We do
learn that core characteristics are at least asymmetric. It is now
suggested that we have two forms of packing and that somehow
temperature has an effect. One wonders just how gravity really works
down there.
Actually
penetrating to depth will require real control of gravity and heat
flow.
In
the meantime we are stuck with our best estimate that has not changed
in decades.
New idea tackles
Earth core puzzle
By Simon Redfern 9
July 2013
The study may explain
a longstanding puzzle about the most inaccessible part of our planet.
It suggests that
differences between the east and west hemispheres of the core are
explained by the way iron atoms pack together.
Details appear in the
journal Scientific Reports.
Lying more than
5,000km beneath our feet, at the centre of the Earth, the core is
beyond the reach of direct investigation. Broadly speaking, it
consists of a solid sphere of metal sitting within a liquid outer
core. [ we very reasonably
presume this to be true but nothing is actually provable - arclein ]
The inner core started
to solidify more than a billion years ago. It has a radius of about
1,220km, but is growing by about 0.5mm each year.
But the stuff that the
core is made from remains a longstanding unresolved problem.
Clues come from the
speeds that seismic waves generated by earthquakes pass through the
core.
These tell us its
density and elasticity, but the precise arrangement of iron atoms
forming the crystalline core controls these numbers.
How those atoms are
arranged remains unclear, since the conditions of extreme pressure
and temperature at the core cannot easily be replicated in the
laboratory.
Seismic data indicate
that the western and eastern hemispheres of Earth's inner core
differ, and this has led some to suggest that the core was once
subjected to an impulse - presumably from the collision of a space
rock or planetoid which shook the whole Earth.
The core, it is
suggested, is constantly moving sideways. As it does, the front side
is melting and the rear side crystallising, but the core is held
centrally by gravity.
With all these seismic
complexities, the link between the crystal structure and the
geophysical observations has yet to be resolved.
In Scientific Reports,
Maurizio Mattesini from the Complutense University of Madrid, Spain,
and colleagues propose a novel possibility for the structure of the
core: that it is composed of mixtures of different iron
arrangements distinguished by the way their atoms pack together.
By comparing seismic
data from over one thousand earthquakes across the globe with quantum
mechanical models for the properties of iron, they suggest that
seismic variations directly reflect variations in the iron structure.
They propose that the
eastern and western sides of the core differ in the extent of mixing
of these distinct structures, and suggest their results account for
the dynamic eastward drift of the core through time.
Their complicated
picture of the core contrasts with earlier suggestions of a more
uniform mineralogy. It has yet to incorporate the effects of minor
amounts of other elements in the iron alloy actually thought to be
there.
But Dr Arwen Deuss, a
seismologist from the University of Cambridge, commented: "This
is a step in the right direction, directly comparing seismology with
mineral physical properties." She added that it should
eventually provide a better understanding of the birth and evolution
of our planet.
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