detailed understanding of the origin of Earth’s magnetic field. We know it comes from the convection of the molten outer core. But if the energy is coming from the growth of the inner core—as gravity pulls the heavy iron down and the lighter materials rise within the outer core—why hasn’t the core become completely solid over the billions of years Earth’s magnetic field has existed?

How is technology helping you?

B: It’s a tough observational game. We’re developing computer models capable of simulating the inner core, but they’re crude now. In the laboratory, we’re using diamond anvils to create high pressures and temperatures, to study the effects of materials in the core. What happens to iron under pressure? We aren’t yet able to reach the conditions of the inner core, though, so we extrapolate the results.

Did the massive earthquake in Sumatra

last year provide you with data?

B: Nothing yet. In studying the core, you need to have seismic stations located opposite the earthquake, collecting data passing directly through Earth’s center. There aren’t many seismometers in the Pacific Ocean.

Are you frustrated by how little we know?

B: It’s an opportunity rather than an aggravation. It would be nice to put all the stuff together: the core anisotropy, the energy question, the source of the core’s heat, Earth’s history. The core is a spider’s web that links to a lot of things. But there aren’t many eureka moments in geophysics.

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