University Of
Illinois At Urbana-Champaign Researchers Confirm Discovery Of Earth's Inner,
Innermost Core
(18 March 2008) Geologists at the
University of Illinois have confirmed the discovery of Earth's inner, innermost
core, and have created a three-dimensional model that describes the seismic
anisotropy and texturing of iron crystals within the inner
core.
"For many years, we have been like blind men touching
different parts of an elephant," said U. of I. geologist Xiaodong Song. "Now,
for the fist time, we have a sense of the entire elephant, and see what the
inner core of Earth really looks like."
Using both newly acquired data
and legacy data collected around the world, Song and postdoctoral research
associate Xinlei Sun painstakingly probed the shape of Earth's core. The
researchers report their findings in a paper accepted for publication in the
journal Earth and Planetary Science Letters, and posted on its Web
site.
Composed mainly of iron, Earth's core consists of a solid inner
core about 2,400 kilometres in diameter and a fluid outer core about 7,000
kilometres in diameter. The inner core plays an important role in the geodynamo
that generates Earth's magnetic field.
The solid inner core is
elastically anisotropic; that is, seismic waves have different speeds along
different directions. The anisotropy has been found to change with hemisphere
and with radius. In the latest work, Sun and Song describe another anomaly - a
global structure - found within the inner core.
"To constrain the shape
of the inner core anisotropy, we needed a uniform distribution of seismic waves
travelling in all directions through the core," Sun said. "Since the seismic
waves we studied were generated by earthquakes, one challenge was acquiring
enough seismic waves recorded at enough stations."
In their analysis,
Sun and Song used a three-dimensional tomography technique to invert the
anisotropy of the inner core. They parameterised the anisotropy of the inner
core in both radial and longitudinal directions. The researchers then used a
three-dimensional ray tracing method to trace and retrace the seismic waves
through the inner core iteratively.
What they found was a distinct
change in the inner core anisotropy, clearly marking the presence of an inner
inner core with a diameter of about 1,180 kilometres, slightly less than half
the diameter of the inner core.
The layering of the core is interpreted
as different texturing, or crystalline phase, of iron in the inner core, the
researchers say.
"Our results suggest the outer inner core is composed
of iron crystals of a single phase with different degrees of preferred
alignment along Earth's spin axis," Sun said. "The inner, inner core may be
composed of a different phase of crystalline iron or have a different pattern
of alignment."
Although the anisotropy of the inner core was proposed 20
years ago, "this is the first time we have been able to piece everything
together to create a three-dimensional view," Song said. "This view should help
us better understand the character, mineral properties and evolution of Earth's
inner core."
The work was funded by the National Science
Foundation.
(source: University of Illinois at
Urbana-Champaign)
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