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Donald Savage
Headquarters, Washington
(Phone: 202/358-1547)
Mary Hardin
Jet Propulsion Laboratory, Pasadena, Calif.
(Phone: 818/354-0344)
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March 6, 2003 |
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RELEASE: 03-094
New information about what is inside Mars shows the Red Planet has
a molten liquid-iron core, confirming the interior of the planet
has some similarity to Earth and Venus.
Researchers at NASA's Jet Propulsion Laboratory (JPL), Pasadena,
Calif., analyzing three years of radio tracking data from the Mars
Global Surveyor spacecraft, concluded Mars has not cooled to a completely
solid iron core; rather its interior is made up of either a completely
liquid iron core or a liquid outer core with a solid inner core.
Their results are published in the March 7, 2003, online issue of
the journal Science.
"Earth has an outer liquid-iron core and solid inner core.
This may be the case for Mars as well," said Dr. Charles Yoder,
a planetary scientist at JPL and lead author on the paper. "Mars
is influenced by the gravitational pull of the sun. This causes
a solid body tide with a bulge toward and away from the sun (similar
in concept to the tides on Earth). However, for Mars this bulge
is much smaller, less than one centimeter. By measuring this bulge
in the Mars gravity field we can determine how flexible Mars is.
The size of the measured tide is large enough to indicate the core
of Mars can not be solid iron but must be at least partially liquid,"
he explained.
The team used Doppler tracking of a radio signal emitted by the
Global Surveyor spacecraft to determine the precise orbit of the
spacecraft around Mars. "The tidal bulge is a very small but
detectable force on the spacecraft. It causes a drift in the tilt
of the spacecraft's orbit around Mars of one-thousandth of a degree
over a month," said Dr. Alex Konopliv, a planetary scientist
at JPL and co-author on the paper.
The researchers combined information from Mars Pathfinder on the
Mars precession with the Global Surveyor tidal detection to draw
conclusions about the Mars core, according to Dr. Bill Folkner,
another co-author on the paper at JPL.
The precession is the slow motion of the spin-pole of Mars as it
moves along a cone in space (similar to a spinning top). For Mars
it takes 170,000 years to complete one revolution. The precession
rate indicates how much the mass of Mars is concentrated toward
the center. A faster precession rate indicates a larger dense core
compared to a slower precession rate.
In addition to detection of a liquid core for Mars, the results
indicate the size of the core is about one-half the size of the
planet, as is the case for Earth and Venus, and the core has a significant
fraction of a lighter element such as sulfur.
In addition to measuring the Mars tide, Global Surveyor has been
able to estimate the amount of ice sublimated, changed directly
into a gaseous state, from one pole into the atmosphere and then
accreted onto the opposite pole. "Our results indicate the
mass change for the southern carbon-dioxide ice cap is 30 to 40
percent larger than the northern ice cap, which agrees well with
the predictions of the global atmosphere models of Mars," said
Yoder.
The amount of total mass change depends on assumptions about the
shape of the sublimated portion of the cap. The largest mass exchange
occurs if one assumes the cap change is uniform or flat over the
entire cap, while the lowest mass exchange corresponds to a conically
shaped cap change.
JPL manages the Mars Exploration Program for NASA's Office of Space
Science, Washington. JPL is a division of the California Institute
of Technology, Pasadena.
For more information about NASA on the Internet:
http://www.nasa.gov.
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