Just like earthquakes help scientists figure out what’s going on inside our home planet, moonquakes have taught scientists a lot about the interior of the Moon. NASA’s Gravity Recovery And Interior Laboratory (GRAIL) mission has also given us a clearer picture of the Moon beneath its surface. Seismic activity on the Moon is one area of scientific interest as NASA makes plans to send the first woman and the next man to the Moon by 2024.
Jim Green: Did you ever wonder what’s inside a terrestrial planet? Well, how about our Moon? Did you know that the moonquakes, and we can find out all kinds of things about its interior?
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: With me today is Dr. Walter Kiefer, a planetary geophysicist. Walter is a staff scientist at the Lunar and Planetary Institute, Houston, Texas. He studies the geophysical evolution of the Moon, Mars, Venus, and even Io. Walter has been a member of the science team for NASA’s GRAIL mission, and that’s the Gravity Recovery And Interior Laboratory, and that explored the structure of the Moon. Welcome, Walter.
Walter Kiefer: Hi. It’s nice to be here. Thank you.
Jim Green: Today, I want to talk about the subsurface of the Moon. Most of our spacecraft always just look at the surface or we land, pick up a few rocks, but we don’t go very deep. What are the ways that we really have that we use to look inside the Moon?
Walter Kiefer: So, one of the things that we’ve done, and this goes back to Apollo actually, is that the Apollo astronauts left a set of seismic stations on the Moon at most of the most of landing sites, and they were used to study the inside of the Moon from 1969 until 1977, when the stations were shut off. And, that provides a great view of the internal structure of the Moon, but it’s located [at] specific places on the Moon, where the landing sites were.
Jim Green: When they deploy these seismic stations, what do they really look like?
Walter Kiefer: Each of the Apollo missions put down between five and eight different experiments. The seismometers were on almost all of them. It’s a box. It doesn’t actually look like much on the surface in the photographs, because you’d want to protect it from changes in temperature during the lunar day and the lunar night. So, it’s got a thick, insulating, silver blanket over it basically.
Walter Kiefer: And what we’ve done since then is the GRAIL Mission, which studied the gravity of the Moon.
Jim Green: Walter, when did GRAIL launch, and how long was it in orbit?
Walter Kiefer: GRAIL launched in September of 2011. It took actually about three months to get to the Moon. Now, you can get to the Moon much quicker, but they chose a very long path, so they could get there with very precise timing, because they had to have two spacecrafts that were going into orbit separately. They went into orbit on New Year’s Eve and New Year’s Day of 2012. It took about two and a half months to get the spacecrafts lined up exactly the way they needed to be in exactly the same orbit.
Walter Kiefer: And then they mapped for three months. Then they had to take a pause, because of the geometry between the Earth and the Moon. And then later that year, the geometry came back appropriately again, and they did another three months of mapping, between September and December of 2012.
Jim Green: When we look at a planet or a Moon, our first thought is that the gravity is uniform in all directions. Is that really true?
Walter Kiefer: No, it’s actually not true. Even on the Earth, as you walk from place to place, the gravity’s a little bit different. It’s a small difference. It’s a small fraction of a percent, so you wouldn’t necessarily notice it, but with good instruments you can measure it and learn about the variations of the density inside of the Earth. It’s an important and powerful tool. On the Moon it turns out that the variations in gravity are actually substantially larger than they are on the Earth. It seems to be a rule from the places we’ve studied that the smaller the planet, the bigger the variations are.
Jim Green: Yeah. That’s really neat. So, then as a satellite orbits the Moon, those little gravity differences, based on that additional mass or reduction of mass at certain locations, tug and pull on the spacecraft, and so we can measure that difference.
Walter Kiefer: Yes. We’re basically measuring the differences in the speed of the spacecraft. GRAIL had two spacecraft, and we were looking at measuring the speed to a fraction of a micron per second. Now, a micron is one one-millionth of a meter, so imagine in the course of the year, imagine the two spacecrafts separating by about 3 meters, or about 10 feet. That’s the rate. That’s the speed that we could measure the differences with GRAIL. That’s why we were so successful at mapping the Moon, is that they could measure very, very small velocity differences.
Jim Green: So, as GRAIL orbited the Moon, these two spacecraft were in the same orbit, but one would feel a tug or a pull based on the gravity in and around it as it was orbiting, and then move away or come closer to the other spacecraft.
Walter Kiefer: Exactly. One spacecraft would get there just a little bit before the other, 30 or 45 seconds, and so you would see the change in velocity as one would get pulled and then the other would get pulled in. Sometimes they’d pull apart. Sometimes they’d come together. We measured those very precisely, using the same technique that cops use to determine if you’re speeding. It’s just that we could do it like a billion times better than the police do when they’re trying to find out if you’re speeding on the freeway.
