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I've seen models in astronomy that show how the Earth-Moon system must have come together after a collision. However, I have not heard whether there is any actual physical evidence on Earth that points to a prior collision. Is there geological (or other physical) evidence here on Earth that confirms the moon originated from a collision on Earth? If so what is that evidence?

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    $\begingroup$ This might be a decent place to start: same oxygen isotope ratio and Moon being iron poor are two pieces of evidence. $\endgroup$
    – user26
    Jul 6, 2014 at 3:48
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    $\begingroup$ The question should say "a moon" not "the Moon". And the Moon was created by a "graze shot" with another planetary body, and not a full collision. $\endgroup$ Aug 19, 2014 at 12:42
  • $\begingroup$ +1 for what Spießbürger said: the moon has never collided with the earth. The best answer to this question, as stated, is 'no'. So can you reword? $\endgroup$
    – Matt Hall
    Nov 20, 2014 at 0:23
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    $\begingroup$ @kwinkunks The question should be remained as it is IMO, as this may be a common misconception. I've edited the answer to clarify this. $\endgroup$
    – Gimelist
    Nov 20, 2014 at 5:08
  • $\begingroup$ Here are some videos: boulder.swri.edu/~robin/moonimpact $\endgroup$
    – Gimelist
    Nov 20, 2014 at 5:14

2 Answers 2

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Is there geological (or other physical) evidence here on Earth that confirms the moon once collided with the Earth?

No, there isn't. This is, however, plenty of evidence that the moon formed due to a collision of a third body (sometimes referred to as Theia) with the Earth, and the moon formed from the ring of debris that resulted from the collision.

This theory is often known as the Giant Impact Hypothesis and searching for this term may help you find other links and references elsewhere.

If so what is that evidence?

To summarise the geological evidence, much of it is indirect evidence, in that it compares geology on the Moon with similar geology or features on the Earth, and draws conclusions to explain the similarities (or discrepancies).

Because of the sheer size of the proposed impact, it would have likely reconfigured the surfaces of both bodies entirely, and so finding direct physical evidence on the Earth would be extremely unlikely. (e.g. a massive hidden crater would simply no longer exist)

Geological Evidence

  • Moon rocks collected from the Apollo missions that have almost identical oxygen isotope ratios to similar rocks found on Earth of the same age.
  • A large portion of the Lunar crust is made up of Anorthosite, which is indicative of a large melting event. (with the energy for this supplied from the impact)
  • Zinc. Lunar rocks contain less zinc, but with heavier isotopes of Zn than those found on Earth, which by contrast has lighter isotopes in greater abundance. This is consistent with zinc being depleted from the moon by evaporation, such as during a massive impact event.

  • Density and volatiles. The Moon is 25% less dense than the uncompressed density of Earth. It is severely depleted in volatiles, with practically no water and less than half the potassium abundance that Earth has. The combination of low density and lack of volatiles implies that the Moon was not a simple accretion of early solar system material, but resembles the Earth's mantle in bulk composition. Volatile material would have been vapourised by the impact.

  • The bulk composition of the Moon's crust. (This one does not actually involve the Earth, but I feel it is still important to mention.) The Moon's mantle and crust chemical composition could be explained if the Moon had a large iron core, but its core is actually quite small.

Other Physical Evidence

  • The ratio of the Earth and Moon's mass far exceeds any other planet in the solar system, and this begs the question of how did so much material become in orbit of the Earth. (Not evidence as such, but raises the question in the first place)

  • Getting more indirect...there are bands of warm silica-rich dust orbiting nearby stars which is interpreted as planet-sized bodies having collided with each other, so there is precedent for similar events happening elsewhere in the galaxy. (Again, I realise this is one is not strictly answering the question, but Earth Scientists often have to look well outside the Earth to answer their questions!)

Hope this gets you started!

Update: I recently came across this short article, which provides a summary of some of the latest thinking on the Moon's formation, including the Giant Impact Hypothesis.

Sources: de Pater and Lissauer (2010): Planetary Sciences; Canup and Asphaug (2001), Nature

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  • $\begingroup$ don't forget the earth is an outlier in the predicted density of the planets based on distance from the sun, that is until you include the density of the moon in the calculation. $\endgroup$
    – John
    Jul 26, 2023 at 3:01
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The theory that planets and moons form from accretion is ridiculous in my opinion. Though I have never used it professionally, I have a degree in Physics. Unless a collision between a Mars sized object and an Earth sized object occurred when both were still molten, the alleged head-on collision event probably would have completely obliterated the smaller planet and left a highly deformed Earth behind. Had the both still been molten, fluid dynamics would come much more into play. However, it's still highly unlikely that the remains of the Mars sized object would have remained in orbit around the Earth.

Want more evidence? Look at Mars. Something huge did impact with Mars, but it was probably smaller than Earth's moon. The evidence is the huge 2km deep basin in the northern hemisphere as well as the boulders strewn across much of the landscape all around the planet. To me, it is obvious that there was a large scale impact event that caused Valles Marineris. This same event caused enough shock to shatter the crust on the opposite side of the planet. The debris likely orbited Mars for a very long time before the overwhelming majority rained back down on the planet. Phobos and Deimos being the only two pieces of debris currently still in orbit. Nevertheless, they will crash back down on the planet many years into future.

Following what I stated above, it's easy to see where something similar would have happened had Earth been struck by a Mars sized object; only the damage would have been much more severe as I stated above.

Is it possible that the moon grazed Earth in the distant past prior to settling into a stable orbit? That's a more likely scenario than a head-on collision in my opinion. There still would have been a lot of interaction between the two bodies; including highly electrical interaction. Thus, that could easily explain our geological findings as well as possibly the tidally locked situation.

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    $\begingroup$ even if they are not molten the impact will make them molten, the same forces that make a planet round in the first place make in round again. our core is lopsided, even if the planet is not. simulations of the impact re quite robust. Grazing is absurd, planets don't act like rigid bodies, if they graze, they impact. nasa.gov/feature/ames/lunar-origins-simulations $\endgroup$
    – John
    Jul 26, 2023 at 3:08
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    $\begingroup$ Several problems with this answer. 1. Appeal to authority. " I have a degree in Physics." Do you know how many physicists have no BS idea about climate change and talk about climate change? Do you know how many biologists talk *words about Relativity and Quantum mechanics, while they shouldn't? 2. "Accretion" is just a synonym for "formation" amongst us formation people. How else would you think a planet formed, than via formation? Did God just plop it there? The scientific question would rather be "how was Earth accreted". 3. Your 'obvious' points for Mars are far from obvious. $\endgroup$ Jul 26, 2023 at 7:14
  • $\begingroup$ You can disagree if you like, but I challenge you to prove me wrong. Thus far, no one has. As far as simulations go, they're fine and dandy to show possibilities. However, the collision theory is just a theory. And, yes, it is entirely possible that it was not a head on collision. I'm not religious. Thus, I don't believe in Creation. No, accretion has a more rigorous scientific definition than that. $\endgroup$ Jul 27, 2023 at 16:21

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