Why do we see only one face of the Moon? What would happen to Earth if the Moon didn’t exist? We dive into questions you asked on social media.

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

Welcome to “Gravity Assist.” This one’s going to be very different. You know, we’ve learned a lot about the Moon in this series, but I want to know what your questions are. So, I’m here with Liz Landau and she is been working Twitter and getting questions, so let’s get at it.

Elizabeth Landau: Thanks Jim. It’s been so fun reading all of the questions that you all had. We won’t get to all of them, but we did select a lot of them that represent a wide range of topics about exploration and science. So let’s dive in.

You know, Jim, something I’ve been curious about is actually something that a bunch of our listeners are curious about. Why is the Moon tidally locked? Why do we only see one face of the Moon?

Jim Green: It’s really all about what’s inside the Moon. You know, when we look at an object, a spherical object, we mentally think it’s uniform everywhere. But it turns out it’s not. Mass concentrations are all over the place. In fact, if you look at the gravity of the Earth, it’s much more pear-shaped than it is actually perfectly spherical, from a gravity point of view. And, the Moon is no different. The Moon has mass concentrations, and those are attracted to the Earth. And, consequently, that is more powerful than the spinning of the Moon itself, and that produces this tidal locking that we see.

Elizabeth Landau: Are you saying that if the Moon were lighter, if it were less massive, perhaps it would not be tidally locked?

Jim Green: No, I’m not saying the Moon has to be lighter. For the Moon not to be tidally locked, it would have to be perfectly spherical in mass. That means everywhere you draw a line from the center of the Moon to the crust has exactly the same mass in its distribution. So, it’s like grabbing a piece of clay and creating a round ball and then taking another piece and sticking it on the side. That’s an uneven mass distribution. And that, that lumpiness is exactly what attracts gravity. Therefore, the parent body that you’re orbiting with a Moon that’s lumpy is going to be tidally locked.

Now, in reality, this is nothing new. When we look out into the solar system, we see many, many, many moons that are tidally locked. In fact, at Jupiter all the Galilean moons are tidally locked, so that’s Io and Europa and Ganymede and Callisto.

Elizabeth Landau: Very cool. Let’s go to another question. @JimTheMcCabe asks, “I’m super interested in hearing more about the giant-impact hypothesis and how planets and their moons can be formed from so many of the same elements around the same time, relatively speaking, in the same relative area of space and yet be so different.” Jim, what do you think?

Jim Green: Well, the giant-impact hypothesis is the leading theory for the creation of the Earth and the Moon. It really goes like this, that early on in the formation of bodies as they orbit the Sun, in our area of our orbit the proto-Earth was coming together and several other objects, one of which we call Theia. Now, Theia ended up being about Mars-size object. Of course, as the object accrete and get bigger and bigger, Theia will be attracted to the proto-Earth because it’s so much more massive, and an impact occurs.

Well, we’ve done the modeling, and we can see the impact is such an enormous collision between a Mars-size object and the proto-Earth that actually they really merge. So whatever material Theia had is merged onto Earth, and whatever Earth had is being thrown all over the place in and around the area. That then re-accretes and creates a new object called the Moon. So that’s why the Moon and the material on the Moon is so much like the Earth, because that merger happened early on.

Elizabeth Landau: Cool. Yeah. The second part of the question is about, you know, how could all these planets be so different if they formed from about the same elements?

Jim Green:When you look at the solar system, you see that you have the rocky planets are close to the Sun, and the more icy bodies are further away from the Sun. What that’s all about is really a differentiation of material that is orbiting the Sun based on several things. One, heat from the Sun, you know, actually teases apart volatiles and molecules and breaks them up if you’re close to the Sun. So you have to be far away from the Sun for the water to exist and for nitrogen and other components of the nebula. In the meantime, what’s left a lot, close to the Sun, are these rocky materials that then accrete and form planets.

Also, you don’t see in our solar system big atmospheres like giant planets close to the Sun, because indeed when the Sun became a star and created its solar wind, it blew away a lot of that material. So what we’re seeing are the rocky planets close to the Sun, which are most affected by the solar wind and the evolution of the early Sun. And then further away, where the wind is not as strong, because it’s dissipating everywhere as it moves away from the Sun, you then see the gas left over from the nebula much more prevalent, and that’s why you get these giant bodies.

Jim Green: And then, of course, we found this debris field of icy bodies that we now call the Kuiper belt, you know, the furthest remnant of our inner solar system that as created in the early 4.6 billion years ago.

