Simulations of magnetic fields produced by asteroid impact: Possible implications for planetary paleomagnetism

The origin and evolution of the Moon's magnetic field, specifically whether it has an endogenic or exogenic origin, has been a major question in lunar science. The lunar field today is a patchwork of magnetic anomalies carried as remnant magnetization in the rocks of the lunar crust. Recent publications suggest some magnetic anomalies may be associated with magnetized impact melt sheets of some large lunar basins cooling in the presence of an early lunar core dynamo or associated with portions of the impactor that formed the South Pole-Aitken Basin. By performing CTH simulations of asteroid impacts incorporating magnetic field generation and thermoremanence magnetization models, this paper helps explain the origin of lunar magnetic anomalies by showing: 1) transient magnetic fields produced by impact events will increase to substantial magnitude at the scale of large lunar basins and 2) magnetization of lunar rocks can occur during crater formation at nearly all scales. If occurring in pristine feldspathic lunar highlands rocks with relatively low thermoremanence susceptibility, the magnetic anomalies resulting from (1) and (2) produce magnetic fields observable in orbital magnetic field data only for large and relatively rare impact craters - large lunar basins greater than similar to 200 km diameter. If occurring in thick units with higher thermoremanence susceptibility, craters as small as 50-100 km may produce orbital anomalies. Craters at nearly all scales may leave behind remnant magnetic fields observable at the surface or in samples. This paper concludes magnetic fields produced by impacts may be an important contributor to the present state of the Moon's magnetic field.