Many mini moonlets merged to make the modern moon – maybe

The moon above may not have formed from a single massive collision between the young Earth and another object, but could have gradually grown as smaller collisions generated many moonlets which clumped together, according to a trio of planetary scientists from Israel.

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The work, led by Raluca Rufu from the Weizmann Institute of Science and published in Nature Geoscience, explains why the moon’s chemical composition is so similar to Earth’s – and not a mixture of Earth and the object that barrelled into it.

The prevailing Giant Impact hypothesis – that around 4.5 billion years ago, our planet was struck by a Mars-sized planetoid called Theia that blasted huge amounts of material into a disc around Earth, part of which coalesced to form the moon – is widely accepted as the moon’s origin story.

Simulations of such a smash show around 80% of the material that formed the moon would have come from Theia, but analyses of Earth and moon rocks show they’ve almost identical chemical composition.

The odds of Theia being made of the same stuff as Earth is possible but highly unlikely – pretty much every object in the solar system has its own chemical fingerprint.

In September last year, US geophysicists found minute differences in heavy and light potassium pointed to an epic smash that vaporised the baby Earth and created a “fog” of material from which the moon and Earth both formed.

To see if there was another explanation, Rufu and his colleagues resurrected a hypothesis from the 1980s: instead of one massive smash, the moon was built as Earth was peppered by lots of smaller collisions. These smaller crashes produced discs rich in Earth debris and which clumped into small moons that, over time, merged into one big moon.

Still, no one knew if these moonlets were big enough to form the current-day moon or how many it would take.

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To test the idea, Rufu and crew conducted nearly 1,000 simulations of planetoids smashing into the early Earth. Moonlets were commonly produced.

And when these collisions were almost head-on, they produces debris discs of debris, many of which were composed of mostly Earth – not impacting planetoid – material.

The discs accreted to form a moonlet that eventually migrated outwards and fused with a growing moon made mostly of Earth material. And to assemble the present-day moon, such moonlet-forming collisions would have taken place around 20 times.

The idea is “appealing from an isotopic composition perspective”, writes Imperial College, London geophysicist Gareth Collins in an accompanying News & Views article, but runs into a few challenges.

While 20 moonlets could feasibly build the moon, some moonlets may have incompletely merged or were lost to space altogether. 

So the likely scenario would need many more of these smaller impacts to create the moon, making “the necessary sequence of events far less probable than any of the more exotic single-impact scenarios”.