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Gravity Assist Podcast: The Moon Rocks! With Barbara Cohen (1)
June 21, 2019

Learn how Moon rocks can reveal all kinds of information about our nearest neighbor, as NASA prepares to send astronauts to the Moon and on to Mars.


Moon rock from Apollo 14 Credits: NASA

Jim Green: Moon rocks from the Apollo missions have taught us a lot, but did you know some are still sealed, and we've never opened them? Why did we do that? Well, we're going to find out by talking to an expert. We're going to find out why we care about these tiny pieces of our nearest neighbor.

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

I'm here with Barbara Cohen. Barbara is a planetary scientist at the Goddard Space Flight Center. Barbara has been analyzing lunar samples that have been returned from the Moon. So, today we're gonna get down to the surface of the Moon and talk about what we've learned.

Jim Green: Welcome, Barbara.

Barbara Cohen: Thanks, Jim.

Jim Green: You know, from the Apollo Missions we brought back about 850 pounds of material from six different missions. But, you know, didn't the Russians also bring back some lunar samples, and how did they do that?

Barbara Cohen: Yeah, the Russians also brought back samples from the surface of the Moon in a way the US never has. And that's fully robotically. So they sent robotic landers to the Moon that collected tubes of lunar samples, rolled them up, blasted them off the surface, and they landed on the Earth for us to analyze. Now we hope to be able to do that someday for the U.S. and to return our own robotic lunar samples as well.

Jim Green:What kind of things are in this lunar material that we bring back?

Barbara Cohen: So, the Moon is a planet just like the Earth is a planet and lots of other planets, and it's covered with rocks. That's what we do as planetary geologists, is we treat rocks like they are history books of the planet they came from. So the rocks on the Moon tell us about the surface of the Moon and the interior of the Moon. We call them different things, we call them breccias. A breccia is just a rock that's made up of tiny bits of other rock. They are sedimentary rocks just like we have sedimentary rocks on the Earth, except they're not made by wind and water like they are on the Earth. They're made by impacts, by impact craters. There's also igneous rocks, they come out of volcanoes, same as they do on the Earth. They come out of volcanoes on the Moon. There's metamorphic rocks, just like on the Earth that are made by heat and pressure in the Moon's interior.

Jim Green: And then of course there's the regolith, the dust that seems to be everywhere on the Moon. How is that created?



Jim Green and Barbara Cohen at NASA's Goddard Space Flight Center, Greenbelt, Maryland. Credits: NASA

Barbara Cohen: The regolith is the very surface of the Moon, and it's very small particles that haven't been welded into a rock yet. So that's all the little debris that's created when there is an impact on the surface of the Moon, and rocks get thrown around and broken up, and they make what sometimes people call the "soil" of the Moon. But it's not really soil like we think of it on the Earth, because it doesn't have any organic compounds. It doesn't have little worms and bacteria in it like we do on the Earth.

Jim Green: Studying this kind of returned material from space, in planetary science there's a special name for it. We call the people that do this kind of study in the laboratories "cosmochemists." So, do you consider yourself a cosmochemist?

Barbara Cohen: Sometimes. I think I consider myself more of a planetary scientist. The reason is, yes, I work in the laboratory, and I work to take the rocks apart, and I work on isotopes, which are very specialized kinds of particles where we try to understand the makeup of the rocks in a very fine atomic level. That is definitely cosmochemistry. But we also need to understand where those rocks came from, the field context. We need to understand the remote sensing to put them in a global context.

Jim Green: The rocks we're brought back, we know where they came from, and so we can get that context. But don't we find lunar samples that have made their way here on Earth other ways? How does that happen?

Barbara Cohen: We do. Those are called lunar meteorites, and they're very near and dear to my heart. I've done a lot of work on lunar meteorites. They get blasted off the surface of a planet, they get blasted off the surface of Mar or an asteroid on the Moon, and they come to us, and they intersect with the Earth's orbit. So it's kind of like if you sat down on a mattress next to someone, and you sit down really hard, and boink they go bouncing off. That's what happens on the surfaces of other planets. An asteroid or a meteorite will come in, blast a rock off the surface of that planet, and it comes to the Earth as a meteorite.

