University Place

When Did the Moon Acquire an Atmosphere?

– Jim Lattis: Welcome to UW Space Place tonight, we have a guest speaker who has many Madison connections. Our speaker is a Madison native and also an alum, got his PhD in physics from the University of Wisconsin and has spent most of his career at Goddard Space Flight Center, the NASA Center, in Maryland, where he is assistant chief of the Planetary Magnetospheres Lab, and is the magnetometer instrument specialist, sorry, magnetometer instrument manager for a number of magnetometers that have found their way onto spacecraft of various sorts, including the one that’s currently orbiting Jupiter, which he’s talked about here before. But tonight, we’re going to hear about a different area of his research and one that, for a lot of us, is a little bit surprising that there’s still some question about this, but we’re gonna get a little historical background about this too, on when did the Moon get an atmosphere. So help me welcome Ron Oliversen. [audience applauding]

– Okay, thank you all, it’s a pleasure to be here. My interest in the Moon goes back to the Apollo mission, you know, as any kid growing up the during the Space Age wanted to be an astronaut, my eyesight sucks, so I just had to be the scientist and work at NASA. And this particular project about the Moon having an atmosphere got started 13 years ago when I was back here visiting colleagues, Fred Roesler in the Physics department, one of my thesis advisors, and then Ed Merkowitz, who was also one of his former students, and we just decided to go out to Pine Bluff Observatory, point an instrument out there, and just take a look to see if we could see some traces of the atmosphere, which we’ll discuss later. But it piqued our interest, and for those of you who would like to do, say astrophotography or have gone to an actual observing site, you know in the desert Southwest or something like that, you generally don’t like the Moon, because it’s up, it’s bright, it makes it hard to see the Milky Way and all the other interesting objects. So when I was going to Kitt Peak to take some of the data that I’ll show at the end of this talk, the people just generally groan because they knew that means the Moon was coming up and they didn’t, you know, I was bad luck to them. [audience laughing] Okay, anyways I just want to deal with the basic here, lunar phases.

Most of you probably all understand this and realize you always just see the same side of the Moon, and the X is pretty much where we are in the lunar orbit right now. This Friday, Friday the 13th, we’ll have the next full moon. But if we could see it, you would find out that 89% of the Moon is illuminated right now. And if you just sort of asked what are the general properties of the Moon, here are some, you know statistics. Basically, it’s you know, a little more than 1% the size of the Earth, radius is a little bit about the quarter. And I have a picture here taken. Anyways, this was a video taken by NASA, a spacecraft called Discover, it’s at about 1 million miles from Earth towards the Sun, and the whole purpose of the spacecraft was just to always look back at the Earth. And so you could see as the Moon went across there, you know the relative sizes of the Moon. And I put for comparison the original picture taken of the Moon by Luna 3, a Russian spacecraft, in 1959. And what you’re looking at was something that you’ve never seen before, right, because it’s the far side of the Moon.

Okay, because since we are looking back at the Earth, we’re looking at the side of the Moon which doesn’t face the Earth. Okay, so anyways, if we back up a little bit and say how did the Earth, excuse me, how did the Moon form, then shortly after the Earth was formed, somewhere within the range of 30 to 100 million years ago, a large object which has been nicknamed Thea, size of Mars, hit the Earth, and had several profound effects. One, it changed the spin axis of the Earth, tipped it to the current 23. 5 degrees, but it also spun up the Earth. And back then you had a five-hour day. Anyways, the material which was kicked off as a result of this rather violent collision, quickly coalesced into one or possibly two moons, became quickly tidal-locked, so you have the same face facing the Earth again on the timescale, just weeks to months, but much closer to the Earth, 10 to 20 times closer than the Earth. And what that meant was, it was 10 to 20,000 miles from the Earth, and has been slowly been receding over the last 4. 5 billion years. Now, this idea that the Moon was formed by this violent collision in the early solar system was first put forth by Reginald Daly in 1946. He was a geologist, but he did not really have a very good PR person, because no one really heard of this idea, and it was then independently come up with in 1975 by Hartmann and Davis, and there’re also two other astronomers, Cameron and Ward, were also working on this idea.

