How did the Moon form? It still remains something of a mystery. Now, a recent paper published in Nature Geoscience has given a new spin to this old debate.
Back at the start of this century it all seemed sorted. Everyone agreed, or nearly everyone anyway, that our Moon was formed when the proto-Earth was struck by a Mars-sized body. This produced a debris disc, dominated by material from the impactor. The Moon later coalesced from this Earth-orbiting disc.
But then the doubts set in.
It turned out that, from an isotopic perspective, the Moon and Earth are near-identical twins. And unless the impactor and proto-Earth had been extremely similar in composition, this evidence seemed at odds with the so-called canonical giant impact model.
One solution to this conundrum invokes extremely high energy impacts. These would have resulted in almost complete mixing between the target and impactor. But this would also have violated the angular momentum constraints that had been such an elegant feature of the canonical model. Although a mechanism by which the system subsequently lost angular momentum has been proposed.
The new model of Rufu et al takes a different tack. They suggest that rather than a single giant impact, the Moon was produce by a series of smaller collisions, perhaps as many as 20. Each of these smaller collisions would have produced a debris disc from which a moonlet might have accreted. Such moonlets would then have moved away from the Earth and finally coalesce to form the Moon. It is suggested that the isotopic problem that bedevils the canonical giant impact is resolved in a multiple impact scenario by a process of blending. In a News and Views on the paper Gareth Collins gives this analogy: “It is rather like mixing colours: the more distinct colours you add, the less change each one makes until the result is dark brown.”
So has this new model finally solved the mystery? Well, probably not. There are still a few outstanding problems.
As Gareth Collins points out, making the Moon from a series of smaller moonlets would have significant implications for lunar structure and provides a way of testing the model. From an isotopic and geochemical perspective, the Moon would most likely be heterogeneous. So far, significant levels of isotopic heterogeneities have yet to be detected in lunar rocks.
Then there is a further implication of the model.
If multiple moonlets formed around the Earth, why not also around the other Terrestrial planets? Venus and Mercury have no natural satellites and the two small moons of Mars, Phobos and Diemos, are often considered to be captured asteroids. Multiple impact origins for Phobos and Diemos have also been put forward, but remain controversial. In 2022 JAXA, the Japanese space agency, is planning to send a probe to Phobos. One of the principal aims of the MMX mission is to return a sample of Phobos to Earth in order to improve our understanding of how it formed, either by capture or giant impact. So paradoxically, if the new theory is correct, by going to Mars we may end up learning more about the origin of our own Moon!