Our Moon May Have Eaten Many Smaller Moons

Over four billion years ago, when Earth was an asteroid-pummeled mess, it's believed that another planetary body the size of Mars - a small hypothetical world called "Theia" - careened into our baby planet, causing the mother of all impacts. From this collision, molten rock was ejected into space and some of the mixed-up Earth-Theia debris solidified to create the moon that we know and love today. But say if there's an alternative moon-formation scenario that doesn't require a singular cataclysmic impact?

Back then, in the solar system's formative years, massive impacts were common as planetary bodies gravitationally jostled each other's orbits. Eventually, the solar system stabilized into relative calm and planets settled into the orbits we see today, but not after this chaos left its mark. The many moons throughout the solar system were formed around this time - some came directly from impacts on the planets they now orbit, others were once asteroids (or exiled moons from other planets) that were captured by the gravities of their now-adopted planetary parents. In the case of Earth, which was rapidly gaining mass from countless asteroid impacts during this tumultuous time, massive impacts were common.

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In research published on Monday in the journal Nature Geoscience, these multiple impacts may have created many moons, which eventually coalesced to create The Moon. Therefore, a massive Earth-Theia impact event probably isn't required.

"Our model suggests that the ancient Earth once hosted a series of moons, each one formed from a different collision with the proto-Earth," said Hagai Perets, of the Technion-Israel Institute of Technology, in a statement. "It's likely that such moonlets were later ejected, or collided with the Earth or with each other to form bigger moons."

This alternative scenario assumes that during our planet's formation, it experienced many massive impacts, each kicking debris into orbit that went on to collect under mutual gravity, forming mini-moons or "moonlets." As each new moonlet formed, it settled into orbit and slowly migrated outward. Then, another impact would kick up new debris into orbit, forming another moonlet. These newer moonlets would have a gravitational influence on the older moonlets orbiting further away and their mutual gravity would cause some destabilization. Some moonlets would be flung away, whereas others would fall back to Earth. But others would merge, creating a growing moon that would form the basis of the moon we have today.

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"It's likely that small moons formed through the process could cross orbits, collide and merge," said Raluca Rufu, of the Weizmann Institute of Science in Israel.

This chain of impacts appears to agree with our understanding of early-Earth formation models and doesn't assume that it was just one cataclysmic impact that formed a single moon. "A long series of such moon-moon collisions could gradually build-up a bigger moon [forming] the Moon we see today," added Perets.

The formation of the moon is a hot topic in astrogeology and the idea that the Earth's only permanent natural satellite was formed by a single massive impact is supported by various lines of evidence. However, many mysteries remain and simulations that assume many impact events gradually contributed to the moon's growth will no doubt be seen as a compelling alternative idea.

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