Jim Green: Yeah. What’s really neat, then is if we then take the orbit and change it over time, I know we had an opportunity towards the end of GRAIL’s mission to reduce the altitude, and so then those changes in distance between the two spacecraft was even more pronounced.
Walter Kiefer: Exactly. So, at the end we were down to about 10 kilometers on average above the surface, but that meant that we were flying in some cases only 2 kilometers above the surface of the highest mountains we were flying over. So, we were really down in the weeds almost. I really wish we could have had a camera to take a video picture of that. The view would’ve been amazing.
Jim Green: Yeah. They eventually crashed, and that ended the mission. So, they’re laying on the surface of the Moon somewhere.
Walter Kiefer: Right. And that was done on purpose. We were out of fuel. They actually deliberately picked a place, and it actually had a science value, because they burned all the fuel and they needed to know the mass of the spacecraft, as they were making the measurements. So, by burning all of the fuel at the end, you could tell how much was left over. It actually improved the analysis as well.
Jim Green: We’re pretty sure that the Moon was once completely molten, as it was forming. Once it cooled enough to be solid, then what’s the overall history of the heat? What happens to that heat being dissipated in the Moon?
Walter Kiefer: We do think that the Moon originated in what we call a magma ocean. That came up initially actually with the rocks from Apollo 11, that theory came about within six months of the first lunar samples coming back. We think that the Moon, it would have solidified from this magma state in probably less than a million years. After that, heat still keeps coming out. The interior of the Moon would have been what we call convecting.
Walter Kiefer: Convection is a process in which we move heat, because material is physically moving. And so, there are places inside the Moon or inside of the Earth, that are hotter than other places. Just as we say that hot air rises, hot rock rises as well. The rock is, if it’s hotter, it’s thermally expanded. It’s a little less dense than the surroundings, so it wants to rise. Other places that are colder are more contracted, it’s denser. Those places want to sink. That motion of some places wanting to come up, other places wanting to go down, creates a flow inside the mantle. Even though it’s solid, it’s moving as a very, very viscous fluid at a rate of potentially a few centimeters per year.
Walter Kiefer: That’s why plates get moved around, why the Atlantic Ocean splits apart and North America and Europe separate, for example. How we put out push up mountain belts on the Earth is because it’s solid, but it is moving very slowly at a rate of a few centimeters per year.
Walter Kiefer: The Moon would have done exactly the same thing, at least early on. It would have been convecting.
Walter Kiefer: Eventually, the Moon would have cooled enough that that now it really isn’t moving very much inside. Mostly the heat is simply coming out by conduction, which is a very slow process. So, it’s still cooling. It creates scarps and actually even earthquakes, moonquakes, on the Moon even today.
Walter Kiefer: By scarp what I mean is it’s a ridge. It’s a structure where it might kind of look like a hill. They actually drove over one of them on Apollo 17 on purpose. It’s a small hill that’s created, where one part got pushed up and one part got pulled down by the motions on the fault.
Walter Kiefer: Actually, there are plenty of places on the Moon where we can see scarps that must have been the signature of faults at some point. Now, we’ve not actually seen them move, but we know that they must have formed in moonquakes in the past.
Jim Green: So, if we were standing on the surface of the Moon during these moonquakes, both the shallow and also the deep ones, would we be shaken?
Walter Kiefer: So, the deep moonquakes are very, very deep, and they’re very, very small. You would not feel them at all. You need a very sensitive instrument to detect them at all. Now, the shallow moonquakes of which Apollo measured I believe 28 in six or seven years, some of them actually get to be very large, and they appear to be associated with thrust scarps on the surface of the Moon, so they’re very close to the surface. Some of them are active today. I would not want to have a lunar habitat near one of those, because I really would hate to have a leak in my in my space station that lost its air. So, yeah. You would feel some of the shallow quakes if you were in the right place. But the good news is there aren’t very many of them.
Jim Green: My analogy is we bake a cake, take it out of the oven, and it’s actually still cooking, because it has to cool off. Our big bodies, our planets and moons that we have, are still in that state. They’re still cooling off.
Jim Green:We’ve newly reanalyzed the Apollo seismic data, as you mentioned earlier. That indicates that the Moon has a solid inner core and then a fluid outer core over on top of that. How big is this core, both the solid inner core and then that fluid outer core?
Walter Kiefer: We think that the core overall is something like 350 or 400 kilometers in radius. That’s to say that it’s about 20% of the radius of the Moon. The inner core, the solid part of that, might be 200 or 250 kilometers, which would leave the fluid outer part maybe a 100 or 150 kilometers. That means that the mass of the core is 1 to 2% of the mass of the planet. That sounds like a big core, but on the Earth, the core extends out more than halfway to the surface, and it’s more than 30% of the mass of the Earth. The Moon is actually very deficient in iron, compared to the Earth.
