The moon was formed 4.6 b.y. ago at about the same time as the earth and meteorites. At the time of accretion, temperatures probably were below the solidus in the deep interior and at, or above, those of the liquids in the outer part. The material that formed the outer part of the moon was richer in some refractory elements and poorer in iron and some volatile elements than was the earth. From 4.6 to about 4.0 b.y. ago, a lunar crust about 65 km thick was formed; this highland crust is rich in anorthositic gabbro and other rocks of anorthositic affinity. During the period of highland formation and differentiation, the moon was bombarded heavily by asteroid-size bodies and circular mare basins were formed by the impact. The heavy bombardment caused considerable mixing of r ck types and breccia formation. Pristine highland rocks and radiogenic clocks were reset during this period, so that crystallization ages greater than 4 b.y. are rare. Partial melting at depths between 150 and 400 km occurred from about 3.8 to 3.1 b.y. ago and produced pyroxenite basalts that filled the mare basins predominantly on the lunar front side.

Controversy exists as to whether the alumina-rich lunar crust is accretional in origin or was developed by widespread differentiation process in which a pyroxene-rich upper mantle and alumina-rich crust were formed. Surface magmatic activity virtually had ceased by 3 b.y. ago.

Lunar heat flow is about half that of earth; the bulk of potassium, uranium, and thorium is concentrated in the lunar crust. From 4 to 3.2 b.y. ago, the lunar surface was exposed to a magnetic field of at least 2,000 gammas, which was produced either externally or internally.

Seismic data indicate that the mantle is ultramafic (velocities 8-9 km/sec) and presently is rigid at temperatures below the solidus to a depth of 800 km. Below 1,000 km the mantle is partly molten and a small metal-rich core may exist. The moon has a bulk density of 3.35 g/cu cm and a coefficient of moment of inertia of about 0.4.

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