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Season 3, Episode 3: Where Could We Go on the Moon? Steve Mackwell

Season 3Episode 3May 2, 2019

With NASA planning to send astronauts to the Moon by 2024, Steve Mackwell chats about the Moon’s exciting unexplored areas.

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With NASA planning to send astronauts to the Moon by 2024, Steve Mackwell chats about the Moon’s exciting unexplored areas.

Scientist-astronaut Harrison H. Schmitt collects lunar rake samples

Jim Green: A lot of cool places on the Moon that we’ve never been. What’s the difference between the near side and the far side? Where would be a good spot for astronauts to camp out? Let’s talk to an expert.

Hi, I’m Jim Green, chief scientist at NASA, and this is “Gravity Assist.” This season is all about the Moon.

With me today is Dr. Steve Mackwell. You know, when I first met Steve, he was the director of the Lunar and Planetary Institute, and now he is at the American Institute of Physics as the deputy executive officer, and I’m really excited to talk to him today. He’s one of the top planetary scientists in the world. Welcome, Steve.

Steve Mackwell: Oh, I enjoy being here.

Jim Green: So this is a perfect opportunity, you know, with the 50th anniversary of Apollo 11, to be talking about the Moon and what happened in the past, but, you know, really take a look at what we want to do in the future. So, let me start out talking about how the Apollo mission planners chose where to go on the Moon.

Steve Mackwell: Well, Jim as you know, the technology back then wasn’t what it is today, and one of the key criteria was safety and landing on a nice, flat spot with no big rocks.

Jim Green: Yeah. I think that was probably number one.

Steve Mackwell: Yeah.

Jim Green:Number one goal.

Steve Mackwell: Yeah. And we didn’t have great imagery of the Moon back then so we didn’t know how big the rocks were and Apollo 11 had some challenges actually finding a place to land safely. But safety was a key criteria. Also, you know, had to be on the near side because communications back to Earth was critical.

Jim Green: Yeah, couldn’t go to the far side. Can’t go to that side that we can’t communicate with. Indeed.

Steve Mackwell: Mm-hmm

Jim Green: So, all that changed. Apollo 11, 12 landed, 13 had to come home, didn’t make it to the surface of the Moon. It was a very famous mission everyone knows about, of course. And then we went on to 14, 15, 16, and 17. So really great set of six fabulous Apollo missions, and they brought back some material. You know, they brought back about 850 pounds of lunar samples. So what are some of the things these lunar samples have told us?

Steve Mackwell: Oh, we learned a lot about, not just the Moon, but also the Earth and the inner solar system. And in fact, we even learned about how the early solar system developed from those samples. One of the key things we learned is, because the Moon doesn’t have the kind of erosional history and everything that we see here on Earth, is, the surface of the Moon is relatively pristine. So we have the history of bombardment of impact cratering on the surface of the Moon that goes back right to the earliest times of the solar system. And we’ve used that information to calibrate the surface ages of Mars, of Mercury, of other planetary bodies in the solar system.

Steve Mackwell: So that was a really critical piece because, you know, we couldn’t unravel the history of our solar system without the information we got from the samples we brought back from the Moon. We also kind of learned a lot about how that the Moon evolved over time, and volcanism on the Moon. You know, there’s a lot of information on the samples that we brought back, even though they were from a relatively limited area on the surface.

Jim Green: You know, having those samples in our hands allowed us to date them. And, some of the key to that is really looking at samples that we brought back from the Moon. So how old is the Earth and the Moon?

Steve Mackwell: About 4.6 billion years. It’s–

Jim Green:Wow!

Steve Mackwell:We’ve been around a while.

Steve Mackwell of the American Institute of Physics talks about the Moon with NASA's Jim Green.

Jim Green: That’s right. You know what’s really neat that’s just happened recently is that we brought back a rock from Apollo that actually has a unique connection to Earth. You remember what that was?

