Traces of another world found on the Moon


By Pallab Ghosh


Researchers have found evidence of the world that crashed into the Earth billions of years ago to form the Moon.

Analysis of lunar rock brought back by Apollo astronauts shows traces of the “planet” called Theia.

The researchers claim that their discovery confirms the theory that the Moon was created by just such a cataclysmic collision.  The study has been published in the journal Science.

The accepted theory since the 1980s is that the Moon arose as a result of a collision between the Earth and Theia 4.5 billion years ago.

Theia was named after a goddess in Greek mythology who was said to be the mother of Selene, goddess of the Moon. It is thought to have disintegrated on impact with the resulting debris mingling with that from the Earth and coalescing into the Moon.

It is the simplest explanation, and fits in well with computer simulations. The main drawback with the theory is that no-one had found any evidence of Theia in lunar rock samples.

Earlier analyses had shown Moon rock to have originated entirely from the Earth whereas computer simulations had shown that the Moon ought to have been mostly derived from Theia.

Alien origin

Now a more refined analysis of Moon rock has found evidence of material thought to have an alien origin.

According to the lead researcher, Dr Daniel Herwartz, from the University of Goettingen, no-one has found definitive evidence for the collision theory, until now.

“It was getting to the stage where some people were suggesting that the collision had not taken place,” he told BBC News.

“But we have now discovered small differences between the Earth and the Moon. This confirms the giant impact hypothesis.”

But the difference, some say, could be explained by material absorbed by the Earth after the Moon formed.

And Prof Alex Halliday of Oxford University, is among many scientists who are surprised that the difference between the Theian material found in the Moon rock and the Earth is so small.

“What you are looking for is a much bigger difference, because that is what the rest of the Solar System looks like based on meteorite measurements,” he said.

Dr Herwartz measured the difference in what is called the isotopic composition of the oxygen contained in rocks on Earth and Moon rock. This is the ratio of different forms of oxygen.

Studies of meteorites from Mars and the outer solar system show that these ratios are markedly different – rather like a fingerprint. So Prof Halliday and others are puzzled by the fact that the fingerprints of Earth and Theia seem almost identical.


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  1. Science is considered complicated, but how should that affect the way journalists cover it? Ghosh didn’t mention what the Earth-moon difference was until his 16th paragraph. Even criticisms of the finding were mentioned sooner. I believe this is called “burying the lead”, and would be anathema on other topics.

    His first sentence is good, but is there any point in the very next sentence not being, “They found that rocks from Earth and the Moon have different ratios of oxygen isotopes”?

    Also, would it kill him to hyperlink to the paper? He wouldn’t have even needed to reword anything; he could just make the journal name a 1-word hyperlink.

  2. I’m not an expert, of course, and I have what could be seen as a (very) silly question: an impact like this would modify the orbit of a planet (proto-earth or whatever we call it, in this case) or not? It was not just a meteor strike, it was a huge collision. If there was a collision, this should mean that the proto-Earth was not on the present orbit: the collision should have ‘pushed’ the planet. I found it difficult to figure out how the resulting planet could have resisted on an orbit in the same plane of the other planets of the solar system.
    Maybe I lack the basic information about physics…can everybody help me?

    p.s.: sorry for my English.

  3. There’s a famous calculation that predicts an orbit will be confined to a plane for a 2-body system. Why all planets would share an orbital plane in our many-body Solar system was a separate question that waited for a later calculation. Planets begin with independent orbital planes that gradually move so as to coalesce, because planets also attract each other. The Earth-Theia collision occurred when the Solar system was far younger than it is today. Earth’s orbital plane did not meet with those of other planets until much later. Indeed, the most likely reason Pluto’s orbital plane is far from those of the planets is that it entered the Solar system far later. (This is part of the reason scientists felt the need to modify the classification scheme so that objects such as Pluto or Sedna would have a name other than planet.)

  4. It’s more complicated than that because, although the bodies were attracting each other, they were ultimately able to largely slide past each other because of their relative velocities at the time of the collision. Gravity’s outcomes are complicated when one body’s relative velocity isn’t parallel to the line connecting them. In particular Theia collided at a 45 degree angle, a bit like complicated snooker shots.

  5. Thanks for that. Your reference includes the following: “….and most of Theia’s mantle accreted onto the Earth’s mantle, …”
    So I’m still baffled as to why there apparently is no trace of it on Earth today while it is readily detectable on the Moon.
    I’m content with the evidence but as yet unconvinced by the conclusion

  6. @jos Gibbons, I don’t know enough about orbital planes to weigh in and help @donnie86dc on that but I did feel like I learned a bit and confirmed a bit about my understanding from your thoughts and its much appreciated. However I think something you said about Pluto being reclassified is possibly incorrect. I studied the debate that preceded the event for a while last year and just freshened up on it.

