Did the Moon Come From Earth?

New findings show that Earth and the Moon have identical isotopic ratios of tungsten — and that's a problem for the widely accepted "big splat" hypothesis.

For much of history, we had little clue about the Moon's origin. Inuit legend claims that the Moon was a stranger chased into the sky by a poker-wielding maiden. In African folklore, the great god Bumba, having a stomach ache, vomited up the Sun, Moon, the stars, and some animals.

But what about scientific speculation? Before the Space Age, various astronomers championed one of three possibilities: that the Moon might formed alongside Earth, was captured into orbit when it passed close by, or was literally torn away from a hyper-spinning Earth.

During the 1970s, scientists proposed that an object the size of Mars could have collided with Earth and thrown enough matter into orbit to create the Moon.
Don Davis / The New Solar System
These days, thanks to analysis of lunar samples, most planetary scientists are comfortable with the notion that the Moon came about after an object the size of Mars (sometimes called Theia) struck the infant Earth with a titanic glancing blow, and that some of the ejected debris coalesced a single large satellite.

It was a radical concept when proposed in 1975 by William Hartmann and Donald Davis. But it's steadily gained standing over the years, particularly after detailed computer modeling by Robin Canup (Southwest Research Institute).

One important outcome of Canup's models is that the Moon would have formed largely from ejected portions of the impactor's mantle, rather than Earth's mantle. And what of Theia's iron-rich core? It fell onto Earth and was gobbled up.

However, the ratios of oxygen's three isotopes are identical in lunar and terrestrial rocks — that is, the two worlds appear to made of exactly the same stuff. Geochemists have finessed this problem by assuming that Theia was in an orbit much like Earth's before it struck, or that oxygen isotopes mingled and mixed freely between Earth and the ejected matter in the collision's chaotic white-hot aftermath.

Now an even bigger "gotcha" has emerged for the collision scenario. In this week's Nature Geoscience, cosmochemist Junjun Zhang (University of Chicago) and four colleagues show that the ratio of two titanium isotopes, 47Ti and 50Ti, are likewise a dead match — identical to four parts in a million — in samples from the Moon and Earth. Unlike the wiggle room afforded by oxygen's volatility, it's hard to imagine how molten titanium-bearing compounds could have ended up equally represented in both worlds — unless all the lunar material came from Earth in the first place.

Moreover, Zhang points out, the Moon and Earth have the same isotopic ratios of tungsten, an important elemental tracer for silicate rocks. But there's a critical complication: tungsten-182 is also created by the decay of radioactive hafnium-182. To get the tungsten ratios to match, in addition to those of titanium and oxygen, the interiors of Theia and primordial Earth would have had to evolve the same way. "It is very unlikely," she says, "although we can't definitely rule out the possibility."

Canup's simulations work best when the Moon-forming impactor strikes at a relatively low velocity, which implies that it must have been in an orbit very similar to Earth. But it doesn't necessarily follow that they're made of exactly the same stuff. In fact, some modeling suggests they'd be chemically quite distinct. "You can't rule out an impactor with a composition just like that of the Earth," Canup says, "but that is not the currently favored view."

On the other hand, new simulations by Julien Salmon and Canup argue that the Moon might have taken a few hundred years to pull itself together — plenty of time for isotopes to slosh back and forth between the just-hit Earth and the disk of matter than formed the Moon.

According to one hypothesis, immediately after the giant impact that formed the Moon, a white-hot disk of silicate vapor would act as the exchange medium between proto-Earth and the proto-lunar disk.
PSRD / Univ. of Hawaii
So I put the question to Caltech's David Stevenson, who with Kaveh Pahlevan conceived the superhot mixing model. Stevenson points out that we still lack a clear understanding of how Earth formed, let alone the Moon. He suggests that lots of sharing took place among proto-Earth and the objects that formed in its vicinity, and he doubts Theia came from very far afield, because it takes only "a bit of Mars-like stuff to mess up the remarkable Earth-Moon similarity."

