The early moon may not have been the sleepy place we know today. Researchers are reporting that our nearest neighbor was once filled with a hot, churning liquid that gave it its own magnetic field. The study, published in tomorrow's issue of Science, sheds light on the moon's early evolution and suggests that other small bodies could also sport magnetic fields.
Earth's magnetic field protects us from harmful solar wind--and gives us stunning auroras (ScienceNOW, 24 July 2008). Most other planets in our solar system also have magnetic fields, and the moon once did too, according to evidence from satellites and lunar rocks. Yet astronomers are split over how the moon's field formed. Some think it arose the same way Earth's does, from the convection of liquid iron in its core. Others, however, think that the moon never had a molten core and that meteor impacts on the lunar surface generated short-lived magnetism.
One extraordinary rock may finally hold the answer. Known as troctolite 76535--"the trocto" to planetary scientist Ian Garrick-Bethell of the Massachusetts Institute of Technology (MIT) in Cambridge and his colleagues who analyzed it--the 4.2-billion-year-old bright green and milky-white rock was collected by the U.S.'s last manned lunar mission, Apollo 17. Using a technique called alternating-field demagnetization, the MIT researchers slowly unraveled the rock's magnetism by subjecting it to a progressively stronger magnetic field. They found that the trocto had been exposed to two stable and intense magnetic events. Previous studies showed that it had also twice undergone prolonged cooling--once for millions of years, the second time for thousands.
Taken together, the findings suggest that the moon sported a stable magnetic field 4.2 billion years ago, just 300 million years after the body formed. That rules out magnetism via meteor impacts, which would have created a short-lived field, and instead suggests that the moon housed a churning molten core. Such a core jibes with a prevalent theory that the moon was conceived when a giant object crashed into the young Earth and blasted material into our planet's orbit.
"People have been attempting to figure out the history of the lunar magnetic field since we first got our hand on [moon] rocks," says planetary scientist David Stevenson of the California Institute of Technology in Pasadena. Many astronomers didn't think a body as small as the moon could support a stable magnetic field, Stevenson says, but these findings show that it could. In that case, other small extraterrestrial objects, such as asteroids and meteorites, may also have magnetic fields, he says.
Still, Dave Stegman, a geodynamicist at the University of Melbourne in Australia, cautions that scientists are inferring a lot from just one rock. "It is really hard to be considered robust [when the study is] based on a single data point," he says.
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