An Impactor Origin for Lunar Magnetic Anomalies

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Science  09 Mar 2012:
Vol. 335, Issue 6073, pp. 1212-1215
DOI: 10.1126/science.1214773

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Bringing Magnetic Materials to the Moon

The Apollo missions to the Moon revealed that portions of the lunar crust are strongly magnetized. Lunar rocks are poor at recording the magnetic field, thus these magnetic anomalies have been difficult to explain. Based on numerical simulations of large-scale impacts, Wieczorek et al. (p. 1212; see the Perspective by Collins) show that the vast majority of lunar magnetic anomalies can be explained by highly magnetic materials that originated outside the Moon and were delivered by the asteroid that hit the Moon and formed the South Pole–Aitken basin, the largest impact basin in the solar system.


The Moon possesses strong magnetic anomalies that are enigmatic given the weak magnetism of lunar rocks. We show that the most prominent grouping of anomalies can be explained by highly magnetic extralunar materials from the projectile that formed the largest and oldest impact crater on the Moon: the South Pole–Aitken basin. The distribution of projectile materials from a model oblique impact coincides with the distribution of magnetic anomalies surrounding this basin, and the magnetic properties of these materials can account for the intensity of the observed anomalies if they were magnetized in a core dynamo field. Distal ejecta from this event can explain the origin of isolated magnetic anomalies far from this basin.

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