Can Impact-generated Plasmas be Responsible for Magnetization on the Moon?

Abstract:

Measurements of the lunar magnetic field have revealed crustal remanent magnetization that is a record of ancient magnetic fields. The source of these magnetic anomalies has been debated. A key question is whether the crustal magnetization is a record of a hypothetical ancient dynamo field (that has since terminated) and/or of external transient fields generated by impact plasmas. This distinction between internal or external sources for the magnetization has broad implications for our understanding of the Moon’s geophysical history and current internal structure. Furthermore, the lunar crustal field serves as a test case for understanding remnant magnetization of other terrestrial bodies, such as Mars, Mercury and asteroids. The identification of strong anomalies at the antipodes of four of the five youngest large impact basins motivated the hypothesis that the crustal magnetization could have been produced by transient external fields created by the impacts. Some studies argued that these transient fields are created by the interaction of the ionized vapor generated by the impact with the surrounding solar wind. Here, we present an examination of this hypothesis, using magnetohydrodynamic (MHD) simulations. This constitutes the first direct MHD calculation of the magnetic fields that can be formed by this mechanism. Preliminary results show that the vapor expansion and wind compression lead to only moderate field enhancements, several orders of magnitude below the required value. Based on these results we propose that impacts probably do not produce the magnetic field responsible for the magnetization and that the source of the magnetic field is more likely to be internal. If this is true, it follows that the magnetic anomalies should be a result of local anomalies in the magnetic properties of the crust. Many of the anomalies are found on the north rim of the South-Pole-Aitkin (SPA) basin, whose creation may have produced iron-rich rocks or which may be associated with mafic dykes. Planned sample-return missions to the SPA may help determine whether such iron-rich rocks exist at the anomalies locations.