Doubling estimates of light elements in the Earth's core

March 5, 2013 by Karen Fox
Doubling estimates of light elements in the Earth's core
Velocity-density plots of the samples at high pressures and temperatures. The top panel shows the velocity-density plot for hcp-Fe at both 300 K and 700 K. The dashed lines shows the linear fit, while the solid line shows the power law fit, which matches the data more closely. The bottom panel shows the velocity-density relation of both hcp-Fe and the iron-silicon alloy at 300 K.

The inner core of the Earth is the remotest area on the globe, mostly impossible to study directly. It is an area of the planet that experiences both extremely high pressure ranging from 3,300,000 to 3,600,000 times atmospheric pressure, and extremely high temperatures somewhere from 5000 to 6000 K. One way to study this area is by recording how sound waves travel across the interior, matching these profiles to known information about how sound waves travel through candidate iron alloys, and attempting to discern which materials must be present. This method requires an understanding of how sound waves travel through the potential materials present in the core.

A team of researchers utilized APS X-rays to develop a new model of how sound waves travel through iron and iron-silicon alloys, showing for the first time that increased temperatures will affect the sound wave profile, and that sound velocity and density correlate in a non-linear way. Their results suggest that the amount of in the inner core could be two times more than estimated in previous studies without considering these effects.

The researchers from the University of Texas at Austin, Argonne National Laboratory, and the Carnegie Institute of Washington studied samples of what is known as hexagonal closest-packed iron (hcp Fe), which is believed to be the high-pressure phase of iron present in the Earth's core, as well as a hcp iron-silicon alloy because silicon is one of the most likely candidate light elements in the core.

The researchers measured the compressional wave velocity (in which the wave has the same direction of vibration as its direction of travel) of the samples using high-energy inelastic x-ray scattering (HERIX) and (XRD) in a resistively-heated at XSD beamline 3-ID of the APS. They made their measurements under simultaneous high-pressure and high-temperature conditions to better simulate conditions in the Earth's core.

Previous studies had suggested that the compressional sound velocity of hcp Fe was generally linear with increased density. In contrast, by subjecting their samples to unprecedented extreme conditions, the team found that the effect of high temperature at a given density on the sound velocity of iron cannot be ignored. As the temperature increased for a given density at high pressures, the slowed down. In addition, the relationship between the sound wave velocity and increased density was not linear, but instead could be better described by an empirical power-law function with concave behavior at higher densities.

The researchers incorporated this new information into models of the Earth's core to provide new estimates of the chemical composition there. The sound wave velocities that have been observed in the core correlate to a profile of hcp-Fe with approximately 8% of silicon by weight at temperatures of 6000 K. This number represents nearly twice the amount estimated in previous studies.

The team hopes to explore additional alloys to further round out estimates of the core's composition and to explain a number of enigmatic behaviors of seismic waves in this most extreme region of the planet. Also, since this study observed temperatures and pressures that are still much lower than those of the inner core, these scientists hope future studies will push the experimental conditions even further. With direct measurements of compressional wave velocities at relevant pressure and temperature conditions on the horizon, such studies may eventually answer the long-standing question of the composition of the Earth's core.

Explore further: Earth's outer core deprived of oxygen: study

More information: Mao, Z. et al. Sound velocities of Fe and Fe-Si alloy in the Earth's core, Proc. Natl. Acad. Sci. 109(26), 10239 (June 26, 2012). DOI:10.1073/pnas.1207086109

Related Stories

Earth's outer core deprived of oxygen: study

November 23, 2011

The composition of the Earth's core remains a mystery. Scientists know that the liquid outer core consists mainly of iron, but it is believed that small amounts of some other elements are present as well. Oxygen is the most ...

Ironing out the details of the Earth's core

December 20, 2011

( -- Identifying the composition of the earth's core is key to understanding how our planet formed and the current behavior of its interior. While it has been known for many years that iron is the main element ...

New scenery at Earth's core-mantle boundary found

September 2, 2010

( -- Using a diamond-anvil cell to recreate the high pressures deep within the earth, researchers at the California Institute of Technology (Caltech) have found unusual properties in an iron-rich magnesium- and ...

Scientists probe Earth's core

April 28, 2010

We know more about distant galaxies than we do about the interior of our own planet. However, by observing distant earthquakes, researchers at the University of Calgary have revealed new clues about the top of the Earth's ...

Recommended for you

Mountain glaciers shrinking across the West

October 22, 2017

Until recently, glaciers in the United States have been measured in two ways: placing stakes in the snow, as federal scientists have done each year since 1957 at South Cascade Glacier in Washington state; or tracking glacier ...

Carbon coating gives biochar its garden-greening power

October 20, 2017

For more than 100 years, biochar, a carbon-rich, charcoal-like substance made from oxygen-deprived plant or other organic matter, has both delighted and puzzled scientists. As a soil additive, biochar can store carbon and ...

Cool roofs have water saving benefits too

October 20, 2017

The energy and climate benefits of cool roofs have been well established: By reflecting rather than absorbing the sun's energy, light-colored roofs keep buildings, cities, and even the entire planet cooler. Now a new study ...


Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.