Martian Liquid Center

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.

The wide angle view of the Martian north polar cap was acquired on March 13, 1999, during early northern summer. The light-toned surfaces are residual water ice that remains through the summer season. The nearly circular band of dark material surrounding the cap consists mainly of sand dunes formed and shaped by wind. The north polar cap is roughly 1100 kilometers (680 miles) across.Credit: NASA/JPL/Malin Space Science Systems

Researchers at NASA’s Jet Propulsion Laboratory, Pasadena, Calif., analyzing three years of radio tracking data from the Mars Global Surveyor spacecraft, concluded that 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.

Artist’s concept of the interior of Mars shows a hot liquid core that is about one-half the radius of the planet. The core is mostly made of iron with some possible lighter elements such as sulfur. The mantle is the darker material between the core and the thin crust.
Image Credit: NASA/JPL

"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 1 centimeter (0.4 inch). 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."

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 of JPL, another co-author of the paper.

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 that 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 we assume the cap change is uniform or flat over the entire cap, while the lowest mass exchange corresponds to a conically shaped cap change.

What’s Next

Mars Odyssey was launched from Cape Canaveral Air Force Station in April 2001 and arrived in Martian orbit in late October 2001. Two Mars Exploration Rovers are targeting what imagery indicates might have been ancient dry lake beds and other geologically interesting sites in early 2004.

Mars Express
Mars Express will itself carry a small lander, the Beagle 2.
Credit: European Space Agency

The UK’s Open and Leicester Universities, together with Astrium, an Aerospace Industry partner, aims to reach Mars at the same time in early January 2004. Beagle 2, a compact, lightweight lander carried on the European Space Agency’s (ESA) Mars Express, will search for signs of life on the red planet. Because of the severe mass restrictions, Beagle 2 has no propulsion system. Instead it relies on parachutes and a slowly deflating balloon in a controlled crash landing. And it cannot move once it lands. Instead it relies on a robotic arm studded with scientific instruments, like digits on the end of a living limb. Scientists call the instrument package the paw.

One of Beagle’s experimental instruments for gas-analysis is a miniaturized version of the lab equipment developed to analyze Martian meteorite samples on Earth. It slowly heats a sample in the presence of oxygen, then analyzes the gases driven off. At each step in the heating cycle, different chemicals burn. During the process, the instrument can detect carbonates produced by water percolating through cracks in rocks and organic matter the chemical signs of life. Because the instrument analyzes gases, it can also analyze the Martian atmosphere. Beagle 2 can also garner isotopic information about individual carbon atoms. Carbon atoms come in two stable forms, called isotopes: carbon-12 and carbon-13. The only difference between the two is the number of neutrons it contains. Carbon 12-and carbon-13 are mixed together in atmospheres. As biological processes build organic molecules, they use more carbon-12 than -13. This distinctive carbon-12:-13 ratio becomes a detectable signature both of living organisms and their leavings. It is this signature that Beagle-2 will look for.

JPL manages the Mars Exploration Program for NASA’s Office of Space Science, Washington, D.C. JPL is a division of the California Institute of Technology in Pasadena.

Related Web Pages

The Mars Exploration Homepage
Mars Global Surveyor
Mars THEMIS Site
MER 2003 Prime Landing Sites
US Geological Survey– Java Maps–MER 2003 Landing Topography
The Greening of the Red Planet (NASA Astrobiology Institute)
Thawing Mars (NASA Astrobiology Institute)
Bring Mars to Life – Chris McKay (Mars Society)
Mars Exploration: Planetary Protection (Mars Now Team and the California Space Institute)
Planetary Protection Provisions for Sample Return Missions (Astrobiology Web)
Mars by Stories