Moreover, geochemists' understanding of the inner solar system is far from complete — just look at the compositional curve balls being thrown their way by Mercury. For all we know, everything from Earth inward has the same isotopic pedigree. In fact, Zhang and her team find that the Earth-Moon titanium ratios are also matched by aubrite meteorites, now considered the best geochemical match to Mercury's surface.

As Stevenson sums it up, "We may be unable to solve this problem until we get a piece of Venus."

14 thoughts on “Did the Moon Come From Earth?

  1. Darrell Heath

    I too had the same question about why shouldn’t we expect Theia to have a similar composition with the young Earth. Thanks for having posed the question to these researchers. Of course their answer does not resolve the issue but at least it highlights the fact we needn’t discard the collision scenario just yet.

  2. Bruce

    This is fascinating. I totally agree with your comment, Darrell. I have two questions: (1) In the simulations what impact velocity gives the best fit to what we have today, and (2) Is a sample return mission to Venus and back even on the drawing board?

  3. Rod

    The Earth and Moon have large differences in mean density. Presumably the larger iron content of Earth came from Theia I take in this collision model while the Moon formed with much less iron. If correct this appears to be an elusive parameter to test and confirm. Q:How much mass is the proto-earth in this model? Q:What was the proto-earth LOD or rate of spin before Theia collides with the proto-earth? It seems the proto-earth would be spinning very slowly, Theia hits it and spins it up, then tidal dissipation works to slow back down to 24 hour LOD over 4.5 billion years. The proto-earth must grow in mass too (and perhaps the Moon too), all of these variables could alter the outcome of the computer model and create something quite different than the Earth-Moon system we see today. Such variables in the simulation appear elusive to test and confirm.

  4. Bruce

    Rod, an accurate computer model of the formation of the Earth-Moon system does indeed seem like a challenging undertaking, but it should certainly be pursued vigorously. I would imagine that many groups are working on this problem, the elusiveness of the answer notwithstanding. Such work may very well work out a convincing solution long before we have any Venusian material to test.

  5. Stan Kerns

    Lately, with the news from Mercury I have been wondering if, with its odd orbit and unusual composition and structure, is there a chance that Mercury could be whats left of Theia?–Also in that all the major outer planets have held on to ring systems, isn’t a bit strange that absolutely nothing of the fragments remain??–not even a piece as big as a sand grain.

  6. Andy ©

    Reading the above article, and in particular the reference to the findings from 1975, I am reminded of an article I found in Popular Mechanics, the January 1987 issue, page 60, entitled Birth of the Moon by Arthur Fisher.

    The heart of that article is reporting on a Los Alamos National Laboratory super computer simulation of a "10-hr. span during a collision between a Mars-sized body and a young Earth…" Six frames of that simulation were published in that article along with four additional frames from a similar simulation "generated by a supercomputer at Sandia National Laboratories by H. J. Melosh and Marlin Kipp…"

  7. Bruce

    Andy, this article made me wish for a scientifically accurate, 3D movie of this colossal collision and it’s fantastic aftermath. (I love math. Why does the English language connect math with disaster?) Just think of what our system might have looked like when the planet and it’s molten ring were enshrouded in silicate vapor!

  8. Rod

    Bruce, I can see you are enthusiastic concerning the computer modeling here and I too can enjoy a good simulation. The computer modeling starts out with a proto-earth about 65% of present mass before Theia collides, thus a previous state which cannot be observed. I urge caution on using computer simulations as a basis for deciding scientific fact concerning the origin of the Moon. A hosts of variables are at play, many that may never be verifiable. I look at the astronomy of Galileo. Observation, repeated observation, and repeatable measurement. For me the computer modeling does not overthrow Genesis 1:16-17 and Psalm 89:37.