Elizabeth Landau: Wow, Jim, so it sounds like the distance from the Sun when these planets were forming really determined their fate.

Jim Green: Yeah, it makes all the difference in the worlds.

Elizabeth Landau: Very cool. Let’s take another question from Twitter. @mundanemariam asks, “Is it true that we can only see one side of the Moon? If so, is that where you guys landed, or was it the other side?”

Jim Green: Because our Moon is tidally locked with the Earth, that means by definition we’re only going to see from the Earth one side of the Moon. In our early Apollo program the first thing indeed is that we want to be able to communicate with anything that’s on the surface of the Moon, so that dictates that we’re going to land on what we call the near side. Now, the far side of the Moon is just as fascinating. It has all kinds of different features that we would love to go and interrogate, and we’re getting to the point where we will start landing a variety of our missions now to the far side of the Moon. Now what’s required is some sort of satellite orbiting the Moon such that on the far side we can communicate up to the satellite, and then that satellite can communicate back to Earth, so that we are in constant contact with anything on the far side of the Moon.

Elizabeth Landau: Very cool. Let’s take a question about exploration. @RosaCeres2 asks, “What is the main reason/motive for exploring the Moon at this time?”

Jim Green: The Moon is such a wonderful object. What we’ve learned from our initial landings and samples we brought back from the Apollo program is that the Moon’s surface has been a witness to 4.6 billion years of solar system evolution and history. And so it’s a matter of bringing back the right rocks, things that have impacted the Moon. Some of the rocks are indigenous to the Moon that we want to collect and look at and understand the Moon itself. And so consequently, the Moon holds a lot of secrets, and that’s what we want to go after.

Elizabeth Landau: Awesome. To follow up, @CRudzinskas asks, “I know we’re sending astronauts back to the Moon, but what are they going to do there, live there?”

Jim Green: Indeed, our plan is to have a sustainable presence on the Moon. That means we’ll have an infrastructure, and there may be a landing area that we will go to on a repetitive basis. We’ll build up a variety of capability there, perhaps some unique habitats that we would go to. But what they would do there is science. They would collect material. We want them to go into some of these permanently shadowed craters and create ice cores. You know, we know that there’s a lot of volatiles, and that includes water that’s trapped in these permanently shadowed craters. That gives us a great idea as to the history of the Moon, and we want to bring these cores back and analyze them in the laboratory. So our next big steps to the Moon with humans will be indeed to do science.

Elizabeth Landau: Awesome. Of course, our ultimate goal at NASA right now is to go to the Moon so that we can go to Mars, right?

Jim Green: Exactly. In fact, what we’re doing by the approach of going to the Moon is learning to live and work on a planetary surface. What we will do at the Moon is practice a number of things that we will do at Mars. So, for instance, the Moon as I mentioned has an enormous amount of trapped water in these permanently shadowed craters. Well, water is just such a fantastic resource. Water is hydrogen and oxygen, so it’s two hydrogens and one oxygen, H2O, and water is water, whether it’s here on Earth or it’s on the Moon or it’s on Mars. So it’s the same thing.

What we’re going to do with that water, of course, is we can drink it. We also can break it apart in its components, hydrogen and oxygen, and with that that enables us to create rocket fuel. In addition to that, of course, we can take the oxygen and breathe it. So water has a great deal to do with being able to sustain constant exploration of the Moon.

This is exactly the same thing we want to do at Mars. We want to go to Mars, we want to go into regions where there’s a lot of water. We see that there are regions on Mars that has buried frozen water, so that’s just exactly what we’re seeing on the Moon. So as we develop capability to tease apart that water on the Moon, that’s some of the same things we’ll be doing at Mars.

Elizabeth Landau: Excellent. I can’t wait. We have a question from @EmperorAaron. He asks, “What would be the main difficulties of establishing a human colony on the Moon?”

Jim Green: Well, the Moon is a lot different than the Earth and even Mars, for that matter, in the sense that it has no atmosphere. Consequently, we’re at a disadvantage. You know, we have to work really hard creating environments where humans can breathe. So this makes it really difficult for us to do. But you know, NASA is in the business of doing these things, knows how to do these things. So we’ll be able to overcome these hurdles and be able to create a fantastic set of missions going to the Moon, staying for long periods of time, and then returning.

Elizabeth Landau: That’s so fun. Tell us what is the timeline for that?

