Scientists believe they finally have an explanation for why magnetized rocks are found on the moon, even though it now has no magnetic field.
Earth's own magnetic field is created by a 'geodynamo', powered by heat from the inner core, which drives complex fluid motions in the molten iron of the outer core. However, the moon is simply too small to support that type of dynamo.
But according to Christina Dwyer, a graduate student in Earth and planetary sciences at the University of California, Santa Cruz, such a dynamo could have existed in the past, created by the stirring of the moon's liquid core, driven by the motion of the solid mantle above.
"This is a very different way of powering a dynamo that involves physical stirring, like stirring a bowl with a giant spoon," she says.
Early in its history, she says, the moon orbited the Earth at a much closer distance than it does today. Tidal interactions between the Earth and the moon caused the moon's mantle to rotate slightly differently to the core. This stirred the liquid core, creating fluid motions that, in theory, could give rise to a magnetic dynamo.
The researchers found that a lunar dynamo could have operated in this way for at least a billion years, but would have stopped working as the moon got farther away from the Earth. "The further out the moon moves, the slower the stirring, and at a certain point the lunar dynamo shuts off," says Dwyer.
More detailed analysis is needed, however, to show that stirring of the core by the mantle would create the right kind of fluid motions to generate a magnetic field. The next stage is to create a working model of a lunar dynamo, combined with more detailed analysis of moon rocks.