Unlike Earth, the Moon does not have a global magnetic field, but it does display regional magnetic anomalies. The magnetic carrier in lunar rocks is metallic iron, but indigenous lunar rocks do not have enough metallic iron to account for the observed field strengths. Joana Oliveira, from the Institut de Physique du Globe de Paris, led a team that modeled the heating, melting, and mixing with lunar crust resulting from impact of an iron-rich asteroid. It was found that such mixing would generate enough metallic iron so that when the rock cooled through the Curie point, it could record an ambient magnetic field and account for the measured magnetic anomalies. Image credit: G. Kramer. Modeled surface magnetic field overlain on Lunar Reconnaissance Orbiter Wide Angle Camera mosaic of mare Crisium.
Link to article: http://onlinelibrary.wiley.com/doi/10.1002/2017JE005397/full