Barbara Cohen: So there are some meteorites that we found that look just like our lunar rocks, and, like, I said they're very special. They have different isotopes and different elements. So when we look at our Moon rocks that we brought back with the Apollo and Luna samples, we know that they're different from the Earth. Then when we find a rock on the Earth that looks like those, we know it's a meteorite from the Moon.

Jim Green: So, how do we get those? I mean, can you just walk outside and pick one up?

Barbara Cohen: If you're in Antarctica you can just walk outside and pick one up. Not everyone can be in Antarctica, though.

Jim Green: Well, why is Antarctica so special?

Barbara Cohen: We have a program called the Antarctic Search for Meteorites. The reason we have that is because when meteorites fall on the Earth they fall randomly over the whole Earth. That means 75% of them will be in the oceans, and we'll never find them. A large portion of the other ones will fall in places that people never are, like the vast wilds of Siberia. So to find one in your backyard would be really, really special. And instead, what we do is we look in places where meteorites can accumulate over hundreds of thousands of years, and those are deserts, like the hot deserts of Africa and the cold deserts of Antarctica. So we have a program that goes to Antarctica every year and finds the meteorites. They accumulate there, and really you can walk outside your tent and pick them up.

Jim Green: Wow. You know, I guess because of the contrast, you know, these rocks are dark the snow on these glaciers is blinding bright and white, the contrast, it's easier to see these.

Barbara Cohen: That's absolutely true. But Antarctica slips you a little curve ball in that there's a lot of black rocks in Antarctica, too. So, we have to learn to distinguish between the terrestrial rocks and the meteorites. Humans are very good at that.

Jim Green: So, when we go down to the Antarctic, it's done during the summer season 'cause it's too cold down there during the winter. How many of these total meteorites do we bring back?

Barbara Cohen: It varies on the location and what the collection site is like. So, some seasons have had as few as a hundred, and some have had over a thousand.

Jim Green: Wow, wow, per season. So we've been doing this for several decades.

Barbara Cohen: We have more than 25,000 meteorites in the U.S. collection from Antarctica.

Jim Green: That's fantastic. Well, what kind of analysis techniques do you do on these samples?

Barbara Cohen: We take them into the laboratories. We look at their elements, so that is the atoms that they're made out of, and then we look at the isotopes, which are the weight of the different atoms. And all of those, and the ratio to each other, tell us about their parent bodies. So we want to know whether something formed in a volcano, or whether it formed in an impact, and then we want to know when those things happened. So the "what" tells us what happened on the surface of the body, and then the "when" puts those events in order.

Jim Green: Recently there was a report that announced that we found an early Earth rock in the Apollo samples. How could that be?

Barbara Cohen: That's amazing, right, that's a really interesting result. That paper did a very careful analysis of different, not just isotopes, but different states of the same element that formed under different conditions. So they went into a mineral called zircon. Zircon is a very resistant mineral on the Earth. It's the oldest mineral that we have on the Earth, is called a zircon. They found some zircons on the Moon. We've been analyzing those for several decades. They found a signature in those zircons that doesn't belong to the Moon, that would be very difficult to understand it having been formed on the Moon. So they speculate that it wasn't formed on the Moon.

In parallel, there's been an idea that's been out there for about two decades now, that maybe early in Earth's history -- Earth also experiences impacts -- the Earth could have launched meteorites off of the Earth. They could have landed on the Moon. That would have happened very early in Earth's history. We don't have big impacts now or a lot of impacts now on the Earth. So, for that to happen, it was probably very early in Earth's history, and these zircons would be consistent with that hypothesis. It's not proven, but it's also very interesting to find anything that's even a candidate for that process.

