[meteorite-list] Earth's New Center: New Region Found, May Be The Seed Of our Planet's Formation

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 10:07:00 2004
Message-ID: <200210111535.IAA25512_at_zagami.jpl.nasa.gov>

Office of News and Public Affairs
Harvard University
Cambridge, Massachusetts

October 03, 2002

Earth's new center: New region found, may be the seed of our
planet's formation

By William J. Cromie, Harvard Gazette Staff
William_Cromie_at_harvard.edu

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.

"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."

Earth's center lies some 3,800 miles below our feet.
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.

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.

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.

Dziewonski and Ishii have published their results in the
Oct. 1 issue of the Proceedings of the National Academy
of Sciences.

Ironing out differences

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."

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.

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.

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.

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.

"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

How did it form?

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.

However, other possibilities exist, albeit not as exciting.
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.

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.

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.

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.

"Some scientists don't believe our findings, although no
one can point out a specific flaw in our data," Ishii adds.

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.

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

Dziewonski want to put temporary networks of recorders on
the ocean bottom in the right places.

"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."

IMAGE CAPTION:
[http://www.hno.harvard.edu/gazette/2002/10.03/photos/06-inner1-450.jpg (79KB)]
Adam Dziewonski and Miaki Ishii discuss the new region
they have found at the center of Earth, shown in the
cross section of our planet behind them. (Staff photo
by Jon Chase)
Received on Fri 11 Oct 2002 11:35:02 AM PDT