Author Topic: Core of the earth  (Read 3772 times)

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Azure Sky

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Core of the earth
« on: July 07, 2012, 07:24:40 am »
A Seismic Adventure
There's a giant crystal
buried deep within the
Earth, at the very center,
more than 3,000 miles
down. It may sound like
the latest fantasy adventure
game or a new Indiana
Jones movie, but it happens
to be what scientists
discovered in 1995 with a
sophisticated computer
model of Earth's inner core.
This remarkable finding,
which offers plausible
solutions to some
perplexing geophysical
puzzles, is transforming what
Earth scientists think about
the most remote part of our
planet.
"To understand what's
deep in the Earth is a great
challenge," says
geophysicist Lars Stixrude.
"Drill holes go down only
12 kilometers, about 0.2
percent of the Earth's
radius. Most of the planet is
totally inaccessible to direct
observation." What scientists
have pieced together comes
primarily from seismic data.
When shock waves from
earthquakes ripple through
the planet, they are
detected by sensitive
instruments at many
locations on the surface. The
record of these vibrations
reveals variations in their
path and speed to scientists
who can then draw
inferences about the
planet's inner structure. This
work has added much
knowledge over the last ten
years, including a puzzling
observation: Seismic waves
travel faster north-south
than east-west, about four
seconds faster pole-to-pole
than through the equator.
This finding, confirmed only
within the past two years,
quickly led to the
conclusion that Earth's
solid-iron inner core is
"anisotropic" -- it has a
directional quality, a texture
similar to the grain in wood,
that allows sound waves to
go faster when they travel
in a certain direction. What,
exactly, is the nature of this
inner-core texture? To this
question, the seismic data
responds with sphinx-like
silence. "The problem," says
Ronald Cohen of the
Carnegie Institution of
Washington
, "is then we're stymied. We
know there's some kind of
structure, the data tells us
that, but we don't know
what it is. If we knew the
sound velocities in iron at
the pressure and
temperature of the inner
core, we could get
somewhere." To remedy
this lack of information,
Stixrude and Cohen turned
to the CRAY C90 at
Pittsburgh Supercomputing
Center
.
Earth's layered structure --
a relatively thin crust of
mobile plates, a solid mantle
with gradual overturning
movement, and the outer
and inner core of molten
and solid iron.
Getting to the Core
Don't believe Jules Verne.
The center of the Earth is
not a nice place to visit,
unless you like hanging out
in a blast furnace. The outer
core of the Earth, about
two-thirds of the way to the
center, is molten iron.
Deeper yet, at the inner
core, the pressure is so great
-- 3.5 million times surface
pressure -- that iron
solidifies, even though the
temperature is believed to
exceed 11,000 degrees
Fahrenheit, hotter than the
surface of the sun.
Despite rapid advances in
high-pressure laboratory
techniques, it's not yet
possible to duplicate these
conditions experimentally,
and until Stixrude and
Cohen's work, scientists
could at best make
educated guesses about
iron's atom-to-atom
architecture -- its crystal
structure -- at the extremes
that prevail in the inner
core. Using a quantum-
based approach called
density-functional theory,
Stixrude and Cohen set out
to do better than an
educated guess. With recent
improvements in numerical
techniques, density-
functional theory had
predicted iron's properties
at low pressure with high
accuracy, leading the
researchers to believe that
with supercomputing they
could, in effect, reach 3,000
miles down into the inner
core and pull out what they
needed.
Three crystal structures of
iron. Yellow lines show
bonds between iron atoms.
Rethinking Inner Earth
On Earth's surface, iron
comes in three flavors,
standard crystalline forms
known to scientists as body-
centered cubic (bcc), face-
centered cubic (fcc) and
hexagonal close-packed
(hcp). Working with these
three structures as their
only input, Stixrude and
Cohen carried out an
extensive study -- more
than 200 separate
calculations over two years
-- to determine iron's
quantum-mechanical
properties over a range of
high pressures. "Without
access to the C90," says
Stixrude, "this work would
have taken so long it
wouldn't have been done."
Prevalent opinion before
these calculations held that
iron's crystal structure in the
inner core was bcc. To the
contrary, the calculations
showed, bcc iron is unstable
at high pressure and not
likely to exist in the inner
core. For the other two
candidates, fcc and hcp,
Stixrude and Cohen found
that both can exist at high
pressure and both would
be directional (anisotropic)
in how they transmit sound.
Hcp iron, however, gives a
better fit with the seismic
data. All this was new
information, but even more
surprising was this: To fit the
observed anisotropy, the
grain-like texture of the
inner core had to be much
more pronounced than
previously thought.
"Hexagonal crystals have a
unique directionality," says
Stixrude, "which must be
aligned and oriented with
Earth's spin axis for every
crystal in the inner core."
This led Stixrude and
Cohen to try a
computational experiment.
If all the crystals must point
in the same direction, why
not one big crystal? The
results, published in
Science, offer the simplest,
most convincing explanation
yet put forward for the
observed seismic data and
have stirred new thinking
about the inner core.
Could an iron ball 1,500
miles across be a single
crystal? Unheard of until
this work, the idea has
prompted realization that
the temperature-pressure
extremes of the inner core
offer ideal conditions for
crystal growth. Several
high-pressure laboratories
have experiments planned
to test these results. A
strongly oriented inner core
could also explain
anomalies of Earth's
magnetic field, such as
tilted field lines near the
equator. "To do these
esoteric quantum
calculations," says Stixrude,
"solutions which you can
get only with a
supercomputer, and get
results you can compare
directly with messy
observations of nature and
help explain them -- this
has been very exciting."

Offline Ningcra

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Re: Core of the earth
« Reply #1 on: November 06, 2018, 12:55:52 pm »
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