Jim Green: Yeah, that's absolutely fascinating to me to be able to bring back a rock that was born on Earth but yet traveled to the Moon. In fact, the analysis seems to indicate--

Barbara Cohen:  That's real sample return.

Jim Green: Yeah, that's real sample return! The sample has been dated to be something like 4 billion years.

Barbara Cohen: Yes.

Jim Green: Now, can we find rocks here on Earth that are 4 billion years old?

Barbara Cohen: It's very rare for us to find those. Like I said, the zircons are the oldest rocks on the Earth. They are, in fact, only single minerals, they're the only things that have been able to withstand all of the Earth's processes that have happened over four billion years of history. All the wind and water and plate tectonics, they really crunch up the rocks on the Earth. So we don't have really old rocks on the Earth, that's why we look to the Moon for some of these very ancient rocks.

Jim Green: Yeah. So the Earth is sort of resurfacing itself. And, so, getting the really old rocks is very hard to do.

Barbara Cohen: It's very hard to do. The oldest rocks that we have on the Earth are in Australia and in Canada, and they're not even 4 billion years old. They're about 3.8 billion years. But some of the minerals in them, those rocks are sediments, which means there had to be a rock before them that got broken up and incorporated. So that's what we look for, are the minerals in those rocks that predated the rocks they're in now.

Jim Green: You know, when we think about it, if this rock is 4 billion years old, originating from the Earth, and it made it to the Moon, when we look at how far the Moon has moved over time, the Moon must have been really close to the Earth at that period of time.

Barbara Cohen: That may not be on a human scale that much closer. It's only moving away from us at about 2 centimeters [less than 1 inch] a year.

Jim Green: Yeah, but 4 billion years ago, I bet it's at least a quarter of that distance. So at least it was closer.

Barbara Cohen: It was a little closer. Eclipses would have looked a lot different from that perspective.

Jim Green: Now, what kind of laboratory equipment do you have that you use today to analyze samples?

Barbara Cohen: I run what we call the Mid-Atlantic Noble Gas Research Lab, and you can pronounce that "Moon girl." And we look at isotopes of noble gasses. Noble gasses are elements that have a full electron shell. That means they don't like to combine with other elements to form compounds. They like to hang out on their own.

Jim Green: Like neon and some of these.

Barbara Cohen: Neon, argon, krypton, xenon, helium. Yep, exactly. We like those elements because they are radioactive decay products of other elements. So I use a process called "radiometric dating." And that's when one atom spontaneously converts into another atom by radioactive decay. And I'm particularly interested in the potassium-argon system. Potassium is a naturally rock-forming element. It's in your bananas, it's in your granite countertops, it is very slightly radioactive. Don't be alarmed. It's very, very slight. But we can measure that in our laboratory. We can measure the decay of that potassium into argon, and from that we can tell when that rock formed or when it was heated up. And that puts those planetary events in order, like I said, volcanoes and impacts.

Jim Green: So some of the things that you find out is the geological history of the Moon. What other things are you learning from the lunar samples?

Barbara Cohen: We also learn about the history of volatiles on the Moon. And by "volatiles" I mean things that are a gas to us or liquid. So things like water on the Moon, that's a really interesting topic these days. We want to know how much water was on other planets. That helps us understand their geologic history, but also helps guide future human lunar exploration. If we could live off the land and get our own water, we wouldn't have to bring it all the way from earth. So one of the things that we're doing a lot, is understanding the water content of lunar minerals and trying to look for places that water may exist on the Moon.

Jim Green: Wow, that concept of water on the Moon, you know, it has no atmosphere. Where are they finding this water?

Barbara Cohen: The water is in multiple different places, and this is one of the great success stories of the Apollo samples, is when the Apollo samples came back and we analyzed them with the techniques that we had at the time, we could not detect water in the samples. But then as time went on and our analytical techniques got better and better, we went back and analyzed those again, and we found very trace amounts of water in them. So there's trace amounts of water in the lunar interior that come out in volcanic samples. There's a little bit that's adhering to the surface of samples. That comes from the solar wind. The solar wind is putting out hydrogen, there's oxygen in the rocks. When that hydrogen slams into those rocks, it makes tiny amounts of water.

