Liquid Mercury: Tiny Planet Has Molten Core

Ben Harder
for National Geographic News
May 3, 2007
Mercury's metallic core is at least partially liquid, say scientists who studied the tiny planet using Earth-based radio telescopes.

The finding could explain a long-standing puzzle: How does the solar system's innermost planet maintain a magnetic field?

Mercury, it seems, could have an electromagnetic dynamo—a phenomenon that occurs as Earth's molten, metallic outer core rotates around a solid inner core, spawning currents that generate a magnetic field.

The new study also indicates that an unknown element that formed beyond Mercury's orbit somehow got mixed into the planet's "batter" as it was forming billions of years ago.

"The prevailing notion is that a planet as small as Mercury would have cooled off and solidified a long time ago," said Jean-Luc Margot, the Cornell University planetary scientist who led the new study.

To stay liquid, Mercury's core must contain significant amounts of a light element—probably sulfur—that would lower the core's melting temperature, scientists say.

But during the solar system's birth, light elements condensed relatively far from the sun.

Small proto-planets moving through the system might have transported sulfur into Mercury's path while the planet was taking shape, Margot said.

The results will be published tomorrow in the journal Science.

Celestial Disco Ball

For the new study, researchers treated Mercury as if it were a spinning disco ball.

"If you shine light at a disco ball, you see a pattern of light traveling across the wall," Margot said. Instead of shining light at the planet, however, his team beamed radio waves toward it.

The researchers then used pairs of Earth-based radio telescopes located thousands of miles apart to monitor the returning patterns as the signals made their way across the surface.

The measurements told the team how quickly Mercury was spinning at various times.

Mercury's spin rate fluctuates slightly as the planet orbits the sun. Scientists have long known that, in theory, they could use those oscillations to determine whether the planet was completely solid.

Until now, though, the oscillations had never been measured.

The new data indicate that the spin rate oscillates about twice as much as it would if Mercury were solid. The planet is therefore at least partially liquid, Margot's team concludes.

Sean C. Solomon, a geophysicist at the Carnegie Institution in Washington, D.C., hailed the study's "spectacular results."

Scientists are now closer to resolving the mystery that arose some 30 years ago with the discovery that Mercury has a magnetic field, he said.

Among the other rocky planets, Venus has no magnetic field and Mars's field has long since become inactive.

(Related news: "Moon Has Iron Core, Lunar-Rock Study Says" [January 11, 2007].)

Like Earth's, Mercury's field may be a by-product of currents flowing through a molten outer core, Solomon said.

NASA's MESSENGER spacecraft, due to fly past Mercury next January, could eventually confirm that idea, he said.

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