Research paper on Origins of the Moon

The age and origin of the moon have been studied with the use of the recently developed short-lived hafnium-tungsten chronometer (182Hf-182W, half-life of nine million years). The tungsten isotopic compositions of 21 lunar samples range from chondritic to slightly radiogenic (EpsilonW = 0.50 +/- 0.60 to + 6.75 +/- 0.42). This heterogeneity may have been inherited from material excavated from Earth and the putative impactor, but it is more likely the result of late radioactive decay within the moon itself; in this case, the moon formed 4.52 to 4.50 billion years ago, and its mantle has since remained poorly mixed.

The most widely accepted model for the origin of the moon is that during the later stages of Earth's accretion the impact of a colliding planet at least the size of Mars generated both the hot debris to form the moon and the angular momentum of the Earth-moon system ( 1, 2). Yet, inconsistencies in this model persist ( 3, 4). For example, it has long been argued on geochemical grounds that most of the material that formed the iron-depleted moon was derived from the silicate Earth after the formation of its core ( 4). This view is disputed by some ( 5), and others have argued, on the basis of their simulations, that the moon must have been derived largely from the mantle of the impactor ( 2, 6). This latter view is considered by some to be inconsistent with the moon's many "Earth-like" features ( 7, 8).

The 182Hf-182W [half-life = nine million years (m.y.)] chronometer can be used to investigate this problem. Because hafnium and tungsten are both highly refractory, planets and planetesimals that formed early in the history of the solar system should have relative proportions of these elements similar to those found in chondrites ( 9-11). However, Hf is lithophile (silicate-loving), whereas W is normally siderophile (metal-loving), such that core formation results in a dramatic intraplanetary fractionation of Hf and W. If this fractionation takes place during the lifetime of 182Hf, the silicate reservoir, which has a high Hf/W ratio, will develop an excess abundance of 182W relative to that found in chondritic bodies.

Impacts after the core has formed can result in entire planetary bodies, such as the moon, with nonchondritic lithophile- to siderophile-element (hence Hf/W) ratios. The W isotopic composition of the moon would be expected largely to reflect that inherited from the particular mixture of silicate-rich debris derived from Earth, and the impactor at the time the moon formed ( 9, 10). However, it is also possible that W isotopic heterogeneity was generated within the moon itself as a result of the decay of live 182Hf. Radioactive decay is an exponential process, such that sufficiently large parent-to-daughter (Hf/W) ratios can still produce observable W isotopic variations at a late stage. It is thought that the earliest history of the moon may have involved a magma ocean, which could have fractionated Hf/W to such extreme values.

The orange glasses display a correlation ...