Washington, Mar 11 : In new study, University of Illinois geologists have confirmed the discovery of the Earth's inner, innermost core.
The geologists have also created a three-dimensional model explaining the seismic anisotropy and texturing of iron crystals within the inner core.
It was U. of I. geologist Xiaodong Song and postdoctoral research associate Xinlei Sun, who carefully probed the shape of Earth's core by using both newly acquired data and legacy data collected around the world.
"For many years, we have been like blind men touching different parts of an elephant. Now, for the first time, we have a sense of the entire elephant, and see what the inner core of Earth really looks like," said Song.
They described that Earth's core, which is mainly composed of iron, has a solid inner core about 2,400 kilometers in diameter and a fluid outer core about 7,000 kilometers in diameter.
The inner core is vital to the geodynamo that generates Earth's magnetic field. This solid inner core is elastically anisotropic; that is, seismic waves have different speeds along different directions. It was found that the anisotropy changes with hemisphere and with radius.
In the study, the researchers have cited 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. Since the seismic waves we studied were generated by earthquakes, one challenge was acquiring enough seismic waves recorded at enough stations," said Sun.
During the study, the researchers used a three-dimensional tomography technique for inverting the anisotropy of the inner core. They parameterized the anisotropy of the inner core in both radial and longitudinal directions. Later, a three-dimensional ray tracing method was used to trace and retrace the seismic waves through the inner core iteratively.
The researchers discovered a distinct change in the inner core anisotropy, clearly marking the presence of an inner inner core with a diameter of about 1,180 kilometers, slightly less than half the diameter of the inner core. They said that the layering of the core is interpreted as different texturing, or crystalline phase, of iron in the inner core.
"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. The inner inner core may be composed of a different phase of crystalline iron or have a different pattern of alignment," said Sun.
Even though the anisotropy of the inner core was suggested 20 years ago, "this is the first time we have been able to piece everything together to create a three-dimensional view. This view should help us better understand the character, mineral properties and evolution of Earth's inner core," Song said.
The findings are reported in a paper accepted for publication in the journal Earth and Planetary Science Letters, and posted on its Web site.