Jun 09 2014

Origins of the Moon

What do Theia, Vulcan, Nibiru, Phaëton, and Antichthon have in common? They are all hypothetical planets that do not currently exist. Antichthon is the “counter-Earth” – a planet claimed to be in the same orbit as the Earth but always on the opposite side of the sun, so we can’t see it.  We know Antichthon does not exist because its gravity would be apparent.

Phaëton was the hypothetical planet between Mars and Jupiter that broke apart to form the asteroid belt. Phaëton likely never existed, and the asteroid belt simply failed to ever form a single planet. Nibiru is the planet, not taken seriously by any scientists, that some believe will collide with the Earth sometime this century (predictions have already failed multiple times). Vulcan was hypothesized to orbit within the orbit of Mercury, invented to explain anomalies in the orbit of Mercury that were later explained by general relativity.

Theia is unique among this list of hypothetical planets in that it probably actually existed. It was the Mars-sized planet that struck the proto-Earth 4.5 billion years ago, creating the current Earth-Moon system. This is, at least, the currently most accepted theory of the origin of the moon. It is supported by computer models, and explains many observations about the moon.

In this model, during the early chaotic period of the solar system, Theia crashed into proto-Earth, although a glancing blow, that threw up a mixture of material that was about 70-90% Theia and the rest Earth material. This mixture then coalesced into the moon. One model holds, in fact, that two moons formed, and then later joined together.

There is one problem with the Theia hypothesis of the moon’s origin – material from the moon appears to be all earth and no Theia (at least until the new study which prompted this post, more on that below). Evidence so far suggests that objects that form in different parts of the solar system will have different compositions, specifically they will have different isotopes.

For a quick background, an isotope is an element with the same number of protons but different number of neutrons in the nucleus. The number of protons determines the element, so oxygen is defined as having 8 protons – its number on the periodic table. The most common isotope of oxygen is O-16, which has 8 protons and 8 neutrons. There are also other isotopes, however, such as O-17 with 9 neutrons, and O-18 with 10 neutrons.

The ratio of oxygen isotopes in rocks is very consistent. Rocks from earth have a certain ratio, and rocks from Mars have a different ratio. Astronomers use this known ratio, in fact, to determine if a meteorite came from Mars.

Therefore, if every planet in the solar system has its own signature oxygen isotope ratio, Theia must have had it too, and we should see that signature in the moon, but we don’t This is a problem for the impact hypothesis of the moon’s origin. It’s not a fatal problem, because there are lots of possible fixes, but still a mystery to be solved.

Now, however, researchers publishing in Science report that they have used a more sensitive technique to measure the oxygen isotope ratio in three samples of moon rock – rocks brought back from the moon during the Apollo missions. Science news reports:

Their conclusion was that the lunar samples had an O-17 to O-16 ratio that was 12 parts per million higher than rocks derived from Earth’s mantle. This difference “supports the view that the Moon formed by a giant collision of the proto-Earth with [an impactor],” the team writes. “It is a relief that a [disparity in ratios] has been found, since the total absence of difference between Earth and moon would be hard to explain,” comments planetary scientist David Stevenson of the California Institute of Technology in Pasadena, in an e-mail.

That’s pretty exciting, and I suppose it’s a relief to proponents of the impact hypothesis. Still, this is a small difference. The measurements also suggest that the moon is a 50-50 mix of Earth and Theia, which is different than most computer models predict.

The difference is also very small. Some say it’s too small to really answer the question. Also, the sample size is very small. Basically we need to get more moon rocks, including from deeper below the surface, to see if this ratio holds up.

Impact proponents also point out that it is possible Theia had a similar isotope ratio to Earth. We don’t currently have samples from Mercury of Venus, so we don’t know how different they are from the Earth. It’s possible that the isotope ratio in the inner solar system is all very similar. Theia may have formed in an orbit very close to Earth’s (which makes sense as they did eventually impact) and therefore have had a similar isotopic signature.


The origins of the moon is a fascinating scientific story, one I have been following for years. It seems that this new evidence is further support for the impact hypothesis, although there is certainly room for further research.

In fact, one of the things I love about this story, more than the question itself, is how scientists go about answering that question. As you can see, every explanation is countered by possible alternate explanations with a description of the type of evidence that should resolve the issue, or at least get us closer.

How different are oxygen isotope signatures in different parts of the solar system?

What are the signatures for Mercury and Venus?

How do different impact models affect the ratio of Earth to Theia in the compositions of the moon?

Will this oxygen isotope ratio recently found in moon rocks hold up with a larger sample size?

Are the surface rocks on the moon contaminated or altered by later impacts, and will deeper rocks have the same ratio?

The question is probed from every possible angle, and answered with various types of evidence. Scientists don’t just focus on the parts of the theory that fit and make sense, but rather they focus particularly on those parts that don’t current fit. Weaknesses in the theory point to areas requiring further research – to more knowledge about the history and nature of our solar system.

