Earth could hold more water
Five times as much water as in all the
world's oceans may lurk deep below its surface.
8 March 2002
PHILIP BALL
Geologists have divined water where you might least expect it:
1,000 kilometres below the Earth's surface. Here, rocks heated to over
1,000o C and squeezed under high pressures may harbour
around five times as much water as in all the world's oceans. This
could give clues to how the Earth formed and how it behaves today.
Between 650 and 2,900 km below the Earth's surface hot, compressed
minerals surround the planet's iron-rich core. Called the lower
mantle, this material may hold up to 0.2 per cent of its own weight in
water, estimate Motohiko Murakami, of the Tokyo Institute of
Technology in Japan, and colleagues1.
Theories of planetary formation take into account how much easily
vaporized material, such as water and carbon dioxide, were originally
present. The findings hint that Earth's starter mix may have been
sloppier than anticipated.
Water would lower the melting point of rocks in the lower mantle
and decrease their viscosity. Over millions of years, the mantle
churns like a pan of hot soup. This moves the tectonic plates and
mixes the mantle's chemical components. A less viscous mantle would
churn faster.
The take-up of water by minerals in the lower mantle might also
affect the ease with which tectonic plates sink deep into the Earth.
As the plates descend, heat up and become squeezed, the water that
they release might soften the surrounding mantle and ease their
passage.
There is already thought to be several oceans' worth of water
slightly higher in the mantle, at a depth of around 400-650 km. This
region is called the transition zone, as it is between the upper and
the lower mantle.
The lower mantle's minerals can retain about a tenth as much water
as the rocks above, Murakami's team finds. But because the volume of
the lower mantle is much greater than that of the transition zone, it
could hold a comparable amount of water.
"The findings will boost the debate about how much water is locked
away in the mantle," says geologist Bernard Wood of the University of
Bristol, UK. Until now, he says, "most people would have argued that
there isn't much water in the mantle". A similar study two years ago
concluded that there isn't much water down there at all2.
Taking on the mantle
Murakami's team mimicked the lower mantle in the laboratory. They
studied the three kinds of mineral thought to make up most of the
region: two perovskites, one rich in magnesium, the other in calcium,
and magnesiowustite, a mixture of magnesium and iron oxides.
To recreate its furious conditions, the researchers used a
multi-anvil cell. This heats materials while squeezing them between
hard teeth. Having baked the minerals at around 1,600o C
and 250,000 atmospheres, the team measured how much hydrogen the rocks
contained using secondary-ion mass spectrometry. This technique blasts
the material with a beam of ions and detects the ions sprayed out from
the surface.
Any hydrogen in the rocks presumably comes from trapped water, an
idea that other measurements support. The researchers found more
hydrogen than previous experiments had led them to expect. |