Steve Mackwell: Oh yeah. There was a lot of hype about that. That was that was the first time we’ve actually seen something that could represent a piece of the Earth brought back in the samples from the Moon.

Jim Green: Wow!

Steve Mackwell: We could have more in the collections, but you know it’s still… You know, with the samples that we brought back, we’ve got a lot of very careful detailed science that we do on them. And, we’re going to see, looking at that sample in more detail, there’s some questions about whether it’s truly lunar material or Earth material. We’re going to see.

Jim Green: What makes it hard to tease that out of course is that Earth rock is embedded in other rocks —

Steve Mackwell:Mm-hmm.

Jim Green:— that are lunar probably in origin.

Steve Mackwell:Yep.

Jim Green:And that’s called the Breccia, and they’re from the impacts that then melt these rocks together.

Steve Mackwell:Mm-hmm.

Jim Green:So now that we’ve recognized one, that’s going to be important as we look over those samples to see if we can find some more. So, why is it important that we find rocks from Earth that have made their way to the Moon?

Steve Mackwell: Well, you know, logically speaking, you’d expect to find some Earth rocks on the Moon, and just because of the proximity and because from the impact history particularly early on in the history of the solar system, we know there was a lot of material that was knocked off the surfaces of planetary bodies, including the Earth and Moon. And the Moon is so close that you would have expected some of these materials to land on the Moon. After all, in our collections that we’ve got at Johnson Space Center, we have samples of Mars — pieces of rock that were knocked off the surface of Mars and eventually landed on the surface of our planet.

Jim Green: So this rock we’ve dated, and it is about 4 billion years old. So, the oldest Earth rock we have found is the one we brought back from the Moon.

Steve Mackwell: Mm-hmm.

Jim Green: That’s really mind-boggling.

Steve Mackwell: Yeah. It’s really quite incredible. And the thing about that, too, is the fact that we have very, very little of the early history of the Earth recorded from any materials we have on Earth. So, in theory, at least, the rocks we get back from another planetary surface could tell us about the very earliest history of our own planet that we cannot get from anything here.

Jim Green: And that’s because of our plate tectonics and the way our surface is still evolving. It’s kind of turning over.

Steve Mackwell: Mm-hmm.

Jim Green:And so some of the oldest stuff we find here on Earth is only about 3.6 billion years old. So that’s really neat.

Steve Mackwell: Mm-hmm.

Steve Mackwell: But, but this really brings up another interesting connection. You know the Apollo astronauts left on the surface of the Moon these reflecting instruments —

Steve Mackwell:Right.

Jim Green:–That allow us to fire lasers at and then get a laser beam back. What do we use those for?

Steve Mackwell: We use those to basically — because we have highly accurate measurements, the distance between us and those retroreflectors that still exist on the Moon, we learned a lot about the interior structure of the Moon as well, and the dynamics of the Earth-Moon system. So, they turn out to be very valuable tools. We had a recent mission, the GRAIL mission, which went and orbited the Moon. It was two orbiting spacecraft that were very, very carefully choreographed in their orbit, to give us very detailed information about the structure, the gravity structure of the Moon. And coupled with the data from the retroreflectors, we were able to learn a lot more than we would have learned based on the GRAIL measurements alone.

Jim Green: One of the most important measurements that they facilitate, when you fire a laser from Earth, it bounces off those laser reflectors, and then comes back to Earth, and we time it. And because we know the speed of light, we can get the distance to the Moon and now we’ve been doing that for 50 years, and we’re finding out the Moon is moving away from the Earth about an inch-and-a-half a year.

Steve Mackwell: Mm-hmm.

And so then that means if we go back in time, we have to move the Moon closer.

Steve Mackwell:Yep.

Jim Green:So the concept of impacts here on the Earth throwing material up and having them land on the Moon actually is easier early on in the evolution of the Earth-Moon system.

Steve Mackwell: Mm-hmm. True.

Jim Green: You know, 850 pounds of lunar material, you’d think we got everything. So, do we really need more samples from the Moon?

