This transcript was generated automatically and lightly edited for readability.

Oliver Strimpel: This is Geology Bites with Oliver Strimpel. In 2025, NASA plans to land humans on the Moon for the first time since the end of the Apollo missions over 50 years ago. While the mission will focus on how to make the Moon habitable, with a long-term goal of establishing a permanent base on the Moon, it will also present us with an opportunity to advance our understanding of lunar geology. So what are some of the major outstanding questions about the origin, evolution and present-day structure of the Moon? And how will the presence of humans on the lunar surface help us tackle these questions?

Mahesh Anand studies the formation and evolution of planetary bodies in the solar system by analyzing samples from the Earth, Moon, Mars and asteroids. With the imminent return of people to the Moon, he has also turned his attention to developing ways in which lunar inhabitants could most effectively take advantage of the resources found on the lunar surface, such as the rocks and water ice. He is Professor of Planetary Science and Exploration at The Open University. Mahesh Anand, welcome to Geology Bites.

Mahesh Anand: Thank you very much for inviting me.

Oliver Strimpel: Can we start by reviewing at a high level what we do know about the geology of the Moon?

Mahesh Anand: Sure. The geology of the Moon is relatively simple. We can see with our naked eye on a clear night that the Moon looks in some places very bright and in some places a little grayer. And essentially those are the two main geological regions on the Moon, which are known as highlands, which are the brighter regions, and maria, which look a little bit grayer. And those two regions are reflecting the two main geological units on the Moon, which formed during the geological evolution of the Moon since its formation. The highland regions appear as high mountains, rugged regions. These are more ancient surfaces of the Moon, whereas the maria are the filling up of the large basins by lava flows that erupted after the Moon had formed.

Oliver Strimpel: What do we know about the internal structure of the Moon?

Mahesh Anand: The Moon is similarly stratified internally to our own Earth, so it has got a crust, a mantle, and then it has got a relatively small core compared to its size. The core may be very similar to Earth's core, where there is an inner solid core and an outer liquid core.

Oliver Strimpel: But no plate tectonics and no volcanism.

Mahesh Anand: That's correct. So that is the biggest difference between the Moon and our Earth: on the Moon there is no plate tectonics, which means that it is essentially a static body that differentiated at some point in its geological history, and since then it has been relatively inactive compared to the Earth.

Oliver Strimpel: But it has had volcanism in the past.

Mahesh Anand: It did. So again, comparing with the Earth, there is no active subduction taking place on the surface of the Moon. But there has been, you're right, there has been volcanism in the past. And as I said, some of these basins were actually filled by vast quantities of lava flows. So clearly the Moon has had various episodes of volcanic activity, and we know that because we have actually collected some of those samples during the Apollo missions. We also have some of those samples as lunar meteorites. And in fact, most recently, China's Chang'e 5 mission collected further samples from the Moon that suggested the volcanism on the Moon persisted until at least as recently as 2 billion years ago.

Oliver Strimpel: Let's talk about some of the major outstanding questions then about the origin, evolution and present-day structure of the Moon. First of all, regarding the origin, while it is generally accepted, I think, that the Moon formed following a collision with a Mars-sized body about four and a half billion years ago, I don't believe there is consensus as to exactly what happened next to form the Moon. In an earlier episode, I talked to Sarah Stewart, who thinks that a large torus-shaped object called a synestia formed around the Earth and that the Moon coalesced out of that. Will the return of humans to the Moon help us narrow down the number of viable scenarios for the Moon's formation?

Mahesh Anand: I think so. The challenge to the science community is: can we actually reconcile all the various characteristics that we know about the Moon, based on our research using the lunar samples, based on the measurements that have been made at the lunar surface, and based on measurements that are made from orbit? Can we actually combine all those measurements and observations together to tie them in with the synestia model? I think that's an open challenge, but it is an elegant model to explain many, if not most, of the observations.

