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Tue, 14th Apr 2015

How the Moon was Made

A waning moon (c) Tomruen @ wikipedia

How the Earth came by its Moon has always been something of a mystery: Scientists had theorised that a Mars-sized planet, called Theia, crashed into Earth and that the moon formed from the debris. But, analysis of the rock chemistry from the lunar surface reveals that the moon and Earth are practically identical. There appeared to be no chemical trace of Theia. So where did it disappear to? This chemical conundrum has thrown a lot of doubt on the impact theory. But a series of papers in the journal Nature suggests that the theory still holds up. Using simulations and isotope measurements, scientists demonstrated that the similarities between the Earthís and the Moons chemistry do not dispel the impact theory,  as Open University planetary scientist David Rothery explained to Chris Smith...

David - Ever since we got the first data back from the Apollo missions, itís been thought generally that the moon is a result of a giant impact, something about the size of the present moon or a bit bigger, maybe Mars-size, hitting the Earth, which was a glancing blow if you will and it threw a lot of debris off from the Earth, which coalesced together to form the moon.

Chris - And this happened fairly early in the Earthís existence?

David - Pretty early than the Earthís existence, yes. I mean the Solar System was growing around about 4.56 billion years ago and thereíd be a series of giant impacts all within a few million years of each other and the last one that hit the earth was the one that wouldíve formed the moon - if that theory works.

Chris - This slew of papers that are in the journal Nature, what do they add to the story and how?

David - What they show is that the moon and the Earth are too similar for the moon forming impact that have come from a very different part of the Solar System. The moon forming impact has come from the same region of space that the Earth formed in.

Chris - And how did they reached that conclusion?

David - There's three papers. One of them is looking at the flavours of oxygen, which are there. oxygen comes in three sort - oxygen 16, 17 and 18. So, youíve got normal oxygen slightly heavier oxygen and even heavier oxygen. The ratio between these is quite a good way of fingerprinting material because if the Earth and the moon were very different in an oxygen isotope ratios, that would tell it would form somewhere different.

Chris - So, thatís one of the papers.

David - Thatís one of the lines of evidence.

Chris - What are the other two?

David - The other two were dealing largely with tungsten. There's an isotope of tungsten, which is tungsten 182, which is produced by radioactive decay of hafnium 182. Hafnium is an element which likes to be inside rock, whereas tungsten likes to go inside iron. So, when you're forming the planet, the tungsten will go and join the iron in the planetís core. But if youíve got hafnium in the rock, that hafnium, after the core is formed can still decay to produce some tungsten. So, you have the tungsten enrichment in the rocky part of the planet because that tungsten was held in the rocky part because it used to be hafnium. So, it doesnít bear on how the moon formed. It bears on when it formed because it was all done by the time that all the hafnium had decayed.

Chris - And do they shed any light on whether the moon is made of cheese in these three papers?

David - We did discover calcium on the moon with an x-ray spectrometer on the Indian satellite Chandrayaan-1 that I was involved in and calcium have never been detected from their new orbit before. So, there's plenty of calcium in cheese, so we can't rule cheese out.

Chris - Food for thought there. That was David Rothery from the Open University.

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