This is the culture of science. You may notice a stark contrast to those who only pretend to be doing science.

25 responses so far

25 thoughts on “Origins of the Moon”

  1. Bruce says:

    Thanks Steve, interesting that the coverage of this in the British media last week spun it out to be almost conclusive evidence of the impact theory. Nice to get some perspective on it.

  2. jasontimothyjones says:

    The only thing I am having difficulties with is that if there is residual isotopes on the Moon, we should assume that there are also some on earth from Theia? or some combination of proto Earth/Theia

  3. jasontimothyjones says:

    Has anyone clicked on the link to the Science Article and read the comments section….. its …. well I don’t know what it is, but you have to read it.

  4. I read it – I thought about commenting on it here for laughs. Reading them supports the hypothesis that there is an inverse relationship between a person’s motivation to comment on a science news article and their scientific literacy and critical thinking skills. Actually I think there is a bimodal distribution including this group and exasperated science fans.

  5. jasontimothyjones says:

    Im holding my breath on the results of the latest reports on this, but I generally don’t have a problem with some sort of collision.


    What I have never got my head around is where the planet came from to collide with the Earth in the first place, if it was forming inside the solar system, its fair to assume that as its mass grew, and as the mass of the earth grew, both planets would have had a semi stable orbit, if either were unstable, the gravitational effect should have destabilising effects on the rest of the solar system. By this I don’t mean that the proto planets were making a nice elips around the sun, but more ordered chaos as in the rotation of the asteroid belt.

    So for another planet to collide with earth, with enough force to eventually form a moon, we have to assume that Newton 2nd law is correct…and it is… so we have mass and acceleration to consider.

    its really the acceleration that I cant come to grips with, its very possible that either, or all the planet were crossing paths and completely unstable, but this give the problem of how they become stable later, which leads me to think that one of the 2 planets may have been a rouge planet that was dragged in via the Suns gravitational pull.

    Perhaps its a long shot, but its a cleaner hypothesis that ‘shit was flaying around all over the place, then settled down a bit’

  6. locutusbrg says:

    The current theory bothers me for a slightly different reason. I think the it is the best hypothesis that fits the observations right now. I am bothered by the implications more than the facts.
    I could be wrong about some of my assumptions, but the large relative size of the moon is why we have significant tides. Which of course in turn had a large effect of the evolution of land animals.

    Assuming my premise holds true. It leads me to believe that the chance of the galaxy making life similar to earth’s even less likely. Making us even more unique, maybe even the platypus of the galaxy. I find the drake equation to be a less and less viable thought exercise as we learn more about how our planet and solar system evolved.
    Leads me to believe that the galaxy and maybe the universe has very little life like us in any significant way. Our data point one the earth may be a complete anomaly. Most of the universe may be silicon based life, or something we haven’t even thought of, or worse vastly empty.

  7. It would be a huge coincidence if we were hit by a rogue planet in the early formation of the solar system, during the heavy bombardment phase. Not impossible, just unlikely.

    Our system likely had many planets, most in an unstable orbit (long term unstable, over hundreds of millions of years). Most were likely kicked out of the solar system, or fell into the sun. There is evidence that other surviving planets also had big collisions.

    Theia could have been slight farther our or in from the Earth, but its orbit was shifting until it crossed the Earth and they collided. This fits with the “glancing blow” model and why the Earth still has a stable orbit. If it were smacked by a rogue planet at a random vector chances are our orbit would have been significant changed, and not the nearly circular orbit we have today.

  8. jasontimothyjones says:

    @locutusbrg consider that our environment as it is now, was not always as it is now. There maybe intelligent life breathing methane if thats how they have evolved

    I would hate to think we were the only ones, but its a possability

  9. jasontimothyjones says:

    Thanks Dr Novella, you just smashed my hypothesis, though it was fun while it lasted. If the orbit stability and the ‘glancing blow’ are considerations, then is it assume that both planets were traveling at a similar speed and direction and sort of ‘come together’ rather than smashing into each other.

    Perhaps I will stick to Philosophy, where there is only questions and no answers 🙂

  10. Everything in the solar system goes in the same direction, so two planets in a close orbit would have very similar speed and direction. Still, it would have been quite an impact, but the net momentum change for the Earth could have been minor. Astronomers believe the Earth spins as fast as it does partly due to the blow, so some of the energy was converted into angular momentum.

  11. If the moon came from the earth, why is there still an earth? Ergo, Goddidit!

  12. Heptron says:

    This may be a more basic question, but one that (I think) is worth asking.

