- LOS ANGELES -- An odd, previously
unknown sphere, some 360 miles in diameter, has been found at the bottom
of the Earth. It was detected by a Harvard professor and a graduate student
who patiently examined records of hundreds of thousands of earthquake waves
that passed through the center of the planet in the past 30 years.
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- "It may be the oldest fossil left from the formation
of Earth," says Adam Dziewonski, Frank B. Baird Jr. Professor of Science.
"Its origin remains unknown, but its presence could change our basic
ideas about the origin and history of the planet."
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- Earth's center lies some 3,800 miles below our feet.
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- Continents and oceans make up only a thin crust that
extends about 20 miles down. Below them lies a huge, 1,800-mile-thick layer
known as the mantle. Slowly moving currents of rock in the mantle cause
continents to drift and the edges of the ocean floors to sink into huge
trenches. The mantle encloses a core 2,100 miles in radius, blistering
hot and made mostly of iron. The outer core is liquid, the inner core is
solid.
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- That's the way Earth has been depicted in textbooks for
the past 66 years. But the work of Dziewonski and graduate student Miaki
Ishii shows that this picture doesn't get to the bottom of things. Engulfed
in the inner core, like a pit in a peach, lies a 360-mile-wide inner inner
core. This core within a core within a core makes up one ten-thousandth
of the Earth's volume.
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- The inner core itself wasn't discovered until 1936. Thirty-
five years went by before Dziewonski and geophysicist Freeman Gilbert proved
that it was a solid enclosed in a liquid. (It doesn't melt because of millions
of pounds of pressure pressing down on each square inch of its surface.)
Now, a careful study of how the speed of earthquake waves changes with
the direction they take reveals that the innermost part of the inner core
is obviously different from the rest of it.
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- Dziewonski and Ishii have published their results in
the Oct. 1 issue of the Proceedings of the National Academy of Sciences.
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- Ironing out differences
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- The inner core, 1,440 miles in diameter and heated to
a blistering 5,000 degrees, has puzzled earth scientists since it was discovered.
In the early 1980s, researchers found earthquake waves traveling parallel
to the axis of Earth's rotation, roughly north-south, went through the
inner core faster than did waves traveling along the equatorial plane,
or east-west. The discrepancy went unexplained until 1986 when Harvard
investigators, including Dziewonski, showed it was due to a phenomenon
known as "anisotropy."
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- Within mineral crystals that make up the core, molecules
and atoms can be spaced differently when looked at in different directions.
Earthquake waves travel faster through closely packed molecules and atoms
than through those with more space between them. If packing is tighter
in the north-south directions, earthquake waves will zip through the core
faster than they would in the east-west direction.
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- Anisotropy explained much but not all of changes seen
in the travel of earthquake waves. In trying to completely close the gap
"things got a little wild," Dziewonski recalls. Some people denied
that anisotropy exists at all, others said that part of the inner core
is isotropic and part is anisotropic, yet others -- including Dziewonski
for a while -- believed that the inner core rotated at a Sdifferent rate
from the rest of Earth.
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- Because earthquake waves pass through the crust, mantle,
and outer core on the way in to the center and on the way out, this introduces
all kinds of complexities in trying to puzzle out their paths. "The
inner core seemed to be the container for all the ignorance we had about
the rest of the Earth," Dziewonski comments.
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- To get the closet look yet into that container, Miaki
Ishii and Dziewonski analyzed 30 years of data that covered several million
records of the travels of earthquake waves. About 325,000 of them passed
through the inner core. At first they saw no change: The inner core looked
about the same from top to bottom. Then they took a shaper look at more
than 3,000 waves that traveled closest to Earth's center. They found an
obvious change in anisotropy, or wave speed with direction, in an area
360 miles in diameter surrounding the very bottom of the world.
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- "It's a very robust effect," they insist. In
the innermost inner core waves travel most slowly at a 45 degree angle
to Earth's axis, as opposed to an east-west direction in the rest of the
inner core
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- How did it form?
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- Dziewonski speculates that this innermost iron ball may
be a leftover from the original kernel out of which Earth separated into
crust, mantle, and core some 4.6 billion years ago. Through subsequent
years of meteoric bombardments and geological upheavals, including the
ripping off of a big chunk to make the moon, the innermost core survived.
If so, it is the oldest unaltered part of our planet.
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- However, other possibilities exist, albeit not as exciting.
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- Most earth scientists believe the inner core is growing
at the expense of the outer core. The solid iron sphere sits in the path
of jets and currents roiling the outer core fluids like a big rock in a
flowing stream. These patterns of flow might have been altered after the
inner core reached a diameter of 360 miles. Afterwards, iron crystals deposited
on the inner core surface in a different orientation, creating a different
kind of anisotropy.
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- A third possibility is that at the higher pressure and
temperature near the planet's center, iron crystals pack differently. The
change in packing pattern could alter the directions of fast and slow speeds
traveled by earthquake waves.
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- Whatever the explanation, Dziewonski and Ishii expect
a lot of heat from their colleagues. "The idea of a new region in
Earth will generate quite a bit of controversy," Dziewonski says.
That's probably a huge understatement.
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- Two of the leading scientific journals rejected the study's
conclusions before the Proceedings of the National Academy of Sciences
decided to publish them. "A lot of people resist new ideas,"
Dziewonski notes.
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- "Some scientists don't believe our findings, although
no one can point out a specific flaw in our data," Ishii adds.
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- Ways exist to test Dziewonski and Ishii's interpretation
of what lies in Earth's basement, but they are expensive and time consuming.
Laboratory devices can subject small samples of rock rich in iron to pressures
and temperatures approaching those at the center of Earth. Powerful beams
of X-rays can map changes in crystal structure forced by such temperatures
and pressures.
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- More definitive tests could be done with new arrays of
seismometers, or earthquake-wave recorders. "Seismometer stations
now in existence are unevenly distributed," Dziewonski explains. "We
don't have the coverage needed to record deep waves passing through the
innermost inner core." To catch these waves
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- Dziewonski want to put temporary networks of recorders
on the ocean bottom in the right places.
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- "For the first time in 66 years, we have good evidence
for a new region inside Earth," he states. "This tells us that,
by gathering data of high quality, we can continue to make new discoveries
about the planet on which we live. We cannot understand the evolution and
dynamics of Earth without knowing its internal structure."
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- William J. Cromie is staff writer for for Harvard Gazette
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- http://www.spacedaily.com/news/earth-02z.html
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