Where did the Moon come from?

And what the theory might reveal about its geological properties...
29 August 2023

Interview with 

Dana Mackenzie




Chris Smith spoke with science writer Dana Mackenzie, the author of 'The Big Splat, or How Our Moon Came to Be.'

Dana - The Moon is one of the most amazing places in the solar system. It's much larger compared to our planet than any other moon in the solar system. Earth's moon is about one 80th the mass of earth, comparable in size to North America in terms of land area. And so this suggests that our moon has a very special origin story. Another thing that I think is really cool about our moon, we're really a double planet. Both of the planets go around each other. So if you are an alien, you probably wouldn't say earth is the third planet. You'd say the Earth Moon system is the third planet from the sun.

Chris - You mentioned that it must have some kind of exciting origin because the Moon is so big.

Dana - This was a mystery for a very long time. And it's finally been resolved to the satisfaction of most lunar scientists. So the current accepted theory is that the Moon was created by a giant impact between earth and another planet. And this impact actually destroyed that planet. It basically liquified it and smeared it around the Earth in a ring. And then the debris coalesced over a pretty short period of time to form the moon. And so this giant impact theory was proposed in the 1970s, became accepted in the 1980s at first, considered really outlandish, but you have to realise that the early solar system was a really chaotic and violent place. And we see evidence of these sorts of impacts all over the place in the solar system. For example, we see the planet Uranus is tipped over on its side. What did that? Probably a giant impact. And the same for our moon. So the moon really has a lot to tell us not only about our origins, but also about what the solar system was like early in its history.

Chris - If the Moon was born from an impact between the early Earth and some other impact tour, does that mean it's made of the same stuff, effectively, that we are? Do we already know what's on the Moon? And we know the earth is replete with all kinds of exciting resources and therefore we can be reasonably sure the same thing is lurking up there waiting for us.

Dana - Well, that's a great question. And actually it's subtle because there are some ways in which the Moon is similar and some ways in which it's different. This giant impact for one thing would have boiled away elements that have a low melting point. So-called volatile elements are less common on the moon. And also, in theory, this is also why the Moon is vastly drier and there's virtually no water on the moon. And this was actually considered one of the main reasons for accepting the theory. Now, since I wrote my book, we've actually discovered that there is water on the Moon in small amounts, but perhaps in amounts that we can use. And it's located in the lunar poles, particularly the South Pole, where you have permanently shadowed craters. So now one of the big questions that the current missions want to address is how much ice is there - what form is it in? Is it mixed in with the dirt or is it more pure ice and how can we get it out? And that's one of the reasons why so many countries are now sending spacecraft there to answer these questions.

Chris - And is the reason they're focusing on the South pole of the Moon is purely because it's always in sunshine? Or is there another motivation for why they want to head down there? They're all going south. Why is that?

Dana - The main motivation is that the known deposits of water ice are the South Pole. Smaller deposits may be at the North Pole. There's a mission called LCROSS that deliberately crashed into the moon in 2010 or so in order to blow up a plume of debris so that they could see what was in the debris. And sure enough, they spotted water ice, water molecules. So we know that the water is there. The reason it's probably there is because, as you mentioned, permanently sunlit areas. But it's the permanently shadowed areas that collect the ice. They're so cold, they're colder than even outer space right now. So these are the areas where the water gets trapped and can't get out. Coincidentally, there are also regions of almost perpetual sunshine near the South Pole where you have mountains. There's one mountain which is almost in perpetual sunlight. Now you won't find water there, but it'd be a great place to build a base. So yes, that is useful as well. You could build your base on that mountain and then have your mining operation down in the craters.


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