The formation of the Moon remains a fascinating topic, with two recent papers providing some important new insights. A numerical modelling study by Nakajima and Stevenson indicates that, despite the very high temperatures involved in the giant impact, only a minor amount of water would have been lost from the newly-formed Moon. This is important because a number of studies have suggested that the lunar interior was relatively wet.
The conditions that existed immediately after the giant impact have been modelled by Lock and co-authors. They suggest that in the aftermath of the collision between the proto-Earth and a planet-sized impactor, a huge, doughnut-shaped mass of vaporised rock was formed. A structure which they call a “synestia”. Their model involves the formation of the Moon within the extended vapor structure of the synestia. The Moon grows as material rains out onto it from the enclosing hot vapor cloud. Lock and co-authors have undertaken geochemical modelling which indicates that a synestia scenario can explain some of the enigmas of lunar geochemistry, in particular the extremely alkali-poor character of the Moon compared with the Earth.
On Wednesday, I gave a presentation about these two papers at our regular Wednesday research group meeting. The general census seemed to be fairly positive with respect to both. But there were certainly a few issues that remain unresolved. In particular, how come alkalies (e.g sodium and potassium) were depleted in the growing Moon and not water? But then the amount of water in the Moon is still a controversial subject. More research needed, of course!