Just about any chef know that by spinning an egg you can find out if the core is solid or liquid, now astronomers have applied a similar test to the planet Mercury and have found strong evidence that the planet closest to the sun has a liquid core.

The research, led by Jean-Luc Margot, assistant professor of astronomy at Cornell, appears this week on the Web site of the journal Science.

Jean-Luc Margot and collaborators conducted a series of observations over five years using a novel technique to detect tiny twists in Mercury’s spin as it orbits the sun. The twists, called longitudinal librations, occur as the sun’s gravity exerts alternating torques on the planet’s slightly asymmetrical shape.

They found that the magnitude of the librations was double what would be expected for a completely solid body, but explainable for an object whose core is molten and not forced to rotate along with its shell.

Mercury is thought to consist of a silicate mantle surrounding an iron core, but because small planets like Mercury cool off rapidly, the core should have frozen long ago. Maintaining a molten core over billions of years requires that it also contain a lighter element, such as sulfur, to lower the melting temperature of the core material.

The researchers used three telescopes, the NASA/JPL 70-meter antenna at Goldstone, California and the National Science Foundation’s Arecibo Observatory in Puerto Rico and Robert C. Byrd Green Bank Telescope in West Virginia to measure slight changes in Mercury’s spin. The system involved sending a powerful radar signal at the planet, then receiving the signal’s echo, which appeared as a unique pattern of speckles reflecting the roughness of the planet’s surface, at two locations separated by about 2,000 miles.

Measuring how long it took for a particular speckle pattern to reproduce at the two locations (about 10 seconds) allowed Margot to calculate Mercury’s spin rate with an incredible accuracy.

Mercury still has its share of mysteries. Some may be solved with the NASA spacecraft Messenger, though, launched in 2004 and expected to make its first Mercury flyby in 2008. The spacecraft will begin orbiting the planet in 2011.

“It is our hope that Messenger will address the remaining questions that we cannot address from the ground,” Margot said.