Jim Green: Well, we are working really hard to put the first woman and the next man on the surface of the Moon by 2024. Not only that, we know where they’re going, they’re going to the South Pole. Now the South Pole is a very large region, we haven’t picked the exact location, but indeed they’re going to be close to some of these water resources I talked about. So right now we’re moving towards identifying the exact location and some of the activities that they’d be doing. So ,we’re really busy.

Elizabeth Landau: Awesome. Let’s take another science question. @WittSteven asks, “Is it true that the Moon is slowly drifting away from the Earth by two and a half inches a year?”

Jim Green: Indeed, one of the first experiments that the Apollo astronauts put out on the surface of the Moon was a retro reflector. It’s just an instrument that allows us to fire a laser beam to the Moon, hit that reflector, and have that beam returned to the Earth. When we use that, and we do that every year, we actually then measure the speed of light from here, the Earth, to the Moon and back. Then, of course, since we know how fast light travels, we can determine the distance to the Moon.

Since we’ve been doing this for 50 years, what we’re finding out is indeed the Moon is moving away from the Earth, and it’s moving away at about an inch and a half a year. Pretty neat.

Elizabeth Landau: That’s so wild to think about. I mean, if you think about millions of years ago the Moon must have been closer in the sky. Right?

Jim Green: Yeah. Actually, the giant-impact hypothesis tells us pretty much where the Moon really came together was about three Earth radii away. When we compare the size of the Moon we would see with the size of the Moon we have today, it would be 16 times bigger than the size of the Moon we see today. So it would really dominate the sky. It would be just absolutely enormous.

Elizabeth Landau: Wow, that sounds like a sci-fi movie in the making right there.

Well, speaking of going inside the Moon, @Splitted_Spark asks, “Are there caves or cave systems on the Moon? If we decide to build underground bases on the Moon, where would the entrance be?”

Jim Green: Oh, that’s a great question. It’s a matter of just the last 6, 7, or 8 years that we’ve really been studying the Moon and its surface to the point where we have found entrances to caves. Now these are special caves. What they really are, if you can imagine early on when the Moon had magma coming up, molten rock, that was then going to pour out onto the surface of the Moon, and it was carving these tunnels and then dumping onto the surface of the Moon, just like they do here on Earth. Then they cool, they create caverns, large lava tubes that then become hollow. Certain times, the roofs collapse, and we can look into these lava tubes.

Now we started seeing these features from orbit from a number of satellites, and we called them skylights, because we knew they weren’t craters. But the more we looked at it, the more we realized when we combined that with the knowledge of the gravity in and around these areas that we got from them GRAIL Mission, that these are indeed lava tubes.

What’s spectacular about these is if we could enter them, if the roof collapses and creates a ramp that we can, and there are one or two that are like that, that we can drive into these large cavernous areas. We then can set up an inflatable habitat. It would be protected from a lot of the intense radiation that our atmosphere and our magnetosphere protect us here on Earth, but on the Moon, which has neither, then would need protection. So that’s a great place to go for that.

But what’s really neat is we believe that the temperature inside these lava tubes remains constant, both day and night. Now on the surface, the temperature change on the Moon between its day and its night is enormous. It’s hundreds of degrees in variation. So if we have a habitat that’s out on the surface, we’d have to bring enough power to accommodate these large variations. But inside a lava tube where we know what the temperature is, it’s much easier to start at a base temperature and then be able to come up with a livable temperature for our habitats.

So these skylights showing us the way to lava tubes, and the lava tubes are giving us some ideas as to potentially where we might future develop some livable capabilities.

Elizabeth Landau: That’s really neat. We have another question, “Speaking of being on the surface of the Moon, would you be able to see the lights from cities on Earth if you were standing on the Moon?” This is a question from @ibe_ie.

Jim Green: Absolutely, if you’re on the near side of the Moon. You know, as I mentioned, one of the things that we’d love to do is also go to the far side of the Moon, not only with our robotic missions but eventually with humans, too. Since the Moon is tidally locked, on the far side the Moon we’d never have an opportunity to see the Earth. But on the near side, we would all the time. Now in fact, if we ended up in these skylights you could stand at the bottom of a lava tube where the roof was collapsed and look up and constantly see the Earth. That’s really neat too. You would set up a communications station there. You know, it’s like being in the Cupola of Space Station, where you always look down and see the beautiful Earth up close. But if you walked into the skylight and looked up, you’d always see the Earth. Now, it would be much further away, but it still would be a beautiful sight, I’m sure.

