These days, a compass on the moon doesn't do much because there is no magnetic field to entice its hands to move. But it may not have always been so. Analysis of rocks recovered during the Apollo missions has uncovered telltale signs of ancient lunar magnetism. A new computer model may help explain the magnetism mystery. According to findings published today in the journal Nature, movement of a radioactive-enriched layer inside the moon--like the rising of the waxen blobs in a lava lamp--could have been responsible for a brief period of internally-created magnetism.

When volcanic rocks cool in the presence of a planet's magnetic field, they can preserve information about the strength and direction of the field. Current theories suggest that the nascent moon could not support the internal dynamo necessary to create a magnetic field. The magnetized moon rocks, which date to between half a billion and a billion years after earth's satellite was formed, thus eluded explanation. Dave R. Stegman of the University of California, Berkeley, and his colleagues adapted a computer model previously used to study planet formation to analyze the moon. By taking into account the different elements present in the young moon, the scientists propose that a layer of titanium and thorium-rich rock surrounded the moon's core and inhibited heat transfer between the core and the overlying mantle. The band eventually heated up and became more buoyant, the scientists say, creating a number of so-called superplumes of molten material that rose to the moon's surface. Maria T. Zuber of Harvard University explains in an accompanying commentary that "with the removal of the thermal blanket, the core is then able to convect vigorously to cool itself, and this can produce a short-lived dynamo." The model suggests that these new conditions persisted for about 300 million years.

The results are by no means the final word on moon magnetism, however. For one thing, the early composition of the moon remains uncertain. In addition, some scientists posit that phenomena other than an internal dynamo--asteroid impacts, for example--might account for the magnetism of the Apollo rocks. Comments Zuber, "Further simulations, continued analysis of the palaeomagnetic properties of lunar samples, and low-altitude global mapping of the Moon's magnetic signature will be required to truly grasp the elusive nature of lunar magnetism."

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