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Season 3, Episode 14: The Moon's Holy GRAIL

Season 3Episode 14Dec 10, 2019

Recent maps of the Moon's gravity have taught us a lot about its overall shape, and have been invaluable for lunar exploration. Maria Zuber, principal investigator of the Gravity Recovery and Interior Laboratory (GRAIL) mission, reflects on the twin spacecraft and their implications for future investigations.

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Neil Armstrong and Buzz Aldrin landed far from where they had anticipated because, 50 years ago, we didn’t have a complete understanding of the Moon’s gravity. Recent maps of the Moon’s gravity have taught us a lot about its overall shape, and have been invaluable for lunar exploration. Maria Zuber, principal investigator of NASA’s Gravity Recovery and Interior Laboratory (GRAIL) mission, reflects on the twin spacecraft and their implications for future investigations.

Jim Green: Soundcheck… 1, 2, 3, “Gravity Assist,” “Gravity Assist.”

Jim Green: So when we look out at night and we see the Moon, it’s spherical. But is it truly spherical? And how do we know how that Moon is put together? Let’s find out from an expert. Let’s understand its gravity.

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

Jim Green: I’m here with Dr. Maria Zuber, the Vice President for Research at MIT. Welcome, Maria, to Gravity Assist.

Maria Zuber: Happy to be here, Jim.

Jim Green: Well Maria, you’ve been involved in more than a dozen planetary missions, and one really special one because you were the principal investigator for the Gravity Recovery and Interior Laboratory, or GRAIL. That mission went to the Moon. Can you tell me about the GRAIL mission and how it worked?

Maria Zuber: Well, the Moon is an interesting place to measure gravity. It’s the first planetary body other than Earth that we ever made a gravity measurement from. But it’s very challenging to get a global map because the way we usually measure gravity is to look at the Doppler shift or the frequency shift of a radio signal, because every spacecraft carries a radio.

Maria Zuber:The Doppler shift is… it’s a frequency shift that happens when you’re either coming towards something or away from something. So it’s like a siren in an ambulance. When an ambulance is coming towards you, you go to higher frequencies. And when an ambulance is going away from you, it goes to lower frequencies. And all radio signals are just like that. You’ll actually be able to tell how fast the ambulance is going if you could actually measure the frequency of the siren.

Maria Zuber: So, in order to get a very good global map, we decided to send two spacecraft that measured the frequency shift or the Doppler shift of each other. And by doing that, we were able to get a spectacular gravity map of the Moon.

Jim Green: Yeah. So as they orbit the Moon, one gets pulled a little, one way or another, based on the gravity it’s experiencing below it before the other one does and that causes them to separate back and forth.

Jim Green: Maria, when was GRAIL launched?

Maria Zuber: GRAIL launched in September of 2011. It took basically three months to get to the Moon. Actually, the two spacecraft inserted into lunar orbit on New Year’s Eve and New Year’s Day 2011, 2012. And it was to map for three months. And then just because of geometry of the way the two spacecrafts talked with each other, we needed to stop for three months and wait for the sun to come around so that we could get it on the solar panels while we were ranging. And then for another three months. And then we did that one more time. So it went through 2012.

Jim Green: Well, you were the very first woman in planetary science who was a principal investigator, and I know that very well as Head of Planetary at the time.

Jim Green: Maria, what does it mean to be a principal investigator?

Maria Zuber: Well, it’s a lot of responsibility. So you have to be following all aspects of the mission.

Jim Green: You’re essentially in charge of everything.

Maria Zuber: Yes. And I’ll tell you, when the spacecraft was sitting on the rocket and it was ready to launch, I realized it was going to succeed or fail on the basis of decisions that I made, with a lot of help from my teammates and a lot of help from NASA. But ultimately, the principal investigator is—

Jim Green: Where the buck stops.

Maria Zuber: If things go well, everybody gets credit and if things go wrong, the principal investigator gets blamed.

Maria Zuber: The launch was where it really hit home about how risky this was.

Maria Zuber, principal investigator of NASA's GRAIL image, with the spacecraft before launch.

Maria Zuber: And it was like a dream because we… The team practiced launches many, many times and went through the whole sequence from the launching and then the spacecraft, both spacecraft, they had to detach from the launch vehicle. And both of them had to work and with their solar panels. And whenever we practiced, something always went wrong. Something was always planned to go wrong so that we would practice figuring out how to deal with it.

