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Planetary geology
Molten martian core

The more researchers study Mars, the more similarities they seem to find between the Red Planet and Earth. The latest parallels come from the planet’s enigmatic interior. As is the case with Earth’s, the martian core appears to be about half the size of the planet and, to some extent, liquid iron, according to new research out of NASA.

Writing in the March 7 online edition of Science, Chuck Yoder and others report on three years of radio tracking data from the Mars Global Surveyor, which reveals a surface bulge on Mars and the first direct evidence of a liquid martian core.

This artist’s concept of the interior of Mars shows a hot liquid core about half the radius of the planet. Like Earth’s, 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. NASA/JPL.

“The gravitational pull of the Sun causes a bulge in Mars toward the Sun and away from the Sun,” says co-author Alex Konopliv. “Because we are measuring the gravity field of Mars, we see the bulge not only on the surface of Mars but also throughout the planet including the bulge in the core.” Yoder’s team found a surface bulge of 0.7 centimeters, much smaller than the tidal bulge on Earth but large enough to indicate that Mars’ core cannot be solid. A solid core would not deform and would show very little gravitational tide.

The findings, however, do not reveal whether the martian core is entirely liquid or has a solid inner core with a liquid outer shell. “It may have a solid inner core, like Earth. However, this measurement is insensitive to that possibility,” Yoder says.

Twenty years ago, David Stevenson, a planetary sciences professor at Caltech, favored a Mars with no inner core, one entirely liquid, but now he sees the core differently, with a solid inner core and a thin liquid outer core, much like Earth’s. The presence of sulfur in the core, he says, would dramatically lower the melting point of iron. “So if the core were pure iron, it would indeed freeze with its high freezing point, but because we expect the core from general chemical considerations to have elements, such as sulfur — which is an antifreeze, like salt on an icy road — the outermost part of it should be liquid,” he says.

On Earth, seismological and geomagnetic evidence give researchers clues to the planet’s interior. No seismological evidence from Mars exists to help determine the composition of its core. And unlike Earth, which has a large magnetic field generated by the motion of its outer fluid core, Mars currently has no magnetic field. This absence has generated speculation on the core’s structure. “One of the possible explanations suggested decades ago for that absence was that Mars has a solid core; if you have a solid core, you cannot generate a magnetic field because it’s not enough to have an electrical conductor — you have to have a dynamo process,” Stevenson says.

Yoder and others’ research rules out that possibility, he says, but further complicates the puzzle as to why Mars has no magnetic field. Stevenson thinks it has something to do with a lack of heat flow needed to set the core in motion. Because Mars is smaller than Earth and has no plate tectonics, the planet lacks an efficient way of losing heat to the exterior, he says — a feature that separates Earth not only from Mars but also from the rest of the solar system. “I’ve speculated that there’s a direct link between whether a planet has plate tectonics and whether it has a magnetic field. Earth is the only terrestrial planet with both a magnetic field and plate tectonics.”

Lisa M. Pinsker

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