Lunar asymmetry and palaeomagnetism

Abstract

The compositional asymmetry between the nearside and farside of the Moon and the natural remanent magnetism (NRM) of lunar rocks are poorly understood. The compositional asymmetry is indicated by the 2-km offset towards the Earth of the centre of mass relative to the centre of figure and the concentration of both KREEP and mare basalts on the nearside. Wasson and Warren recently noted that these asymmetries may be better explained by an asymmetric crystallization of a primordial magma ocean than by the often proposed greater thickness of farside anorthositic crust. The NRM5 has been attributed to an ancient lunar dynamo. I propose here a model for the early lunar evolution in which the preferred gravitational energy state consisted of an asymmetric accumulation of a liquid iron alloy (Fe–Ni and a small amount of sulphur) which displaces upwards the cold, primordial, undifferentiated core. The resulting depth asymmetry of the outer partially molten zone leads eventually to the subcrustal accumulation of light, magnesium-rich pyroxenes and olivine, preferentially in one hemisphere, sufficient to explain the offset and also indirectly providing a possible explanation for the nearside concentration of KREEP and mare basalt. Meanwhile, slow downward migration of the iron releases gravitational energy sufficient for convection and dynamo generation in an iron layer for about 10^9 yr. The proposed present state of the Moon has a symmetrically placed iron core (radius ≃ 500 km), unlike a previous model for the lunar asymmetry.