for National Geographic News
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.
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