Earliest History Of The Earth-Moon System
Authors: Ringwood A E 
Published: November 2023

The thermal and chemical development of the Earth during its accretion is considered, particularly in the context of the single giant impact hypothesis for the origin of the Moon. Impact of a martian­ sized planetesimal during accretion of the Earth would have caused complete or extensive melting of the mantle. Differentiation of a molten mantle would then have produced strong chemical and mineralogical stratification, causing it to become gravitationally stable. The resulting composition and mineralogy of the upper mantle would have been quite different from those that have existed during the past 3.8 Ga. The absence of these fractionations implies that the upper and lower mantle possess similar bulk compositions. The formation of metallic cores within planetary bodies is accompanied by drastic fractionations of siderophile elements. Because of the complexity of the core-formation process within a given body and the multiplicity of chemical and physical processes involved, the resultant abundance pattern of siderophile elements that remains in the silicate mantle is expected to be uniquely characteristic of the particular body. Thus, the siderophile signature of Mars and of the eucrite parent body are quite distinct from that of the Earth’s mantle. Because of the tiny size of the lunar core and the vastly different conditions under which it formed, as compared to the Earth’s core, it would be expected that the siderophile signature of the lunar mantle would be quite different from that of the terrestrial mantle. However, a detailed review of lunar siderophile geochemistry demonstrates that many moderately siderophile elements possess similar abundances in the terrestrial and lunar mantles. This similarity implies that a major proportion of the material now in the Moon was derived from the Earth’s mantle after core formation. Chromium, vanadium, and manganese are of particular significance since these elements are depleted in the terrestrial mantle and Moon to similar degrees, but are undepleted in the mantles of Mars and the eucrite parent body. High-pressure experimental studies show that Cr, V, and Mn are lithophile under conditions of core formation in a martian-sized planetesimal. However, at the much higher pressures accompanying core formation in the Earth, Cr and V ( and possibly Mn) become siderophile. Thus, the depletions of these elements in the Moon are consistent with derivation of protolunar material from the Earth’s mantle, but not with its derivation from the mantle of a martian­ sized planetesimal. An assessment is made of the single giant impact hypothesis, which maintains that the Moon was formed as a byproduct of the impact of a martian-sized planetesimal upon the Earth, and that this event was also responsible for the high angular momentum of the Earth-Moon system. This giant impact must have occurred very late during Earth’s accretion, otherwise the Moon would be contaminated with siderophiles to a far greater extent than is observed. However, numerical simulations of the Earth’s accretion indicate that most giant impacts occur relatively early, and also indicate that the probability that a giant impact occurred both very late and with an impact parameter capable of producing the angular momentum of the Earth-Moon system is less than 1 in 300. The single giant impact model is also incapable of explaining the siderophile geochemistry of the Moon and the lack of evidence for total melting of the Earth’s mantle at a late stage of accretion. It may be possible to avoid these difficulties by relinquishing the requirement that a single giant impact was responsible for both the angular momentum density of the Earth-Moon system and the origin of the Moon. A giant impact by a planetesimal ( e.g., 0.2 ME) occurring when the Earth had accreted to about 60% of its present size could provide the required angular momentum. It would also cause melting and differentiation of the mantle. However, subsequent accretion of the remaining ---0.2 ME in the form of small, dense, Fe-rich planetesimals could cause convective rehomogenization of the mantle, without extensive melting. Protolunar material may have been ejected into orbit from the Earth’s mantle at a very late stage of accretion by collisions of relatively small high-velocity (20-30km/sec) icy planetesimals scattered by Jupiter from the outer solar system. The Moon may have accreted from the family of moonlets thereby placed in Earth orbit.