As well as being a fascinating subject in its own right Chemistry can be used as a tool to solve many problems. One interesting example that has emerged recently is “What is the origin of the moon?”

The most usual explanation is that during the formative period of the Earth some 4.5 billion years ago a giant asteroid collided with the Earth. The moon, which has a diameter roughly one quarter of that of the Earth and contains1/ 81 of its mass, is thought to have been flung off from the Earth as a result of this collision. This is known as the Giant Impact Hypothesis. However, there is no real hard evidence to support this and several reasons to question its validity. For example, the energy produced in such a collision should have been enough to produce a huge global ocean of molten magma and yet there is no evidence of the heavier material sinking into the Earth’s mantle. Similar collisions during the formation of Venus are also thought to have occurred during its formation and yet Venus does not have a moon.  

So how can Chemistry help to solve the problem? In an earlier blog I talked about how the accurate measurement of isotopic mass has led to relative atomic masses being quoted as between two values rather than being given precise values. This is because the relative atomic mass of an element can vary slightly depending on the original source of the element. Titanium is an element with a very high boiling point and so would be less likely to be vaporised during the huge release of energy following a cataclysmic collision. Asteroids that have fallen to Earth have been shown to have a different ratio of ⁵⁰Ti to ⁴⁷Ti so it would seem reasonable to assume that if the moon was formed from such a collision the ratio would be different on the moon to that on Earth, A team from the University of Chicago using samples of rock brought back from the Apollo missions has shown that the ratio of ⁵⁰Ti to ⁴⁷Ti is in fact exactly the same on both the Earth and the moon. This of course is achieved using high resolution mass spectrometry and could be used as a nice example to support the teaching of the IB Assessment Statements 2.2.2 and 2.2.3  The findings suggest that the moon only had one ‘parent’, the Earth, and not two as previously proposed and hence falsifies the Giant Impact Hypothesis.

As well as the concept of falsification you can of course bring in a bit more Theory of Knowledge and find some possible flaws in the argument. It may be that the huge asteroid hitting the Earth did not contain any titanium so of course the ratios would then be the same. Coincidence may play a part and the asteroid may by chance have had exactly the same ratio of the two isotopes as the Earth. It is also possible that the temperatures reached were so high that the titanium (probably in the form of its oxides) did in fact vaporise during the impact and hence distribute itself equally between the two bodies. There are serious counter arguments to all of these possible hypotheses. So at the moment the evidence is now in favour that the entire moon came from the Earth alone. More than 40 years after the Apollo missions there is still no good theory as to exactly how that might have happened.