    Would the density of a rock increase with an increasing amount of isotopes? I know it wouldn’t be much, but I could not find numbers on it.
    I ask because the comparison between the Earth and the moon only used rocks which I’m assuming were taken from the surface of the moon. Heavier elements and isotopes would settle deeper into the moon, would they not?
    The next question becomes: How does the ratio of isotopes change when comparing rock on earth at the surface versus rock from 1 or 2 km below the surface.
    This is off the cuff so I hope it’s not a stupid question. :s

  13. jasontimothyjones says:

    Ok so lets assume just for now that Dr Novella is right, mostly because he may be right, if there was little momentum change its possible that the 2 planets were in a binary configuration rather than having a similar speed and direction/orbit, so common centre of gravity keeps Newton happy as far as the rest of the solar system is concerned (mostly). Assuming that the sum of mass of the 2 original planets is the same the sum of the mass of the Moon and the Earth, point of the common centre of gravity is unchanged, the rest of the solar system would not be greatly effected.

    We must remember to keep Newton happy or we have big problems

  14. nice related discussion of the “Big Splat” theory of moon creation, very intriguing topic!:

  15. jasontimothyjones says:

    this is a bit of fun if you want to see what minor changes to size and velocity can do…. the best part is watching planets explode http://phet.colorado.edu/en/simulation/my-solar-system

  16. Steven Yenzer says:

    Sort of a side question I’m wondering if anyone can answer: what are our current “stores” of moon rocks like? Is this kind of research limited by that? Or do we have enough to continue work in this area?

  17. a_haworthroberts says:

    My latest post here refers to somebody who claims to know what is NOT science (a young earth creationist ideologue who calls himself ‘Cowboy Bob’ and ‘the Question Evolution Project’ and ‘Piltdown Superman’ and ‘Stormbringer’s Thunder’…):

    This being the link in question:

    He also likes banning critics from his facebook page.

  18. Paulz says:

    “If the moon came from the earth, why is there still an earth? Ergo, Goddidit!”
    Checkmate, atheists!

    Regarding the Asteroid Belt, a category question. Even though we know that Phaeton never successfully formed, can we say that the asteroid belt is still the material that would have composed the potential planet Phaeton?

    I suppose it’s far too fraught to consider POTENTIAL planets, though.

  19. Sylak says:

    WOW, That bongorock guy is something, and he can’t even spell right. I’m not that good in english ( hey my language is French after all), and my spelling is better!

    Anyway, The moon subject is really fascinating, back when I read my first astronomy book when i was 8, the moon formation and the Death of the sun and stars were the thing that fascinated me the most

    I always wondered if it could have from the same dust/rock disk that formed earth. Maybe the disk was large enough to let it form father away and not being pull right away by gravity. They probably thought of that one i guess, and that’s is why the main accepted theory is the impact. But what are the alternative ones?

  20. jasontimothyjones says:

    This is an interesting related article, even though they keep calling it the “Dark Side” of the moon, and not the far side, If I can find the original paper ill post the link


  21. EmmainItaly says:

    I remember a professor talking about this in college and it left me with my jaw on the floor. He emphasized the implications of this moon formation theory: without the collision model the Earth’s gravity would not have been strong enough to capture a body of such relatively large size, and that the mixing and sludge produced by our large moon’s tidal forces may have played a large role in life taking form on Earth. Given these specific and unlikely events, life could be a lot more rare than we think. I’ve been captivated ever since but am not sure if more current research agrees with this theory.

  22. grabula says:

    Steve Novella said:

    “I read it – I thought about commenting on it here for laughs. Reading them supports the hypothesis that there is an inverse relationship between a person’s motivation to comment on a science news article and their scientific literacy and critical thinking skills. Actually I think there is a bimodal distribution including this group and exasperated science fans.”

    I LOL’d a ton reading through those. For me I think the issue is this: When I read a rational and plausible scientific article I think ‘interesting’, then I move on. I find this kind of thing fascinating but it’s just an article supporting a theory so I don’t really feel the need to comment yeah or nay. I guess those only driven to comment are those who are either defending some sacred cow, or fighting those defending their sacred cows like Steve N. says.

  23. grabula says:


    I think that’s a trap really, one man people fall into. They look around and see how ‘perfect’ our surroundings are for supporting life. I tend to believe that regardless of the environment (within of course certain parameters) life will find a way. In reality our environment can be pretty unfriendly to life. Take Oxygen for example, supports most of the life on this planet, it’s also a corrosive, and probably one of the reasons we don’t live forever.

    As science moves forward I think we’ll expand our understanding of where and how life can adapt. Just look at extreme-philes, life forms a few decades ago we couldn’t fathom, so to speak. Now we know life can exist in just about every nook and cranny on earth.

  24. BillyJoe7 says:

    grabula: “In reality our environment can be pretty unfriendly to life”

    And the universe is almost antagonistic to life.
    A vanishingly small fraction of the universe actually contains life.
    Taken as a whole, you are not far off wrong when you say the universe is dead.

  25. BillyJoe7 says:


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