Backgrounding Apollo and Beyond: The GRAIL Mission at 2+ Years

Events of March 31, 2014

One of the exciting things about living in the Boston area is that the high number of colleges and universities means I'm always just a few miles (or less) from scientists conducting fascinating research -- and many of them are glad to talk to interested members of the public about the work they do! Back in October of 2012, I had the good fortune to sit down for a 45-minute conversation with MIT's Maria Zuber, the principal investigator for NASA's Gravity Recovery And Interior Laboratory (GRAIL) mission. (You can read the first part here and the second part here.)

At that time, the twin GRAIL probes, nicknamed Ebb and Flow, had been orbiting the moon for a little over ten months, since January 2012. By measuring small changes in the distance between the two probes, scientists were able to map local variations in the moon's gravity with incredible precision. Two months after I spoke with Professor Zuber, Ebb and Flow ended their one-year mission with controlled crashes into a mountain near the moon's north pole. But while the spacecraft themselves had finished their mission, the work was just beginning for the scientists back on Earth. Analyzing the data returned by the twin probes, scientists have learned about the moon's composition, interior, its origin, and its place in the solar system's history.

During our last interview, Professor Zuber had told me there was only so much she could tell me aboard GRAIL's science results, because the mission team had three papers pending approval with the journal Science that had yet to be published. About a year after we spoke, I saw a paper based on GRAIL data--"Asymmetric Distribution of Lunar Impact Basins Caused by Variations in Target Properties"--in the Science issue for November 8th 2013--and Professor Zuber was one of the co-authors!

Excitedly, I e-mailed her back to offer congratulations and ask if it would be possible to do a follow-up "one-year-later" interview about what had been learned since. Unfortunately, that wasn't possible because of scheduling issues on both sides of the Charles River, but we finally worked out a meeting day in time for a "one-year-and-five-months-later" interview on the last day of March. And so, tape-recorder-in-tow, it was back across the river...

Me: We last spoke in October of 2012, around that time. That was when the GRAIL probes had been orbiting for several months, and it was just a few months before the end-of-mission. (holds up issue of Science) This is the issue of Science for November 8th, so it's about a year afterwards. You had said there were certain things you couldn't say until they were published--

MZ: Sure.

Me: So I contacted you to ask about meeting again now that some things had been published. So, now that it's been a year and a half, what would you say are the big things that have happened since the last time we spoke?

MZ: Okay, well, so first of all, the operational part of the mission came to an end. The two spacecraft crashed into the moon--on purpose, what we decided to do is have them dedicate their demise to science. Over the course of the mission, we brought them down [to lower- and lower-altitude orbits], we started at an altitude of 55 kilometers (34.17 miles), orbiting in a polar orbit around the moon, then we brought that down a factor of 2 to an average altitude of 22 kilometers (13.67 miles) above the surface of the moon, then we took it down again to an average altitude of 11 kilometers (6.84 miles) above the moon, in which case we were as little as two kilometers above the rim of one of the major basins around the moon.

It's the closest a spacecraft has ever observed a planet from orbit. On purpose, anyway. And so we did that so that we could make a very, very, very high-resolution gravity map. I've just made the first presentation of that map a week ago at the Lunar and Planetary Science Conference in Texas.

But we've also made many other discoveries. There were some things that we hoped we would discover [about the moon] that we've discovered, and there were some things that we hoped we would discover that we haven't discovered yet because the analysis of the data and the corrections that you have to apply are so complicated. And then there were a number of things that we discovered that we had no idea were ever there. So all of those things are part of our ongoing science investigation.

I should say that the system was two satellites sending radio signals back-and-forth to each other. The data quality was about a factor of four better than we had designed, so as a consequence of that, we have to make much more detailed corrections in order to be able to take full advantage of that quality of the data. So we're in the process of doing that, and it takes a long time to do.

Me: What surprised you?

MZ: One of the surprises is that while we knew the crust of the moon had been broken up by impacts--if you look at the moon in a telescope or you look at a picture of the moon taken with a telescope, you can see that there are many craters, and craters are caused by impacts of asteroids into the moon early in the moon's history-- it was thought that the lunar crust was fractured because this occurred, but what we learned from GRAIL is that the crust of the moon isn't just fractured, it's absolutely pulverized.

It's much, much more fractured than we had previously thought. And we've been able to look at how the fracturing and the porosity of the rocks actually varies with depth. We now know that faults penetrate all the way through the lunar crust and into the mantle.

Me: So the crust is in many more, smaller, pieces than you imagined? It's not continuous?

