Some 4.5 billion years ago, an object the size of Mars smashed into the embryonic Earth. This devastating collision created a debris field around our young planet that eventually coalesced into the Moon.
This idea, called the giant impact hypothesis, has accumulated compelling evidence, but there are still missing pieces of the puzzle. For instance, it does not fully account for the near-identical isotopic signatures of Earth and the Moon (meaning they are made of the same material). If a Mars-sized impactor really did wallop our planet during its infancy, it's odd that it didn't contribute more of its own debris to the fallout.
A study published on Monday in Nature Geoscience suggests that this incongruity could be explained if the Moon was formed as a result of around 20 smaller impacts, instead of one colossal dust-up between worlds.
A team led by Raluca Rufu, an astrophysicist based at the Weizmann Institute of Science in Rehovot, Israel, ran 864 simulations of the infant Earth getting bombarded by objects much smaller than Mars over a period of millions of years. The results showed that such impacts could produce debris fields that in turn fuse into tiny "moonlets," objects no more than ten percent the mass of the Moon. These mini-moons could gradually migrate away from Earth and become building blocks of the developing satellite.
According to Gareth Collins, a planetary scientist based at Imperial College London and the author of a News & Views article accompanying the new research, Rufu's moonlet hypothesis resolves the isotopic problem because "the Moon becomes a blend of multiple compositional signatures, rather than two, and so the effect of each impactor on bulk composition is reduced."
"It is rather like mixing colours: the more distinct colours you add, the less change each new one makes until the result is dark brown," Collins said.
The paper posits a promising alternative to the giant impact hypothesis, but it will take much more evidence to determine which scenario is more likely to have forged the Moon.
For her part, Rufu plans to run further simulations with a tighter focus on the process by which moonlets might accrete into a proto-Moon. "We will test the merging process of these moonlets to understand what is the mixing efficiency of individual moonlets within the final Moon," she told me over email.
In addition, lunar rocks collected from different regions of the Moon could shed more light on our natural satellite's origins. Fortunately, lunar sample return missions are back in style, with China set to use its Chang'e 5 mission to send home some Moon samples by the end of 2017. Hopefully, it will be the first of many 21st century missions to bring back these lunar keepsakes, which are essential to understanding our planetary history.
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