Geochemical constraints on the origin of the Moon

Hypotheses for the origin of the Moon involve variants on capture, double-planet, and fission processes. Double-planet and fission hypotheses are examined in the light of siderophile trace elements. The siderophile trace elements chosen (W, Re, Mo, P, Ga, Ge) have well understood geochemical behavior such that appropriate metal/silicate partition coefficients are available and their abundances in the lunar and terrestrial mantles 4.4–4.5 × 10⁹ years ago may be reasonably inferred. The fission hypothesis of Ringwood (1979) is not consistent with the behavior of Re, Mo, and P. The hybrid fission hypothesis of Wankeet al. (1983) overcomes many of the deficiencies of ringwood's hypothesis, but is not readily reconcilable with the behavior of Re and Ir. The double-planet hypothesis as most recently advanced by Newsom and Drake (1982, 1983) appears to be consistent with siderophile element behavior in the Moon.