The biggest issue about the Moon that remains is that we really don't know its age. So when we say the Moon formed around four and a half billion years ago, it matters whether it actually formed within a few million years of the solar system formation itself, or 50 million years later, or 100 million years later. Right now there is no consensus, and there are two main groups: one that says that actually the Moon is old and it formed very close, almost within 50 to 60 million years, to the solar system formation itself, which, by the way, happened 4,568 million years ago, whereas the other group says no, no, the Moon is young; it actually formed at least 100 million years after the solar system formed. And the only way to confirm which of these theories is right is to go back to the Moon with the new information that we have collected in the intervening 50 years since the Apollo missions brought samples back to Earth, and target areas where we think we are going to get the best samples that will help us address the age of the Moon.

Oliver Strimpel: Where do we expect to be able to find such samples?

Mahesh Anand: So sometimes you have a serendipitous discovery, but we can target regions that appear very old based on crater counting. So if you are looking at the south polar region of the Moon, people might have heard about the South Pole–Aitken basin, which is believed to have formed around 4.3 billion years ago. Now, it is the largest basin on the Moon. It's about 2,500 kilometers in diameter. So imagine an impact creating that basin: it could have excavated material from as deep as the mantle of the Moon. And in fact, right now we have absolutely no confirmed sample of the lunar mantle. And one of the things that a sample from the lunar mantle could help us figure out is when actually the lunar mantle took shape, and that indirectly could tell us about the origin of the Moon itself. Alongside, we could be looking at some of the earliest basalts that might have erupted onto the lunar surface, and again using them as an indirect tracer of the lunar interior and getting back to the origin of the Moon.

Oliver Strimpel: That's fascinating, but do all these theories still assume that the Moon formed very soon after, or at some point maybe 100 million years after, the formation of the Earth through this collision with Theia, as they call it, a Mars-sized body?

Mahesh Anand: Almost everybody agrees that the Moon did form within a couple of hundred million years of the solar system formation. The difficulty is that because the Moon's geological history is so intertwined with our own Earth's geological history, and because right now we have no better explanation for the origin of the Moon other than the Earth being hit by an impactor, what that means is that when that big impact happened, it must have disturbed whatever was there on Earth. So if we want to understand when Earth became habitable, when the Earth's atmosphere actually became what we think of as an atmosphere, when water on Earth might have started existing as liquid water, all of those questions, it's absolutely critical that we know the timing of the Moon's formation, because if we know the timing of the Moon's formation, from there on we can actually get the Earth's clock ticking for the variety of things that I just listed.

Oliver Strimpel: Just coming back then to the presence of humans on the Moon, so we've been able to scout out a few more areas. You mentioned this area near the South Pole where we think we might be exposing some really ancient rocks. So why would the presence of astronauts walking on the surface of the Moon help us snag those rocks, if you like, and identify which ones we're interested in?

Mahesh Anand: Having astronauts at the lunar surface is going to be absolutely pivotal for how we advance our understanding of the Earth–Moon system collectively. For example, the south polar region of the Moon is very rugged. It's very difficult to have a rover trundling around and actually doing the measurements. The speed at which a robot can actually cover some of those areas is going to be several orders of magnitude slower than a human can manage. Secondly, humans can take a decision in real time when they see an interesting outcrop, and they can then communicate with the team back here on Earth and discuss in real time what should be done. A trained geologist can look at a particular type of rock that they might have seen on Earth, and just out of curiosity, they might pick it up, because they can make certain assumptions in a few short seconds and think, oh, this rock might actually tell us something. So some of these things can only be done, at this moment in time, and most efficiently, by humans. And that's why I think that if humans are returning to the Moon, it is going to be very good news for lunar science.

Oliver Strimpel: When we spoke earlier, you mentioned the story about Jack Schmitt and Apollo 17.

