The Gravity Assist Podcast is hosted by NASA’s Director of Planetary Science, Jim Green, who each week talks to some of the greatest planetary scientists on the planet, giving a guided tour through the Solar System and beyond in the process. This week, he is joined by lunar expert Sarah Noble to discuss how the Moon was formed, lava tubes and moonquakes, the “dark side of the Moon,” and mysteries we have yet to solve about Earth’s nearest neighbor.

Here’s a short teaser of this week’s podcast:

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You can listen to the full podcast here, or read the first part of the transcript below.

Jim Green: The Moon is a fabulous object in the sky that humans have always observed, and we’ve been studying it up close and personal for several decades. What do we really know about how it was formed?

Sarah Noble: Our best understanding of how the Moon formed is that, early in the formation of the Solar System, a big planetesimal [an object formed from rock, dust or other materials]– something about the size of Mars – crashed into the Earth, and that spewed a lot of material off [Earth into space], which then came back together to form the Moon. So the Moon really formed very hot and very violently.

Jim Green: My understanding of the initial part of that theory is, as the Moon began to coalesce and come together, it actually was pretty close to the Earth.

Sarah Noble: Yeah, it was much closer than it is today. The Moon actually continues to move away from us at a rate of about four centimeters a year. So, back in those days, yes, the Moon was much closer and would have been much bigger and brighter in the sky.

Jim Green: The Moon also affects the Earth in terms of messing with our tides – tugs and pulls. So if it was so close to us, the tides must have been enormous.

Sarah Noble: Exactly, they would have been much bigger than we see today.

Jim Green: Another thing about the fact that the Moon is slowly moving away is that we live in a wonderful time where the Moon is just at the right distance to pass completely in front of the disk of the Sun during solar eclipses.

Sarah Noble: Yeah, it’s perfect right now [but] there’ll come a day when it will no longer cover up the Sun and we won’t have total eclipses anymore.

Jim Green: One of the things we’ve tried to learn about the Moon since the Apollo days is more about its structure. What do we know about its structure today?

Sarah Noble: The Apollo missions actually left behind seismometers [and] for a while we had a seismic network on the Moon and measured a lot of moonquakes, [which are] the lunar equivalent of earthquakes. We understand that there are both shallow moonquakes and deep moonquakes, and this begins to help us understand what the internal structure of the Moon is. It turns out that the Moon actually has a core just like the Earth does.

Jim Green: Do we know if the core was ever liquid [i.e. molten]?

Sarah Noble: We don’t know, but presumably it was when the Moon formed, and the heavy elements, the iron and nickel, [sank] to the bottom to form the core.

Jim Green: One of the things that we know about Earth’s molten core is that it facilitates [an electric] current that runs around the Earth, which generates a magnetic field, which gives us a magnetosphere. Is there any indication that happened in the Moon, too?

Sarah Noble: The Moon doesn’t have a magnetosphere. It does have local areas where there are magnetic fields, probably caused by very large impacts.

Jim Green: Those remnant fields that are trapped in the crust of the Moon really change the environment [around them]. What happens?

Sarah Noble: That’s correct. They form something we call swirls, which are these really cool patterns of light and dark markings in sort of swirly patterns that we find in particular places on the Moon where we have this remnant magnetism.

Jim Green: As the Moon started to form, it was probably very volcanic. What do we know about how the Moon evolved over that period of time? Could it have had an atmosphere like the Earth?

Sarah Noble: There’s actually some very exciting research that came out recently indicating that there were so many volcanoes about 3.5–3.8 billion years ago that [the Moon] might have had a temporary atmosphere, not like our atmosphere, but something more akin to Mars’ atmosphere, sort of thin but enough to have wind even on the surface of the Moon. This atmosphere would have been full of volatiles, including water. We think it’s possibly one of the sources of the water [found] at the lunar poles.

Jim Green: If there was an atmosphere and some sort of [atmospheric] circulation, are there places on the Moon that we could actually go and make measurements that could tell us about that ancient atmosphere?

Sarah Noble: There are permanently-shadowed craters at the poles and we know that they have volatiles in them, including water. We have measurements from space, but we also have in situ measurements. We actually crashed a piece of a rocket into one of those areas and measured what came off [NASA’s LCROSS mission]. It turns out that we know for sure that water is one of the things that is hidden in those poles. We don’t know for sure what the source of that [water] is, but volcanism is one of the possibilities.

Jim Green: What is the concept behind permanently-shadowed craters? How can they be possible on the Moon?

Sarah Noble: Unlike the Earth, which is tilted in its orbit, the Moon is almost exactly straight up and down. [Sunlight can’t reach] down into the bottoms of deep craters at the poles, so there are places that haven’t seen sunlight for over a billion years.

Jim Green: That’s absolutely fascinating. In fact, there’s an indication from some missions that by looking at certain things, such as escaping neutrons, that there may be water trapped underneath the surface, and that indicates all kinds of different things about the evolution of the Moon’s poles over time. What can you tell us about that?

Sarah Noble: It turns out that the permanently-shadowed regions don’t match up perfectly with where we find evidence for volatiles, and if you work it backwards, it turns out that the actual pole of the Moon seems to have shifted so that it’s in a different orientation now to what it was a couple billion years ago.

Jim Green: How could the Moon have shifted its pole after it started spinning?

Sarah Noble: You’ve maybe noticed that the Moon looks different on one side than the other, right? The side that faces the Earth has all these volcanic fields, all of this dark lava that has flowed out onto the Moon. The other side does not. The two sides are different. They have different thicknesses in their crust, and they probably weren’t always perfectly coordinated with one side toward us and the other. Over time, that has turned out to be the lowest energy position, and so the pole moved in order to get into the correct position.

Jim Green: One of the really fascinating areas on the Moon that we have discovered in the space age is on the far side. It’s called the South Pole Aitken Basin. What is that, and what can it tell us about the Moon’s evolution?

Sarah Noble: The South Pole Aitken Basin is one of the biggest impact [basins] that we have found in the Solar System. It’s big enough that [the impact] actually probably cut all the way through the crust and down into the mantle of the Moon and probably, if it had been even a little bigger, it might have blown the Moon apart. So it gives us an opportunity, a place where we can see deep down into the Moon where we can’t anywhere else. We are hoping one day to be able to go and take samples from there and see what those deep rocks look like.

Jim Green: The Earth has a core where most of the heavy elements like iron and nickel are found, and then the next major area is called the mantle, and then on top of the mantle is the crust. The mantle is under enormous amounts of pressure, which really changes the mineralogy of the rocks. We can’t get to Earth’s mantle, yet there might be mantle material on the backside of the Moon. From a planetary geologist’s point of view, that’s really spectacular.

Sarah Noble: Yeah, it’s very exciting.

Jim Green: I recently heard about the discovery of large lava tubes and from my perspective, they’re very fascinating – the tubes tell us about the past geology of the Moon but they also could be a future home for astronauts. What can you tell us about them?

Sarah Noble: Lava tubes are created when you have lava flowing and the top surface cools but the lava underneath it continues to flow. So you end up with basically a tunnel left over where the lava has flowed through. It turns out that these are places where we could actually consider putting people. They’re protected from the space environment, which is very harsh and you have to worry about everything from radiation to micrometeorite impacts. If you put your people underground, they are much safer there, and so that’s a very exciting thing. But [the lava tubes] also tell us about the Moon itself and what the lava was like and how it flowed. So we can learn about the geology of the Moon, as well.

You can read the second part of the transcript here.