Lunar asymmetry and palaeomagnetism

The compositional asymmetry between the nearside and farside of the Moon, 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, and the natural remnant magnetism of lunar rocks, are considered. A model is proposed 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 109 yr. (U.K.)