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^1–6, and several interesting suggestions have been made concerning the origin of such high magnetic fields during the early history of the Moon. Runcorn and Urey⁷ 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.⁵ 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.