Maria Zuber: But when the launch actually happened, everything went perfectly and none of us knew quite what to do. We just sat there. But the—

Jim Green: And let it do its thing.

Maria Zuber: But the mission actually operated pretty flawlessly. Although I will say, later in the mission when we brought the altitude down very low, I said, “It was like having a newborn baby that was up all night.” You had to just watch it constantly, constantly, constantly, to make sure that nothing happened that we didn’t know about because you would have to do a close-to-immediate maneuver to avoid having one of the spacecraft crash into the surface.

Jim Green: So why do we care about the gravity, in such great detail, of the Moon?

Maria Zuber: So if we look at the Moon, these dark areas, the mare, there have been lava flows that have extruded on the surface and you see very, very little of this on the far side. So the near side of the Moon and the far side of the Moon are extremely different. And at the time that we were proposing GRAIL, the countries of Earth collectively had sent over 100 missions to the Moon. And we still didn’t know why the front side of the Moon looked different than the back side of the Moon.

Maria Zuber: So that told us that maybe the answer isn’t on the surface, maybe the answer is somewhere that we haven’t looked before. And so we decided to look inside the Moon.

Jim Green: And that’s the gravity?

Maria Zuber: And that’s what we measure gravity for, is to see… essentially, map the inside of the Moon.

Jim Green: So are there regions on the Moon where the mass is larger than the surrounding areas?

Maria Zuber: If a planet was a billiard ball, if it was just completely uniform and if it was completely round and spherical, then we would have a very, very simple gravity field. But planets are complicated. Planets formed by the accumulation of dust and gas from the solar nebula and this material came together and it coagulated into asteroids of different sizes then came together and then melted and we had volcanic eruptions and comets hitting and liberating water.

Maria Zuber: So it was just all kinds of stuff going on planets and this causes the gravity field to be what I call, lumpy. That’s what our students would call it, lumpy.

Jim Green: So even though it looks spherical, there are huge mass variations?

Maria Zuber: That’s correct. And the type example of mass variations on the Moon was recognized from the first satellites that we sent to the Moon to do reconnaissance for where we should land astronauts. When the satellites were over the dark areas of the Moon, these are the “maria,” the spacecraft would get accelerated inward towards the Moon.

Maria Zuber: So they would deflect inward, and it was because there was more mass in those areas that were tugging on the satellites and having them come a little bit closer to the Moon. These anomalies were called “mass cons,” for mass concentrations. And one of the goals of the GRAIL mission was to determine what these mass concentrations were due to, that it turns out they’re due to a combination of lava that filled crater basins.

Maria Zuber: So the large circular areas that are dark on the near side of the Moon are all impact basins. And when a large impactor hits a planetary surface, it excavates the crust and throws it away to some other place. And the mantle comes in and fills in underneath the crater. And this mantle tends to be more dense than the crust.

Jim Green: As you say, we really needed to figure out what the mass distribution of the Moon was for humans to be able to explore. In fact, my understanding is when Neil Armstrong landed Apollo 11, and we’re in the 50th anniversary for that, he landed downstream from where they thought they were going to be because they didn’t understand the gravity. And in fact, it took Collins to find them first.

Jim Green: So it’s too bad they didn’t have GRAIL data before they did the Apollo program.

Maria Zuber: Well, it’s a good thing we had Neil Armstrong at the controls

Jim Green: OK, that’s true too!

Maria Zuber:…for Apollo 11, but now, any person, any robot who wants to land on the Moon now, not knowing the gravity field will not be a problem. So we can now navigate very precisely. In fact, we know the gravity field of the Moon now considerably better than we know the gravity field of the Earth.

Jim Green: Well, what do you think was the most surprising result from the GRAIL mission?

Maria Zuber: We knew that the crust of the Moon would be broken up because when an impact hits a planetary surface, it breaks up the crust.

Jim Green: Shatters it pretty good.

Maria Zuber: I think none of us in the scientific community had any comprehension that the lunar crust was as shattered as it is. So the way that we were able to tell that was it turns out at short wavelengths that are consistent with the shallowest crust. 98% of the gravity is gravity due to topography and only 2% of the gravity that we see is gravity due to anomalies or mass concentrations within the crust, and this is outside the maria.

