Whither Theia?

Another theory of Lunar formation could be about to bite the Terran dust.

Take an interest in such things, and the chances are that you have read magazine articles and seen television documentaries which explain Earth’s moon as being the result of a collision around 4.5 billion years ago between our planet and a smaller body going by the name of Theia, after the minor deity of Greek mythology who was mother to Selene, goddess of the moon.

Artistic depiction of the giant impact hypothesis

This cataclysmic event is known as the “Giant Impact Hypothesis”, with or without the dramatic capitalisation. For several decades it has been the most plausible explanation for the formation of the moon, and the 23.4 degree tilt in Earth’s axis of rotation which gives rise to the seasons.

The moon stabilises Earth’s axial tilt, raises ocean tides, and is gradually slowing the planet’s rotation, thereby lengthening the day. It is also slowly receding from Earth.

The giant impact hypothesis has survived numerous computer simulations, and its basis as a model of planetary formation is physically sound. As with any functional hypothesis, it leads to a predictive theory which can be tested against observation.

Numerical modelling of the Theia collision has shown that no more than 60% of the material in the impact debris which coalesced over time into the Moon can have come from Earth’s interior. The remainder must be from Theia, which we can reasonably assume was geochemically distinct from Earth’s mantle. The isotopic composition of the moon should reflect this chemical mix, and so far it has, according to our best understanding of the situation.

It appears now that Earth and Moon are identical to within 0.0004% in their isotopic compositions of the metal titanium. This comes from a paper by Chicago University geoscientist Junjun Zhang and others. It is not the first time that the giant impact hypothesis has been challenged. Studies of oxygen, silicon, chromium and tungsten have constrained but not been inconsistent with the Theia impact, but the latest results can most easily be explained by assuming that the moon formed almost exclusively from Earth’s mantle. This runs contrary to the Theia hypothesis.

While Theia could have been compositionally similar to Earth, only a few of the known meteorite groups are sufficiently similar in oxygen isotope balance. The new data also rule out an icy Theia with its origins in the far reaches of the solar system. Unless, that is, Theia’s rocks contributed less than 2% to the proto-moon.

Another question concerns the rate of exchange of material between Earth and the accreting impact disk. This could over the aeons have led to the erasure of all remaining isotope differences, but only if the post-impact Earth system cooled exceptionally slowly, or large-scale turbulent mixing was unrealistically large.

Further reading

Zhang et al., “The proto-Earth as a significant source of lunar material”, Nature Geoscience (2012)