Scientists Try To Explain Magnetic Moon Rocks

The moon has no magnetic field, but Apollo astronauts brought back magnetic moon rocks. Scientists have a possible explanation.(PHYSORG)- THE PRESENCE OF MAGNETIZED ROCKS ON THE SURFACE OF THE MOON, WHICH HAS NO GLOBAL MAGNETIC FIELD, HAS BEEN A MYSTERY SINCE THE DAYS OF THE APOLLO PROGRAM. NOW A TEAM OF SCIENTISTS HAS PROPOSED A NOVEL MECHANISM THAT COULD HAVE GENERATED A MAGNETIC FIELD ON THE MOON EARLY IN ITS HISTORY.The "geodynamo" that generates Earth's magnetic field is powered by heat from the inner core, which drives complex fluid motions in the molten iron of the outer core. But the moon is too small to support that type of dynamo, according to Christina Dwyer, a graduate student in Earth and planetary sciences at the University of California, Santa Cruz. In the Nov. 10 issue of Nature, Dwyer and her coauthors -- planetary scientists Francis Nimmo at UC Santa Cruz and David Stevenson at the California Institute of Technology -- describe how an ancient lunar dynamo could have arisen from stirring of the moon's liquid core driven by the motion of the solid mantle above it."This is a very different way of powering a dynamo that involves physical stirring, like stirring a bowl with a giant spoon," Dwyer said.Dwyer and her coauthors calculated the effects of differential motion between the moon's core and mantle. Early in its history, the moon orbited the Earth at a much closer distance than it does today, and it continues to gradually recede from the Earth. At close distances, tidal interactions between the Earth and the moon caused the moon's mantle to rotate slightly differently than the core. This differential motion of the mantle relative to the core stirred the liquid core, creating fluid motions that, in theory, could give rise to a magnetic dynamo."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 researchers found that a lunar dynamo could have operated in this way for at least a billion years.