Astronomers have used a classic technique for checking how well an egg is cooked to discover that Mercury has a molten core. Chefs will tell you that watching how a boiled egg spins will reveal whether its contents are solid or not.

Planetary scientists managed to apply the test across millions of miles of space by bouncing radar signals from three radio telescopes.

These revealed minute changes in the spin of the closest planet to the Sun. The subtle twists, called longitudinal librations, were twice as strong as would be expected from a solid “hard-boiled” world. But they matched one with a liquid centre.

The three telescopes used for the discovery were Nasa’s 70-meter antenna at Goldstone, Californis, the Arecibo Observatory in Puerto Rico and the Green Bank Telescope in West Virginia.

The research was led by Jean-Luc Margot, assistant professor of astronomy at Cornell University, New York, and was published on the journal Science’s website.

Mercury is a rocky, terrestrial world like Earth. In size and appearance, however, it more closely resembles the Moon. It has a heavily cratered, mountainous surface that has been untouched by weathering because the planet has only a very tenuous atmosphere.

Mercury, which has an average distance of 58 million km from the Sun, is believed to have had a particularly violent birth. The planet is known to contain unexpectedly high levels of iron. The theory is that this is because it was formed from an impact between a giant asteroid and a much larger world that was orbiting the Sun more than four billion years ago.

A Nasa mission, called Messenger, has been en route there since 2004 but will not go into orbit around the planet until 2011. Another mission, BepiColombo, will carry two orbiters to Mercury, one European and one Japanese. The craft is not due for launch until 2013 and the probes will not arrive until six years later.

Photo: Jean-Luc Margot with soft-boiled and hard-boiled spinning eggs. Credit: Lindsay France, Cornell University.

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