Demagnetization Signatures of Lunar Impact Craters

Observations of lunar crustal magnetic fields by the Apollo subsatellites and the Lunar Prospector (LP) spacecraft have shown many indications that the distribution of crustal fields may be largely controlled by impact processes. The largest concentrations of strong magnetic fields (>100 nT) are correlated with the antipodes of large impact basins, and some smaller concentrations are associated with impact ejecta. Large impact basins themselves, however, tend to be areas of low magnetic fields (<2 nT), suggesting that impacts can demagnetize the crust. High resolution LP measurements have allowed us to investigate the magnetic properties of impact basins and craters down to ~50 km in diameter. We have found that even the smallest craters in this observable size range often show demagnetization signatures similar to those associated with large basins. Many such signatures are symmetric and clearly associated with individual craters, leaving little doubt that we are actually observing the results of demagnetization by crater impacts. The demagnetization signatures tend to extend more than one radius from the center of the crater or basin, showing that the shock and/or thermal effects responsible for demagnetization affect regions past one radius. Some craters show the edge effects that one would expect if an area smaller than the local magnetic scale size was demagnetized. Most larger craters and basins, on the other hand, do not show such an effect. This suggests that the crustal magnetism is jumbled on a scale smaller than ~100 km. There are clear differences in the demagnetization of craters and basins of different ages, and there are also hints of differences in the demagnetization of craters and basins of different sizes. This suggests that careful studies of crater demagnetization can help to determine both the age and depth of lunar crustal magnetism.