| Kitchen physics dictates that a raw egg will spin slower than a hard-boiled one. Scientists using this same logic have discovered the planet Mercury has a fluid core of molten iron.
The finding, detailed in the May 4 issue of the journal Science, solves a 30-year-old mystery but raises another.
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To figure out whether Mercury's core was liquid or solid, a team of scientists led by Jean-Luc Margot at Cornell University measured small twists in the planet's rotation. They used a new technique that involved bouncing a radio signal sent from a ground telescope in California off the planet and then catching it again in West Virginia.
After 5 years and 21 such observations, the team realized their values were twice as large as what would be expected if Mercury's core was solid.
"The variations in Mercury's spin rate that we measured are best explained by a core that is at least partially molten," Margot said. "We have a 95 percent confidence level in this conclusion."
A polluted core
Mercury, named after the Roman gods' fleet-footed messenger, is the closest planet to our Sun. One year there is equal to 88 Earth-days. Mercury is thought to consist of a thin silicate mantle encasing an iron core. Because it is so small-its mass is only 5 percent of Earth's-scientists thought it cooled rapidly early in its formation, essentially freezing any liquid core it had into a solid.
But 30 years ago, a flyby of the planet by the Mariner 10 spacecraft detected a weak magnetic field, about 1 percent as strong as Earth's, within the planet. Magnetic fields are generally associated with a dynamic molten core.
Margot and his team speculate that sulfur or some other light element got mixed with Mercury's iron core when the planet was forming and lowered its melting temperature.
"If you had such a lighter element polluting the iron, it could explain why the core has remained fluid up to the present time," Margot told SPACE.com.
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