A new study concludes that the core of Mars is the consistency of the syrupy goop found inside chocolate-covered fruit candy.
The inference was made simply by noting minor changes in the position of a Mars-orbiting spacecraft, caused by tides.
Yes, tides on Mars. While one commonly thinks of tides having to do with oceans on Earth, and being generated only by the Moon, the inner parts of heavenly bodies endure tides, too. On Earth, gravity from both the Moon and the Sun fuel ocean tides and simultaneously stretch and pull the entire planet by less noticeable amounts.
Mars, too, is tidally tweaked by the Sun.
Scientists have not until now had any firm evidence whether the core of Mars was solid or liquid, though they had suspected it would be at least partly liquid, and they knew it was mostly iron. The new study finds that at least the outer portion of the core is indeed molten, making it similar to the cores of Earth and Venus. The conclusion is based on three years worth of data showing changes in the orbital characteristics of the Mars Global Surveyor.
The study was led by Charles Yoder at NASA's Jet Propulsion Laboratory and was published today in the online version of the journal Science.
Yoder and his colleagues tracked a radio signal emitted by the spacecraft to monitor the craft as its orbital plane tilted gradually. This shift is caused by the planet's tidal bulge -- the extent to which the Sun physically distorts the planet. A continually moving "tidal bulge" on Mars exerts an ever-changing gravitational influence on the spacecraft.
The spacecraft's movements, in turn, revealed the extent of the bulge.
Knowlege of the amplitude of the tidal bulge was compared with models of Mars' tidal deformation assuming a liquid or a solid core, explained Alex Konopliv, a JPL scientist who did data analysis for the study. A smaller bulge would indicate a solid core whereas a larger bulge would suggest a more flexible planet and, thus, a liquid core.
"The amplitude is consistent with a fluid core, not a solid core," at least in the outer portion, Konopliv said.
Additional information for the study came from previous measurements of the precession of Mars' rotation. Like Earth, the Red Planet's axis of rotation is not steady, but instead proscribes an imaginary cone-shaped feature in space as it shifts over time.
It takes 170,000 years for the Martian axis to make one revolution, Konopliv said, effectively drawing a circle on some imaginary plane above the pole. "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."
In addition to iron, the new study indicates there is also some lighter element in the core, possibly sulfur, Yoder told SPACE.com. The study also pinned the core's radius down to between 945 and 1,143 miles (1,520 and 1,840 kilometers).
The average radius of Mars is 2,106 miles (3,390 kilometers), just more than half that of Earth. But Mars is less dense -- containing only 11 percent as much mass as Earth.
More study is needed, the researchers said, to understand how the apparently liquid core might have affected evolution, size and composition of the core as well as higher layers of the planet's innards.
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