What Lies Beneath -A Mystery at the Earth's Center

Surprisingly, we know very little about what lies beneath the Earth's surface. The scientific community is in generaly agreement that the world beneath our feet is made up of four layers: a rocky outer crust, a mantle of hot viscous rock, a liquid outer core, the seat of magnetism, and a solid, spinning inner core.

The liquid core, creates the Earth's magnetic field in concert with the spinning solid core, which acts like an electrical motor, reverses itself about 200 times in the last 100 million years. But we don't have the slightest idea why; it's one of the great unsolved mysteries of science.

The Earth's solid inner core is hotter than 1043 K, the Curie point temperature at which the orientations of spins within iron become randomized. Such randomization causes the substance to lose its magnetic field. Therefore the Earth's magnetic field is caused not by magnetized iron deposits, but mostly by electric currents in the liquid outer core.

It is thought that we might be going through a reversal of the magnetic field now. Recorded measurements show that it has diminished as much as six percent in the last century alone, which along with global warming spells potential trouble for the planet. The Earth's magnetic field deflects dangerous cosmic rays away from the planet's surface into two zones of near space called the Van Allen belts.

When the Earth's in a possible hotspot, Hollywood can't be far behind, right? Well, in 2003 a Hitchcockian movie The Core (checkout the cool trailer), starring Hillary Swank, a bird "incident" and a crash landing of the space shuttle Endeavor in downtown Los Angeles leads Carl Sagan wannabe scientist to a devastating conclusion: the Earth’s magnetic field is collapsing and will soon vanish, resulting in the death of every living thing on Earth.

But we are chipping away at our ignorance of inner Earth bit by bit. A recent Laboratory measurements of a high-pressure mineral believed to exist deep within the Earth, show that the mineral composition may not, as geophysicists hoped, have the right properties to solve the long-standing mystery of the layer lying just above the planet's core, known as the D" layer, from 1700 to 1900 miles down.

Led by Sébastien Merkel of the University of California-Berkeley, now at CNRS/the University of Science of Technology of Lille, France, an international team of scientists made the first laboratory study of the deformation properties of a high-pressure silicate mineral named post-perovskite, which appeared in the June 22 issue of Science.

The team included Allen McNamara of the Arizona State University School of Earth and Space Exploration. McNamara, a geophysicist, modeled the stresses the mineral typically would undergo as convection currents deep in Earth's mantle cause it to rise and sink. “This the first time the deformation properties of this mineral have been studied at lower mantle temperatures and pressures,” McNamara says. “The goal was to observe where the weak planes are in its crystal structure and how they are oriented.”

The results of the combined laboratory tests and computer models, he says, show that post-perovskite doesn't fit the known about conditions in the lowermost mantle.

Earth's mantle is a layer that extends from the bottom of the crust, about 25 miles down, to the planet's core, 1,800 miles deep. Scientists divide the mantle into two layers separated by a wide transition zone centered around a depth of about 300 miles.

The lower mantle, known to earth scientists as D" (dee-double-prime), lies below that zone. This layer averages 120 miles thick and lies directly above Earth's core. D" was named in 1949 by seismologist Keith Bullen, who found the layer from the way earthquake waves travel through the planet's interior. But the nature of D" has eluded scientists since Bullen's discovery.

“Our team found that while post-perovskite has some properties that fit what's known about D", our laboratory measurements and computer models show that post-perovskite doesn't fit one particular essential property,” McNamara says.

That property is seismic anisotropy, he says, referring to the fact that earthquake waves passing through D" become distorted in a characteristic way.

“Down in the D" layer, the horizontal part of earthquake waves travel faster than the vertical parts,” McNamara says. “But in our laboratory measurements and models, post-perovskite produces an opposite effect on the waves. This appears to be a basic contradiction.”

McNamara's work modeled the slow churn of the mantle, in which convection currents in the rock rise and fall about as fast as fingernails grow – roughly an inch a year. He calculated stresses, pressures and temperatures to draw a detailed picture of where post-perovskite would be found. This let him profile the structure of the D" layer.

“All these computations have been in two dimensions,” he says. “Our next step is to go to three-dimensional modeling.”