Earth's getting 'soft' in the middle

Since we can’t sample the deepest regions of the Earth, scientists watch the velocity of seismic waves as they travel through the planet to determine the composition and density of that material. Now a new study suggests that material in part of the lower mantle has unusual electronic characteristics that make sound propagate more slowly, suggesting that the material there is softer than previously thought.

The results call into question the traditional techniques for understanding this region of the planet. The authors, including Alexander Goncharov from the Carnegie Institution’s Geophysical Laboratory, present their results in the January 25, 2008, issue of Science.

The lower mantle extends from about 400 miles to 1800 miles (660-2900 kilometers) into Earth and sits atop the outer core. Pressures and temperatures are so brutal there that materials are changed into forms that don’t exist in rocks at the planet’s surface and must be studied under carefully controlled conditions in the laboratory. The pressures range from 230,000 times the atmospheric pressure at sea level (23 GPa), to 1.35 million times sea-level pressure (135 GPa). And the heat is equally extreme—from about 2,800 to 6,700 degrees Fahrenheit (1800K–4000K).

Iron is abundant in the Earth, and is a major component of the minerals ferropericlase and the silicate perovskite in the lower mantle. In previous work, researchers found that the outermost electrons of iron in ferropericlase are forced to pair up under the extreme pressures creating a so-called spin-transition zone within the lower mantle.

“What happens when unpaired electrons—called a high-spin state—are forced to pair up is that they transition to what is called a low-spin state. And when that happens, the conductivity, density, and chemical properties change,” explained Goncharov.

“What’s most important for seismology is the acoustic properties—the propagation of sound. We determined the elasticity of ferropericlase through the pressure-induced high-spin to low-spin transition. We did this by measuring the velocity of acoustic waves propagating in different directions in a single crystal of the material and found that over an extended pressure range (from about 395,000 to 590,000 atmospheres) the material became ‘softer’—that is, the waves slowed down more than expected from previous work. Thus, at high temperature corresponding distributions will become very broad, which will result in a wide range of depth having subtly anomalous properties that perhaps extend through most of the lower mantle.”

Source: Carnegie Institution

Related stories:

Researchers discover unexpected properties of materials in lowermost mantle
Materials deep inside Earth have unexpected atomic properties that might force earth scientists to revise their models of Earth's internal processes, a team of researchers has discovered.
Improved Ion Mobility Is Key to New Hydrogen Storage Compound
A materials scientist at the National Institute of Standards and Technology has deciphered the structure of a new class of materials that can store relatively large quantities of hydrogen within its crystal structure for later release. The new analysis may point to a practical hydrogen storage material for automobile fuel cells and similar applications.
Researchers identify pressure effects on nanomaterials
Transistors, lasers and solar-energy conversion devices may be easier to manipulate because of recent research by Lawrence Livermore National Laboratory scientists. The researchers defined the role high pressure plays in precisely tuning the fundamental properties of nanomaterials and, in particular, nanoparticle assemblies that are important for device applications.
'Dynamic duo' develops framework for Earth's inaccessible interior
A new model of inner Earth constructed by Arizona State University researchers pulls past information and hypotheses into a coherent story to clarify mantle motion.
Inexpensive roof vent could prevent billions of dollars in wind damage
Hurricanes often lift the roofs off buildings and expose them to havoc and damaging conditions, even after the worst of the wind has passed. A local roofer, Virginia Tech faculty members from architecture and engineering, and a graduate student have devised an inexpensive vent that can reduce roof uplift on buildings during high winds, even a hurricane.
Laser experiments offer insight into evolution of 'gas giants'
By shooting the high-energy Omega laser onto precompressed samples of planetary fluids, scientists are gaining a better understanding of the evolution and internal structure of Jupiter, Saturn and extrasolar giant planets.
Researchers solve decade-old mystery of hydrogen storage material
Environmentally friendly hydrogen gas fueled vehicles can dramatically reduce greenhouse gas emissions and lessen the country’s dependence on sources of fossil fuel. Though several hydrogen vehicles exist on the market today, there is still much room for improvement in the way that hydrogen is stored on-board the vehicle. With current technologies, hydrogen gas storage tanks have to be as large as or larger than the trunk of a car to carry enough gas to travel only one to two hundred miles.
Journey to the center of the Earth -- Scientists explain tectonic plate motions
The first direct evidence of how and when tectonic plates move into the deepest reaches of the Earth is published in Nature today. Scientists hope their description of how plates collide with one sliding below the other into the rocky mantle could potentially improve their ability to assess earthquake risks.

News discussion:

Space & Earth science news