Jim Green: Then on top of that is the mantle and then the crust. How big are those regions?
Walter Kiefer: The easier answer is probably the crust. The crust is the outermost layer of the Moon. It’s formed in the solidification of this magma ocean phase. It’s mostly a rock called anorthosite, very rich in the mineral plagioclase, which is a light gray mineral. That’s what we see when we look at the Moon at night, look at the full Moon. The lighter parts are this anorthositic crust. The combination of GRAIL and the seismic data from Apollo tells us that on average that crust is between 34 and 43 kilometers thick. In some places it’s much thinner, especially on the near side, and on the far side it gets much thicker, some places as much as 80 kilometers thick on the far side of the Moon.
Jim Green: Wow. Well, let’s switch gears a little bit and go back to some of the Apollo observations. The seismic observations indicated that there were not only shallow but deep moonquakes. Where are they coming from?
Walter Kiefer: The deep moonquakes are the most common kind of Moon quake on the Moon. They were much of a surprise I think. There are, it turns out, about 300 places deep in the Moon, we’re talking 700 to 900 kilometers below the surface, that become seismically active about once a month. They repeat in many cases every month. It appears that they’re driven by the Earth’s tide. Just as the Moon creates tides on the Earth and it makes the ocean go up and down, it also makes our rocks go up and down on the Earth. The earth does exactly that same thing to the Moon. So, the Moon goes up and down by a little bit. It’s a small amount of stress, but for reasons we don’t fully understand these parts of the deep Moon are weak enough that those stresses make moonquakes in those places sometimes once a month.
Walter Kiefer: We see the same signature month after month in the seismograms, so we know that they’re coming from exactly the same place. That actually turns out to be useful, because even though we don’t know where they’re coming from, the fact that they’re a signal coming from the same place, we can add up that signal maybe 30, 40, 50 times in the Apollo data and we can use it to look and see how the seismic waves from those events propagated through the Moon bounced off the core of the Moon, and therefore we can look and use that data. That’s actually the data that that was used to estimate the size of the core is the seismic waves from these deep moonquakes helped us to understand the structure of the moon’s core.
Jim Green: One of the things that I had heard that was done I think starting with Apollo 12 was they actually staged an impact. After they left the lunar limb, they let the limb go down and hit the surface, and then that caused quakes that they could then uh know the time and location. What did they learn from that?
Walter Kiefer: It was a good way to calibrate the response of the Moon to a known event. Actually, they did it not just with the lunar module ascent stages, but they actually, starting on Apollo 13, they deliberately crashed the third stage of the Saturn rocket into the Moon I think probably as many as five times. When they did that on Apollo 13, remember this was the very early stages of lunar seismology, and the Moon, they kept seeing it for almost an hour. They described the Moon as ringing like a bell. The Moon is a colder than the earth, so the seismic waves do not attenuate as well as they do on the Earth. They just kept going around and around and around the Moon. They literally, they saw them for like an hour. It was the one science result from Apollo 13, because obviously they didn’t get to land on that mission, but it was a surprise. The Moon is, because it’s colder, it responds to seismic waves somewhat differently than the Earth does.
Jim Green:Yeah. That crust sort of rings, and any impacts then then go around the Moon, rather than through the Moon, as we had hoped to be able to tease out the size of the mantle and the core.
Walter Kiefer: They actually do go through the Moon as well.
Jim Green: Oh. Do they?
Walter Kiefer: It’s not just going around, but it’s going through. On the Earth, the waves would be attenuated, because the rocks are warm. If they’re hot enough to flow, like we were talking before, they will start to damp the amplitude of the seismic waves down over time, but on the Moon it doesn’t do that so much, and so the waves just kept going and going and going. It’s like the Energizer Bunny.
Jim Green: Wow. That’s fantastic. Well, what do you think was the most surprising results from the GRAIL data?
Walter Kiefer: Seismology is great at determining depths, but it was near the limits of what it could do in terms of seeing the core of the Moon. The GRAIL data let us confirm, in a completely independent way, the structure of the deep part of the Moon. one of the real surprises is that we confirm that the Moon has a liquid outer core. That means the Moon has not cooled down as much as we might’ve thought. Small bodies cool off more quickly than large bodies. If we’re going to keep the cooking analogy going, hamburgers cook quick, pot roast takes longer, and Thanksgiving turkeys take much longer. Big things take a long time to get energy in them. They take longer to cool off as well. The Moon is much smaller than the earth. We expected it to cool off, and yet the deep part of the Moon,, there is still a liquid core probably related to having some sulfur that acts like an antifreeze in it.