Steve Mackwell: Well, you know, we were talking about that earlier on, that the Apollo astronauts, the missions themselves landed in parts of the Moon that were the safest and easiest land. Well, those are the lunar mare, and the mare themselves are pretty flat regions, so they’re relatively safe to land on, but they don’t represent anything like the true surface of the Moon. And so, because of reasons of safety, we have a lot of the Moon left to look at. And even when you look at the landing sites, you see they’re not distributed very broadly on the Moon itself. So there are many places left to go and many types of rocks that exists on the Moon that we know are there based on orbital measurements from spacecraft that we haven’t sampled yet. So there’s a lot left to do and many, many questions about the Moon that would be answered by collecting more samples.

Jim Green: You know, missions like Clementine and Chandrayaan, which was from the Indian Space Agency, those instruments on those spacecraft had the ability to look in different wavelengths, and one of the sets of wavelengths they looked at allowed us to tease out different minerals that are on the Moon. And so when you look at that data and they can color that data in different colors, you really see the Moon, its various areas are completely different than other areas.

Steve Mackwell: Mm-hmm.

Jim Green: So that’s the areas we want to go to and bring back samples and understand what that material is and its origin because likely that stuff was made somewhere else in our solar system and brought to the Moon.

So, where would we like to go to the Moon? Is it just the near side or are there different things that we’re seeing on the far side?

Steve Mackwell: The near and the far side are quite different. On the near side, you can see, if you look up at the night sky, you can see the lunar mare, which were big volcanic basaltic deposits on the near side of the Moon. You don’t see those on the far side. The far side is dominated by older, more solicit crust. A lot of plagioclase, which is a relatively light mineral, makes up those rocks. So they represent the early crust of the Moon. So there’s a lot of reasons for going to the far side.

Steve Mackwell: Also, the deepest basin on the Moon, which may penetrate all the way down and provide access to pieces of the mantle on the Moon, as well as the lower crust of the Moon, is on the far side, the South Pole-Aitken basin, which is an important target for us to go and collect materials from.

So, there are many reasons to look at different parts of the Moon. I should add that if we go to the far side of the Moon, we will no longer be the first there. The Chinese Chang’e 4 lander is already there, and the Yutu 2 rover is exploring new areas on the backside.

Jim Green: Well, that’s great. As more countries have the ability to launch probes, gather scientific data, it allows our scientists as a world community to be able to interact and exchange information about what we find.

Steve Mackwell:Mm-hmm.

Jim Green: Well, you know, South Pole-Aitken basin, you know, this huge, deep impact hole: That was not filled up with volcanic material.

In this multi-temporal illumination map of the lunar south pole, Shackleton crater (19 km diameter) is in the center, the south

Steve Mackwell:Mm-hmm.

When we look at the near side, we see the mare, and the mare has impacts, but then material — molten rock — has filled that in. The volcanic material.

Steve Mackwell:Mm-hmm.

Jim Green:And that differentiates a lot between the near side and far side. So what are some of the ideas on why we see that on the near side, the volcanic mare, and not so much on the far side?

Steve Mackwell: There are many questions about that and there are various debates about, why that, why that came to pass. Some have to do with impacts on the Moon, some have to do with just the fact that the Moon has been tidally locked to the Earth, so we always see the same side of the Moon. And so —

Jim Green: So, there’s a gravitational pull that must be affecting the volcanic material?

Steve Mackwell: One would imagine so, yeah. So, it’s curious. We haven’t closed on that. We actually need more information, and a lot of that information will be derived from increasing a collection of samples over a wider distribution.

Jim Green: Well, you even pointed at the GRAIL mission. And, one of the great things about the GRAIL mission: It was really able to understand the thickness of the lunar crust.

Steve Mackwell: Mm-hmm.

Jim Green: And it’s different from the near side than the far side.

Steve Mackwell: Mm-hmm, yes.

Jim Green: So on the far side, the crust is 30 kilometers thicker in general.