Mahesh Anand: Jack Schmitt was doing the extravehicular activity on the Moon. He came across something that didn't look in place. It looked very different to the rest of the surrounding area, and he stumbled upon what we know as orange glasses. And these orange glasses are the ones in which water was discovered for the first time in lunar samples, back in 2008. And it opened up a whole new area of research, reanalyzing Apollo samples and other lunar samples for the possibility of detecting water and other volatiles.

Oliver Strimpel: You also mentioned when we spoke earlier that had we known what we know today, we would probably not have picked all those landing sites for the Apollo mission, because they, it turned out to be special in some peculiar way. In what way were they special, and why would we have chosen differently today?

Mahesh Anand: So that's quite fascinating, because in the Apollo days, the overriding desire at that point was: how can we get humans to the lunar surface as soon as possible? And for that we looked at the near side of the Moon, the safest landing area, and that was the equatorial region of the Moon, those flat plains in the maria region. And so that was the overriding objective. We landed there, we collected the samples, and then we repeated it a few times. In the following couple of decades, when we started sending satellites to make global-scale measurements of the Moon, we started noticing that the area of the Moon where the astronauts landed is geochemically very distinct compared to the rest of the Moon.

And what I mean by that is that if you look at the near side of the Moon, in those dark patches that we can see with our naked eye, most of that dark, patchy area is characterized by some elements which together are called KREEP, which stands for potassium, rare earth elements and phosphorus. Now, these are incompatible elements that are thought to have concentrated in the final bits of the lunar magma ocean crystallization, and somehow they were brought to the lunar surface during an impact process. And this is the external expression of this incompatible-element-rich layer in the upper part of the lunar mantle that is defining the near side of the Moon. And we just went and collected those samples and ignored the rest of the Moon. So if you want to fully understand the geological evolution of the Moon, we must go to other areas that are not contaminated by this so-called KREEP.

Oliver Strimpel: And such areas would include, for example, the South Pole.

Mahesh Anand: Absolutely. So the South Pole–Aitken basin is another region where we see a slightly higher concentration of incompatible elements such as thorium, but it's not as high as in the near side of the Moon. But you can go to other areas of the far side where there is hardly any expression of these KREEP signatures, as we call them. Or you can go to the north of the area where Apollo landed. So there are many other areas of the Moon, on the far side and in the polar regions, which can provide additional samples.

Oliver Strimpel: But given that these areas might be very rugged, as you pointed out, not the best place for a lander to come down, how would the astronauts reach these areas?

Mahesh Anand: So there are two different things here. For the exploration of the Moon, the technology is improving quite rapidly, such that you can actually have more precise, smaller landing ellipses, as they are called. So you can now target areas, perhaps, which are 100 meters by 200 meters, and you can still land in the center of it, as opposed to kilometer-size landing ellipses. Now, landing is one thing, but then to traverse the terrain nearby is another. So what I was referring to is that if you land a lander, it will have a rover. For that rover to be able to negotiate those terrains will be much harder than it would be for astronauts.

Oliver Strimpel: One of the reasons that the lunar South Pole was selected as the landing site for the Artemis mission was to do with the fact that we believe that there's some water ice surviving there.

Mahesh Anand: That is probably the most important reason why humans are planning to land in the south polar region of the Moon. There are other reasons why the South Pole is good for addressing other scientific questions, in addition to confirming the actual presence of water ice, which has been indicated by a number of remote sensing spacecraft, right from the early 1990s to the present day. It will be absolutely fantastic if we can confirm that there is indeed water ice present in one of these polar locations, and it's most likely to be in the south polar region, because that's where the majority of the missions are going.

Oliver Strimpel: But is it likely to be in quantities sufficient to support long-term community of humans?

Mahesh Anand: At the moment, all we know from remote measurements is that there may be some water ice. So first we need to confirm: is there water ice? Then the next thing would be to confirm its distribution, both laterally and vertically, because remote sensing measurements at best can give us information about the top meter or 2 meters, but most of them are only telling us about the top millimeter. But with humans, we would be able to sample deeper levels, and if we can confirm that water ice is present in those locations, then we can be a bit more confident about extrapolating that and finding out the actual reserve that might be there.

