No. 78: Nov-Dec 1991
Astronomers have long puzzled over the origin of localized magnetic anomalies on the moon. These magnetic concentrations (called "magcons") are located precisely on the opposite side of the moon from the larger lunar basins. How could an impact on the moon magnetize the antipodal region?
The impact of a large silicate meteoroid at speeds of 10 kilometers/second would not only blast out a big crater but it would also create a huge cloud of hot, partially ionized gas. This hot gas or plasma will conduct electricity and interact with lunar magnetic fields. As the plasma cloud spreads away from the impact site, it acts like a bulldozer, compressing the lunar magnetic fields ahead of it, as it envelopes the whole moon and rushes towards the antipodal point. It drives the compressed mag netic field into the surface, permanently magnetizing the rocks at the antipodal point. Voila! Magcons. (Hood, L.L., and Huang, Z.; "Formation of Magnetic Anomalies Antipodal to Lunar Impact Basins: Two-Dimensional Model Calculations," Journal of Geophysical Research, 96:9837, 1991.)
Comment. The earth also sports scars from the impacts of large meteoroids. Are there magnetic anomalies opposite these craters? Even more interesting to check out would be the holes blasted in the earth's biosphere by the converging masses of hot gases at the an tipodal points. Wouldn't there be extinctions seen in the fossil record at these antipodal points?
Reference. Magcons are cataloged as ALZ1 in The Moon and Planets. This catalog is described here.