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Gravity Assist Podcast: Where Could We Go on the Moon? With Steve Mackwell (1)
May 2, 2019
With NASA planning to send astronauts to the Moon by 2024, Steve Mackwell chats about the Moon’s exciting unexplored areas. 11 December 1972 -- Scientist-astronaut Harrison H. Schmitt collects lunar rake samples at Station 1 during the first Apollo 17 extravehicular activity (EVA) at the Taurus-Littrow landing site. Schmitt is the lunar module pilot. The Lunar Rake, an Apollo Lunar Geology Hand Tool, is used to collect discrete samples of rocks and rock chips ranging in size from one-half inch (1.3 cm) to one inch (2.5 cm). Credits: Eugene A. Cernan, Apollo 17 CommanderJim 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.
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 pole is located approximately at 9 o'clock on its rim. The map was created from images from the camera aboard the Lunar Reconnaissance Orbiter. Credits: NASA/GSFC/Arizona State UniversitySteve 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.
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