Researchers have cracked a mystery at the core of Mercury — and say there's molten fluid inside the tiny planet.

They did it by using the same trick as cooks who want to know if an egg is raw or hard-boiled.

The finding helps explain the unexpected discovery several years ago that Mercury has a small magnetic field.

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The discovery of the magnetic field by the Mariner 10 spacecraft puzzled scientists, who believed that because of its small size the planet's core had long ago solidified.

But the most common explanation for a magnetic field is a molten interior, such as on Earth. The Moon and Mars, for example, show evidence only of ancient magnetic fields.

The Messenger spacecraft is on its way to Mercury and is expected to arrive next year, but in the meantime researchers led by Jean-Luc Margot, an assistant professor of astronomy at Cornell University, launched their own attempt to learn about the planet's core.

Cooks wondering if an egg is raw or has been cooked have used a simple trick for many years — spin the egg.

A hard-boiled egg with a solid interior will spin smoothly, a raw one with a liquid center will wobble.

Of course, Mercury may be small, but they obviously couldn't spin it themselves.

Instead, they closely studied its movements using telescopes in California, Puerto Rico and West Virginia.

They sent powerful radar signals at the planet, then received the echo, which appeared as a unique pattern of speckles reflecting the roughness of the planet's surface, at widely separated locations.

By measuring how long it took for a particular speckle pattern to reproduce at different locations they were able to calculate Mercury's spin rate with an accuracy of one part in 100,000.

The observations, conducted over five years, allowed the scientists to calculate twists in Mercury's spin, called librations, caused by the effect of the sun's gravity.

The magnitude of the librations was twice what would be expected for a completely solid body, they concluded. But it matched what would be expected for an object whose outer core is molten and not forced to rotate along with its shell.

Sean C. Solomon of the Carnegie Institution of Washington, who was not part of the research team, said in a commentary that the measurements "constitute a triumph" of theoretical ideas developed in the past.

Margot's research was supported by NASA.

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