What's Inside the Earth's Core?

The inner core of the Earth appears to hold an innermost secret. From a report: Geology textbooks almost inevitably include a cutaway diagram of the Earth showing four neatly delineated layers: a thin outer shell of rock that we live on known as the crust; the mantle, where rocks flow like an extremely viscous liquid, driving the movement of continents and the lifting of mountains; a liquid outer core of iron and nickel that generates the planet's magnetic field; and a solid inner core. Analyzing the crisscrossing of seismic waves from large earthquakes, two Australian scientists say there is a distinctly different layer at the very center of the Earth. "We have now confirmed the existence of the innermost inner core," said one of the scientists, Hrvoje Tkalcic, a professor of geophysics at the Australian National University in Canberra. Dr. Tkalcic and Thanh-Son Pham, a postdoctoral researcher, estimate that the innermost inner core is about 800 miles wide; the entire inner core is about 1,500 miles wide. Their findings were published on Tuesday in the journal Nature Communications. While the cutaway diagram appears to depict clear-cut divisions, knowledge about the deep interior of Earth is unavoidably fuzzy. It is nearly 4,000 miles to the center of Earth, and it is impossible to drill more than a few miles into the crust. Most of what is known about what lies beneath comes from seismic waves -- the vibrations of earthquakes traveling through and around the planet. Think of them as a giant sonogram of Earth. Two Harvard seismologists, Miaki Ishii and Adam Dziewonski, first proposed the idea of the innermost inner core in 2002 based on peculiarities in the speed of seismic waves passing through the inner core. Scientists already knew that the speed of seismic waves traveling through this part of the Earth varied depending on the direction. The waves traveled fastest when going from pole to pole along the Earth's axis and slowest when traveling perpendicular to the axis. The difference in speeds -- a few percent faster along polar paths -- arises from the alignment of iron crystals in the inner core, geophysicists believe. But in a small region at the center, the slowest waves were those traveling at a 45-degree angle to the axis instead of 90 degrees, the Harvard seismologists said. The data available then were too sparse to convince everyone.

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