Steve Mackwell: Yes.

Jim Green: So that, that’s — We have to figure out why or how that happens.

Steve Mackwell: That’s true, yes. We also need to get a better quantification of the crustal thickness, and while the GRAIL mission was outstanding in providing high-resolution gravity data, which allowed us to figure out roughly the global distribution of crustal thickness, we really need additional work to be able to measure the crustal thickness more accurately. And we’ll get that when we send more seismometers back to the Moon.

Jim Green: So why are the seismometers important? I mean, didn’t we do that with the Apollo missions, have seismometers and set them down on the surface? Wasn’t that enough?

Steve Mackwell: Oh, well, they were only on the surface for seven years. We did actually get a lot of really interesting data. It’s clear that the lunar interior is more complicated than we ever imagined, and the lunar crust itself is quite distinct from what we had imagined. Unfortunately, a decision was made about seven years after Apollo to turn those seismometers off. So we lost a lot more information. The seismic, you know, information we get, tells us about Moonquakes, but it also tells us about impacts and tells us a lot of interesting stories about the interior structure and dynamics of the Moon. Interestingly enough, we have very deep Moonquakes that are very poorly understood, coming from so deep in the Moon, you wouldn’t have imagined it was possible for the rocks to break down that deep.

Also, the Apollo landings were not over a big geographic area. So, if you’re going to be able to figure out where these things are going on in the Moon, you need to have a seismic network, which is spread out over a larger area on the surface of the Moon. Putting more seismometers on the Moon and having a better distribution would provide us with some of the really key information that we’re currently lacking.

Jim Green: You know, other things in the samples that we brought back, some of the scientists are talking about a variety of materials, and one thing they talk about is KREEP. K-R-E-E-P, by the way.

Steve Mackwell: Yeah. K-R-E-E-P is: K for “potassium,” R-E-E is “rare Earth elements,” P is “phosphorus,” and these are what the geochemist would call incompatible elements. And the material that makes up the KREEP terrain that we’re seeing on the Moon is also enriched in thorium. In fact, it was the thorium that first alerted us to the presence of these materials. And this is, is material that was kind of the last gasp of liquid material in the interior of the Moon that was erupted onto the surface, and it’s rather unusual, and on the basis of this material, we learned a lot about the, the final history of the solidification on formation of the crust of the Moon.

So the KREEP terrain is rather unique, but it is very, very important in understanding the history of bodies like the Moon that had an early magma ocean, where the entire Moon was liquid on this near surface region and then–

Jim Green: Liquid rock.

Steve Mackwell: Liquid rock, yeah.

Jim Green:Yeah, Liquid rock, wow. Yeah, in fact, it’s during those time periods that, you know, the water that was inherent in the materials in the Moon were being baked out.

Steve Mackwell: Right.

Jim Green: You know, and so another result that came back from the Apollo astronauts is the rocks seemed to be dry.

Steve Mackwell: Oh, yes.

Jim Green:But that’s changed recently. You know, we’ve made measurements now of the rocks to finer detail because our instrumentation has gotten better —

Steve Mackwell:Yep.

Jim Green: —And we now are detecting small parts of water in some of these rocks. But there are places that we’ve heard about now on the Moon where there might be 100 or 200 million tons of water. Where is that?

Steve Mackwell: Oh, that’s at the poles. Because it doesn’t have an atmosphere, and because of its orbital dynamics, there are places on the Moon, the craters on the south and north poles, where the temperature in the bottom of those craters is colder than anywhere else in the solar system. And within those regions, which never see sunlight, the– any volatile deposits, particularly water, is trapped and can build up over time.

And so, we believe based on measurements that were done from spacecraft, and also from crashing the LADEE spacecraft into one of the south pole craters, we understand that there are some fairly major deposits of water ice and maybe quite high concentrations that we can then use for other purposes, such as, we can use it to extract oxygen for humans to work on the Moon. We can use it for breaking into, cracking into hydrogen plus oxygen for fuel for spacecraft to move out further into the solar system. So, these deposits are really important and make the Moon not just a place we want to go do science, but a place that can be a commercial venture to enable further solar system exploration.

