What is the currently-accepted theory for how the moon was formed?

Submitted by CopperGenie t3_zv61wc in askscience

In 1976, the "Big Whack" model was proposed that combined aspects of three other incomplete models (intact capture, co-accretion, and fission). The Big Whack model proposed that a planetoid impacted the Earth while still in a semi-molten phase, scattering particles into Earth's orbit. These particles then coalesce into a body that is now the moon. Is this still the currently-accepted theory, and has any new evidence been revealed since then to prove/modify it?

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sciencedthatshit t1_j1oa4ny wrote

Geologist here...yep, the impact theory is still the accepted theory on the formation of the moon.

Since the 70s, better computer modeling, improved geophysical studies of the earth and moon and isotopic evidence have added pretty convincing evidence in support of the impact hypothesis.

The wikipedia article is pretty good.

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dukesdj t1_j1q1xaa wrote

It should be noted that despite it being the well accepted theory there are still many unknowns around the exact details of the process (Cattaneo and Hughes 2022).

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YujiroDemonBackHanma t1_j1q8l7r wrote

Question... if something external hit the planet, ejecting debris that eventually formed into the moon, why is the moon so far away? Or, shouldn't the ejected pieces eventually fall to the planet and merge with earth again?

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Game_Minds t1_j1qh57r wrote

The moon is actually really, really close in cosmic terms for a moon of its size relative to earth. Saturn and Jupiter's moons are much smaller AND much farther away. and it is in fact slowing down veeeeeeeeery marginally over time, so if the sun didn't explode eventually it might make it back to earth

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sciencedthatshit t1_j1r3hsr wrote

The moon is slowing migrating away from Earth, not towards it. And, to be pedantic, stars like the sun don't explode. They swell, puff off outer layers and then shrink into a white dwarf.

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Game_Minds t1_j1r5o47 wrote

Ahhh, okay. I remembered the 1.5 inches per year but in the wrong direction-- I think i mixed up decaying orbits / Lagrange points for closer satellites that are impacted by atmospheric friction. I had forgotten that the weird phenomena of rotation and tidal gravitational forces combine to speed up the moon's orbit, functionally the same thing as escaping. And yes, explode was a convenient fill in word for "expand into a red giant, assimilating most of the inner bodies in the solar system, then shed the outer layers in a not-quite-explosion, leaving a white dwarf core", but you're right, that's incomplete

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zekromNLR t1_j1v2qkw wrote

A recent simulation suggests one possible way is that the impact debris initially coalesced into two bodies. One of those did indeed fall back into Earth, but the other one, due to momentum exchange with the first one, got enough momentum to get a stable orbit. And from there, tidal interactions slowly transferred momentum from the Earth's rotation to the Moon's orbit, widening its orbit and slowing down the rotation.

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superbob201 t1_j1p44qw wrote

Still the generally accepted answer. One fairly recent modification is that it looks like Theia hit Earth twice. Still not a perfect theory: some isotope ratio studies don't quite match the model, and our computer models of the collision suggest Theia was going slower than we would reasonably expect (Which itself could mean that Theia started life as an Earth-Trojan

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Game_Minds t1_j1qi2wm wrote

I've seen some other stuff recently that suggests the other inner planets like Mars and Mercury have slightly unexpected composition, like Earth and the moon. This has led to speculation that there was potentially a "billiard ball era" where chunks of protoplanets were smashing into each other, distributing heavier elements more evenly across the inner planets, and explaining the traces here and there that don't line up with Theia. A rogue body like a pluto- sized comet passing through and impacting one protoplanetary mass could lead to a whole series of impacts. There's still a ton of theories about the asteroid belt, and a bunch of new studies on the composition of the larger bodies in it. Exciting stuff!

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