Lunar surface magnetic field concentrations antipodal to young large impact basins

We find that the major regions of strong surface magnetic fields detected by planetary electron reflection magnetometry (PERM) measurements of the Apollo 15 and 16 subsatellites are located near the antipodes of four young large ringed impact basins: Orientale, Imbrium, Serenitatis, and Crisium. Statistical analysis of the 26 impact basins whose antipodes fall within the 35°S to 35°N latitude band covered by PERM measurements shows that the antipodal regions of older basins have median surface magnetic fields of {less-than or approximate}0.2 nT compared with ∼0.5-1.5 nT for the four youngest basins. Although the mechanisms for producing the surface magnetic field concentrations antipodal to impact basins are not well understood, these results indicate a period of strong lunar magnetic fields between ∼3.85 and 3.6 aeons, consistent with lunar sample paleomagnetic data. The strong surface magnetic fields near the basin antipodes may have resulted from concentration of a preexisting ambient magnetic field at the antipode by the partially ionized vapor cloud produced in hypervelocity basin-forming impacts and acquisition of shock remanent magnetization from the impact of secondary ejecta.