Constraints on the nature of the ancient lunar magnetic field

Abstract

Many lunar rocks possess a stable component of natural remanent magnetisation (NRM) with specific intensities around 10^-6 e.m.u. g^-1 (refs 1-6). Most of them also have compaction ages of more than 3 × 10⁹ yr (refs 5 and 6). Ancient lunar surface magnetic fields of the order of 10^-2-1.2 gauss have been postulated to explain the observed NRM in lunar rocks, and several interesting suggestions have been made concerning the origin of such high magnetic fields during the early history of the Moon. Runcorn and Urey have proposed an intrinsic dipolar field, acquired by the Moon during its initial stages of formation. Such a dipolar field could result either from a primaeval magnetisation of the Moon by an external field or from the action of an ancient lunar dynamo. Collinson et al. supported that theory with the observation that the palaeofield required to explain the stable NRM in lunar rocks decreases with the decreasing formation or compaction ages of the rock samples. I point out here some constraints that can be put on the origin and nature of the ancient lunar magnetic field, based on the observation of solar-wind ions in lunar breccias with compaction ages of more than 3.2 × 10⁹ yr.