Elizabeth Landau: Incredible. Here’s a question from @IMProductionUK. This sounds like a sci-fi movie in the making as well. “Hypothetically, what would happen to the Earth if the Moon got destroyed?”

Jim Green: Okay. Well, it would take a lot to destroy the Moon. You know, another huge impact, and that of course is unlikely. You know, we have a variety of hazards we call near Earth objects. These are asteroids that have been thrown out of the asteroid belt by Jupiter in their gravitational interaction with Jupiter. They come inward, they also go outward, Jupiter throws them out of the solar system. But those that come inward that then orbit the Sun and then eventually cross the orbit of Earth have the opportunity of hitting the Earth but also the Moon. In fact, for every one impact on the Moon, we expect that there’s 20 impacts on Earth because of just the size difference. But none of these objects are of any size that would bust up the Moon. So indeed, that would be pretty extraordinary science fiction.

Elizabeth Landau: Indeed. But let’s say hypothetically that the Moon suddenly disappeared. What would happen to Earth? Would our oceans be the same? What would happen?

Jim Green: Okay, that’s a good question. Let’s just wave a wand, okay, and eliminate the Moon. What happens to the Earth? Actually, several things. One, even though we know tides in the ocean are connected with the gravitational pull of the Moon as it moves around the Earth, once the Moon is gone the tides are radically reduced. Now it turns out they don’t go away, because 30% of the tides, the size of these tides, are actually contributed by the Sun’s gravitational interaction with the Earth. Okay? So there will still be some tides.

But perhaps the biggest problem we’ll have is because the Earth is not uniform in mass, just like the Moon is not uniform in mass, the pull from the Sun on these mass concentrations on Earth will yank our pole around. So like a top that’s, you know as you spin a top, and it starts to run out of energy and all of a sudden that spinning axis moves around, that’s what will happen to Earth. So our axis will now change from 23 and half degrees to some other number. We call that change in that axis “obliquity.” It could be quite severe. So, over the next tens of thousands of years, you know, maybe the equator will be where our current pole is. It could literally have huge climatic effects on Earth. So it would not be a fun Earth to live on if we lost our Moon.

Elizabeth Landau: That’s so wild. Thanks for explaining that. We have some more questions about exploration. @alexpri19570045 says, “When humans do colonize the Moon, what sort of practical issues would arise from the lower gravity?”

Jim Green: Okay. So the practical issues, some will be solved easily. You know, we’ll be able to move more freely on the Moon, so that means building habitats or structures will be a tad easier on the Moon than on the Earth. The bigger problems might be how the body adjusts to that low gravity. We know the body goes through an enormous number of changes on Space Station, where there’s virtually no gravity. And so on the Moon, even though we’ll have some, there will be some effects. There may not be anywhere near as severe as what we experience on Space Station with bone loss and with pressure on the eyes, because the gravity still will pull or liquids in our bodies down towards our feet as we do here on Earth and we’re used to that. So the pressure variation in our bodies is gauged by that gravity. On the Moon we’d still have that, so that would be very helpful. But I’m sure there will be some things that we’ll uncover that we never, never thought of. I can’t think of them, but we’ll just have to experience them.

Elizabeth Landau: Cool. Does that mean, let’s say there’s a whole bunch of astronauts, would they have to avoid knocking each other around?

Jim Green: Well, you know, when our Apollo astronauts were on the Moon, they found that skipping was a lot easier than walking. They could skip, and therefore literally lift off the Moon and move, you know, several feet quite easily. So it just was more of an effortless movement for them to do that. So, that’s kind of fun. I’m sure everyone will figure out a way that they will enjoy walking on the Moon, and it will be different than the walking that we get here on Earth.

Elizabeth Landau: Wow, that sounds super fun. I think those are all the questions that we got. Actually, let’s do one more. Let’s do one more question from @sobek42. “Does the Moon have an atmosphere, and if so what does it consist of?” Thanks!

Jim Green: Okay. When we look at the Moon, we basically say it has no atmosphere, but technically that’s not quite correct. All right? The Moon outgases, it vents some interior gases as it’s continually cooling from the time it was made. In fact, all planets are cooling from the time they are made. It’s just like you have an oven, you bake a cake, you take it out, and the cake is still cooking because it’s hot, and it’s cooling. Well, the Moon is cooling, too, and so consequently it will out gas. But that out gas creates a very thin atmosphere. It’s almost like it’s not even there.

