The Origins of the Moon

17 August 2014

Interview with

Alex Halliday, University of Oxford

The moon seems a timeless feature of our night sky, but there was a time before it existed. So how was the moon first formed? New research published this week might change our understanding of our planets largest satellite. Alex Halliday from the Head of 'Mathematical, Physical and Life Sciences Division' at Oxford University spoke to spoke to Ginny Smith about the findings. The moon

Alex -   The giant impact theory basically proposes that the Earth was hit when the Earth was about 90% formed. Planets like the Earth grow over a period of time millions of years by accreting other planet by gravity so they get bigger and bigger. And when the Earth was about 90% of its current size, it was hit with another planet about 10% of the size of the Earth which is about the same as planet Mars.  And this Mars-size planet hit the Earth with a glancing blow which gave the Earth and the debris all around the Earth from the impact the spin.  It also would've heated the planet up.  And finally, it would've actually formed a disc around the Earth and that disc of debris and gas would've condensed and coalesced to form the moon that we have today. So the giant impact theory came about in the 1980s as the 'least worst' explanation for the origin of the moon. One thing is, the moon is larger relative to the size of its host planet than any other moon in the solar system, the other thing is that the moon moves around the Earth as the Earth spins- what we actually see in terms of the movement appears to be coming from the moon itself and that's something that's a bit hard to explain.  The moon is gradually moving away from the Earth over time.  But there is also one other striking thing that was not clear until the Apollo astronauts went to the moon and brought back samples.  And that is, that the moon appears to have a slightly lower density to the Earth, suggesting it's not quite the same  stuff as the Earth.

Ginny -   So effectively, a smaller planet knocked the moon off the Earth.  It knocked a chunk of the Earth off and that became the moon.  Is that the idea?

Alex -   So, that's where the problem lies.  This impact would've been so energetic- think about the collision of a 10-kilometre sized object, wiping out the dinosaurs. But this was something actually another planet colliding with the Earth, so it wouldn't have just knocked a bit of the Earth off.  It would've actually vaporized parts of the Earth and caused massive damage.  If you model this with supercomputing which we can now do, you can track the particles that make up the Earth and the particles that make up the other planet as they collide.  What you show in these simulations is that most of the stuff that ends up forming the moon, comes from this other planet that we sometimes call Theia.  When we look at those samples that the Apollo astronauts brought back, we find that this doesn't fit at all.  If you look at the atoms in those samples, they have a diagnostic fingerprint that suggests they came out of the Earth, not out of this other planet.

Ginny -   So, the theory doesn't quite match up with what we actually see when we test it.  So, do we need a new theory?

Alex - Over the last decade, there've been a lot of new computer simulations, lots of new measurements of the composition of samples brought back from the moon, providing new constraints. And so, one of the key things was that people had started questioning whether this spin of the moon around the Earth and the spin of the Earth itself, the orbit of the moon around the Earth rather, whether this was actually a result of the giant impact or whether the Earth was already spinning before the giant impact took place.

Ginny -   What do we need to do in order to find out how the moon was formed?  What research would you love to see happen?

Alex -   Well, the main problem we've got at the moment is that this issue about where the atoms come from.  If you look at what we call isotopic fingerprints of the atoms in the moon, they look just like the Earth.  And so, it looks like a bit of the Earth has formed the moon.  If you do the simulations, it suggests that the moon should come out of this other planet which should have a different composition from the Earth.  So, there's one possible thing that either those simulations are wrong or alternatively, there's a possibility that maybe this planet Theia actually came from inboard of the Earth rather than outboard of the Earth.  It's possible that the compositions of the atoms in that region actually may look more like the Earth in that region.  And if we could get samples of Venus or Mercury, we could test that theory.

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