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Thursday, March 6, 2003
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.
