It has long been thought that the lunar body resulted from an impact between the early Earth and another planet-sized object 4.5 billion years ago. But this theory predicts Earth and its satellite should have a quite different chemical make-up - and the data shows in fact they are very similar.
Now, new modelling reveals that if the Earth had a much faster spin before the impact, the theory fits the chemistry. It is one idea to fix the composition conundrum explored in two papers published this week by the journal Science. The giant impact hypothesis was first posited in the 1970s. It holds that the Moon formed from the debris kicked into orbit by the collision of a smaller proto-planet with the infant Earth.
But the early models indicated that much of this debris would have originated from the impactor, whose composition would most probably have differed substantially from that of Earth. This is not reflected in the analysis of Earth and Moon rocks – for example, their oxygen isotope, or atom type, compositions are identical.
Matija Cuk, from the Seti Institute, and colleagues ran new simulations in which the early Earth was rotating on its axis in just a few hours prior to the impact - compared to the present 24 hours.