Our knowledge of how Earth's natural satellite formed is increasingly being challenged by observations and computer simulations. Two scientists outline our current understanding from the point of view of the satellite's geochemistry and its early dynamical history.
A chip off the old block
Not since NASA's scientists definitively announced that the lunar white stuff was non-dairy has the Moon faced such an identity crisis. Ironically, it seems that our satellite is compositionally too similar to Earth for a simple explanation of its origins. Most dynamical and even some chemical attributes of the Earth–Moon system have been successfully explained by a 'giant impact' scenario, in which a Mars-sized impactor collided with the proto-Earth. Yet the standard version of this model produces a Moon that is mainly made of the impactor and not the target (Fig. 1). As emphasized in a Royal Society meeting1 in September that debated the origin of the Moon, the compositional differences between Earth and the Moon that would be expected as a consequence are increasingly at odds with diverse, high-precision isotopic observations.
The isotopic kinship of Earth and the Moon was initially apparent in their indistinguishable oxygen isotope ratios2, which contrasted with analyses of meteorite samples from most other planetary objects in the Solar System. The dilemma of this matched isotopic composition has deepened with more-recent measurements — notably, analyses of tungsten3 and silicon4 isotopes, which are controlled by very different processes from oxygen.
The radiogenic-tungsten isotope ratios of different planetary mantles should vary because they record the stochastic growth of the parent bodies and the formation of their cores. For the impactor and the proto-Earth to have the same oxygen isotope ratio is unlikely2, but for them also to have the same tungsten isotopic composition is highly implausible. The distinctive silicon isotopic composition of Earth's silicate mantle reflects the consequences of silicon sequestration by a core formed at high temperatures on a large planetary body. Despite its moniker, the Mars-sized impactor of the standard giant-impact model is not large enough to have experienced conditions that would generate an Earth-like silicon isotope ratio. Thus, differences in oxygen, tungsten and silicon isotope ratios between target and impactor seem inevitable, and so the standard model predicts isotopic differences between Earth and the Moon that are not observed.
These various isotopic embarrassments might potentially be explained away by rapid isotopic re-equilibration of Earth and the Moon in the vapour-rich aftermath of the Moon-forming collision5. But recent work has shown6 that the isotopic similarity of the two bodies extends to refractory elements such as titanium, which should not remain in the vapour phase long enough to allow such re-equilibration.
New dynamical models that can produce the Moon from the proto-Earth do not have the inherent simplicity of the canonical giant-impact scenario, and some argue that there are crucial flaws in such models. The sequence of conditions that currently seems necessary in these revised versions of lunar formation have led to philosophical disquiet. From a naive geochemical perspective, however, the isotopic similarity of Earth and the Moon holds an obvious appeal; the proto-Earth represents an abundant local source of material from which to build the Moon. Whether or not this comfort of availability can be meshed with the rigours of celestial mechanics remains to be seen.
See Comment p.27