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APS X-rays reveal secrets of Mars' core

Cutaway image of Mars' core.

NEAREST NEIGHBOR — Based on research at the Advanced Photon Source and measurements by spacecraft, scientists have inferred the likely interior structure of Mars. The red planet's core is probably composed of an iron-sulfur alloy and about 3,000 kilometers (1,860 miles) across.

While astronomers peer at the surface of Mars, now making its closest approach to Earth in 60,000 years, scientists are learning the secrets of its deep interior using the Advanced Photon Source at Argonne.

Scientists Abby Kavner and Thomas Duffy of Princeton University and Guoyin Shen from the University of Chicago have been using the nation's most brilliant X-ray beams to study iron sulfide (FeS) under the extreme pressures and temperatures thought to exist in the core of Mars. The experiments were the first direct density measurements of FeS at pressures and temperatures corresponding to conditions at the martian core. Along with observations made by spacecraft, the experiments have given scientists a better idea of the red planet's interior structure — and raise new questions about how the solar system's inner planets formed.

The research was performed at GeoSoilEnviroCARS, a synchrotron-based research facility at the Advanced Photon Source dedicated to research on earth materials and open to the entire scientific community.

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The data support previous models of the martian core. If the core contains about 14 percent sulfur by weight, it has about 15 percent of the planet's total mass and is about 1,480 kilometers (918 miles) in radius. The results also place a limit on the average thickness of Mars' crust: it can't be thicker than 125 kilometers (78 miles). The most likely average thickness is probably about 50 kilometers (31 miles), similar to that of the Earth.

Recent observations from the Mars Global Surveyor spacecraft have placed tight new constraints on the Mars' core, indicating that the core of Mars is at least partially fluid and roughly 50 percent of the planet's radius. Taken together with the high-pressure experiments, these findings imply the core of Mars may be even richer in light elements, more than 17 percent by weight, if predominately sulfur, and about twice the concentration of light elements thought to exist in the Earth's core.

"The nature of the core of Mars is a critical unresolved issue that holds the key to an understanding of the evolution, structure and dynamics of the planet's interior," said Shen, a senior research associate. Shen has been studying the density and chemistry of iron sulfide at high temperatures and pressures. "The behavior of iron sulfide at high pressures and high temperatures plays a key role in understanding the state of the martian core."

Such research could also help explain some of the awe-inspiring surface features of Mars, such as a volcano three times as high as Mt. Everest and a canyon system three times as deep and four times as long as Earth's Grand Canyon.

Geologists have inferred the interior structure of the Earth from the way vibrations from earthquakes travel through the planet's interior. There are currently no seismographs on Mars, so scientists must rely on other means to model the planet's structure.

Recent clues to the structure of Mars' interior came from the Pathfinder spacecraft, which helped establish the planet's "moment of inertia." Objects with mass concentrated at their centers will have lower moments of inertia and will spin faster than objects with mass distributed more to the outside, even if the size, shape and total mass are the same. Based on the moment of inertia of Mars, estimates of the radius of the central metallic core range from 1,300 to 2,400 kilometers (806 to 1,488 miles), compared to the Earth's 3,500-kilometer (2,170-mile) core.

Iron alloys are believed to make up the bulk of the cores of "terrestrial" planets (as opposed to gas giants like Jupiter). Sulfur is likely to be a major alloying component of the martian core, based on its abundance in meteorites believed to originate from Mars, theoretical models of how the planets formed and sulfur's ability to dissolve in iron.

"Sulfur chemistry is very interesting," Shen said. "A small amount of sulfur leads to an iron alloy at high pressures and temperatures, and significantly affects its physical properties. Melting temperature, for example, can be reduced by adding sulfur."

To determine the properties of iron sulfide at the temperatures and pressures of the martian core, Shen and co-workers placed a small sample of iron sulfide in a diamond anvil cell, an instrument that generates high pressures by squeezing a sample between two diamonds. The 10-micrometer (0.0004-inch) speck of iron sulfide was then heated with lasers. X-rays from the Advanced Photon Source (APS) probed the structure of the sample as pressure and temperature were increased.

The high brilliance of APS X-rays was vital to the experiment, Shen said. To reach high temperatures and huge pressures required the use of a small sample. It would have been impossible to compress and heat bulk material due to the limitations of the hardware: the diamonds used to squeeze the sample only get so big. "Getting data from such a tiny sample requires a brilliant source of X-rays," Shen said.

The scientists analyzed how X-rays were scattered as they passed through the sample at temperatures up to 4,000 degrees Kelvin (6,740 Fahrenheit) and 35 billion Pascals (GPa) — about five million pounds per square inch.

The scientists found that when heated at martian core pressures, FeS undergoes a phase change from a crystal structure with low symmetry to a hexagonal form called FeS-IV, and determined its density under those extreme conditions.

In addition to the studies of the structure of FeS, other University of Chicago scientists, Takeyuki Uchida and Yanbin Wang, used the APS to study how FeS separated from the surrounding mantle minerals under pressures and temperatures thought to have occurred early in Mars' history. Compressing and heating a cubic millimeter of an iron metal and iron sulfide mixture with representative mantle minerals using 250-ton and 1,000-ton presses, they found the iron separated from the silicate materials and sank rapidly to the bottom of the material.

"The iron sulfide lowered the melting point of the mixture," Uchida said. "The sample had a low viscosity, which means Mars' iron-rich core could have formed very quickly in its history."

GSECARS is supported by the National Science Foundation, the U.S. Department of Energy, the W.M Keck Foundation, the U.S. Department of Agriculture and the State of Illinois. GSECARS, together with BioCARS and ChemMatCARS, make up the three APS sectors of the Consortium for Advanced Radiation Sources (CARS) managed by the University of Chicago. Use of the APS was supported by the DOE's Office of Basic Energy Sciences.

The nation’s first national laboratory, Argonne National Laboratory conducts basic and applied scientific research across a wide spectrum of disciplines, ranging from high-energy physics to climatology and biotechnology. Since 1990, Argonne has worked with more than 600 companies and numerous federal agencies and other organizations to help advance America's scientific leadership and prepare the nation for the future. Argonne is operated by the University of Chicago as part of the U.S. Department of Energy's national laboratory system.

For more information, please contact Dave Jacqué (630/252-5582 or info@anl.gov) at Argonne.

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