The natural remanent magnetization (NRM) of the Apollo and Luna rocks gave evidence for an early core dynamo field, palaeointensity measurements show that it decayed exponentially from 1 G 3.9 b.y. ago to about 0.02 G 3.2 b.y. ago. The nonhydrostatic figure of the Moon is explained by a solid-state convection of second-degree harmonic pattern implying the existence of an iron core. This 2 cell convection develops only if a substantial part of this heat flow comes from the inner boundary: the same requirement for fluid convection in the core. That the time over which a field was generated was the same as the volcanism which flooded the mare basins is probably no accident; both imply substantial heat transport through the lunar mantle. Palaeomagnetic directions derived from the Apollo 15 and 16 sub- satellite magnetic anomalies yield 3 pole positions for Pre-Nectarian, Lower Nectarian and Upper Nectarian-Imbrian ages. Their palaeoequators fall close to the multi-ring basins of corresponding ages. Because the Coriolis force was a dominant term in core magnetohydrodynamics, the dipole axis would have been aligned with the rotation axis. The 3 pole positions form a 90 degree spherical triangle, which is only explainable if the Moon had 3 satellites, each of which disintegrated at the Roche limit resulting in a number of impact basins near the equator, which, because tidal friction is inversely proportional to the 6th power of the distance, occurred within a relatively short interval of time. The subsequent reorientation of the Moon through 90 degrees occurred over a time dependent on its solid-state creep parameters. It follows that the multi-ring basins were formed by low velocity projectiles; some evidence for this is that many of them show asymmetries from which the velocities of the impacting bodies can be found. They are roughly parallel to the corresponding palaeoequators. Reorientation of the Moon along its present axis would appear to have resulted from the lava flooding the mare basins with the formation of the mascons. While lunar palaeomagnetism has provided a consistent picture of early lunar dynamics key questions remain: What was the source of energy which maintained the dynamo field until some time after 3.2 by ago? Can the mechanics of low velocity impacts result in the negative contribution to the moment of inertia tensor required for "polar wandering?" Does the regolith contain information about the chemical composition of the satellites? Are there clues to the origin of the Moon?
- Pub Date:
- July 1992