So Hartmann and Davis generally are the people who get credit, but they weren’t the first ones to think of it. Okay, so after the Moon was created 4. 5 billion years ago, variety of things have happened, and this is a short video that’s put together by the Lunar Reconnaissance Orbiter mission, a NASA mission, which has been orbiting the Moon for the last 10 years. And so after the Moon formed, you know, it started out in the very molten state, the Earth was also molten at that point, so both these bodies were very hot, and so 4. 3 billion years ago, you had the largest meteor strike on the Moon to create the south polar Aitken basin, that’s the area gets a lot of attention nowadays. Gets a lot of attention nowadays ’cause that’s where there’s ice. And so when you think about human exploration, if you go where the ice is, ’cause that means you can get water. So, what the animation is showing you is in the initial bombardment period 3. 8 to 4. 1 billion years ago, you created most of the major features on the Moon, particularly the marias, because these meteors were then releasing the molten rock below the surface, and so the volcanic activity that you’re talking about is not the type of like a big spouting volcano, but just molten rock flowing out on the surface, creating this much more flat surface of the Moon.

And then we entered a period from 3. 8 to 1 billion years ago where there was not as heavy, let’s say a reasonable amount of bombardment, which is still affecting the face of the Moon. And this is also the period in which you created certain of the famous Ray craters, most notably Tycho. Okay, so after all this, meteor activity, you basically got the two phases of the Moon that we see nowadays; the near side and the far side, and what is just obvious, and it was a first surprise that you can even tell from that very low resolution picture from Luna 3 that I showed, is the far side looks radically different than the near side. Okay, and part of that difference is not just that the near side has all these extra marias where you had the volcanic activity creating these planes, but as a result, that is the result of the fact that the Moon on the far side has a much thicker crust, so even when the meteors were hitting it, they weren’t as likely to bust through to get to where the molten rock was. And so the question becomes why is the far side of the Moon thicker? I don’t think they really know for sure, a couple of reasons that have some bearing on it. One, when the Moon first formed, it would have been very hot, it would have been close to the Earth which is also very hot, and so that the front side is gonna stay hot on the Moon, and so the back side is gonna cool, or the far side is gonna start to cool much more rapidly, plus the fact that they speculate that actually was a second moon created at the time of the initial impact, and that that moon eventually then hit the far side of the Moon and then added extra materials. And so that might be the reason why it is thicker. Okay, so we already know the answer about what caused those craters, but it was a debate that really was not resolved until about 60 years ago. The idea that the craters were first created by meteors was suggested by geologist Gilbert in 1983, but then this guy Barrington came along, one of the premier geologists of the time, he was the head of the USGS, U.

S. Geological Survey, he went to a meteor crater in Arizona, incorrectly determined that that was the remnant of a volcano, and so his opinion held sway for over 40 years, until Ralph Baldwin started to look at the issue and to question that no, maybe meteor crater was not a volcano, and then finally Gene Shoemaker, around 1960, was able to prove that the meteor crater was indeed the result of a pretty sizable object hitting the Earth and creating that crater about 50,000 years ago. Okay, so as we have already mentioned, these marias are the result of meteor impact exposing molten rock, which came to the surface, and a couple of key features about the rocks itself is they’re rich in iron and manganese, but these do not have any water or hydrated minerals. So when they talk about areas on the Moon that have water ice, you’re talking shadow regions or talking about the hydrated minerals, you’re talking more like the highlands. And if you wanted to go find titanium, that’s actually in the maria regions. So if anyone can figure out a way to mine that and bring that back economically, that’s where you would go. The Chinese have expressed a lot of interest on their probes going to, rovers going to the Moon about what can they do in the mining respects. So what does all this volcanic activity do? Well, it actually does give you an atmosphere, and so over the period from one billion to four billion years ago, you’ve got a chart here showing you peaks that you had an atmosphere and how much of an atmosphere you had by the pressure. And you can see around maybe 3. 4, 3.

5 billion years ago, the Moon actually had atmosphere almost 1% of the Earth’s atmosphere, which would mean that its atmosphere was more dense than Mars’s current atmosphere, which is a little less than 1% of the Earth right now. So it never had a really big atmosphere, you can see I’ve marked on the left, the 1%, the 0. 1% marks, and then on the right, I’ve marked where 0. 01% is. But what you notice is, you’ve got peaks anytime at a lot of volcanic activity, yeah, you get an atmosphere, but once the event volcanic activity stopped, the atmosphere just seems to go back to zero. So, the question that we’re gonna address for the rest of the the talk is; does the Moon have an atmosphere now? And I titled it like this, when did the Moon get an atmosphere, not so much that it one day decided to have an atmosphere, but the endeavor for people to figure out that the Moon really did have an atmosphere, because they really wanted to believe that the Moon had an atmosphere, and they just couldn’t prove it. The idea of the Moon having an atmosphere at least goes back to the 1st century AD or CE, depending on your preferences, when a Greek philosopher Plutarch, in the form of a dialogue that he wrote, just discussed the idea that when he looked at the Moon, thought that those dark regions were oceans, and he thought all those light regions were land. And they even speculated maybe that the Moon was habitable. Now bear in mind, for a century they had no idea that the Moon is a rocky body, they just knew that it was something up in the sky. They also speculated maybe the Moon is the place where all the souls went after people died, but nevertheless, they took what they saw around them, which was obviously water and land, and said, “Maybe that’s what we’re seeing in the sky.

