The bulk composition of the Moon was determined by the conditions in the solar nebula during its formation, and may be quantitatively estimated from the premise that the terrestrial planets were formed by cosmochemical processes similar to those recorded in the chondrites. The calculations are based on the Ganapathy-Anders 7-component model using trace element indicators, but incorportate improved geophysical data and petrological constraints.
A model Moon with 40 ppb U, a core 2% by weight (1.8% metal with
35% Ni and 0.2% FeS) and Mg/(Fe
2++Mg)
0.75 meets the trace element restrictions, and has acceptable density, heat flow and moment of inertia ratio. The high Ni content of the core permits low-Ti mare basalts to equilibrate with metal, yet still retain substantial Ni. The silicate resembles the Taylor-Jake
composition (and in some respects the waif Ganapathy-Anders Model 2a), but has lower SiO
2.
Minor modifications of the model composition (U=30–35 ppb) yield a 50% melt approximating Apollo 15 green glass and a residuum of olivine plus 3 to 4% spinel; the low SiO2, favors spinel formation, and, contrary to expectation, Cr is not depleted in the liquid. There may no longer be any inconsistency between the cosmochemical approach and arguments based on experimental petrology.
Paper dedicated to Professor Hannes Alfvén on the occasion of his 70th birthday, 30 May, 1978.
Unless otherwise stated, this ratio refers to themolar ratio throughout this paper.