Formation of magnetic anomalies antipodal to lunar impact basins: Two‐dimensional model calculations

Abstract

The production of large‐scale magnetic fields and associated crustal magnetization in lunar basin‐forming impacts is investigated theoretically. Two‐dimensional numerical models of the partially ionized vapor cloud produced in such impacts show that the low‐density periphery of the cloud expands thermally around the Moon and converges near the antipode in a time of the order of 400 to 500 s for silicate impactor velocities of 15 to 20 km/s. Fields external to the impact plasma cloud are produced by the magnetohydrodynamic interaction of the cloud with ambient magnetic fields and plasmas. For the most typical case in which the Moon is immersed in the solar wind plasma and its embedded magnetic field, an MHD shock wave forms upstream of the cloud periphery separating the shocked solar wind from the free‐stream solar wind. For impacts occurring on the downstream (antisunward) hemisphere, convergence of the impact plasma cloud and associated MHD shock waves occurs on the upstream side and results in a large antipodal field amplification. For impacts occurring on the upstream (sunward) hemisphere, some antipodal field amplification is still expected due to the finite electrical conductivity of the lunar interior (requiring an induced external magnetic field) and the likely presence of some residual plasma in the wake of the impact plasma cloud. During the period of compressed antipodal field amplification, seismic compressional waves from the impact converge at the antipode resulting in transient shock pressures that have been calculated to be as large as 2 GPa (20 kbar). This is near to the range of 50–250 kbar at which stable shock remanent magnetization has been found experimentally to occur in lunar soils. Significant crustal magnetization anomalies antipodal to lunar impact basins are therefore expected, consistent with orbital mapping results. Weaker magnetization observed peripheral to the Imbrium basin may also be explained by shock effects together with compressed ambient fields in a surface boundary layer. Although other processes such as cometary impacts and a former core dynamo may have contributed significantly to the observed paleomagnetism, meteoroid impact plasmas appear capable of explaining a major part of the large‐scale magnetization detected thus far from lunar orbit.