How Earth got its moon: 2 new ideas compete

earth, earth's moon, moon theories, science news

One theory suggests series of small impacts followed by merger, another proposes single hit by mass of Earth-like composition.

The moon has long held on to one of its most sought-after secrets: when it was formed, and how. Various theories have abounded over the centuries but none of them has been conclusively proved; many, in fact, have fallen out of favour because of various inconsistencies. Two new theories, each independent of the other, have now been published, almost back to back. One of them questions a prevailing idea while the other seeks to address inconsistencies in that idea.

Until now, a leading theory has been that the moon was a result of a collision between a Mars-like object and Earth, which sent debris from both bodies into space, where the parts eventually merged to form the moon. If that were true, however, the composition of Earth and the moon should have been different; instead, studies of the moon’s rocks have shown an almost identical match with Earth rocks.

One of the two new studies, published in Nature Geoscience, now proposes that the moon was formed with material that came largely from Earth itself. Research student Raluca Rufu and Prof Ohed Aharaonson of the Weizmann Institute of Science, Israel, along with Dr Hagai Perets of the Technion-Israel Institute of Technology, suggest that billions of years ago, a number of small objects collided with Earth at high velocity, over a period of millions of years. Such small collisions could mine more material from Earth than a single large collision, say the scientists, who carried out simulations to suggest that the debris agglomerated into small moonlets that, in turn, merged to become the moon.

Weeks after this report came the other study, published in Nature. Going back to the single-impact theory, Prof Nicolas Dauphas of the University of Chicago seeks to explain the similarity in the composition of Earth and moon rocks, based on a study of the abundance of oxygen isotopes, the yardstick for determining a rock’s origin.

“The moon is isotopically similar to the Earth,” Dauphas is quoted on his university’s website. “Therefore the giant impactor that struck the Earth soon after it was created, thereby forming the moon, most likely had a similar isotopic composition to the Earth.”

Asked about the competing ideas, Rufu said her multiple-impact theory is a more natural way to explain the moon’s formation. “We do not require a specific impactor with a specific velocity, but a chain of natural processes,” she told The Indian Express by email. “We know that Earth experienced several of those impacts, but these were overlooked as each impact could not have formed the moon. We are the first group to dynamically test these smaller impactors and also to see their impact on the planet.”

Neither Dauphas nor his university had responded to emails when this report was being written. Dauphas’s research, as detailed on the website, involved deciphering the isotopic nature of the material that formed Earth. Along the way, he found that a rare type of extraterrestrial material, known as enstatite meteorites, formed half of the first 60 per cent of Earth; after that, the rest of Earth was formed entirely by enstatite-type impactors.

“By studying high-precision measurements, we have shown that Earth, the moon and meteorites with a high concentration of the mineral enstatite have almost indistinguishable isotopic compositions,” the website quotes him.

“Dauphas’s paper, which was published right after our paper, suggests that the last 40% of material accreted by Earth is from a distinct family, named enstatite meteorites,” Rufu said in reply to a question. “He performed chemical modelling… It would be interesting to find a dynamical solution why the last building blocks were all from the same family.”

Current dynamical models, she noted, suggest a chaotic mixing between the terrestrial bodies. “Probably, the dynamical models do not incorporate important physical processes in the protoplanetary disk,” she guessed. “Either way, assuming this is true, this will only strengthen our conclusion by increasing the success rate of forming the moon, but it may be harder to prove whether you will need several impacts to form the moon or only one is enough.”

Historically, theories about the moon’s formation have been hard to prove. “Scientists have been heard to mutter that if the evidence for the moon’s origin is carefully considered, then the only possible conclusion is that the moon is not really out there,” the late Isaac Asimov wrote in Asimov’s New Guide to Science, where he discussed a number of older theories and their inconsistencies. Yet this conclusion, he asserted, only meant that the search must continue. “There is an answer,” he wrote, “and it will be found.”