 |
|
Layers and layers
Most of us tend to think of the structure of the Earth,
if we think of it at all, as a three-layered affair: a
hot metallic core surrounded by a vast stone mantle topped
by a thin crust. Geophysicists, more sensitive to the structural
subtleties beneath our feet, have identified distinct layers
in the mantle. They long ago identified discontinuities
in the seismic wave velocities of the mantle at depths
of 410 kilometers and 660 kilometers, and researchers determined
that those changes result from phase transition in the
crystalline structure of the same basic material: magnesium
silicate. Another, minor phase transition occurs at a depth
of around 520 kilometers, but that is inside a layer generally
characterized as the transition zone between the upper
mantle and lower mantle.
By 1983, researchers had identified yet another discontinuity
in the mantle, this one at a depth of between 2,600 kilometers
and 2,700 kilometers. That's getting extremely close to
the boundary—2,890 kilometers beneath the Earth's
surface—between the mantle and the core. Scientists
initially attributed that discontinuity to a fundamental
difference in chemical composition and to a large temperature
differential. But Hirose demonstrated that it, like the
other discontinuities, is a phase transition. He did that
by discovering a new mineral, post-perovskite.
"People had good reason to doubt that the deep-mantle
discontinuity was the result of a mere phase change," observes
Hirose. "The lower mantle consists mainly of magnesium
silicate in a crystalline structure known as perovskite
[named for the Russian mineralogist L. A. Perovski (1792–1856)].
But seismic behavior in the deepest stretches of that enigmatic
realm is inconsistent with what we would expect with perovskite.
Since perovskite has an extremely dense structure. a phase
transition to an even-denser polymorph seemed unlikely.
Seeking explanations in chemical differences was therefore
only natural. The post-perovskite that we identified is
denser than perovskite, however, and it appears to solve
several long-standing seismic riddles."
Hirose and his colleagues made their discovery by using
laser-heated diamond-anvil cell techniques. Basically,
they squeeze a mineral sample between the flattened tips
of two opposing diamonds in a screw-clamp fixture. With
just a small screwdriver, they can produce deep-mantle
pressures of more than 125 gigapascals at the 200-micron-diameter
tips of the diamonds. To produce deep-mantle temperatures—around
2,500 degrees kelvin—the researchers direct a high-power
laser onto the sample. They then use the powerful X-ray
beam at the SPring-8 facility to analyze the resultant
mineral material. Squeezing and heating the sample resulted
in a material that exhibited "odd diffraction patterns," as
reported in Nature.
"Chemistry professors told me there must be something
wrong with my data," Hirose told Nature, "or
something wrong with me." Welcome assistance arrived
at that crucial juncture in the person of Professor Katsuyuki
Kawamura, a mineralogist in Tokyo Tech's Department of
Earth and Planetary Sciences. Hirose persuaded his colleague
to have a look at the diffraction data. Kawamura determined
that it contained a dense, previously unknown mineral,
now known as post-perovskite, and the Earth gained a new
phase transition. Subsequent work with Dr. Toshiaki Iitaka,
of Japan's Institute of Physical and Chemical Research
(RIKEN), revealed that the elasticity of post-perovskite
explains some enigmas of the deep mantle. |
|
|
 |
| Kei
Hirose |
|
 |
|
 |
| Hirose has achieved pressures
and temperatures characteristic of the Earth's lower mantle
with laser-heated diamond-anvil cell techniques. He dreams
of being first to reproduce the ultrahigh pressures and
temperatures that prevail at the center of the Earth. "It's
a huge challenge," acknowledges Hirose, "but
one well worth undertaking. Tackling that challenge successfully
could unlock long-held secrets of the deepest interior
of our planet." |
|
