Glitches
in the Earth's wobble help geophysicists probe the planet's
core
22
Jan 2001
By
Robert Sanders, Media Relations
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Schematic
cross-section of Earth's core and lower mantle.
Click here
for a larger view with detailed description.
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Berkeley
- Millimeter deviations from the expected wobble of the Earth's
axis are giving geophysicists clues to what happens 1,800
miles underground, at the boundary between the Earth's mantle
and its iron core.
A new theory
proposes that iron-rich sediments are floating to the top
of the Earth's core and sticking like gum to the bottom of
the mantle, creating drag that throws the Earth's wobble off
by a millimeter or two over a period of about 18.6 years.
"The wobble
is explained by metal patches attached to the core-mantle
boundary," explained Raymond Jeanloz, professor of geology
and planetary science at the University of California, Berkeley.
"As the outer core turns, its magnetic field lines are deflected
by the patches and the core fluid gets slowed down, just like
mountains rubbing against the atmosphere slows the Earth down."
The theory,
first proposed by Bruce A. Buffett of the Department of Earth
and Ocean Sciences at the University of British Columbia,
also explains a peculiar slowing of seismic waves that ripple
along the core-mantle boundary.
Buffett
laid out the theory at the December meeting of the American
Geophysical Union and in an article with Jeanloz and former
UC Berkeley post-doctoral fellow Edward J. Garnero, now at
Arizona State University's Department of Geological Sciences
in Tempe, in the Nov. 17 issue of Science. Much of the work
was done while Buffett was on sabbatical at UC Berkeley.
The wobble
values that the theory explains have been adopted by the International
Astronomical Union as its standard for calculating the position
of the Earth's axis into the past as well as the future.
As the
Earth spins on its axis the moon and sun tug on its bulging
equator and create a large wobble or precession, producing
the precession of the equinoxes with a period of 25,800 years.
Other periodic processes in the solar system nudge the Earth,
too, creating small wobbles - called nutations - in the wobble.
The principal components of the nutation are caused by the
Earth's annual circuit of the sun and the 18.6 year precession
of the moon's orbit.
While these
nutations have been known for many years, extremely precise
geodetic measurements of the pointing direction of the Earth's
axis have turned up unexplained deviations from the predicted
nutation.
An annual
deviation that lagged behind the tidal pull of the sun first
suggested to Buffett 10 years ago that strange processes may
be going on at the boundary between the mantle, made up of
viscous rock that extends 1,800 miles below the crust, and
the outer core, which is thought to be liquid iron with the
consistency of water. The inner core, made of very pure, solid
iron, rotates along with the outer core, dragging the Earth's
magnetic field with them.
"The Earth
is getting pulled and tugged at regular periods, but we observe
a difference in the way the Earth responds to these tugs and
pulls and what we predict," Buffett said. "One of the ways
you could explain that is by having some dissipation in the
vicinity of the core-mantle boundary as the fluid moves back
and forth relative to the mantle. But the viscosity of the
fluid core is comparable to water, and having water slosh
back and forth relative to a rigid mantle wasn't going to
produce the kinds of dissipation we needed to see."
He hit
on another way the rotating core could dissipate energy: via
electrical drag.
Based on
experiments Jeanloz had performed on the chemistry of rocks
at the high temperatures and pressures characteristic of the
core-mantle boundary, Buffett suggested that silicon-containing
minerals would float to the top of the liquid outer core,
carrying iron with it. Together they would form an iron-rich,
porous sediment at the mantle boundary that would stick to
the mantle, settling into depressions.
Because
the Earth's core rotates about a slightly different axis than
the mantle (due to the tug of the Sun and Moon), the core's
magnetic field is dragged through the mantle, passing unhindered
because the mantle does not conduct electricity. The porous,
iron-containing sediment stuck to the mantle, however, would
resist the rotation of the magnetic field, creating just enough
tug to perturb the Earth's rotation.
"As the
core rotates it sweeps the magnetic field with it, which easily
slips through the mantle with no resistance," said Buffett.
"But if the bottom of the mantle has conductivity, then it's
not so easy to slip the magnetic field lines through the mantle.
The magnetic field tends to stretch and shear or pull out
right across the interface. That generates currents, and those
currents damp out the motion and create the kind of dissipation
we need to explain this lag in response."
The sediment
layer would have to be less than a kilometer thick (about
half a mile) in order to have the observed effect, and would
probably cover only patches of the outer core.
Support
for the idea that a thin layer of iron-rich silicates may
be plastered to the underside of the mantle came from the
work of Arizona State University's Garnero and his colleagues,
who use seismic waves to probe the mantle and core. They had
observed very thin layers at the core-mantle boundary in which
seismic waves slow to a crawl. Using Buffett's ideas, Garnero
modeled what a thin silicate layer would do to seismic waves
and found agreement with the data.
The team
subsequently predicted where these patches are located, based
on where seismic waves slow down substantially and where they
do not.
"Think
of it as a fuzzy boundary between the mantle and the core,
with patches perhaps 10 to 20 kilometers across and up to
a thousand meters thick," Jeanloz said.
The rising
sediment eventually would squeeze out the iron, leaving the
silicate sediments tucked to the bottom of the mantle as the
iron falls toward the solid iron inner core. The rising of
the silicate contaminants and the subsequent fall of metallic
iron would create a convection in the outer core consistent
with what geologists think to be the source of the core's
magnetic field. Thus, the rising sediments and falling iron
could rev up the Earth's dynamo.
"In one
of the popular models, created by Gary Glatzmaier and Paul
Roberts, the dynamo is powered mainly by the growth of the
inner core as light elements get excluded and float up through
liquid iron, driving convection that powers the dynamo," Buffett
said. "If this idea about sediments is right, the sediments
would add a component to drive flow from the top down. This
is going to have a pretty important effect on the style of
fluid motions in the core, and even in the way in which the
magnetic field gets generated."
The silicates
stuck to the mantle also might be caught up in mantle convection
and carried to the surface, accounting for reports of core
material in lava erupting from hot spot plumes like that under
Hawaii.
Though
Buffett first proposed his theory 10 years ago in his PhD
thesis, the data to prove it were not available. In particular,
long-term measurements were needed to accurately determine
an out-of-phase anomaly in the 18.6 period wobble.
"Now, with
more than 20 years of data, we can confirm that the discrepancy
is there and is explained very nicely by the Earth's magnetic
field causing friction at the bottom of the mantle," Jeanloz
said.
The work
was supported by the National Science Foundation, the University
of California Institute of Geophysics and the Natural Sciences
and Engineering Research Council of Canada.
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For images,
check out http://www.eos.ubc.ca/people/faculty/buffett/sediment.html.
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