Study crashes main Moon-formation theory

January 9, 2017
This is a composite image of the lunar nearside taken by the Lunar Reconnaissance Orbiter in June 2009, note the presence of dark areas of maria on this side of the moon. Credit: NASA

The Moon, our planet's constant companion for some 4.5 billion years, may have been forged by a rash of smaller bodies smashing into an embryonic Earth, researchers said Monday.

Such a bombardment birth would explain a major inconsistency in the prevailing hypothesis that the Moon splintered off in a single, giant impact between Earth and a Mars-sized celestial body.

In such a scenario, scientists expect that about a fifth of the Moon's material would have come from Earth and the rest from the impacting body.

Yet, the makeup of the Earth and the Moon are near identical—an improbability that has long perplexed backers of the single-impact hypothesis.

"The multiple impact scenario is a more 'natural' way of explaining the formation of the Moon," said Raluca Rufu of the Weizmann Institute of Science in Rehovot, who co-authored the new study published in the journal Nature Geoscience.

Such multiple hits would have excavated more Earth material than a single one, which means the moonlets would more closely resemble our planet's composition, said the study authors.

Rufu and a team created nearly a thousand computer simulations of collisions between a proto-Earth and embryonic planets called planetesimals, smaller than Mars.

Every collision would have formed a disk of debris around the proto-Earth which would, in turn, clump together to form a "moonlet", they found.

Moonlets would eventually coalesce to form the Moon.

"In the early stages of the Solar System, impacts were very abundant, therefore it is more natural that several common impactors formed the Moon rather than one special one," Rufu told AFP.

Discarded theory revived

Our Solar System is thought to have formed 4.567 billion years ago, followed by the Moon about 100 million years later.

Numerous "impactors" would have excavated more Earth material than a single one, which means the moonlets would more closely resemble our planet's composition, said the study authors.

About 20 such crashes would have been required to build the Moon, they concluded, while conceding that further study is needed into the mechanics of Moon formation from "moonlets".

The giant-impact hypothesis was first proposed in the mid-1970s, followed in the 1980s with the first suggestions that several collisions may have given the Earth its tide-creating satellite.

The latest study has "revived the hitherto largely discarded scenario that a series of smaller and more common impacts, rather than a single giant punch, formed the Moon," Gareth Collins of Imperial College London wrote in a comment carried by the journal.

"Building the Moon in this way takes many millions of years, implying that the Moon's formation overlapped with a considerable portion of Earth's growth," he added.

Explore further: Image: Mars Reconnaissance Orbiter captures the Earth and its moon

More information: Nature Geoscience,

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1.6 / 5 (5) Jan 09, 2017
They said the moon must have come from a collision, because the compositions of earth and moon are so alike. Then they said the compositions were too similar, and a collision would have left unusual concentrations of elements fro the other body. They said, if the moon formed from accretion, it would be at the equator, not tilted to the equator, but the collision story has the moon forming from the accretion of a cloud of debris, which likely would have been pulled toward the equator. And, among other things, there is still the issue that a collision with a Mars sized body would have thrown the earth off its orbit. As likely universal as such meteorite collisions as mentioned in the article were, why didn't they create huge moons for Venus and Mars?
3.4 / 5 (7) Jan 09, 2017
Good question about Venus and Mars lacking an Earth-sized moon, Julian. But if the scenario they posit is correct I think it ups considerably the chances of finding Earth-like planets with Moon-sized satellites providing the sort of stability that evolution needed to generate our vast assortment of life forms. I would think that solar system evolution is similar across most of the universe.
4.3 / 5 (7) Jan 09, 2017
Julian, do we know that earth was not in fact thrown off its orbit? Can we tell, with confidence, that the earth has been in its current orbit since its creation?
5 / 5 (2) Jan 10, 2017
There is a theory that Venus had a large moon, that it was destroyed by a subsequent collision, and that the combination of the two collisions is why Venus rotates backwards.

As for Mars, it definitely had very large collisions, as evidenced by the Hellas Basin, among other structures. Maybe they occurred at the wrong times, so the debris from one either dissipated or reentered before the next. Maybe, with a lower escape velocity, the debris didn't stick around. Maybe it's just unlucky.
3 / 5 (4) Jan 10, 2017
If a single large impactor had the same isotopic composition as Earth, then the subsequent moon would also have a similar composition. If an impactor formed at an L4/L5 Lagrange point of the Earth-Sun system, it would have a very similar isotopic composition to Earth's and would have become unstable as it approached some 10% of Earth's mass (like most models posit the Theia impactor to have been roughly the size of Mars). I'm not discounting the possibility of smaller impacts creating the moon, but a single Theia-sized impactor seems more probable.
5 / 5 (2) Jan 11, 2017
You're (probably) right about an object forming at the same distance form the Sun as Earth. I'm not an expert on planet formation, but it makes sense. And it is possible.

On the other hand, I'd argue that multiple smaller impacts are more likely, for two reasons:
1: A second large object in our orbit isn't certain. It's not impossible, but statistically it's unlikely. Given that orbits weren't fixed, due to gravitational interactions with the gas and dust, as well as momentum exchanges in collisions, two objects probably wouldn't STAY in the same orbit. More likely, they would collide before one grew large, or they would approach, loop around each other, and recede again,or one would be ejected.
2: There were a lot more small objects than large ones, as in today's asteroid belt, and objects crossing orbits are more likely to collide than those in similar orbits.
1 / 5 (4) Jan 11, 2017
I know humans do not want to acknowledge a creator and that the moon was put here as part of a grand design to make the earth a home for the human race. Think about it this way, he was making the earth and the moon at the same time and since he can create matter it makes sense that they are made out of the same material.
If you take the accident out of the picture like the big bang and evolution, and posit a designer, then all things become easier to understand. How can you not realize this?
Then all that is left is to study them the way there are instead of by trial and error.
The human race has no FAITH and take it on themselves to explain the evolution of everything when they will never get it right.
Someone wiser and, who is perfect and has power beyond our understanding made everything.
Humans are small in understanding if you look at the big picture.
More power to you but the answer will not be yours.
2.6 / 5 (5) Jan 11, 2017
Yes, well, when the Goddess made you She obviously screwed up-- you've clearly got shit-for-brains. Get lost.
5 / 5 (3) Jan 12, 2017
"Humans are small in understanding if you look at the big picture."

True, but that's why we STUDY things, to learn more about the big picture, instead of simply saying "that's the way it was made".
3.4 / 5 (5) Jan 12, 2017
"Someone wiser and, who is perfect and has power beyond our understanding made everything."
If such a wise, perfect creator existed, the world wouldn't be the mess it is today. War, disease, poverty, Trump, etc. prove that the creator either was deeply flawed or didn't exist.
3.2 / 5 (5) Jan 13, 2017
Nothing wrong with the creation....should have hired better janitors.

If there's something wrong with the janitors - who are part of the creation - then there's something wrong with the creation. If we were created in God's image, then God's got problems.
5 / 5 (3) Jan 13, 2017
The human race has no FAITH and take it on themselves to explain the evolution of everything when they will never get it right
-And yet 99% of all the species god created are extinct. Looks like we are getting it right a lot more often than he did.
1 / 5 (2) Feb 10, 2017
Pseudoscience at its best. First they said single impact, then double-impact, now multiple impact! Haha...

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