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You know how cop cars pile into each other in old comedy movies? That's how the Moon was built, say boffins

Moonlets glommed together

The birth of the Earth’s Moon didn’t begin with a single huge collision – rather it grew as lots of baby moons from smaller impacts fused together, according to a new theory published in Nature Geoscience.

Scientists believe that a proto-Earth and a Mars-sized body smashed together in the earlier days of the Solar System to create Earth’s only satellite. The leading theory, however, doesn’t explain why the Moon is mostly made up of Earth-like material, rather than a jumble of stuff from Earth and the other colliding body, called Theia.

Of course, the protoplanet that smacked into the young Earth could have had similar composition to our home world. Although that scenario is possible – some scientists suspect that Theia formed near Earth – researchers from the Weizmann Institute of Technology and the Technion–Israel Institute of Technology believe that the chances are slim.

That leaves the possibility that lots of relatively minor collisions between space rocks and Earth kicked up enough material to form baby moons that slammed into each other like the police in The Blues Brothers to create the Moon as we know it.

“Deriving more disk material from the proto-Earth occurs in impact scenarios with increased angular momentum beyond the present value, which is later dissipated by an orbital resonance or an associated limit cycle,” the paper [paywalled] said.

After a series of simulations modelling the collision, the impacts leave debris rings of Earth-like material instead of impactor material. Out of the 864 simulations ran, 750 resulted in debris disks with a “discernible mass.” The biggest disk was 1.2 times larger than the mass of the Moon.

The trail of debris grows with each impact to form something called a moonlet. Tidal forces from the Earth cause the moonlets to migrate outward to merge with a larger proto-Moon. It would take approximately 20 moonlet-forming impacts to make the Moon.

“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, coauthor and assistant professor at Technion.

Getting rid of the single giant-impactor event means that strict constraints on the debris disk mass and angular momentum can be released.

“Freedom in impact geometry and velocity allows mining more material from Earth, and the sum of such impact-generated moonlets may naturally lead to the current values of the Earth Moon system,” the paper noted. ®

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