MZ: Exactly. In fact, it's been beaten up and blasted so much that it's very homogenous. Everywhere you look, except the dark parts, which are a rock called basalt.

Me: The maria.

MZ: The maria, right. But the white parts, the white highlands are just fractured, fractured, fractured. We see very little in the way of subsurface gravity at short wavelengths that corresponds to the crust. The only way that the crust of the moon could be so homogenous is if it's been broken into so many small pieces. So it actually tells us that the early moon was a much more violent place than we thought it was.

And that tells us something else, because the Earth is right next-door, and the Earth is an even bigger gravitational target, so it got hit by even more impacts than the moon did. So we now also know that in the same period of time when the moon was accumulating all of these impacts, the Earth was accumulating even more impacts than this, and so the Earth's crust was very, very pulverized. And this is important because the earliest period of time after the formation of the planets, when all of this bombardment was taking place--

Me: The heavy bombardment.

MZ: The Late Heavy Bombardment, yes. When all of this was taking place, after the accretion of the planets, still very early, this was the period when the first single-celled animals were forming in the Earth's oceans. We know from some very isolated mineral samples that the Earth had oceans back then--

Me: Right.

MZ: --and this was the time when life was starting to develop. So goodness knows how many times life started, and then got wiped out by an impact, and re-started. We did this mission to study the moon, but it's actually causing scientists to think about the conditions on the early Earth as well.

Me: And then that's is why some people have suggested that life began in the deep ocean perhaps, where it would have been more protected from the surface impacts.

MZ: Perhaps, it looks like life started in the ocean and then developed and crawled out, but I'll tell you, even if you're in the ocean, big impacts, the kind you see on the moon, will ruin your day. (laughs)

Me: (laughs) Yes. I've seen some of the craters that are the size of cities. [The prominent lunar crater Tycho is 86 kilometers across, while the Boston zip code, from Dedham to the outermost harbor island, is 28.5 kilometers, and Manhattan Island is 21.6 kilometers long.]

MZ: Yes.

Me: Tremendous.

MZ: Yes. So that has been a surprise, it's been a very welcome surprise. It's gotten scientists thinking about the role that impacts have played in the early evolution of planets. Like I said, we're studying the moon, we're learning something about the Earth, but all of the planets have been bombarded like this. So the crust of the moon is so homogenous that we were able to see very subtle mass anomalies in the subsurface.

There was a theory back in the 1970s that the moon had actually expanded. The way this would have occurred, is that because all of these impacts had hit the moon, the outer part of the moon melted. This is something called the Lunar Magma Ocean. It was actually discovered during the analysis of the Apollo samples. The idea of the Lunar Magma Ocean, the fact that the exterior of the moon was almost entirely melted, was actually one of the big scientific discoveries from the Apollo missions.

The theory behind this was that the exterior of the moon was very hot, and it heated the inside of the moon, and when it heated up the inside of the moon, the inside of the moon expanded. And this caused the whole moon to expand. This had been theorized and models had been developed that suggested that the moon had expanded. But when we looked at the surface of the moon, we didn't see any evidence for this expansion having taken place. But in the GRAIL data we saw dikes--essentially magma-filled cracks-- beneath the surface. And these are actually evidence for the expansion of the moon.

And either these cracks never got to the surface, or they had gotten to the surface but the surface got so beat-up by impacts that it wiped out the record of it. So these dikes never get any closer than 5 kilometers to the surface.

Me: Did it expand as much as thought?

MZ: It expanded about as much as thought, as much as five kilometers. But maybe the record was wiped out because of all of the impacts, because the surface got all beat up and homogenized. What is interesting is that that was found in the data by Jeff Andrews-Hanna, who's a professor at the Colorado School of Mines, a former post-doc of mine, and he never really liked that theory about the expansion of the moon, but he wound up proving that it was correct on the basis of the data! (laughs)

That's okay. Whether you like something or don't like something, you want to be the first person who finds the truth, and that's the wonderful thing about science.

Me: Now, if the moon had been as tectonically and volcanically active as the Earth, would the evidence of the pulverization not be as apparent?

MZ: This bombardment occurred very early. If this expansion had continuously occurred throughout much of lunar history, the evidence for that would have been apparently, because it would have just intruded through all of this pulverized material. And the fact that the crust was so broken-up would have made it very easy for the magma to get to the surface.

Me: Can you make an estimate for when the moon stopped having these processes?

MZ: This was older than four billion years ago. For all the HuffPost readers who are experienced with lunar stratigraphy, this is pre-Nectarian.

Me: Pre-Nectarian.