Oliver Strimpel: Are there any theories as to the origin of this water?

Mahesh Anand: There are many, many theories. This is another area of research which I think will blossom once we have samples returned from the polar locations. So far, from the work that we have done, we have speculated that one of the sources for this water ice, or the hydrogen in this water, could have been the Sun itself, through the solar wind implantation of protons into the lunar regolith, which then combine with the oxygen that is already present there to form water. That's one way you could have produced water.

The second is due to cometary impact. So we know that comets are full of ice and carbon-rich material, and the Moon, as I said before, has been subjected to intense bombardment by all sorts of meteorites or impactors, and comets must have been part of it. And in the polar locations on the Moon, the temperature is so low, in some places as low as 25 kelvin, so now we are talking about minus 250 degrees Celsius, that any water that gets near it is going to stick there. So if cometary water was being delivered to the Moon through impact, the place where it would probably end up would be in these permanently shadowed regions.

And then you also have asteroids. Even today we have so many asteroids that send meteorites our way on Earth. Likewise, they send lots of meteorites to the lunar surface, and they can contain as much as 20% of their mass as water. So again, asteroids could have been a source. Now, we can actually differentiate between these sources if we have our hands on that water ice. If we can bring it into our lab and make the isotopic measurements for hydrogen, oxygen and various other things, we should be able to tell which sources are contributing to this water ice. That will then allow us to speculate how much of that water ice could have been concentrated there over the past four and a half billion years of lunar history.

Oliver Strimpel: And that origin of water would be the same as how water got to Earth as well.

Mahesh Anand: Absolutely. We would not just understand what the sources for this water were; we would also hopefully understand the timing of this water delivery. And understanding the timing of water delivery on the Moon is going to be intimately linked to the history of water on Earth. And then we start addressing that long-standing unresolved issue of when the Earth became habitable. When did it get its water? Did it have it right from the beginning? Did it arrive during the giant impact? Did it arrive after the Moon had formed from a giant impact? At the moment we are not able to distinguish between those theories, but this will help us if we can actually touch and analyze the water that is present at the lunar poles.

Oliver Strimpel: The surface layer of the Moon has been affected by weathering because, they don't have weather on the Moon, but still exposed to micrometeorites and radiation and so on. How far down below the surface do you need to get before you get to pristine lunar rock? And how will these human missions help us get to that rock?

Mahesh Anand: This is another fascinating area of lunar geology. Because there is no atmosphere on the Moon, the surface of the Moon has, since its formation, been exposed to meteoroid impact. It has been exposed to solar wind. It has been exposed to galactic cosmic rays. And all of these act just like weathering agents here on Earth, where our rocks get affected by the action of water, wind, et cetera. Over the last few billion years, because these surfaces have been constantly bombarded by these elements, there is a top layer a few meters thick at most places, where you have a loose material called regolith, and this regolith is a mixture of broken-up bits of rocks and remelted rocks, et cetera. To get to the actual bedrock, you have to drill at least a few meters below. Now, either humans can actually take the drilling equipment with them, or, in some remote sensing imagery, there is some evidence that there are some open lava pits where you can see the layering in the bedrock. So it may be possible that some of those lava pits become places for exploration where you can directly sample bedrock.

Oliver Strimpel: You just mentioned that China's Chang'e 5 mission discovered that volcanism has been happening much more recently than previously thought, as recently as 2 billion years ago. And we know that's an accurate date, as it was obtained by radiometric dating of a sample returned to Earth. How does that affect our understanding of lunar evolution? And will that, in turn, have implications for the return-to-the-Moon missions we're talking about?

