January 11, 2017
Our Moon could have formed from ‘Moonlets’ merging
Long believed to have been the result of a single object crashing into the Earth, causing a large chunk to break off and enter orbit around the planet, the origin of the moon may be far different and could involve the amalgamation of multiple “moonlets,” according to a new study.
As the New Yorker explained, since the 1980s, scientists believed that the moon formed nearly 4.5 billion years ago after a planet-sized object typically referred to as Theia crashed into Earth. The ensuing large chunk of debris entered orbit, thus becoming the planet’s natural satellite.This explanation was generally accepted, and as the Washington Post pointed out, it did help to explain the moon’s ongoing recession at a rate of approximately 4 centimeters per year. But this model had its problems as well, such as the fact that moon rocks collected by Apollo astronauts had an almost identical composition to Earth, with no chemical trace of Theia whatsoever.
“The whole giant impact model had been put into crisis several years ago, to the point where people thought it might be completely wrong because we couldn't make it work in its details,” University of California, Davis planetary physicist Sarah Stewart told the newspaper last year.
In an effort to help explain things, additional hypothetical elements were added. Perhaps Theia and Earth were chemically identical, or maybe both objects were vaporized after the impact and their ingredients mixed together before they condensed back into solid objects. However, as the Post noted, “every tweak seemed to make the giant impact model even more improbable.”
Exploring a new alternative involving multiple ‘moonlets’
In their new study, Raluca Rufu, a planetary scientist at the Weizmann Institute of Science in Israel, and her colleagues explored an alternative possibility: what if the moon was the result of not a single impact, but several – at least a dozen, in fact – that the object left behind was a conglomeration of the several smaller objects involved in those impacts?
Rufu’s team conducted more than 800 simulations and discovered that it is possible to create a satellite the size of the moon from 20 smaller impacts, each of which would have caused a chunk of the molten Earth to be ejected. Those fragments would slowly coalesce into a “moonlet,” then once enough of these impact took place, the moonlets came together to form the moon itself.
Envisioning a scenario in which the moon is “the product of a succession of a variety of smaller collisions,” the authors explained in their study that each of the smaller collisions caused a debris disk to form around the Earth. This debris then “accretes to form a moonlet. The moonlets tidally advance outward, and may coalesce to form the Moon.”
“We find that sub-lunar moonlets are a common result of impacts expected onto the proto-Earth in the early Solar System,” they added, “and find that the planetary rotation is limited by impact angular momentum drain.” Rufu’s team concluded that “assuming efficient merger of moonlets, a multiple-impact scenario can account for the formation of the Earth-Moon system with its present properties.”
However, as the lead author told the New Yorker, this hypothesis is based on the assumption that the moonlets would have survived instead of being lost or reabsorbed by the Earth. The next step is to investigate the impact-and-accumulation process itself, to determine how the moonlets may have plausibly ended up becoming the modern moon that we all have come to know and love.
Image credit: Hagai Perets; real images of Mars and Ganymede and an artist’s image of a planet courtesy of NASA were used in the picture construction