    ‘Pluto’s orbital plane is far from those of the planets is that it entered the Solar system far later. (This is part of the reason scientists felt the need to modify the classification scheme ‘

    That claim is likely incorrect. Pluto failed the third requirement of the new planet standards made by the IAU. The third requirement for an object to be considered a planet is;

    It must have cleared the neighborhood around its orbit.

    Due to Plutos Mass it can’t do this. Please correct me if I am wrong about any of this. I like to know the truth. Thanks for your time.

  7. I’m not an expert on the giant impact hypothesis, but I suspect part of the reason may be that, while Theia has always been cool due to a lack of radioactive decay in its core, Earth’s then molten surface has subsequently changed quite a lot. Some materials may have sunk before the surface solidified.

    Whatever the technical details, it takes mathematical models whose equations are soluble only by computer to work out what should happen. There’s only so much we can achieve when discussing it with words here.

  8. Earth is tectonically active – the mantle has been churned up/mixed over time and largely homogenised. Furthermore, any crust on the Earths surface at the time has long-since been re-incorporated back into the mantle and recycled. In contrast, the moon is tectonically dead, much of the surface is incredibly old and there has been far less chance of homogenisation (also very limited weathering). Therefore there is pretty much no chance of isolating samples of Theia on Earth but the moon retains the possiblitiy of preserving distonct samples of Theia.

  9. PS – Mods, please add the edit function again like the good old days – It drives me mad spotting a typo that I can’t do anything about after hitting the post comment button!

  10. Steve_M @ 4.30 am 12.6.’14:

    Although I’m not qualified to enter the debate in detail, your comment about the active nature of Earth as opposed to the inactive characteristics of the Moon would, prima facie, explain what we find; in fact what we would expect to find.

    Incidentally, I endorse your request for the edit facility to be reinstalled; oh, and while you’re about it Mods, how about reloading the reply button link between comments too please?

  11. What I find difficult to comprehend is why after such a catastrophic impact is Theian debris only found on the Moon? Would not the greater mass of the Earth have attracted a proportion as both bodies coalesced from the rubble?

    A great deal of debris would have rained down on to both bodies, with much of it molten or vapour and well mixed in.
    With decaying orbits we should expect the rain of debris to be spread over an extensive period of time.

    As for finding original material on the Moon, we should remember that much of the surface of the moon is the “maria” of solidified larva flows, while much Moon-dust is from meteorite impacts. It is perhaps not very surprising that material from the impacts of formation are very scarce on the Lunar surface.

    The Earth is constantly being resurfaced by the subduction of its tectonic plates, so has none of its original surface material on its present-day surface.

  12. I was discussing not the IAU criteria that were eventually published, but what inspired the choice of them. Until Sedna et al were discovered, scientists put up with calling Pluto a planet, but they noted how peculiar it was. The orbital plane, as well as its orbit’s eccentricity, both made people suspect it joined the Solar system late. It was then of no surprise to discover that the matter source from which Pluto was derived also contained some other… well, as they would have had to call them back then, very small planets. Two things were then obvious: the definition of planet needed to be restricted, and it should be such as to exclude Pluto. One of the criteria eventually chosen concerns orbital intersection. Pluto would have been able to fail this test even if its orbit was in the same plane as planetary orbits, but the difference in planes gave cause to see what other facts about relations between orbits made Pluto and Sedna unlike planets. The orbit-intersection issue was ultimately just about the most plausible choice they could make to justify planets “outranking” dwarf planets in terms of their regional dominance.

  13. ‘Pluto’s orbital plane is far from those of the planets is that it entered the Solar system far later. (This is part of the reason scientists felt the need to modify the classification scheme ‘

    That claim is likely incorrect. Pluto failed the third requirement of the new planet standards made by the IAU. The third requirement for an object to be considered a planet is;

    The Oort Cloud objects being small, lacking mass and being far from the Sun’s gravity are not yet in an ecliptical plane of the inner planets. The Kuiper Belt has partially progressed towards one. Pluto probably originated as one of these outer Solar-System bodies, before being deflected inward.

    It must have cleared the neighborhood around its orbit.

    While Pluto has captured some moons, it has not “cleared its orbit” – it crosses Neptune’s orbit, because of Pluto’s elliptical bringimg it inside and outside the orbit of Neptune. The orbit of Sedna is much more highly elliptical. All the planetary orbits are slightly off circular.

    Due to Plutos Mass it can’t do this. Please correct me if I am wrong about any of this. I like to know the truth. Thanks for your time.

    If you want a good clear explanation of the current scientific thinking on the evolution of the Solar System, Look at this link.

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