The facts are these: the Earth's magnetic field is produced by the planet's spinning solid core that generates the magnetic field, the moon is too small and cold to have that magnetic field generating core. So how did magnetized rocks appear on the surface of the moon if it doesn't have a global magnetic field?

This mystery has been puzzling astronomers since the Apollo missions brought back the first moon rocks that displayed this magnetized quality. Now a team of scientists from California has proposed a possible solution to this decades-old case. Christina Dwyer, and her coauthors--planetary scientists Francis Nimmo at UC Santa Cruz and David Stevenson at the California Institute of Technology think the answer could be found in an ancient lunar geodynamo could have provided the electromagnetic field to magnetize the rocks in the lunar surface. The moon's core and mantle rotate at different axes and since their boundary is not spherical, their relative motion is enough to cause the liquid core to move around.

"The moon wobbles a bit as it spins--that's called precession--but the core is liquid, and it doesn't do exactly the same precession. So the mantle is moving back and forth across the core, and that stirs up the core, " explained Nimmo, a professor of Earth and planetary sciences at UCSC.

The strength of this movement is also influenced by the tidal gravitational pull from the Earth which causes the moon's mantle to rotate differently than the core. The Earth and the Moon were closer in distance billions of years ago, this tidal pull would have been enough to generate a dynamo on the moon. But as the Earth and moon drifted further apart, the gravitational tug wasn't enough to keep the core from moving to generate the magnetic field. The scientists estimate that the moon had a magnetic field for about a billion years, somewhere between around 2.7 billion and 4.2 billion years ago, after which the distance became too great to keep the dynamo from going on.

"The further out the moon moves, the slower the stirring, and at a certain point the lunar dynamo shuts off," Dwyer said.

However another group of scientists proposed another solution to the ultimate culprit of the moon's missing magnetic field. Michael Le Bars of the French National Center for Scientific Research and the Université Aix-Marseille in France, along with his colleagues, published in the same issue of Nature another possible solution. Although they agree with Dwyer's team that it was the motion of the mantle and core that generated the electromagnetic field, Le Bars' team believes that it wasn't the tidal interaction between the Earth and moon that stirred the moon's core but it was the impact of large space rocks in the moon's surface that caused the core to rotate differently from the mantle.

The resulting magnetic field would have been short-lived instead of the billion year span proposed by Dwyer's team. More data from the magnetized Apollo rocks is needed to determine which of the two theories is correct. Dwyer however believes the two models could be combined with tidal forces pushing the mantle steadily for a time and giant impacts from space speeding up the motion occasionally.