IRON IN EARTH'S INNER CORE BECOMES WEAKER

Researchers have used a
diamond anvil cell to squeeze iron at
pressures as high as 3 million times that
felt at sea level to recreate conditions at
the center of Earth.

The findings could
refine theories of how the planet and its
core evolved.
Through laboratory experiments,
postdoctoral researcher Arianna Gleason,
left, and Wendy Mao, an assistant
professor of geological and environmental
sciences and of photon science,
determined that the iron in Earth's inner
core is about 40 percent as strong as
previously believed.
The massive ball of iron sitting at the
center of Earth is not quite as "rock-solid"
as has been thought, say two Stanford
mineral physicists. By conducting
experiments that simulate the immense
pressures deep in the planet's interior,
the researchers determined that iron in
Earth's inner core is only about 40
percent as strong as previous studies
estimated.

This is the first time scientists have been
able to experimentally measure the
effect of such intense pressure -- as high
as 3 million times the pressure Earth's
atmosphere exerts at sea level -- in a
laboratory. A paper presenting the
results of their study is available online in
Nature Geoscience.
"The strength of iron under these
extreme pressures is startlingly weak,"
said Arianna Gleason, a postdoctoral
researcher in the department of
Geological and Environmental Sciences,
and lead author of the paper. Wendy
Mao, an assistant professor in the
department, is the co-author.
"This strength measurement can help us
understand how the core deforms over
long time scales, which influences how we
think about Earth's evolution and
planetary evolution in general," Gleason
said.
Until now, almost all of what is known
about Earth's inner core came from
studies tracking seismic waves as they
travel from the surface of the planet
through the interior. Those studies have
shown that the travel time through the
inner core isn't the same in every
direction, indicating that the inner core
itself is not uniform. Over time and
subjected to great pressure, the core has
developed a sort of fabric as grains of iron
elongate and align lengthwise in parallel
formations.

The ease and speed with which iron grains
in the inner core can deform and align
would have influenced the evolution of
the early Earth and development of the
geomagnetic field. The field is generated
by the circulation of liquid iron in the
outer core around the solid inner core
and shields Earth from the full intensity of
solar radiation. Without the geomagnetic
field, life -- at least as we know it -- would
not be possible on Earth.
"The development of the inner core
would certainly have some effect on the
geomagnetic field, but just what effect
and the magnitude of the effect, we can't
say," said Mao. "That is very speculative."

Gleason and Mao conducted their
experiments using a diamond anvil cell --
a device that can exert immense pressure
on tiny samples clenched between two
diamonds. They subjected minute
amounts of pure iron to pressures
between 200 and 300 gigapascals
(equivalent to the pressure of 2 million to
3 million Earth atmospheres). Previous
experimental studies were conducted in
the range of only 10 gigapascals.
"We really pushed the limit here in terms
of experimental conditions," Gleason said.
"Pioneering advancements in pressure-
generation techniques and improvements
in detector sensitivity, for example, used
at large X-ray synchrotron facilities, such
as Argonne National Lab, have allowed us
to make these new measurements."
In addition to intense pressures, the
inner core also has extreme
temperatures. The boundary between the
inner and outer core has temperatures
comparable to the surface of the sun.

Simultaneously simulating both the
pressure and temperature at the inner
core isn't yet possible in the laboratory,
though Gleason and Mao are working on
that for future studies. (For this study,
Gleason mathematically extrapolated
from their pressure data to factor in the
effect of temperature.)
Gleason and Mao expect their findings
will help other researchers set more
realistic variables for conducting their own
experiments.
"People modeling the inner core haven't
had many experimental constraints,
because it's so difficult to make
measurements under those conditions,"
Mao said. "There really weren't
constraints on how strong the core was,
so this is really a fundamental new
constraint."

                        Source: Science daily