Mahesh Anand: We did have some inkling before the Chang'e 5 mission that the area they were trying to sample represented much younger volcanism, between 1 and 3 billion years. And that was on the basis of what we call crater counting techniques. So if a surface has more craters, it's relatively older; if a surface has fewer craters, it's relatively younger. So based on the crater counting, using the remote sensing data, our colleagues had already prepared a map of the Moon where we knew what the ages of different units were, and the unit that Chang'e 5 was targeting was in the estimated age range of 1 to 3 billion years. Now, it was not until they landed and collected the sample and brought it back to the Earth, and it was analyzed in the lab, that they confirmed that it was exactly 2 billion years old.

It provided an anchor point for what we call the cratering curve for the inner solar system. And much of the inner solar system cratering record is based on the cratering record that we have learned from the Moon. For example, the ages we ascribe to the different regions of Mars are based on the cratering chronology derived from lunar studies. The ages we ascribe to surfaces of Venus or Mercury, those are all ages derived from the lunar cratering record. So the Chang'e 5 sample helped us put an anchor point in that cratering curve, which was anchored by some very old ages that were reported from the Apollo missions and some very young ages that were also reported from some of the samples collected by the Apollo missions. And in between, we didn't know whether the lunar cratering record had a spike, whether it was monotonically decreasing, or what the bombardment history was. And now we know that actually it almost follows a nice curve, and we can confirm that we know a bit better than we knew two years ago.

Oliver Strimpel: So given that that's helped us calibrate the age of the surface of the Moon, have we been able to then locate regions of the Moon that are still younger than 2 billion years?

Mahesh Anand: Yes, indeed, there are areas on the Moon which appear to be as young as 100 million years. Can you imagine? I mean, we thought that the Moon is a fairly cold object where volcanism ceased shortly after 3, 3 and a half billion years ago. But now we are finding that volcanism at least happened 2 billion years ago. Now, if we are seeing some tantalizing evidence from remote sensing that there are surfaces which may be as young as 100 million years, and we think that they formed through volcanic eruptions, then I think there is every reason to actually target these places, go there and collect the samples, and confirm by radiometric dating back here on Earth.

Oliver Strimpel: If we confirm that 100-million-year-old volcanism, would that really turn our theories of the evolution of the Moon on their head?

Mahesh Anand: Well, I think we would have to go back to the drawing board. There is going to be an attempt to do some in situ radiometric dating. We have improved over the past 50 years in our ability to make certain measurements using advanced instruments at planetary surfaces. Now, to date something which is 100 million years old, even if your error bar is 100 million years, it doesn't matter, because you are trying to differentiate something which is either hundreds of millions of years old or billions of years old. So to get a first pass, you can conceive of sending a robotic instrument that can do a crude radiometric dating that will establish whether it is 100 million years, plus or minus 100 million years, or it's actually a billion-year-old surface. And then in the next phase, you collect that material, because it's not just the age that we are interested in. We are also interested in knowing the chemical compositions. We are also interested in knowing the other radiogenic characteristics, such as the strontium isotope, neodymium isotope and hafnium isotope compositions. And there's a whole plethora of measurements that you can make of stable isotope compositions that we are using nowadays to ascertain how, in this case, the Moon, but in other cases, how planets actually form and evolve.

Oliver Strimpel: Wow, that's fascinating. So through the use of all these isotopes and geochemical analysis and isotopic analysis, we'll be able to do some of the similar kind of forensic history, if you like, that we've been able to do on the Earth.

Mahesh Anand: Exactly. And given the technological advancement and our ability to now measure a single grain at a time, we can utilize a very small amount of material. So nowadays we are using perhaps less than a few tens of micrograms of material, not even milligrams. We are using micrograms of material from which we are able to decipher a global-scale phenomenon, in this case the origin and evolution of the Moon.