There's also water at the lunar poles. The lunar poles are a really special place, because they're places that are permanently shadowed. There's places that sunlight has never gotten to for 2 billion years or maybe even 4 four billion years. If anything gets in there like a comet or a meteorite that's bringing in water, or water that's hopping around on the surface, it sticks there and it stays there and it builds up into big deposits. We're very interested in those as places where we might be able to dig those up and use them for astronauts.

Jim Green: Well, you know, the Apollo missions were all pretty much centered around the equator or at least low latitudes. So this concept of having water in the poles, how can that be? Why are we seeing accumulation of water there?

Barbara Cohen: Because at the poles it's very cold, and there are these very, very cold places, these permanently shadowed craters. The Lunar Reconnaissance Orbiter has measured the temperature of those permanently shadowed craters, and it's about 20 degrees Kelvin [400 degrees Fahrenheit, 250 degrees Celsius], that's colder than the surface of Pluto. So they're very cold, and they're right in our backyard. They serve as traps that have those molecules like water, carbon dioxide, ammonia, methane. They build up in those traps. Whereas, at the equator it gets very hot. Because the Moon has no atmosphere, the sun just beats down on it, and those molecules turn into gasses and they escape the Moon.

Jim Green: You know, we've been analyzing samples that we returned from the Moon for 50 years. You would think we'd have known everything we can possibly know from that kind of analysis. But is that true? Is there other things that we can find out?

Barbara Cohen: With the samples that we have, you're right, they came from the near side near the equator. That was because we wanted astronaut safety, we wanted them to come back to us safely. So we wanted to put them in places that were not dangerous. But there's lots of places on the Moon that we have not sampled. We haven't sampled the whole far side of the Moon. There's an enormous basin on the far side of the Moon called the South Pole-Aitken basin that may have punched all the way down to the lunar mantel. That would be a very exciting place to analyze samples from.

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Gravity Assist Podcast: The Moon Rocks! With Barbara Cohen (2)


Barbara Cohen studies lunar samples at NASA's Goddard Space Flight Center, Greenbelt, Maryland. Credits: NASA

Jim Green: When you say "basin," how is that created?

Barbara Cohen: A basin is a gigantic impact crater. When you look up at the Moon on a clear night on a full Moon, there's big round black areas. Those are giant impact craters on the Moon. We call those basins. If you could see the far side with your naked eye, which you can't but you can look on LRO and you can see the data. The South Pole-Aitken Basin is the biggest impact crater in the inner solar system. It's 2,500 kilometers [1,600 miles] across.

Jim Green: Yeah. It's huge. In fact, it gets its name from the fact that it extends from the South Pole to a small crater called Aitken.

Barbara Cohen: That's right.

Jim Green: South Pole-Aitken Basin.

Barbara Cohen: That's right. Basins on the Moon are named for features either side. So, hyphenated.

Jim Green: Yeah. Right. How do you get these lunar samples? We store them our curation facility at the Johnson Space Center. Is this a loan agreement, or how does it work?

Barbara Cohen: Yeah. So the way that we get and analyze lunar samples is a pretty long odyssey. It's not something that happens overnight. First, a researcher would have an idea of something that they wanted to test with a lunar rock or something they wanted to investigate in the lunar samples. That comes from going to conferences, reading papers, talking to our colleagues, and trying to generate ideas that we can test with lunar rocks. We have 800 pounds as you said, but we're not getting any more anytime soon. So we treat that as a very precious resource.