Maria Zuber: And the only reason that that could be the case is if the crust has been completely broken up and homogenized because we talked earlier about all these processes going on that make planets lumpy and uneven, so the impact process has just evened everything out. But that 2% of the gravity within the crust is very, very interesting.

Maria Zuber: For example, we found massive, massive volcanic dikes that point to an earlier time on the Moon when the Moon heated up and expanded. And it had been hypothesized decades ago and no evidence for it was ever seen at the surface. So it turned out that these dikes at the surface were all broken up by the impacts, but the dikes still exist very deep in the crust.

Jim Green: So the dikes are those things that squirt out lava?


Maria Zuber: That’s right.

Jim Green: And form a wall?

Maria Zuber: The rocks melt deep and form the lava and it rises up and essentially like a crack, it fills up the crack. And that material is more dense than the crust, so it is a mass concentration and we see it. But they’re massive, but they’re really intruding upon a crust that has been all broken up and homogenized.

Maria Zuber:
When you only see one side of the Moon, that’s something called synchronous rotation. And it turns out that synchronous rotation is a minimum energy state in a planet-Moon system. So people are lazy and it turns out planetary systems are lazy also.
Jim Green:
Just as lazy. Yes, they are.
Maria Zuber:
They want to do the least amount of work they could. So a planet with a Moon will want to get to the synchronous configuration and the Moon got to this configuration pretty quickly. The question is, did this have something to do with why the lava flows are on the near side? If we had more of the masses there, then it could be that that was the favored place for this minimum energy configuration to be worked out.

Jim Green: Well, you know, one of the really unique imaging instruments you had on GRAIL was called MoonKAM. What was the purpose of that instrument?

Maria Zuber: So MoonKAM, the sole purpose of that experiment was educational outreach. Our outreach leader for the mission was Sally Ride.

Maria Zuber: Sally was the first American woman to fly in space.

Maria Zuber:She had a vision that we could put cameras on the GRAIL spacecraft and that students in classrooms across America and actually internationally could download the software, learn how to use the software and take their own images of the Moon.

Maria Zuber: And the challenge with that is that when you put an instrument on a spacecraft, this instrument wasn’t required for the science part of the mission, therefore it couldn’t undergo as much testing and scrutiny and use the same kind of high level parts for cost reasons. So it was a challenge to make it work. But I’ll tell you, everybody on our team, most of the people had kids and grandkids and everybody treated the cameras associated with MoonKAM just like they did the radio systems on the spacecraft. So everybody thought it was important.

Maria Zuber: MoonKAM took over 200,000 images of the Moon.

Jim Green: Wow.

Maria Zuber: And Sally and I, we went out to classrooms across America and we sat down in classrooms. There was a fear, is it only going to be affluent classrooms on either coast of the United States? Classrooms in every state adopted MoonKAM.

Jim Green: Wonderful.

Maria Zuber: And we went to some of these classrooms and every kid in the class knew how to run the software. There was always one child or two who taught everybody else how to do it.

Jim Green: Sure.

Maria Zuber: And some students would say, “Well, I want to take a picture of a crater,” or, “I want to take a picture of a lava flow.” But then some students would say, “Oh, I would like to take a picture of wherever the camera is when it’s my birthday,” okay?

Jim Green: Okay. Sure.

Maria Zuber: So the concept of MoonKAM was that there are many images of the Moon, but there’s nothing like your image of the Moon.

Jim Green: Correct. Correct.

Maria Zuber: So if you take your own image of the Moon, it becomes deeply meaningful to you. And kids are smart. They will learn whatever they need to learn to do something interesting. So Sally Ride Science, the nonprofit that led that experiment, they’re actually tracking the MoonKAM students now to find out what these students do in their careers.

Maria Zuber: And I think we’re going to have—

Jim Green: Well, they have to know the Moon really well to pick out what they want to do.

Maria Zuber: We’re going to have a lot of budding scientists, I think. I think and I hope.

Jim Green: That’s great.

Maria Zuber, professor at MIT, with NASA's Chief Scientist Jim Green.

Jim Green: Well, the Moon is so important to the Earth. What are some of the reasons why we are so lucky to have a Moon?

Maria Zuber: Well other than the fact that it’s beautiful and it gives us something we can observe at night and something that we can aspire—

Jim Green: And we can sing about.

