If Earth were a candy, there would be a nut inside that creamy filling.
Confirming a long-held scientific notion, a Northwestern University seismologist and a colleague at the French Atomic Energy Commission have provided the first direct evidence that -- inside a liquid core -- the very center of the Earth is solid.
The long sought finding, which had been hinted at but never proven, came from analysis of seismic waves generated by the June 1996 earthquake in Indonesia and recorded at a large-array seismic network spread across France. The finding will be presented Thursday at the American Geophysical Union meeting in San Francisco and will appear in the Dec. 15 issue of Earth and Planetary Science Letters.
For decades, seismologists have used seismic waves as a sort of probe of the Earth's insides. They look at how the waves created by an earthquake at the surface of the Earth reverberate through the interior before being detected on the other side.
"The general picture of the Earth at the turn of the last century was that it had a rocky mantle floating on a liquid core of molten iron," says Emile A. Okal, professor of geological sciences at Northwestern and an author of the new study. The fluidity of the iron explained the existence of the Earth's magnetic field, he said.
But geophysicists also assumed that at some great depth, the pressure would be so high that even at temperatures of thousands of degrees the iron would freeze solid. In the 1930s, seismologists did find a "discontinuity" in the velocity of waves propagated through the center of the Earth, suggesting some sort of stratification of the core.
The problem, for 60 years now, is that those waves never carried the signature of a solid.
"A solid has a very distinctive mechanical property, which is that it can sustain two different kinds of waves," Okal said. "It can transmit a wave that oscillates in the direction of travel, sort of a pulsing compression-and-relaxation, and it can transmit a wave that vibrates perpendicular to the direction of travel, like a guitar string."
A liquid can propagate only the first type of wave, which corresponds to a change of volume and pressure, as it propagates, he said. "The second type requires memory of a shape for its restoring force, and a liquid has no shape."
Only the first type of wave, characteristic of liquids, had ever been observed coming from the Earth's core.
Okal and his colleague in France, Yves Cansi, used an eight-station French seismic network to study the Indonesian earthquake, and for the first time detected the telltale second vibration.
"The 1996 Flores Sea earthquake, which was a big earthquake at about 600 kilometers depth, was perfect in geometry for recording in France," Okal said. "If you want to sample the deepest part of the Earth, you need a big, deep earthquake," he said. "And they are rare." A deep earthquake gives rise to cleaner signals, he said.
Improvements in instrumentation over the last 15 years were crucial to the new finding, Okal said, as were computer capabilities, developed in France, to extract signals from noise.
Okal's expectations for the significance of the finding are, well, down to earth.
"We look at the interior of the Earth because we would like to know what is below us," Okal said. "But this may turn out to be interesting to the field of materials science because it indicates that under tremendous pressures, iron is behaving in a different way," he said. "Understanding how the qualities of materials are affected under extremely high pressures -- millions of times the atmospheric pressure -- might be applicable for different materials at not-so-heavy pressures."
The research was supported by the National Science Foundation.
Journal
Earth and Planetary Science Letters