The Earth's inner core is hot, under immense pressure and snow-capped, according to new research that could help scientists better understand forces that affect the entire planet.
The snow is made of tiny
particles of iron -- much heavier than any snowflake on Earth's surface -- that
fall from the molten outer core and pile on top of the inner core, creating
piles up to 200 miles thick that cover the inner core.
The image may sound like an alien
winter wonderland. But the scientists who led the research said it is akin to
how rocks form inside volcanoes.
"The Earth's metallic core
works like a magma chamber that we know better of in the crust," said
Jung-Fu Lin, a professor in the Jackson School of Geosciences at The University
of Texas at Austin and a co-author of the study.
The study is available online and
will be published in the print edition of the journal JGR Solid
Earth on December 23.
Youjun Zhang, an associate
professor at Sichuan University in China, led the study. The other co-authors
include Jackson School graduate student Peter Nelson; and Nick Dygert, an
assistant professor at the University of Tennessee who conducted the research
during a postdoctoral fellowship at the Jackson School.
The Earth's core can't be
sampled, so scientists study it by recording and analyzing signals from seismic
waves (a type of energy wave) as they pass through the Earth.
However, aberrations between
recent seismic wave data and the values that would be expected based on the
current model of the Earth's core have raised questions. The waves move more
slowly than expected as they passed through the base of the outer core, and
they move faster than expected when moving through the eastern hemisphere of
the top inner core.
The study proposes the iron
snow-capped core as an explanation for these aberrations. The scientist S.I.
Braginkskii proposed in the early 1960s that a slurry layer exists between the
inner and outer core, but prevailing knowledge about heat and pressure
conditions in the core environment quashed that theory. However, new data from
experiments on core-like materials conducted by Zhang and pulled from more
recent scientific literature found that crystallization was possible and that
about 15% of the lowermost outer core could be made of iron-based crystals that
eventually fall down the liquid outer core and settle on top of the solid inner
core.
"It's sort of a bizarre
thing to think about," Dygert said. "You have crystals within the
outer core snowing down onto the inner core over a distance of several hundred
kilometers."
The researchers point to the
accumulated snow pack as the cause of the seismic aberrations. The slurry-like
composition slows the seismic waves. The variation in snow pile size -- thinner
in the eastern hemisphere and thicker in the western -- explains the change in
speed.
"The inner-core boundary is
not a simple and smooth surface, which may affect the thermal conduction and
the convections of the core," Zhang said.
The paper compares the snowing of
iron particles with a process that happens inside magma chambers closer to the
Earth's surface, which involves minerals crystalizing out of the melt and
glomming together. In magma chambers, the compaction of the minerals creates
what's known as "cumulate rock." In the Earth's core, the compaction
of the iron contributes to the growth of the inner core and shrinking of the
outer core.
And given the core's influence
over phenomena that affects the entire planet, from generating its magnetic
field to radiating the heat that drives the movement of tectonic plates,
understanding more about its composition and behavior could help in
understanding how these larger processes work.
Bruce Buffet, a geosciences
professor at the University of California, Berkley who studies planet interiors
and who was not involved in the study, said that the research confronts
longstanding questions about the Earth's interior and could even help reveal
more about how the Earth's core came to be.
"Relating the model
predictions to the anomalous observations allows us to draw inferences about
the possible compositions of the liquid core and maybe connect this information
to the conditions that prevailed at the time the planet was formed," he
said. "The starting condition is an important factor in Earth becoming the
planet we know."
The research was funded by the
National Natural Science Foundation of China, Fundamental Research Funds for
the Central Universities, the Jackson School of Geosciences, the National
Science Foundation and the Sloan Foundation.
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