NSF PR 95-44 - June 29, 1995
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Geophysicists Explore Interior of Mars -- from Earth
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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