Liquid Center: Mercury has a molten core, radar reveals

Mercury is hot stuff. That’s the conclusion of a new radar study demonstrating that the core of the solar system’s innermost planet is at least partially molten. The finding settles a long-simmering debate about the least studied of the planets. It may also provide insight about how the solar system created its planets.

With Mercury averaging just a third as far from the sun as Earth is, it might seem clear-cut that Mercury’s core would be molten. But sunlight has a negligible effect compared with the heat left over from the planet’s formation. And considering Mercury’s small size, only 40 percent the diameter of Earth, astronomers calculate that the planet ought to have cooled and solidified long ago.

That thinking was challenged in 1974, when the Mariner 10 spacecraft found that Mercury has a magnetic field. The magnetic field of a rocky planet—such as Earth—is usually generated by the sloshing of charged material within a liquid core. It’s also possible, however, that the magnetic field is a remnant in the crust, frozen in place when Mercury’s core solidified.

In the new research, Jean-Luc Margot of Cornell University, Stan Peale of the University of California, Santa Barbara, and their colleagues bounced radio waves off Mercury’s surface to measure small variations in its spin.

Peale had proposed 3 decades ago that a close study of Mercury’s spin could determine whether the planet has fluid in its core. Mercury completes three rotations about its spin axis for every two 88-day revolutions around the sun. In that lock-step configuration, the sun’s gravity causes the spin of the slightly out-of-round planet to vary. Solar gravity has a greater effect on a planet’s spin if the core is at least partially liquid.

After 6 years of recording radar echoes from Mercury back to Earth, the researchers found that variations in the spin were about double what would be expected if Mercury were solid.

The most likely explanation is that, at minimum, the outer core of Mercury—the boundary between the heart of the planet and its overlying mantle—must be liquid, Margot, Peale, and their collaborators report in the May 4 Science.

“It is clear that Mercury is not solid throughout,” comments planetary scientist David Stevenson of the California Institute of Technology in Pasadena.

Scientists propose that Mercury remains molten because sulfur infiltrated the planet’s iron core and lowered its melting temperature.

But at the distance from the sun where Mercury formed, the high temperature would have kept sulfur as a vapor and so prevented it from being incorporated into the planet. That suggests that Mercury, and perhaps other planets, grabbed sulfur and other material from beyond their immediate surroundings, Margot says.

To provide further information about the planet’s core, scientists are now looking to NASA’s MESSENGER spacecraft, which will settle into orbit around Mercury in 2011, and a Japanese-European mission scheduled to arrive at the planet in 2019.