Moonstruck Magnetism

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Science  09 Mar 2012:
Vol. 335, Issue 6073, pp. 1176-1177
DOI: 10.1126/science.1217681

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A surprising result of the Apollo missions was the discovery of strong, localized magnetic fields emanating from the lunar crust (1). One reason these fields are so enigmatic is that endogenous lunar rocks contain a low abundance of metallic iron, making them very weakly magnetic (24). How, then, did the Moon record such strong magnetic signatures? On page 1212 of this issue, Wieczorek et al. (5) propose that many of the Moon's magnetic anomalies originate from highly magnetic deposits of a giant asteroid that collided with the Moon early in its history. These readily magnetized, extralunar deposits subsequently recorded magnetic fields that may have been generated by an ancient lunar core dynamo and/or transient impact-generated fields.