View of the Moon’s eastern limb and far side taken by Apollo 16 (Image: NASA)

It’s now over 40 years since the first lunar samples were returned by Apollo 11. Almost immediately after their arrival on Earth these precious materials were subjected to an unprecedented level of scientific scrutiny. By all accounts the first Lunar Science Conference, where the initial results of this work were announced, was a tense, but exciting event. Nothing like it had been seen before.

You might imagine that with this level of scientific activity, there would be little left to do on lunar rocks. You might also think that in the years since Apollo all the outstanding questions concerning the origin of the Moon would have been done and dusted. Amazingly, nothing could be further from the truth. In fact, with each passing year the mystery of just how our Moon formed gets increasing perplexing. Here’s how the current state of play stands……

There are lots of theories about how the Moon formed, but only one currently explains the main dynamic characteristics of the Earth-Moon system. This is the well-known 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 here is the really strange thing. In a number of important ways the Earth and the Moon are not just twins, they are identical twins.

Oxygen is a major constituent of both the outer layers of the Earth and Moon. In agreement with previous studies, two recent investigations of lunar oxygen isotopes by teams at The Open University and in the US (led by Yang Liu, University of Tennessee) both show that the Moon and the Earth have identical compositions. The study led by Yang Liu also showed that the lunar and terrestrial mantles have overlapping iron isotope compositions. Silicon isotopes in the Earth and Moon also seem to be identical in composition.

One possible way out of this dilemma was suggested by Kaveh Pahlevan and Davis Stevenson of the California Institute of Technology, who suggested that following collision high temperature equilibration took place effectively obliterating any isotopic differences between the early Earth and the Mars-sized impactor. But objections to this possibility have been raised by H Jay Melosh of the University of Arizona, who suggests that such equilibration would effectively destroy the dynamic characteristics of the system.

So, while an impact origin for the Moon seems to fit most of the evidence, doubts persist. It would seem that after 40 years of study it’s a case of more work needed.

To see what lunar rocks look like under the microscope why not visit the OU-NASA Virtual microscope