” That idea certainly continued. And Plutarch was actually quoted by several scientists and philosophers up until the 1600s. He was quoted by Galileo and also Kepler. And so the advent of the telescope, something that Galileo did not invent, but was one of the early users and made an improvement, he drew one of the early maps of the Moon. He does get credit for recognizing mountains and making measurements of how tall those mountains were, but he wasn’t the first person to draw the map, he’s just a person who gets the credit because he wrote Starry Messenger in 1610. The person who did it before him was Thomas Harriot, an English astronomer, but he didn’t publish. He worked for Sir Walter Raleigh and then later, another earl named Henry Percy who kept him employed, but the guy never published anything, and so even though he is a well-respected scientist of the time, no one knew what he did outside of the circles that he talked in. So using more than, you know, people using telescopes, you had an English clergyman, John Wilkins, come along who wrote this book entitled Discovery of a World in the Moone. And he then put in, in print actually that he thought the Moon had an atmosphere. And this book had several propositions, and I’ve just highlighted a couple of them relevant to this talk, and the other one being that he also thought it was very probable that the Moon had inhabitants, and he actually proposed, I don’t wanna say a space vehicle, but he proposed some means by maybe we could get to the Moon, thinking ahead.

And so he came up with a name for the inhabitants that you would meet, Selenites. And so he was forward-thinking on that, but again, the basis of the idea was “Okay, “I’ve got these things around me here on Earth, “I now know that the Moon actually is a, is a body “that is going around the Earth, “and so if I think it’s Earth-like, “then it should have an atmosphere like I have on Earth, “and it should have creatures, animals and plants. ” And we’re all familiar however with the another notion that the Moon is cheese, [audience laughing] and it actually turns out that in 1546, a writer from England, John Heywood, actually came up with the idea, who was then quoted later by John Wilkins, basically making fun of what he considered simpletons or people who were just ignorant, who would believe if they saw the reflection of the Moon in a body of water, or a lake or stream, that that somehow represented a wheel of cheese. So that’s not meant to be a compliment. [audience chuckling] Okay, so in 1792, we have an English gentleman, he was a lawyer, independent means. And so to him, he got his own telescope, he may have built it himself, and then he made observations looking through the telescope. And one of the conclusions that he came to, and it wasn’t an uncommon thing, is you stare at a star, and he watched the Moon as it goes across occulting that star, and when that happened, he saw instances in where the star appeared to be in front of the Moon. And the only way that that could happen, he reasoned, was if the Moon had an atmosphere. It’s called refraction, a couple slides I’ll show a demonstration of that, and he was very passionate about, this proved the Moon had an atmosphere. And he wrote a long article.

I have read many, many pages of this to try and get an understanding for his frame of mind, but the one thing you got is he was very enthusiastic, and he believed what he saw demonstrated the Moon had an atmosphere. You had a premier– premier, premier? Scientist mathematician named George Airy, who basically didn’t wanna commit to the Moon having an atmosphere. And so he sort of said, maybe it has an atmosphere, but it could also be attributed to maybe something about the optics you’re using, or maybe it’s the Earth’s atmosphere, or maybe it’s just a physiological issue with the eyes’ perception. In 1897, a UW astronomer here at Washburn Observatory just echoed this belief that the Moon just had to have an atmosphere, it just had to. We just hadn’t found it yet, but it’s got to be there. And you can see from his, his quote is basically saying, everyone who studies the Moon of consequence recognizes that the Moon has to be an atmosphere, you just have this other scientist guy named Peschel who’s putting physics into the equation, and sort of demonstrating that if the Moon has an atmosphere, it can’t be much of one, and it’s gonna be really hard to detect. And so they did make an observation right here that indicated that if the Moon had an atmosphere, it’s less than 1/5,000th of the Earth’s atmosphere. In this particular case, again it’s a matter of using refraction, and as you look there, you see your line of sight. As you’re looking at a star, if there is an atmosphere, then the light to the star looks like it’s bent, and so the position of the star then seems to change. And so Comstock’s approach was he put that device I showed you on the previous slide, here, that filar micrometer, and it has two crosshairs on it.

