The Moon-Forming Giant Impact – going, going, gone!

The currently accepted view of how our Moon formed is known as the giant impact hypothesis. The most recent formulation of this model developed by Robin Canup of the Southwest Research Institute, Colorado, is largely in agreement with previous studies. It suggests that the Moon formed from the debris disc created when a Mars-sized body struck the early Earth. This event probably took place between about 50 and 150 million years after the Solar System was formed.

One consistent feature of modern simulations of the giant impact event is that at least 60% of the Moon is made from material that came from the Mars-sized impactor. So, as most large Solar System bodies are believed to have fairly distinct compositions, the Moon ought to be significantly different from the Earth. But there is increasing geochemical evidence that such differences do not exist. The extent to which this geochemical reality contradicts the predictions of the giant impact model is leading an increasing number of researchers to question the validity of the model itself.

The first significant evidence that the Earth and the Moon were much closer in composition than predicted by the giant impact model came from oxygen isotope measurements, which showed that lunar and terrestrial rocks are essentially identical at the present levels of analytical uncertainty. Silicon isotopes in the Moon and Earth are also very similar in composition. More recently the composition of Titanium isotopes has been found to be identical in the two bodies.

These results now raise significant questions about the validity of the giant impact model. In an abstract presented at the Lunar and Planetary Science Conference earlier this year van Westrenen and co-workers concluded: “Moon formation models have to be consistent with lunar chemistry. Current versions of the giant impact model are not. Alternative models in which the Moon is formed from terrestrial material deserve more detailed study. We provide one (radical) alternative hypothesis –we hope others will follow”. The model presented by van Westeren and co-workers is certainly radical. They suggest that the Moon was essentially produced as the result of a “deep nuclear explosion”.

Less controversial alternatives to the current giant impact model are suggested by Zhang and co-workers. They return to models based on formation of the Moon by fission from a rapidly rotating Earth or alternatively impact from an icy body that delivered water but minimal amounts of rocky material. While such models currently lack numerical precision they demonstrate that scientists are beginning to search for alternatives to the giant impact model.

As usual the ever excellent PSRD Discoveries website provides an in-depth perspective written by Jeff Taylor on the implications for the giant impact hypothesis of the new Zhang et al. (2012) Ti-isotope data.

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