Maria Zuber:…to study, And we have a big Moon compared to other planets that have Moons because ours is formed by a giant impact into the Earth, we believe. It stabilizes our climate because the tilt of the Earth is limited because of the force of the Moon there. It’s hard to overcome the force of the Moon being nearby.

Maria Zuber: And so for example, the tilt of the Earth causes our seasons. The tilt of the Earth changes over time, but over many, many thousands of years on the Earth, it changes less than a degree, okay, whereas if we didn’t have a Moon, the tilt of the Earth could change faster. And even though our tilt changes only very slightly, it’s still enough to give us ice ages.

Jim Green: Yeah.

Maria Zuber: So imagine if the tilt changed much quicker.

Maria Zuber: So interestingly, Mars right now has a tilt almost the same as the Earth’s. It’s just coincidence because both the Earth’s tilt and Mars’ tilt change. But Mars’ Moons are tiny so they don’t have that stabilizing influence that Earth’s Moon does. So Mars goes through variations in its tilt, chaotic cycles where there are times when the current poles of Mars get more heating from the sun than the equator does. So that obviously has serious climatic consequences.

Maria Zuber: And so, there have been papers written where people have proposed that the Moon — the presence of the Moon actually helped facilitate life to develop. We don’t know that life wouldn’t have developed —

Jim Green: At least complex life.

Maria Zuber: If the Earth doesn’t have a Moon. But for whatever beings or microbes, they would have had to be more adaptable in terms of their ability to handle different climatic conditions if the Earth did not have a Moon of the size it does.

Jim Green: Thinking of Mars now, do you think it would be important to do a GRAIL-like mission at Mars?
Maria Zuber: Yeah. We’ve been thinking about that.

Maria Zuber: GRAIL was based on a mission concept called GRACE which mapped the Earth. And on the Earth, the two spacecraft couldn’t get down real low because of the atmosphere. So you couldn’t get the high resolution… the very, very super high resolution that we got for the Moon.
Maria Zuber: But when GRACE mapped the Earth, it measured changes in gravity. Changes in gravity due to water outflow and aquifers and ice sheets melting. So there are actually really interesting changes that go on Mars. And Mars is a dynamic planet. So for example, every year, and one Mars year is 687 Earth days, on every year on Mars, a third of its atmosphere exchanges with the polar cap. So it snows out on the polar caps.
Maria Zuber: And this causes the gravity field of the Moon to change by a part in a billion. And that’s actually pretty big.
Jim Green: Yeah.
Maria Zuber: It sounds like a small number but a part in a billion, we could measure with a GRAIL-like mission. So you could track the seasonal changes. And if there are underground aquifers on Mars in the present day, you would… If there are any changes in those aquifers, so if for example in the summer season, if water thawed and flowed underground, it would be conceivably possible to track that.

Jim Green: Well, our next step in lunar exploration is the Artemis program where we’re going to have the first woman and the next man step foot on the south pole of the Moon in 2024. So Maria, how excited are you about that?

Maria Zuber: Send me.

Jim Green: Well, I can assure you the GRAIL data will be extremely important for that to happen.

Maria Zuber: I’m delighted to contribute in any small way that I can.

Jim Green: Well Maria, I always like to ask my guests to tell me what was that activity, that person place or thing in their life that really got them so excited about planetary science that they changed their direction and it was accelerated to be the scientist they are today. And I call that a gravity assist. So Maria, what was your gravity assist?

Maria Zuber: So actually, I’m an unusual case because I have wanted to study space since I was a baby in my playpen watching the first Mercury rocket. So that could be it, deciding to study space. But deciding to study planetary, when I applied to graduate school, I applied to both astrophysics programs and planetary science programs. And it was around the time that Voyager was flying past Jupiter.

Maria Zuber: And I just said, “When you go someplace and you look at something that you’ve never seen before or you look at it at higher resolution or with a different set of eyes, discovery is assured.”

Jim Green: Sure.

Maria Zuber: So it was really seeing those images come back from Voyager that I think really drew me towards planetary science as opposed to the rest of the universe.

Jim Green: Well, that’s great. Well, thanks so much for joining me on “Gravity Assist.”

Maria Zuber: Well, thanks Jim. I’m happy to be here. And actually, let me just say that I love the title of the show.

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


Lead Producer: Elizabeth Landau
Audio Engineer: Emanuel Cooper