One of the four rocky planets in the inner Solar System, Mercury is believed to generate its magnetism in the same way as Earth -- by a dynamo, caused by the rotation of molten iron at its core.

But question is why Mercury's magnetic field is so puny, as it amounts to just one percent of the strength of Earth's.

If the dynamo theory is right, the planet's magnetism should be 30 times stronger than it is.

Ulrich Christensen of the Max Planck Institute for Solar System Research in Katienburg-Lindau believes the answer lies in the structure of Mercury's core.

The core's outer layers are "stably stratified," which means they are largely insulated from the heat of the swirling inner core.

As a result, only the inner core rotates effectively to generate the magnetic field.

This braking effect is relatively important because Mercury has a very slow rotation, which also affects the dynamo's power. The planet takes 59 Earth days to complete one spin on its axis, the slowest rotation in the Solar System except for that of Venus.

Christensen says that two scheduled missions to Mercury, NASA's Messenger (launched in 2004 and scheduled to rendezvous in 2008) and the European Space Agency's BepiColombo (due for launch in 2013), should be able to put his theory to the test.

The paper appears on Thursday in Nature, the weekly British science journal.

The only trip to Mercury was made by the US spacecraft Mariner 10, which recorded the planet's unusual magnetic field in fly-bys in 1974 and 1975.

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