Walter Kiefer: The other really fun part of GRAIL was that we could see the structure of some of these large impact basins. We can see the thickness of the melt sheet that was created from these impacts, which tells us something about the size. We can see the faults that are created by the impacts. We’re learning something about the impact process. There are impact basins on the Moon that are so old and so degraded that you can hardly see them in typography now, but they still have the gravity signature. GRAIL was able to show that there are about twice as many of these large impacts as we actually knew about from the typography. We changed the impact history of the Moon by a factor of two, and of course the Earth was being hit by the same amount of things, so we’re learning also about the Earth by doing this.
Jim Green: What have we learned about the Moon that helps us understand these other rocky bodies? You’ve started talking about the fact that they all have cores, and maybe even since the Moon has a liquid outer core, perhaps things like Mars or Mercury or even Venus still do.
Walter Kiefer: The two things that the Moon has told us about I think the most, one is the role of impacts, which I touched on a few moments ago, that large impacts really sculpted the early solar system in a way that… We were starting to understand, the role of impacts before Apollo, with work by people like Gene Shoemaker on the Earth. But, the Apollo data really told us the importance of large impacts shaping the surfaces of all of the planets.
Walter Kiefer: But the other part is the — and this was a complete surprise until we got the Apollo samples — was the extent to which early planets were partially or probably fully molten. That was a surprise. When we went to the Moon, leading scientists, like Harold Urey, thought that the Moon formed cold and that we would see what the early solar system was like, because the Moon had not changed since it formed. That turned out not to be true. What we understand now is that that the Moon and probably the Earth, and Mars, and Venus, all went through this magma ocean phase that we don’t see the evidence for on the Earth, because so many other things have overprinted it. But on the Moon we still see it, and yet we know that the Earth must have gone through that. So, we see a part of Earth’s history recorded in the Moon that we can no longer see on the Earth.
Jim Green: If you were to put a set of scientific hardware back on the Moon, what would it be and why?
Walter Kiefer: I really hope that some time, as we’re starting to try and go back to the Moon, both with a robotic spacecraft and eventually with a human crew, that we will go to a larger range of places. I know that there’s real interest in going to the South Pole, but I hope we will go to other places. And I would like to see a network of seismometers and heat flow probes globally distributed around the Moon, because what we know from the Apollo data is really focused on the central part of the near side.
Walter Kiefer: A globally distributed set of seismic stations could tell us so much more about the deep structure of the Moon, tell us about the core, and the magnetic fields, and things like that, but also there are signatures left behind there of this magma ocean phase that we can only start to glimpse in the current data. When seismometers are more widely separated, they see deeper, and so it really will be important to get them globally distributed. That will tell us a lot that we can only start to speculate on right now.
Jim Green: Well, you know, I think that era is coming up.
Jim Green: You know, NASA’s Artemis program is planning to go forward to the Moon and put the first woman and the next man on the surface by 2024. It’s going to be an exciting time.
Jim Green: As we begin to execute the Artemis program, one of the elements, the Gateway, will allow us to, because it orbits the Moon in such a unique way, allow us to put instruments on the far side of the Moon. That then blows open I think a whole new area of scientific research. So, we’re all really looking forward to that.
Walter Kiefer: I am too.
Jim Green: Well, Walter, I always ask my guests to tell us how they became the scientists they are today. What happened in their career that really accelerated them forward, perhaps even change their direction? What was your gravity assist?
Walter Kiefer:I would say my gravity assist, Jim, actually was the Apollo 11 landing. I was seven years old. I was captivated listening to that and the thought that people could explore other worlds. I wanted to be one of those people. I discovered, as I went through junior high and high school, that I was very good at math and science, and so I put that thought together and I thought, “Well ..” I’m in high school when NASA announced the first set of mission specialists for the space shuttle. I thought, “Okay. I can be one of those.”
Walter Kiefer:I was pretty clear I wasn’t going to be a pilot. It turned out well. I turned out to be too tall to be an astronaut at all, but I realized that I could do the next best thing. If I can’t actually go to the Moon, I can at least actually study planetary science and learn not just about the earth, but about our whole solar system. I attribute that really to Apollo 11 and what Neil Armstrong and Buzz Aldrin did 50 years ago.
Jim Green: Well, thanks so much for joining me here on Gravity Assist. You’ve really allowed us to appear inside the Moon and get a deeper look.
Walter Kiefer: Thank you for having me, Jim.
Jim Green: My pleasure. Well, join me next time, as we continue our exploration of the Moon. I’m Jim Green, and this is your “Gravity Assist.”
Credits:
Lead Producer: Elizabeth Landau
Audio Engineer: Emanuel Cooper