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Donald Savage
Headquarters, Washington           March 6, 2003
(Phone: 202/358-1547)

Mary Hardin
Jet Propulsion Laboratory, Pasadena, Calif.
(Phone: 818/354-0344)

RELEASE: 03-094

SCIENTISTS SAY MARS HAS LIQUID IRON CORE

     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.

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