Moon turned itself inside out while forming

Large parts of North America witnessed a total solar eclipse this week - which occurs when the Moon passes between the Earth and the Sun, blocking our view of the Sun as it passes. The Moon’s ability to block our view of the Sun is an incredible cosmic coincidence. It happens because - by chance - the Sun and our Moon appear almost exactly the same size in Earth’s sky. So it’s perhaps a good time then to reappraise how the Moon was formed 4.5 billion years ago and what’s happened to it since, including the possibility that it’s turned itself inside out in the past! Here’s Jeff Andrews-Hanna at the University of Arizona…

Jeff - We're basically trying to understand the earliest evolution of the Moon and how the Moon got from what we think was initially an entirely molten body to the body we see today orbiting the Earth.

Chris - Because the Moon is thought to be the product of a giant cosmic collision. In fact, some people have written books called The Big Splat and things like that some four and a half billion years ago, isn't it?

Jeff - The Moon formed in some manner of giant impact. The details are uncertain. In one version of the story, a Mars sized body crashes into the proto Earth and the outcome of that is material gets ejected out into space and that material out in space very quickly, very rapidly and violently coalesces to form the Moon. But it's such a rapid and violent process, that initial Moon that formed orbiting the Earth would've been mostly or entirely molten. We say that the Moon had a magma ocean that may have extended to hundreds of kilometres depth, and may have gone all the way down to the core.

Chris - What is the evidence that that's either true or may be uncertain?

Jeff - The story of the magma ocean really begins with rocks that were brought back by the Apollo astronauts more than 50 years ago. That's where we really confirmed that the crust of the Moon, the bright surface of the Moon that we look up at at night, is made of a mineral called plagioclase. And plagioclase is really important because as that magma ocean crystallised, at some point plagioclase starts to form and it's light, it's buoyant, it floats. So it floats to the top of the magma ocean. That's why much of the surface of the Moon looks so bright when we look up at it at night. And that really, I think, clinches the story for the magma ocean. But the rest of that solidification process is really interesting because, as the Moon crystallises, different minerals form at different times and the final outcome basically leaves us with a Moon that is just wildly unstable and out of equilibrium. Because aside from that bright crust that we see sitting on the surface today, below that crust is the lunar mantle. And in the mantle after the magma, the ocean solidifies the most dense material, minerals rich in titanium and iron, are all sitting at the top of the mantle and the least dense material is all sitting at the bottom of the mantle. And that's just not where it wants to be. And so there's long been this idea of an overturn of the lunar mantle. That after the Moon solidifies, basically the mantle turns itself inside out. Everything on the bottom floats to the top. Everything at the top sinks to the bottom. And that's basically going to be the biggest thing that happens to the Moon between the Moon's formation and today. And it really sets the stage for everything else that happens.

Chris - This is a theory, of course. You've done some modelling to come up with this idea. Does this give us some testable hypothesis though? Some experiments we can now do in order to see if what you are proposing is what really happened?

Jeff - You know, there are a number of different models that have looked at this process of mantle overturn on the Moon. And this is a problem that goes back decades. For us though, this story really begins with a paper that some of my colleagues wrote a few years ago. So Nan Zhang had a model of this magma ocean overturned, and he published that work. And I looked at the figures, he showed the patterns of what he showed, and it reminded me of something that I'd seen in data from the Moon, in gravity data in particular. In his model as this really dense titanium rich material at the top of the mantle sinks into the interior, it sinks into this really unique pattern. It basically makes these sheetlike downwellings, it's basically like waterfalls or avalanches of titanium rich material. But it's not quite perfect. At the end of that process in his models, there are these long linear zones of dense, titanium rich material that are still sitting at the top of the mantle that didn't quite manage to sink down into the interior. And I saw those results and it immediately reminded me of something I'd seen in gravity data. About a decade ago I was analysing gravity data from NASA's GRAIL mission. And here we're looking at just subtle, subtle variations in the lunar gravity field. It's really important data because it tells us about what's going on below the surface. It basically tells you the distribution of mass and density beneath the surface. And when I analysed that gravity data 10 years ago, I saw this really interesting pattern on the near side of these long linear gravity anomalies intersecting in this polygonal pattern. And what I was seeing in the gravity data 10 years ago looked exactly like what Nan was showing in his Geodynamics models today,

Chris - You say on the near side of the Moon because the Moon is a bit weird, isn't it? Because although it's tightly locked, it always shows us the same face and that's where we get this whole concept of the dark side of the Moon. It's not dark at all, it's just that we don't see it. The other side looks very different to the side that we see now. Has that got anything to do with this?

Jeff - Yes, very much so. The Moon is fundamentally lopsided in almost every respect. When we look up at the Moon, we see these bright and dark patches and the dark patches are these mare volcanic lava flows. And there are very few of those lava flows on the far side. But, but this, this asymmetry, this lopsidedness goes beyond that. The near side is low in elevation and the far side is high in elevation. The near side has a thin crust. The far side has a thick crust. And very interestingly, the near side is really rich in titanium and the far side is generally poor in titanium. And so in these models, there's a giant impact on the far side that makes this giant impact base we call South Pole-Aitken. And that actually triggers, it catalyses, this migration of dense, titanium rich material towards the near side. And then it's on the near side that that all sinks into the interior.