And he put one on one star, and one on another star, and he measured the distance between the two stars; you’ve got a micrometer, it gives you the distance, and it kept measuring those distances as the star was being eclipsed by the Moon. Happens very quickly, and he basically couldn’t prove that the distance between these two stars ever changed, and so he just got what he determined as an upper limit. So then in 1946, another UW Washburn astronomer used a slightly different technique. Any of you who are amateur astronomers may be familiar with the diffraction limit of a telescope that’s determined by the size of the telescope. And so if you have good optics and you have good seeing, then it’s possible to see this diffraction pattern around a star. So what Whitford did is, using a high speed photometer, in just measuring what the total light output from the Sun was, from the star, but electronically measuring it very fast, and watching as that star got occulted by the Moon. So if you do that, what you see is if you have a diffraction pattern, you’ll see that you have a bunch of secondary peaks. And so he measured this, and found that the pattern that he got exactly matched what you would have if the Moon had no atmosphere, ’cause if the Moon had an atmosphere, it would smear out this diffraction pattern. And so in Science Newsletter, as it says there, September 21st, 1946, very bold letters, “Moon Has No Atmosphere,” we’re done, thank you for coming to my talk. There’s nothing left to say.

[audience chuckling] But people didn’t quit, 1955, you had Elsmore and Whitfield said “Okay, we’re gonna try this, “we’re gonna look for that diffraction again, “but we’re not gonna do it in the visible “like everyone has been trying to do. “We’ll try to do it in the radio. ” And so in this case, they had IC 443, that’s an extra galactic galaxy and in the Crab Nebula, they watched the Moon go over these, they looked at the radio and tried to see whether or not there was any indication that the radio waves were being refracted. We can say that they got a positive answer, but it was a positive answer pretty much consistent with slightly better than zero. So they could sort of say, okay, if it’s got something rather than, you know the earlier number that I quoted from Comstock, 1 part in 5,000, they now said, “Okay this is a really really small number. ” 10 to the minus 14th, I’ll show another description of that in a bit. But they kept looking. Well, then basically what we have, and as I grew up, a product of the Space Age, and the textbook telling me exactly as Whitford had said, “The Moon has no atmosphere. ” So the standard argument that you would have made or seen in a textbook at this time, is that if you looked at star occultations, you’ve got no fading of the signal. Nothing that gradually causes it to blink out, and so there is no atmosphere.

Obviously, there’s no clouds, nothing in the weather. And even if there were, people made arguments going back to Bessel, pretty much saying that the Moon, because of its mass, has a low escape velocity, it gets very hot during the day. In one of the early slides there, it gets up to 250 degrees Fahrenheit during the day, and so as a result, if you do have any gas, it’s going to escape eventually. If the gas becomes ionized, then the gas is going to be swept away with the solar wind. So it’s just like, even if there was an atmosphere, it’s just not going to just hang around. So this is the second time in which we’re done, right, there’s just no atmosphere. Well, okay, as I mentioned, you know, product of the Space Age here, and we did have the Apollo program, and so we’re sending spacecraft to the Moon ’cause we got to understand the Moon. And then you get something like this from Surveyor in 1967. In the lower right there, you’re looking at a picture of the limb of the Moon and the, excuse me, yeah, the limb of the Moon, but the Sun has set. If the Sun has set and there is no atmosphere, you cannot have any glow of the sunlight over your horizon, it’s just got to be dark.

Well, it’s not dark. So there’s something above the surface of the Moon. You don’t know if it’s gas or you don’t know if it’s dust, but there’s something. So now there’s something, and you start asking yourself the question about, okay, what is it? Is the source of the material you’re looking at something that comes from the solar wind? The solar wind is just a bunch of high-energy charged particles that are flowing off the Sun at all times. Is it material that’s somehow being released from the regolith of the Moon’s surface, what is it? So on Apollo 12, 14, and 15, they put what they call a cold cathode gauge, which basically is this cold plate, and you make it cold plate because then things will settle on it and stay there, right. And so they could measure the fact that something was settling on that plate, but they didn’t know what it was. And so in Apollo 17, they actually had an experiment to try and determine out some of the actual composition of this atmosphere. And they couldn’t determine the composition during the day, because during the day, as the Sun heated up everything, the Apollo spacecraft, the sent module which was left as well as all the experiments with outgas, right. And for those of you who may not be familiar with outgassing, think about your car, right. You’ve got the plastics in your car that get heated up during the summer, UV radiation, and you come in and you end up with this thin film of gas on your windshield on the inside, right.