  9. Bruce

    Rod, thanks for the above, for it prompts me to explain a few points about how Genesis chapter 1 can be harmonized with modern astronomical findings. (1) The “beginning” of Genesis 1:1 includes everything that led up to God’s earthly creative works during the six creative “days” of verses 3-31. The “heavens and the earth” of verse 1 is the whole physical universe, including the sun and other stars, the earth and the moon. According to cosmological findings this “beginning” could have been about 10 billion years long. (2) The Genesis creative “days” were not literal 24 hr days, but rather they are periods of undetermined length during which steps were taken resulting in what we might call the terra-forming of the planet. That Genesis uses the word “day” at times to mean periods longer that 24 hrs is shown by Gen. 2:4 in which the entire universal formative period is called “the day that Jehovah God made the earth and heavens.” (3) The Genesis creation account was written from the point of view of earthly observers and in terms that could be understood by ancient peoples. Thus, when there “came to be light” in verse 3 on the first day we can understand this to be the period during which the inner solar system was being cleared of dust so that light and dark periods could be discerned from the earth’s surface. Verses 16 and 17 which Rod mentions are in the passage describing the fourth creative day, and evidently during this period further clearing of the earth’s atmosphere occurred, thereby making the already existent sun, moon and stars visible from the surface of the earth. So, Rod, I definitely don’t believe that the verses you referred to can or will ever be overthrown by any scientific idea. But, perhaps, the One who made all things caused a titanic collision 4.5 or so billion years ago, thereby forming the earth-moon system. The Bible reveals why God made the world, science can help us see how he might have done it.

  10. Bruce

    To Stan Kerns, your question about whether there could be a Theia/Mercury connection got me to thinking. (Dangerous, perhaps, but this is fun!) I would have to guess that the chance that Mercury could be a remnant of the proposed Proto-earth/Theia collision would be very low, maybe zero. In the impact modeling being discussed most of Theia’s iron core merges with the earth’s core, and the velocity has to be slow enough so that a large amount of the ejecta would have been retained by the earth-moon system. If this really happened then actually the moon itself would be the biggest part of what’s left of Theia. And, look again at the illustration in Kelly Beatty’s article, taken from “The New Solar System.” Ejecta would have sprayed out in a giant dispersal cone, but how much would have escaped from the system? (This is why I asked what is the impact velocity.) Also, consider that at this early stage in the history of the solar system the molten fraction of Earth’s (and Theia’s) interiors would have been much greater than the Earth and the Moon today. (Picture the collision of two molasses filled balloons, that’s why it’s referred to as the “big splat.”) Some of this material would have left the earth-moon system, launched into orbits around the sun. Could, after 4.5 billion years, some of the remaining earth crossing asteroids be remnants of this proposed collision? Do any meteorites have the right chemistry and age to have been from this impact? Thanks for your comment, Stan.

  11. James

    Though the idea that Mercury could not have formed from a collision between a Proto-Earth and Theia from the resent scientific moldels, science changes when new facts and information presents itself. I believe that when more information is discovered that Mercury will infact be proven to be a left over fragment of the said collision.

  12. James

    Though the idea that Mercury could not have formed from a collision between a Proto-Earth and Theia from the resent scientific moldels, science changes when new facts and information presents itself. I believe that when more information is discovered that Mercury will infact be proven to be a left over fragment of the said collision.

  13. James

    Though the idea that Mercury could not have formed from a collision between a Proto-Earth and Theia from the resent scientific moldels, science changes when new facts and information presents itself. I believe that when more information is discovered that Mercury will infact be proven to be a left over fragment of the said collision.

  14. Philip Bruce Heywood

    I am an Australian but couldn’t log in without claiming to be an Oklahoman or something American.. Lunar origin was close to solved even before APOLLO and was subsequently mired in confusion by people talking through their hats. The late Prof. Andrew Ringwood of the Australian National University got it largely correct on the basis of mineralogy back when APOLLO had only just terminated. Please see my publications, at http://www.creationtheory.com , go specifically to COMMON DONOR CAPTURE MOON ORIGIN. The donor was Mercury and the common isotope chemistry has to do with the donor supplying quantities of material to Earth and Moon. Looking at my site, you will deduce that it was extremely likely that Aubrites would have the same titanium isotopy as Earth – Moon. It’s all over bar the loose ends. It was all over, almost before APOLLO got there — that’s how obvious it is. Good post, Sir, and thanks for including this:
    "In fact, Zhang and her team find that the Earth-Moon titanium ratios are also matched by aubrite meteorites, now considered the best geochemical match to Mercury’s surface."