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.
