Activity in the planet Mercury's molten core has played a central role in the shaping of the planet, said scientists studying data sent from the latest NASA spacecraft to pass the planet.

Images and sensor readings from the Messenger space probe, which passed Mercury on Jan. 14, lend support to the theories that volcanic activity early in the planet's history helped smooth the planet's surface, while a cooling but still active core is responsible for the contraction of the planet.

The findings were reported in a series of 11 studies published in the Friday issue of the journal Science.

Mercury, the smallest planet in the solar system and the closest to the Sun, has long puzzled scientists: while it resembles the Earth's moon on the surface, the planet's density and magnetic field suggest a planet with a core that is still active.

Mercury's core makes up at least 60 per cent of its mass, according to the Messenger findings, a figure almost twice as large as any known terrestrial planet.

The latest analysis of the magnetic field suggests that, like the Earth, Mercury's core is similar to a modern dynamo, with movement of an active core acting as an energy source for the field. Scientists previously weren't sure if the planet's magnetic field was a remnant of a now dormant core.

The still active core may be responsible for most of Mercury's unique surface features as well, said principle investigator Sean Solomon at the Carnegie Institution of Washington.

Solomon said the cooling of the core led to a shrinking of the planet at least one-third greater than originally thought, leaving behind telltale "wrinkles" — huge cliffs called lobate scarps that mark the tops of crustal faults.

"They tell us how important the cooling core has been to the evolution of the surface," he said in a statement.

Researchers also found evidence of volcanic vents along the margins of the Caloris basin — a prominent remnant of an impact with a giant space rock — suggesting that Mercury had a more active geological past.

These vents may help explain the planet's smooth plains, since such plains could have been formed in the aftermath of lava eruptions on the planet's surface.

Messenger will be making two more passes of the planet, with each pass designed to slow and alter its trajectory, until finally settling into orbit in 2011.

Messenger, which stands for Mercury Surface, Space Environment, Geochemistry and Ranging, was launched in 2004. It's only the second Earth probe to visit the planet and the first since Mariner-10, which passed the planet three times, the last time in 1975.