By Thomas

The Moon formed in a Giant Impact of a planet called Theia with Earth (Figure 1). You can find a nice simulation of this event at the end of this post or watch the orginal here. In a post a few weeks back I mentioned that there are some problems with the details of this theory and that I would write about that soon.

The Giant Impact
Figure 1: The Giant Impact
Source: National Geographic

As a paper published last week in Science [1] is exactly about that topic, I thought it is time to follow up on this promise. The mentioned paper shows that technology has developed far enough to allow science to tackle the problems lurking within the Giant Impact Theory. Responses in the media (e.g. here, or for german speakers here) about the paper seem to focus on the fact that the paper supports/confirms the Giant Impact Theory.

Yes, the results are in agreement with the theory and therefor support/confirm it!

But to focus on this, implies that the main question we are facing, when it comes to the Moons origin, is:

Was there a giant impact?

Of course one is allowed to ask this question. However, in science this question is only taken seriously if you`ll offer an alternative hypothesis, that works equally well or better in hindsight of the known facts. When it comes to the origin of the Moon, all other origin hypotheses have already been discarded for various reasons and to my knowledge there are no rival hypotheses to the giant impact left.1 The Giant Impact is a fact, and was one before this paper!2

The question is not: Did it happen?

The question is: How did it happen?

The Giant Impact Theory came a long way since it was first proposed in 1975 [2]. It solved a lot of problems regarding chemical similarities and differences between Earth and Moon. But in recent years it got stuck in a bottleneck.

What had happened?

Measurements of the elements oxygen, chromium, titanium, tungsten and silicon had shown that the ratios between the isotopes of those elements were the same in the Earth and the Moon. [3]

Why was that a problem?

Those ratios are fundamentally different in other rocky bodies in our solar system like the asteroids and – more important – Mars. Therefor it is likely that Theia and the Proto-Earth (the Earth before the Giant Impact) differed in these isotopic signatures as well. If that was the case, the standard model of the Giant Impact would predict that those ratios would be different in the Earth and the Moon (Figure 2).

F2.medium
Figure 2: Different Models of the Giant Impact.
Source: Science

Different changes in the model were proposed to achieve the similarity in isotopic ratios, but all of them require very special conditions, such as a fast spinning Proto-Earth or Theia being the same size as Earth (Figure 2) or complex processes following the impact [3-5].

Or maybe Theia didn`t had a similar isotopic signature to the Proto-Earth? For that line of thinking we would have to revise what we assume about the distribution of isotopic signatures in the solar system. That would be a great and enlightening thing to do, but would require samples from a rocky planet other than Mars – preferentially our twin planet Venus. [3]

So what`s the great news?

Actually, the great news are small – very small. That best describes the differences between the isotopic signatures of Earth and Moon rocks which now were uncovered.

But didn`t I said above that there were no such differences?

Indeed, but I should have added “within error”. The error is used to describe the area of uncertainty around a measured result. For example, I know I`m 185 cm tall. But I have never measured that very accurately, so it could easily be that I`m actually 1 cm taller or smaller. The 1 cm is the error on the measurement of my height. If I now meet a person who has measured his or her height (equally lax) to be 185 cm, we would have to conclude that we are equally tall (within error), even though he or she might be 186 cm and I only 184 cm. How can we find out? We have to measure more precise, let’s say with a mm-scaled tape.

In a nutshell, that is what the new study did – measured more precise. They found that when you compare 1 million oxygen atoms on Earth to 1 million oxygen atoms on the Moon, you`ll find that 123 of those atoms are different isotopes4.

This means a great relief, for some reasons:

a.) It means there is a difference in those isotopic signatures and we don`t have to invoke very special conditions5 for the Giant Impact.

b.) It promises that we`ll find similar differences in the other isotope systems apart from oxygen.6

c.) When we know how the Earth and the Moon differ, we can infer more on the nature of Theia, the Proto-Earth and the details of the impact.7

And that is much more, than just to confirm that there was a Giant Impact!

 


A simulation showing the Giant Impact.
Source: Nature


1 That doesn`t mean someone might come up with a new one in the future.

2 Imagine the media would title every article about new advances in evolutionary biology with “Evolution Confirmed!” – Technically true, but still missing the point.

3 If we`re including the error: 9-15

4 Don`t worry about the isotopes. It basically is like two boxes filled with 1 million red and blue balls, where one box has 12 more of the blue balls. Even finding that out, would be hard. Now imagine that on an atomic scale, where you can`t “see” the atoms and where the difference between the balls is the equivalent of blue and very-slightly-darker blue.

5 Special conditions are often a sign that there`s something wrong with your theory.

6 Oxygen isotopes were the first to indicate the similarity-problem [6], it`s good that they make up by showing a way out.

7 In the presented paper, the authors speculate on the basis of their data, that Theia might have had a enstatite chondrite composition, which is material we find in the asteroid belt. But we have to see what the future (and other isotopic systems) will bring.


Literature

1. Herwartz, D., et al., Identification of the giant impactor Theia in lunar rocks. Science, 2014. 344(6188): p. 1146-1150.
2. Hartman, W.K. and R.D. Davis, Satellite-Sized Planetesimals and Lunar Origin. Icarus, 1975. 24: p. 504-515.
3. Canup, R., Lunar conspiracies. Nature, Vol. 504, 2013(7478): p. 27.
4. Elkins-Tanton, L.T., Planetary science: Occam’s origin of the Moon. Nature Geoscience, 2013. 6(12): p. 996-998 (2013).
5. Clery, D., Impact Theory Gets Whacked. Science, 2013. 342(6155): p. 183-185.
6. Wiechert, U., et al., Oxygen isotopes and the moon-forming giant impact. Science, 2001. 294(5541): p. 345-348.