Barbara Cohen: So, when I want to request a lunar sample, I make sure that I cannot do the same kind of work on any other kind of rock, I can't do the same kind of work on a terrestrial analog, that I really need that lunar rock, and that I've given a lot of thought to the analytical protocol that I'm gonna put into it, so that I can use as little as possible. Then, I write a request, and the request goes to a committee, and there's a committee of other scientists who read all of the requests. Twice a year they take requests. They read them all, and they review them, and they say: Is this important science? Can this be done in this laboratory? Are they making the best use of the lunar samples they can? And the answer isn't always yes, but they will try to put you in touch with someone who can help you better your request.

Barbara Cohen: If the answer is yes, then the curation staff at Johnson Space Center works to fulfill your request by cutting up samples or doling out samples. They put them in a little box or a little vial, and then, yes, it is a loan agreement. We sign security protocol so that we will take care of the samples, we will not let them out of our possession, and then we can only do the things that we said we were going to do. We can't just willy-nilly do any analysis we want.

Barbara Cohen: And then, when we're done with the analysis, we send those samples back to Johnson Space Center, and they keep them. And maybe someone else would want to use them someday.

Jim Green: Well, you know, some of the analysis that's done is destructive. Can you actually propose to get access to a small sample and then destroy it that analysis process? Is that okay?

Barbara Cohen: Yes, that is okay if you have a very good reason. So you need to demonstrate to the committee that you have a really good reason for doing destructive analysis. But yes, many of our analyses are destructive of very small amounts of material. And when I did my dissertation, I actually calculated the monetary value on the street of the lunar material that I destroyed during my dissertation. And, it turned out to be only a few hundred dollars, because the samples we use are so small and we're so careful with them.

Jim Green: Are there some samples in our archive that we've never really analyzed to any level?

Barbara Cohen: There are. They're called "pristine lunar samples." When the Apollo samples came back, we took a portion of them and we put them into permanent storage for future generations to use when they come up with techniques that we couldn't even think about. In fact, we're going to open some of those. We just had a call. Thank you, Jim, and others at NASA headquarters.

Jim Green: You're welcome.

Barbara Cohen: They decided to open some of those for the community now. That was prompted in large part by some of the analysis techniques that we talked about earlier, that found water in the lunar rocks. We have more analysis techniques that we can use now on lunar rocks. And hopefully, we will see some of those results soon.

Jim Green: So how many samples go out to the science community a year, do you think?

Barbara Cohen: There are hundreds of active Apollo--

Jim Green: Wow.

Barbara Cohen:   --sample researchers. So I think there are tens of samples, maybe 50 or so samples that go out every year. They go all over the world, not just to the U.S., but to every country, all countries that want to. Anyone can request a sample, and if they have a good reason they can receive that sample.

Jim Green: It sounds like we haven't brought samples back from every place that we would like. So, where on the Moon would you like to get some additional samples? And, what will that tell us?

Barbara Cohen: Personally, I'm very interested in the history of impacts on the Moon. And, that is something that the Apollo samples opened up for us in a major way. We did not understand the role of impacts or the history of impacts before we got the Apollo samples. But they've been a blessing and a curse. We've formulated some really interesting hypotheses based on those samples, but there was a spike in impacts that affected the Earth and the Moon. But now we know that those samples, they all came from the lunar nearside. They may all have been affected by one big basin that formed at that time. So now we're reopening that idea. We're questioning our original hypothesis, and personally I would like to have more samples from actual basins, basin impact melt sheets.

Barbara Cohen: That's very difficult because a lot of those have been covered by basalts. So we're looking for places where maybe a little bit of melt sheet is peeking through, or maybe where you hook up a 2-meter [7-foot] drill and go up there and really get some good samples.

Jim Green: Sounds great. You know, as NASA is moving now towards a really important program going back to the Moon, is sample return part of that thinking?

Barbara Cohen: Absolutely. Like I said in the beginning, Russia, the USSR, sent robotic rovers that got samples for us. That is a lot less expensive than sending humans, although you don't get the benefits of having humans on the Moon. They're different from each other, but they're complementary. But I think that going to the far side, going to lots of different places on the Moon, if we could have robotic sample return to some of these places, it would just be a huge benefit not only to the lunar community but to the planetary science community for figuring out how the Moon works and, by extension, how other planets work.