Jim Green: Yeah. That’s really a fantastic idea. You know, the tough thing is, it’s in a permanently shadowed crater, so that means we got to crawl into that and get in there to not only sample it and really understand what’s there, but then to be able to figure out a way to be able to use it.

Steve Mackwell:Mm-hmm

Jim Green:So there’s a lot of engineering that really has to come into play. Now, these regions, these permanently shadowed regions of course, as you point out, are in the poles —

Steve Mackwell:Mmm hmm.

Jim Green: — The north pole and the south pole. You know, and so there’s other advantages for human exploration in particular to be able to go to those poles. And that’s why they’re important and that’s because of the access of the sunlight.

Steve Mackwell:Yes.

Jim Green:How’s that happen?

Steve Mackwell: Well, as I mentioned, the Moon rotates on its axis as it goes around the Earth. Essentially the deep, deep craters do not see sunlight, but there are regions right on the rims of some of those craters that see permanent sunlight. And because of the orbital dynamics of the Moon, the temperature variation over time near the poles, in those permanent sunlit regions, is pretty benign.

The region near the poles is actually a much better place to put astronauts on the surface of the Moon because the day/night temperature variation there is pretty small. And even though it’s about minus 80 degrees Fahrenheit in that region of the Moon, the fact that it’s reasonably constant makes it much easier for them to be comfortable there. By contrast, if they were in a habitat in the equatorial regions, the day/night variation is about 470 degrees Fahrenheit, which is a real challenge to be able to keep them healthy in that kind of an environment.

Jim Green:Yeah and the temperature swing is so big that you have a completely different set of systems that you have to kick in.

Steve Mackwell:Exactly right. And you need a lot of power to keep them warm.

Jim Green:Yeah. That’s right. You know, when you talk to people, some people have the misconception that, you know, the far side of the Moon is always dark, but as you point out, because the Moon is rotating, and it does so once a month and constantly has one face towards the Earth, there are times, of course, that the far side is completely lit —

Steve Mackwell:Mm-hmm.

Jim Green:And we call that “New Moon.”

Steve Mackwell:Yep.

Jim Green:So, you have to think about the dynamics and then being able to go to the pole to be able to get access to sunlight all the time, to power your habs and do all kinds of stuff, is really important.

Steve Mackwell:Mm-hmm.

Well, those are really key places, not only for scientists to go to understand the chemical environment and how the water gets there, but there are also other neat places on the Moon. Where else could we go?

Steve Mackwell: Well, there are a couple of places that we know about that would be really very useful to go to bring back samples from. One of them of course is the South Pole-Aitken Basin on the far side of the Moon, where we have material that, as I said, that comes up from the, potentially, from the lower crust or mantle. And so we could sample the mantle and see what the rocks or the minerals are like. We can also see whether or not there’s water in those minerals, there’s water dissolved within the minerals that is an important resource for us, but also tells us about the early history of the Moon. And the South Pole-Aitken Basin is arguably the oldest basin on the Moon, but we don’t know its age because we don’t have rocks that we know definitively came from there.

So, bringing back some material from the melt sheet that was associated with that, that basin formation event would be tremendously valuable, enabling us to understand the earliest part of the cratering history of the inner solar system, which would allow us to be much more robust in our understanding of the early solar system.

Jim Green: Well, you know, that impact region is enormous.

Steve Mackwell:Mm-hmm.

Jim Green: We had a good day. The Moon run block for us and took the hit.

Steve Mackwell:Mm-hmm.

Jim Green: And that hit didn’t come to the Earth. Now, there’s other places on the Moon that are quite fascinating. One of these things that we talk about are skylights.

Steve Mackwell: Oh, yeah.

Jim Green:What are those?