MZ: Very, very, very old.

Me: What are the different names for the periods of geologic history on the moon? I know that on Mars, there's Amazonian, and um--

MZ: Oh, on Mars, let's see, there's Amazonian--

Me: --Noachian--

MZ: --Noachian, yeah, and Hesperian. The youngest period on the moon is the Copernican, and then the Eratosthenean is before that, the Imbrian is before that, and the Nectarian is before that. So this is pre- the oldest period.

Another thing that I think would be of interest to the science-oriented readers out there is this--I was hoping that GRAIL would do something to elucidate our ideas about the formation of the moon, and I think we talked about that last time--

Me: Okay.

MZ: I wasn't quite sure what that would be, but I knew in my heart that something would come out of it. So what happened is, we were able to make a map of the lunar crust that gave us the thickness of the crust and the volume. Other models of the thickness of the lunar crust had been developed before, by some of the members of our team. But none of those models ever agreed with the point measurements of crustal thickness that were made by the seismometers the Apollo astronauts put out.

So the question was, when the seismometers saw a velocity discontinuity in the moon, was it really the crust-mantle boundary, or was it something else that was just very local and not a global effect? [That is, when the seismometers recorded differences in the velocity of moonquake waves, did this reflect something about the interior of the moon, just as our current model of the interior of the Earth owes much to changes observed in the speed of earthquake waves?]

When we made our crustal-thickness map, we found that the thickness of the crust was almost a factor of two less than previous maps had it.

Me: Wow.

MZ: Yes, we were wrong. 'Cause I published some of those previous maps, and so did others on our team. (laughs) The reason that we and others had it wrong was that we used the wrong density [value] for the lunar crust when we made the model. It turns out that because the moon's crust is so broken up, the density is lower than we thought it was.

Me: But it was still the same material [you had thought it was]?

MZ: It was still the same material, but it was broken up. Basically, everyone who had done these models had thought that the breaking-up was close to the surface and most of the crust was really the density of intact rock. And so the value that was used for all of these crustal density models was the value for intact rock. Wrong.

Me: It was pulverized.

MZ: It was pulverized. So when you use the correct density, you wind up with a value that's nearly a factor of two less, and so the volume is much lower. This turns out to be extremely important for the origin of the moon.

The idea for the origin of the moon, well, I will say that no one was there when the moon formed--

Me: (laughs) No.

MZ: No, so the idea of how the moon formed is a theory. But a theory in science is actually a hypothesis that has been tested many times and has withstood those tests. A theory is actually something very good in science, it's withstood a lot of scrutiny, but it's not positive.

Me: I think it's important to establish that the scientific meaning of "theory" is different than the colloquial meaning.

MZ: Exactly, in the scientific meaning of theory, there's had to be a lot of hypothesis testing that has gone into it for an idea to survive. There's plenty of ideas that are around out there and many of these ideas are not so good and can be dismissed pretty easily, but a theory has withstood a lot of scrutiny.

That doesn't make it absolutely correct, but for our theory on the origin of the moon, the only theory that does not have a fatal flaw is the idea that there was an impact into the Earth, and material was thrown out and formed the moon.

So the leading variant of that theory is that it was a Mars-sized body that hit the Earth at a strafing angle and the iron core of the impactor wrapped itself around the Earth's core. Material from the mantle of the impactor and the mantle of the Earth was thrown off and formed the moon, in the Earth's orbit. The reason that that theory was put forward and has survived is that the moon has very little iron in comparison to the Earth. Earth's core is about half its radius and mostly iron, about 90% iron, while the lunar core is probably about a tenth of its radius, it's very small. If both of these things just accumulated in the same part of the solar system, they would have accumulated out of about the same stuff and therefore had about the same amount of iron. Not so.

So this Giant Impact Theory can account for the difference in iron. Alright, that explains the iron, but to explain whether or not this impact theory is really how the moon formed, you have to explain not only iron, you have to explain all the elements. One of the important elements that scientists measure is aluminum. Earth has a fair amount of aluminum, and the moon's crust has a lot of aluminum.

There were different variants of this impact theory that said that either the moon has the same amount of aluminum as the Earth or the moon has twice as much as the Earth. The one where the moon has twice as much aluminum as the Earth is the one where you have this Mars-sized impactor that does the strafing impact. So it turns out, we now know that's wrong, the moon does not have a factor of two as much aluminum, it has about the same amount of aluminum as the Earth.

But the reason people thought it would be a factor of two more is because they had the wrong value for the crustal thickness.

Me: Right.

