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QF Focus Magazine


Where did the moon come from?


From the biggest cheese factory in the solar system....


Obviously! Well, perhaps not. The ideas about how the moon was formed have changed quite dramatically over the years, until today, now we have a solid theory based on fact and proof.


I'll pause a moment here to clarify what a scientific theory actually is. It is not 'an idea' or 'a suggestion' it is a clearly defined explanation of something in the natural world, confirmed by observation and fact.


When I was young, the most popular ideas about the moon were that it formed at the same time as the Earth from all the detritus surrounding the Sun in the early solar system, or that it was a minor planet captured by our gravity. If the moon had formed at the same time as the earth, it should have an iron core like the earth... it hasn't. If it was a captured planet, it should have different rock types to the earth, it hasn't. There are a lot more observations like this, but I won't bore you with them.


In the 1970s some researchers, evolved a new suggestion, based on what had been learned from the moon rocks brought home by the Apollo


astronauts. This is now accepted by most scientists. The giant impact theory.


The basic idea is this: around 4.45 billion years ago, a young planet Earth, still molten rock, had the largest collision of its history. Another planet with roughly the size and mass of Mars had formed with an orbit that placed it on a collision course with Earth - the solar system was cluttered in those days. When the young Earth and this rogue body collided, the energy involved was 100 million times larger than the meteor strike that probably killed most dinosaurs. The early giant collision destroyed the rogue body, vapourised the upper layers of Earth's mantle, and ejected large amounts of debris into Earth's orbit. Our Moon formed from this debris.


Sounds fanciful and fantastic doesn't it? But there is now so much evidence for this that you can feel quite safe in accepting it. The early solar system was a violent place.


Charles Oates. Vega Baja Astronomy group. Email: vegabaja.astronomygroup@gmail.com.


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