Jim Green: Yeah. That's fantastic. Our astronauts that will be going to the Moon in the future, they just have to be trained in lunar geology. They just have to be able to be able to land and look for the right material and bring the right stuff back.

Barbara Cohen: Yes. And we at Goddard are doing some of that astronaut training alongside JSC.

Jim Green: That's great. Well, you know, I always ask my guests what happened in their life, what was the person, place, or thing or activity that really got them excited about becoming the planetary scientist they are today. So, Barbara, what was your gravity assist?

Barbara Cohen: Well, Jim, I wasn't really interested in science as a kid. I wasn't one of those nerdy kids who had a telescope at age 3 or anything like that. But I do trace this back to a program called Olympics of the Mind. It's now called Odyssey of the Mind. It was a problem-solving competition. I was always super interested in logic problems and puzzles and strategy games and things like that, which you may not think has anything to do with science. But that really got me into a mindset where I could solve a problem creatively. And that to me is the heart of science. We try to identify problems that will advance science, but we don't do the same thing over and over again. We really need to bring creative solutions into the workplace to do things that have never been done before, to use new techniques or new samples and think about things in a new way to get answers that improve on our current understanding. So the things that we did in that program were things like: Use 50 mousetraps to set off a solenoid and pop a balloon and move things from one place to the other, where you could only touch the first mousetrap, and they had to do its own thing.

Jim Green: Problem solving.

Barbara Cohen: Problem solving.

Jim Green: Yeah.

Barbara Cohen: Then we had to build a vehicle that only had human power, and we had to pick up objects and move them around. And, so, you could really let your creative flag fly. You could do this in a million different ways, but you had to solve the problem. So, you could do it in this fun way, but you had to ultimately solve a problem. That kind of logic combined with creativity, set me on the path to being a scientist.

Jim Green: Well, in particular, a lunar sample analysis, that's still hands on activity.

Barbara Cohen: Absolutely.

Jim Green: Yeah. So, I can see where that helped you.

Barbara Cohen: Yeah.

Jim Green: Well, Barbara, thanks so much for joining me today to talk about the Moon, our nearest neighbor. And, it was so fascinating. I really appreciate it.

Barbara Cohen: Thanks for having me, Jim.

Liz Landau: What's up, Gravity Assist listeners? Producer Liz Landau here. Our colleagues at NASA Goddard Space Flight Center are working on a new podcast called NASA Explorers: Apollo, a series about the people behind past, present and future lunar science. They've also been collecting stories from people like you, reflecting on the 50th anniversary of the Apollo 11 Moon landing. If you'd like to share your story, record an audio clip and send it to apollostories@mail.nasa.gov. Today's Gravity Assist concludes with a memory from a listener named Jennifer Butz:

Jennifer Butz: My name is Jennifer Butz. I grew up in Ann Arbor, Michigan, but today I make my home in San Miguel de Allende in Mexico.

It was July 20, 1969. My brothers and I were sitting around a campfire just outside Halifax, Nova Scotia, in a campground we called home as Dad did a sabbatical at the University of Halifax. Mom had just brought out a tray of bananas as the fire roared brightly, and the full Moon hung overhead.

It was magic. We could just make out the images on our little black and white television.

We looked overhead and munched thoughtfully on our bananas, swearing that we could see the lunar module as it touched down. We were transfixed as Neil Armstrong and Buzz Aldrin floated down the stairs in to that puff of lunar dust and planted the U.S. flag.

To this day, whenever I see a full Moon, a little part of me is that 6-year-old girl in the woods in Nova Scotia, looking up at the Moon and wondering, did the astronauts see me as clearly as I saw them that July night?


Credits:
Lead Producer: Elizabeth Landau
Audio engineer: Emanuel Cooper

Source: https://www.nasa.gov/mediacast/gravity-assist-podcast-the-moon-rocks-with-barbara-cohen