MZ: They had the crustal volume wrong. And so we now know that the moon has about as much aluminum as the Earth. So it gives us different scenarios for the origin, we still believe that the moon formed from a large impactor hitting the Earth, but now the impactor might have been smaller and faster, or it might have been a bigger, slower impactor nearly the size of the Earth.

So there are different scenarios now that are being investigated because we've gotten this good refinement of the aluminum content. We did indeed contribute something to our understanding of the origin of the moon, but scientists are still studying the origin of the moon. It's a very, very difficult question, and no one measurement is going to clarify it. It will require lots of measurements, lots of simulations, lots of analysis of samples. I would love us to bring more samples back for scientists to study.

Me: Is there any correlation between the location of these fractures and faults and the location of moonquakes?

MZ: No, they're all over the place. From our early analyses of the lunar seismic data, it looked like all of the moonquakes were on the side where Earth is. [AKA the near side.] You know, because the moon always shows the same face towards the Earth, all the time.

Me: But all the seismographs were on that side. [Because they were all left by the Apollo astronauts and all of those missions went to the near side.]

MZ: Yes, all of the seismometers were on that side! There were a lot of questions about whether that was just incidental [because of the location of the instruments]. Several tens of years subsequent to the Apollo landings, scientists have now looked more closely at the seismic data and they have now found some far side moonquakes. So we know that there are moonquakes all over. We know that the number of moonquakes is correlated to when the moon is closest to Earth in its orbit, and that's related to tides.

We would expect more [moonquakes] on the side facing Earth because the tidal forces would be greater, but we don't know, because we had the ability to measure them on the near side more easily than the far side.

Me: It's really incredible that you've been ground-truthing Apollo like this. That was what you said you wanted to do, and you've been comparing this data with their data to understand the moon as a whole.

MZ: The Apollo samples that were brought back, they are a priceless treasure. They are still being analyzed and they are becoming even more valuable. One of the ways that we figured out how fractured the moon was was to take intact mineral grains from Apollo samples and measure their density. We then had the density of actual moon samples that were intact, and then if you measure the density of the crust from a satellite and it's less, you can tell how much of what is there is fractured and void space.

Me: We have to go soon, but what so far has been the proudest moment of the mission, either of the operational phase or of this scientific phase?

MZ: The proudest part of the mission for me was when we were able to orbit around the moon at an average altitude of ten kilometers. The moon is a really, really bumpy gravitational target, and for our team to be able to get down that low... If you're 20 kilometers above the moon and you put a spacecraft in a circular orbit there, it immediately becomes elliptical. The apoapsis, which is the far side of the orbit, gets farther away, and the periapsis, which is the closest point to the planet, gets closer and eventually hits it.

So I say that it was like just having a baby, where the baby gets up every two hours and you have to take care of it and feed it. We had to take care of these spacecraft constantly.

Me: It's like being on a teeterboard, I guess, where you have to keep making corrections to stay balanced?

MZ: Exactly. Satellites around Earth orbit at 500 miles or 25,000 miles (804.67 or 40,233.6 kilometers), but around the moon we were able to orbit at less than 10 kilometers. The only way we were able to do that was that, first of all, our team was very good, but also because we had measured the gravity field so well that we were able to navigate that bumpy environment.

I'm of course also proud of all of the science that's coming out of the mission and continues to come out of it. In terms of the maps and the resolution that is possible because we were able to make these measurements from so low. It will be very difficult to orbit any other planet that low in anywhere close to our lifetime or even your lifetime.

Me: Have you heard back from the MoonKAM users about how that went? [MoonKAM was a project allowing students to control cameras about GRAIL and take their own images.]

MZ: Oh yes! In fact, I saw some in Texas last week when I gave a talk at the University of Texas Austin. Some MoonKAM students came to my talk.

MoonKAM images are still being analyzed in the classroom, and they're archived so that students can continue to use them. The other thing about it is that I have gone to scientific conferences where there are students presenting original research using the MoonKAM images.

Me: Students in what grade?

MZ: These are middle-schoolers at these student conferences. New things are being discovered by students using MoonKAM images. It's so thrilling that that has happened, and I think it's going to continue to happen.

Me: It's so great that you were able to do that through Sally Ride Science. The impact site was named for her, wasn't it?

MZ: That's correct, we found an unnamed mountain in the northern hemisphere of the moon, because we had to stay away from impacting near any historical sites, obviously. We wanted to honor her contribution to education, so we chose this unnamed mountain on the polar near side.

Me: That's a great tribute. Thank you so much for taking the time to meet with me.

MZ: You're welcome, I'm very happy to do this.