- For 60 years geophysicists have suspected
that the Earth's inner core was solid - now they have proved it.
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- By detecting special seismic tremors
in the aftermath of a massive Indonesian earthquake, the seismologists
from Northwestern University and the French Atomic Energy Commission have
shown without doubt that at the heart of the Earth is a solid iron-nickel
ball, 2400km in diameter.
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- Emile Okal and his French colleague,
Yves Cansi, used an eight-station French seismic network to study the earthquake
occurring on the other side of the world.
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- "The 1996 Flores Sea earthquake,
which was a big earthquake at about 600km depth, was perfect in geometry
for recording in France," said Okal. It was a rare opportunity as
massive, deep earthquakes are needed to probe the necessary 5000km below
the surface of the Earth but only happen infrequently.
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- Breakthrough applauded
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- Experts hailed the research, announced
at the American Geophysical Union meeting in San Francisco, as a breakthrough.
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- Professor Kathy Whaler, a geophysicist
at Edinburgh University told BBC News Online, "No-one has unambiguously
taken these waves from the inner core before. And there is a group in Utrecht
who are getting the same result but using a different analysis of a different
earthquake, 1994 in Bolivia - that gives us confidence."
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- As long ago as the 1930s, scientists
predicted that despite a temperature of thousands of degrees, the crushing
pressure at the Earth's centre would cause the iron-nickel alloy to freeze.
But because the telltale signals are so weak they have taken 60 years to
detect.
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- The key to the breakthrough is the behaviour
of the two types of seismic waves. Pulse waves can travel through both
liquids and solids as they move by compressing and then relaxing the material
in the direction of travel.
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- Shear waves, in contrast, can only pass
through solids. They vibrate at right angles to the direction of travel
and as liquids have no material strength the signal rapidly dissipates
in the fluid.
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- So detecting a shear wave coming all
the way from the inner core would prove it was solid. But the scientists'
task was made even more difficult by the fact that the liquid outer core
surrounding the inner core blanks out all shear waves.
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- Wave energy converted
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- So they then had to exploit the different
speeds at which the waves travel. Imagine a seismic wave rumbling down
through the Earth. When it reaches the outer core, all shear waves are
lost and only pulse waves continue. When the pulse waves reach the inner
core the waves are partly refracted and reflected.
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- This allows part of the energy to convert
into shear waves that then travel through the inner core. Shear waves travel
more slowly than pulse waves so they reach the opposite side of the inner
core later. Here they are partly converted back to pulse waves.
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- It was these delayed pulse waves that
the scientists detected. The reason it has taken 60 years to do so is because
the conversions from pulse to shear to pulse sap the energy of the waves
so the signals are exceedingly weak.
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- Improvements in instrumentation over
the last 15 years were crucial to the new finding, Okal said, as were computer
capabilities, developed in France, to sort the signals from the noise.
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- "We look at the interior of the
Earth because we want to know what is there," explains Okal simply.
"But it may be interesting material scientists because it shows that
under tremendous pressures, iron is behaving in a different way. This understanding
might be applicable for other materials at not-so-heavy pressures."
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