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News Release 95-44

Geophysicists Explore Interior of Mars -- from Earth

June 29, 1995

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Scientists believe that planet Mars, like the Earth, has a largely iron core. But without seismological evidence, the dimensions, composition, and physical state of the core of Mars have been difficult to assess. The presence in many meteorites of iron sulfide (FeS) suggests that sulfur is a possible component (along with iron) of the cores of Mars, and other planets.

Writing in the journal Science (30 June 1995), Yingwei Fei, Charles Prewitt, Ho-kwang Mao, and Constance Bertka of the Carnegie Institution of Washington, all working at the National Science Foundation (NSF)'s Center for High-Pressure Research, reported on their recent x-ray diffraction experiments.

In these experiments, they applied synchrotron x- radiation to FeS samples, employing newly developed capabilities for achieving simultaneous high pressure and high temperature in a diamond-anvil cell. Capabilities for "in situ" measurement were essential because high-pressure, high-temperature forms of FeS are "nonquenchable." (They change phase upon return to room conditions.) The new instrumentation is capable of achieving pressures in excess of conditions inside Mars' core.

In their experiments, Fei and colleagues discovered and characterized a previously unknown high-pressure/high temperature form of FeS, called FeS IV, which exists at the pressures and temperatures believed to prevail in the core of Mars. They also found an electronic transition in FeS IV at intermediate pressures, indicating metallization of FeS at higher pressures; pinned down boundaries of pressure and temperature dividing FeS IV from other FeS forms; and applied their data in a fresh evaluation of the depth of Mars' core mantle boundary. Their conclusion is that if FeS is a major component of the planet's core, it is in the FeS IV structure.

The discovery of the FeS IV form bears directly on calculations of the depth of Mars' core-mantle boundary. Such calculations depend on density profiles of the mantle and core. Previous calculations indicated a core-mantle boundary between 1370 and l990 kilometers in depth. The new calculations with FeS IV show that the core-mantle boundary of Mars is probably some 2,000 kilometers down.

The Center for High-Pressure Research is funded by NSF's division of earth sciences.


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