It now appears increasingly likely that the Moon is the offspring of the single most devastating event in the history of our planet.
Three new studies into the formation of our natural satellite, each published in the journal Nature, indicate that a collision between Earth and a very similar, planet-like object roughly 4.5bn years ago set in motion the process that created our orbiting Moon. The new evidence looks set to resolve why the geologies of the Moon and Earth are so similar, when there is no obvious reason why they should be.
This general idea of a proto-Earth collision with another world (the Giant Impact Hypothesis) was first proposed in the 1970s. But it was always assumed that a Mars-sized world (‘Theia’) striking the early Earth would have resulted in two different chemical signatures on each world’s surface. Happily, we eventually made it to the Moon to check. Unhappily, the rock samples gathered by the Apollo missions indicated a stunning similarity — to a difference of just a “few parts in a million”. Explaining how the Giant Impact Theory could co-exist with the realities of Lunar geography has been a puzzle ever since.
In a new, key study, researchers Alessandra Mastrobuono-Battisti (Israel Institute of Technology), Hagai B Perets and Sean N Raymond (Laboratoire D’Astrophysique de Bordeaux) report that they have been able to build a model of the inner solar system so accurate that it can precisely simulate what types of collisions occurred on Earth in its very early history. By studying it, they are able to show mathematically why the Earth and the Moon are so similar, despite the strange mixture of isotope signatures contained in the bodies of the early solar system.
By simulating collisions between 85 to 90 objects, each of about 10 percent of the Earth’s mass, with many more smaller bodies, they demonstrated how the first few hundred years of the Solar System probably played out. After running the simulation many times, they observed that it was generally expected within 100 to 200 million years that three to four rocky planets would form in the inner solar system. There would also be a 20 percent to 40 percent chance that one of these planets would resemble closely the last protoplanet with which it collided — just like the Earth and the Moon. That might sound low, but in fact it’s far higher than had been previously assumed. Their modelled evidence helps to explain how a collision between Earth and an object ten times smaller — but still massive and similar in composition — created the Moon. It gives the Giant Impact Hypothesis a real chance of turning out to be true.
Two further studies, also published in Nature, offer new evidence that after this collision, the Earth collected a larger “veneer” of debris from meteorites than the Moon — which again, scientists always suspected should be the case, but were not able to demonstrate.
Each of the new studies (one based in the United States, the other in Germany) looked at traces of tungsten in rocks gathered by the Apollo crews, and found that there was a small — but significant — difference with levels on Earth. The results suggest that the Moon and Earth were indeed similar prior to impact, but gradually diverged as the Earth’s greater mass pulled more asteroids and other rocks onto its broiling surface.
“It is the first time that we can resolve such a small difference,” cosmochemist Thomas Kruijer at the University of Münster in Germany told Space.com. “Defining this value precisely is a very important step forward.”