SYDNEY: Astronomers have discovered that Mercury has a molten core, solving a 30-year-old mystery and providing insights into the planet's history and magnetic field.

"The question of the presence of a liquid core is central to our understanding of Mercury," said Clark Chapman, of the South West Research Institute's Department of Space Studies in Boulder, Colorado.

A team led by Jean-Luc Margot, from Cornell University in Ithaca, New York, report the find today in the U.S. journal Science.

Much of our knowledge of Mercury comes from the Mariner 10 mission, which flew by the closest planet to the Sun three times between 1974 and 1975. One of the most surprising findings was that Mercury has an internal magnetic field.

Mariner mystery

"It was generally concluded that Mercury must have a liquid core, since magnetic fields – like that of planet Earth – were thought to be generated from motions in a liquid core," said Chapman, who is not part of the team behind the new find.

However, researchers also believed that Mercury's relatively small size meant any molten core must have cooled and solidified long ago.

Shortly after the Mariner 10 flyby, researchers came up with a method for determining the state of Mercury's core based on measuring subtle deviations in the way solid and liquid-core planets spin. But experts thought they would have to wait for NASA's MESSENGER mission – which flies by Mercury in January 2008 and settles in its orbit in 2011 – to put the idea in to practice.

In the meantime however, experts at the Space Research Institute in Moscow, Russia, came up with an innovative method for measuring the spin of Mercury using the 'speckle pattern' of irregularities in radar signals returned to Earth.

"Analysis of radar signals recorded at two stations on Earth can detect this rotation as the speckle pattern sweeps coherently across the Earth's surface," said Sean Solomon of the Carnegie Institute in Washington, whose commentary on the paper also appears today Science. "[Lead author Jean-Luc] Margot and his team applied these two theories with spectacular results."

The experiment consisted of repeated measurements of radar signals bounced off Mercury and recorded at pairs of radio antennas in California, West Virginia and Puerto Rico over a period of four years.

The results showed that Mercury displays tiny twists in its spin as it orbits the Sun, which are twice what would be expected in a completely solid body. "This result indicates that Mercury has a molten outer core at 95 per cent confidence," said Solomon.

Liquid, longer

These twists – or longitudinal librations – were explained by the fact that molten cores are not forced to rotate along with the solid surface of the planet. They occur as the Sun's gravitational pull affects the planet's slightly asymmetrical shape.

To maintain a molten core for such a long time after the planet has cooled, the core must contain other elements besides iron, researchers said. That makes it likely that the core contains a lighter element such as sulphur which would lower its melting temperature.

"We thought that the planet would have cooled off and solidified by now, but the molten core indicates otherwise… [it] tells us something very important about the planet's body temperature," said Jean-Luc Margot, from Cornell University. "Figuring out the interior properties and the thermal evolution of Mercury helps us understand how habitable worlds – planets like our own – form and evolve."