Geochemical evidence for a close genetic relationship of Earth and Moon

Samples from Earth and Moon plot exactly on the same oxygen isotope fractio nation line. The abundance of the now extinct manganese-53 was identical fo r Earth and Moon but varied for other objects with the distance from the Su n. The composition of the Earth's mantle is the result of the complex accre tion history of the Earth. Very special features of the Earth's mantle are the depletion of V, Cr, and Mn and the high abundances of Ni and Co. Simila rly V and Cr and Mn are low on the Moon and Ni and Co are high. The depleti on of the moderately siderophile element W in the silicate phase of the Moo n requires equilibration with metallic iron in an amount exceeding what is permissible for the Moon. The most plausible model for the formation of the Moon is an impact induced ``fission'' of the proto-Earth. Such a model can also easily account for t he depletion of moderately volatile and volatile elements on the Moon by lo ss of these elements during recondensation and accretion of the Moon in Ear th orbit. The higher FeO content of the Moon might be explained by addition of material from the impactor.