Oliver Strimpel: You mentioned that our ability to conduct in situ measurements on the surface of the Moon, and hopefully on Mars and elsewhere too, is improving rapidly. But still, you're especially interested in analyzing samples that we've been able to bring back to Earth. It's an awful lot of trouble to collect a sample and bring it back safely to Earth. So why do we go to all the trouble to do that? What exactly do we need to measure on Earth that we cannot actually measure in situ?

Mahesh Anand: The biggest issue with doing things in situ on another planetary body is that we are constrained by resources. For running some of these instruments in our labs, we need a huge amount of space, we need energy, we need sample preparation techniques. So it is almost impossible to prepare the samples in the right manner and then have the right equipment present at the lunar surface or at the Martian surface to make the measurements that we need to make today to advance our understanding. What these remote instruments are able to do is to make some relatively crude measurements to act as a pathfinder. Otherwise, we would be spending two or three orders of magnitude more to still get probably a tenth of what we can get here on Earth.

Oliver Strimpel: You've recently become interested in technology for building a colony on the Moon and study the effect of microwaves on lunar rock. What did you discover, and why are you interested in that?

Mahesh Anand: We experimented with microwaves to see what type of material changes they bring about when you actually process the lunar material. In this case, we had to start with lunar simulants, because of course we could not burn the precious lunar material that was collected. And what we started finding was that microwaves were much more energy efficient in processing lunar regolith. And not only that, we then conceptualized that potentially you could use microwaves to melt the lunar regolith, which, by the way, happens in less than 10 minutes. And we are using the same microwave power as in our kitchen microwave. So we put about 50 grams of powdered lunar simulant in the microwave and we heat it up for 10 minutes, and within 10 minutes we reach a temperature of over 1,500 degrees Celsius, which is the temperature at which you melt a solid rock. So we are melting this lunar simulant in the hope that one day we can handle it and put it through the nozzle of a 3D printer, which can then print objects for us, which can then go on to help build a lunar habitat.

So we have demonstrated that microwaves are much more efficient in heating and sintering. Sintering is a process before melting where you just fuse the grain boundaries, and things become a bit more solid than where you started. And something else that I found, that I didn't set out looking for, was that by microwaving, I'm changing the material properties in such a way that I'm concentrating some of the things that can be considered critical metals.

Oliver Strimpel: So potential twofold benefit, both as a way of sealing together blocks or consolidating blocks of lunar material for habitation, and then also extracting valuable metals.

Mahesh Anand: Absolutely. So multiple purposes. With microwaves, we can process the regolith. We can make bricks, as you alluded to; you can build your habitat. If there is any water present in that regolith, you can extract that water, so you get your water. That water can then be electrolyzed into oxygen and hydrogen. If the microwaving is also helping us to concentrate certain metals such as iron, you can set up a process to extract that iron, and we know how iron has been used throughout human history. And at the end, whatever is left is going to be used in building your shelter. So a zero-waste approach.

Oliver Strimpel: So when the Artemis mission returns and manages to successfully land people on the Moon, let's say it's 2025, what aspect of that will you be looking at most closely and be most excited to learn about?

Mahesh Anand: I really hope that this is going to be an international effort. So this is for humanity to extend its presence to the Moon. Secondly, we also want to see human endurance, because this time the Artemis astronauts are going to stay there for longer periods of time. They are going to be taking experiments with them. They themselves are experiments in their own right: how humans can cope in a very different environment to the one that the Apollo astronauts experienced when they landed there. So I think human endurance, the teamwork, the exploration they do and the discoveries they make. And as I said, I'm a firm believer in serendipitous discoveries. No matter how much planning we do before we go somewhere, we always find something new. That's where the opportunities are unlimited and nothing is beyond limits.

Oliver Strimpel: Mahesh Anand, thank you very much.

Mahesh Anand: Thank you. I really enjoyed talking to you.

Oliver Strimpel: To see pictures and illustrations that support this podcast, go to geologybites.com, where you'll also find transcripts and a subject matter index of all the episodes there. You can also give me feedback, which I welcome, as well as sign up to get my emails about new episodes.