The early magnetic field and primeval satellite system of the Moon: clues to planetary formation

Abstract

From the stable remanent magnetization of the Apollo igneous rocks and high-grade breccias the existence of a primeval lunar magnetic field was inferred. The palaeointensities of the samples rise rapidly to a maximum at 3.9 Ga, then decrease exponentially to 3.2 Ga, strongly suggesting that the Moon had a field generated in a core, the existence of which was inferred from its non-hydrostatic figure. Modelling of the Apollo 15 and 16 subsatellite magnetic anomalies, by P. J. Coleman, L. L. Hood and C. T. Russell, gave palaeomagnetic directions of crustal strata. This enabled N pole positions to be calculated, which were empirically found to form three bipolar groups, the mean poles of which define (on the core dynamo hypothesis) three axes of rotation different from the present. These were dated as Pre-Nectarian, Lower Nectarian, and Upper Nectarian-Imbrian. Multi-ring basins of these ages were found to lie close to the corresponding palaeo-equators. The impacting bodies were therefore satellites, not asteroids or comets. Their velocities, before collision, can be shown (from basin asymmetries) to be nearly equatorial. The consequent changes in the moment of inertia tensor by basin formation caused these successive reorientations of the Moon relative to its axis of rotation in space. The three mean poles form a 90° spherical triangle. The explanation is that the Moon had three satellites: the orbits of each decayed, they broke up at the Roche limit into smaller bodies, which produced impact basins near the equator. The Moon then reorientated according to Euler’s principle before the next group of impacts. Lunar palaeomagnetism, and especially the inferences that the Moon has an iron core that segregated late and had a primeval satellite system, may provide important constraints on theories of lunar and planetary formation.