Now we had a mission a few years ago called LADEE, which was the lunar atmosphere and dust explorer. That spacecraft had very sensitive instruments on it that was designed to measure that out gassing, measure that what we call “tenuous” atmosphere. It found out all kinds of really neat things. One of the things that it found out is that during certain times the Moon’s tenuous atmosphere would be dominated by water. That was really startling, that was a brand new discovery.

How that comes about is during those times, they are also the time that the Earth and the Moon are moving through an area in space where there’s a lot of small dust left over from a comet dissipating, and it creates this trail of small debris and material that orbit the Sun, and we pass through those orbits every once in a while and create meteor showers. And so, what’s happening then is these micro meteors that hit the Earth, we see them and we, “Oh, falling stars, just beautiful.” But they hit the Moon, too. When they hit the Moon, if they can really penetrate into the upper crust, and shock waves go down, we estimate, about 10 feet, that liberates the water that’s below that surface and creates that little tenuous atmosphere than has a lot of water in it.

So we see these puffs of atmosphere really intensifying many times a year. You know, we get six or seven a year. So, the Moon’s got such a small atmosphere we tend to neglect it or not talk about it, but it turns out it’s really important to understand it, because that water, once it’s generated, will come back down to the Moon and go into these permanently shadowed areas and contribute to the water that we’re going to get access to when we go to that South Pole, because we’re going to need that water.

Elizabeth Landau: Awesome, can’t wait! Well, I think that’s all the time we have this week. I want to thank everyone who submitted questions on social media. You all had really good ideas and really good questions about lunar exploration and lunar science. Thank you, Jim, for answering these questions.

Jim Green: Well, my pleasure. We’ve got more exciting discussions coming up about the Moon. And, so I’m hoping that you’ll enjoy this segment and others.

Elizabeth Landau: Awesome. Before we sign off, I want to plug another NASA podcast called NASA Explorers: Apollo, a podcast from the Goddard Space Flight Center. They are actually asking listeners for memories of the Apollo 11 mission. If you would like to send your story, record an audio clip and send it to: apollostories@mail.nasa.gov.

We’ll end with a clip from John Oliver Smith from Ohio.

John Oliver Smith:I recall it was hot and humid, a typical July day in Western Ohio. It was one of those summers I spent swimming at the pond, doing chores with Dad, playing out in the fields and making my own discoveries. My older brother Jim was out traveling the world somewhere. My teenage sisters were busy with their friends. And one of our local natives was in a small spaceship headed for the Moon.

Mission Control (in 1969):Eagle, you’re looking great! Coming up 9 minutes.

John Oliver Smith:I had a reel-to-reel tape recorder — a household item before VCRs, DVDs and iPhones. I carried it everywhere. This was obviously a day to be recorded.

John Oliver Smith (in 1969):July 20, 1969. It is 10 p.m. The following are the voices of Neil A. Armstrong, Air Force Colonel Edwin Aldrin Junior. They are the first men to ever walk on the Moon.

John Oliver Smith:Neil Armstrong was a local boy. He grew up no more than 20 miles away and his parents still lived nearby. Grandma and Grandpa were coming over. Mom had cooked something special, and we were getting ready to watch Neil Armstrong do what no man had ever done before. Grandpa was sipping his drink — wine or scotch, I don’t remember which. But he already on his second when Mom said, ‘He’d better land on the Moon or Grandpa will get there first!’

Smith’s Mother(in 1969): Hurry up, or Grandpa will land first!

NASA Mission Control(in 1969): We copy you down, Eagle.

Neil Armstrong(in 1969): Houston, Tranquility Base here. The Eagle has landed.NASA Mission Control(in 1969): Roger, Tranquility. We copy you on the ground. You got a bunch of guys about to turn blue. We’re breathing again. Thanks a lot.

John Oliver Smith:My father, the engineer, was explaining the events on TV moment by moment. We all held our breath at 9:56 p.m.

Armstrong(in 1969): That’s one small step for a man, one giant leap for mankind.

John Oliver Smith:When we mark the anniversary this summer, I will fall back to the day I first heard that phrase “a giant leap for mankind.” But I won’t be thinking only of Neil Armstrong. I’ll see perfectly my mom, dad, Grandma and Grandpa, all sitting in the living room around the television, in our little house in Western Ohio. I’ll also be sitting with my beautiful wife, children and grandchildren, thinking of the future, imagining discoveries yet to come, and wondering what they will remember.

Credits:

Lead Producer: Elizabeth Landau
Audio Engineer: Emanuel Cooper