Steve Mackwell: The skylights are pits on the surface and you can see down into the pits. You could even see the layering of the lava flows. And we believe that these pits, these structures were formed due to collapsed to crustal material over the top of a lava tube. And we see plenty of lava tubes on Earth. We see them in Hawaii and other places and it is reasonably conceivable that you would break down material above and create a cavity. And those cavities could be really important for us as we think about sending humans back to the Moon, because they present an opportunity for putting astronauts below the surface somewhat, which is a more benign temperature environment. And also, habitats in the bottom of these structures would also allow them to be protected from solar radiation, much more than putting them in a habitat on the surface of the Moon.

Jim Green: That sounds really neat. I mean and those skylights, and they’ve found quite a few of them, the skylights on the near side of the Moon, if you can think about this, of course, standing right at the bottom of this collapsed lava tube, where the hole is and looking out and seeing the Earth and you see the Earth all the time.

Steve Mackwell:Mm-hmm.

Jim Green:So it also facilitates communication and —

Steve Mackwell:Yep.

Jim Green: —makes it a really important and special place for consideration.

The fact that it goes down tens of meters, and we have all kinds of stratigraphy that we could look at scientifically, could be quite important for us.

Steve Mackwell:Mm-hmm.

Jim Green:So I’m really excited about pulling all these ideas together and seeing how we’re going to use them to continue to help humans explore our Moon as we go forward.

Well, you know, one of the things I always ask my guests when we when we sit down and talk is about their gravity assist: That that thing that happened in their life in the past that really got them excited about the field and allowed them to become the scientists they are today. And if you haven’t noticed, Steve’s got a great accent, and it’s a New Zealand accent. So, Steve, I’m really interested in your gravity assist. How did that happen? How did somebody from New Zealand get up here and, working with NASA?

Steve Mackwell: Oh well, I would say I’m a bit like the Galileo mission to Jupiter. I didn’t have just one gravity assist, I had two. The first one of mine was back when Apollo 11 landed because I can still remember very, very clearly, sitting in the classroom, and, with a grainy black-and-white TV set, and seeing the first steps of Neil Armstrong on the surface of the Moon. It was fascinating to see, you know, as people were moving around, it was clearly so different. And it was us stepping away from this environment we’ve been so comfortable with forever, here on Earth. So, that was one of the things that really impacted me.

I also had another a gravity assist that came quite a number of years later. I was finishing up my master’s degree in astrophysics at University of Canterbury in New Zealand, and the first images came back from Voyager 1 out at Jupiter.

Jim Green: Wow, yeah.

Steve Mackwell: And one of the images that sticks in my mind more, probably, than anything was the image of the limb of Io, and seeing this cloud, this volcanic eruption putting a cloud of material up, and thinking, “Wow! I always thought that the solar system was a kind of a static place, and here was this crazy volcanic activity on a body way out in the cold realms of the solar system.” So, you know, it really kind of made me think a lot that we live in a much more dynamic environment than I ever imagined.

Jim Green:Yeah, we do.

Steve Mackwell: And so that was what got me thinking about it. And so when I took over the Lunar and Planetary Institute, for me, that was really coming back home. That was getting back and, and, looking out into the solar system and thinking, “Wow! We need to understand what’s going on here much, much more.” And in the last 20 years or so, wow! We’ve made such progress. It’s really amazing.

Jim Green: It’s been amazing. It really has. Yeah, so you were the kid in the candy store at the Lunar and Planetary Institute. That’s for sure.

Steve Mackwell:Oh, yes I was. Mm-hmm. Oh, yeah.

Jim Green:Well, I really enjoyed chatting with you this afternoon. Thanks so very much for joining me, Steve.

Steve Mackwell:Oh, it was a pleasure, Jim.

Jim Green:Well, join me next time as we continue our exploration of the Moon. I’m Jim Green, and this is your Gravity Assist.

Producers:JoAnna Wendel and Elizabeth Landau

Audio Engineer:Emanuel Cooper