So that’s what’s happening on the Moon. So they’d have to wait until they got night time, so all the outgassing of what man had left in the Apollo program would go away, and then they could start to see there was some stuff left over that was lunar in origin. But I’m gonna put lunar in quotes there because what they detected was helium, hydrogen, argon, and neon. And you’ll notice I listed as one of the sources captured solar wind. All four of those elements are very prominent in the solar wind. So it wasn’t obvious that what you were detecting was coming from the Moon, it could just be coming from the solar wind and just temporarily creating an atmosphere there. Okay, so now we have some evidence of something, and so as scientists, we wanna be motivated, and we have to convince some funding agency to continue our work so you got to give them reasons, right. Okay, so number one reason is that it’s actually the most common form of an atmosphere in the solar system. So if we wanna understand the atmosphere around say Mercury or a large satellite, or icy moons around Jupiter and Saturn, then the Moon would be a good place to start, because they all have this type of atmosphere in common. Very thin, but nevertheless an atmosphere.

And that atmosphere at this level is called a surface bounded exosphere. And all that really means is that the density of the atmosphere is so low that the gas particles never interact with one another. Right now you have gas particles in this room, I believe in the range about 20 quatrillion, bumping into one another. So when you get to an exosphere– And the Earth does have an exosphere, right. It’s that transition region from atmosphere to outer space. So typically on the Earth, we say above 50 to 60 miles, we enter into, to outer space, and so that’s where our exosphere begins, but our exosphere goes out 10, 20,000 miles. Okay, so reason number one, just basic knowledge, we wanna understand this type of environment that is found on the Moon, in other bodies around our solar system. Reason number two, space weather. We’re very much interested in studying the time varying conditions due to our Sun, that includes radiation, solar wind, as well as corona mass ejections. These are the large explosions on the surface of the Sun that send out billions of tons of material that then have the ability to create aurora.

I understand that week and a half ago, they were possibly aurora seen in Wisconsin, I don’t know if anyone saw any, whether or not it was clear, I know there was an alert to do so. And it’ll affect satellites and communications, has potential to affect the power grid. So the Moon just represents another platform of which we can study these events besides the satellites that we already have out there to study space weather, and then the observations that we made on the ground. But the Moon represents a good laboratory to look at these effect. And then the third reason, sort of why NASA exists, is human exploration. Okay, so there’s a lot of talk about going back to the Moon, hopefully putting a man and woman back on Moon by 2024, that’s an ambitious program, but that’s the goal. But to do that, you really wanna understand what’s going on at the surface of the Moon. And so there are various explanations for this glow that was first seen in Surveyor 6, but that is not completely understood. There’s a lot of charging that goes on at the surface of the Moon, so you have electrostatic issues. And this was demonstrated in the Apollo program, because what they were not expecting were the spacesuits to get dirty, okay.

It’s not that they didn’t realize that there was dust on the Moon, what they didn’t expect was all this dust to stick to their spacesuits. And so by sticking to their spacesuits, you have got a problem that you have to deal with if you’re gonna build a habitat on the Moon, right, because you effectively, you need a mud room, someplace to come in and make sure that this dust does not get into your living quarters, because otherwise it’s gonna clog up your air system, okay. So you need to understand the surface processes. Okay, so how do we create an atmosphere? So now we’ve got an atmosphere on the Moon, not very thick, but we do have one. And so there’re various processes that could be used to create that atmosphere, okay. I’ve already mentioned, thermal desorption, that idea is just like snow sublimating in the wintertime, just going from solid to gas. Photostimulated absorption, that’s what you’re seeing when you see that residue in your car windows, UV radiation liberating some of the gases from that polymer. And okay, so that’s a direct energy transfer from an energetic photon. Iron sputtering, now you’re talking about material hitting the surface of the Moon and releasing the particles, solar wind constantly going on, except when the Earth is near full moon, then the Moon is in the Earth’s magnetotail. I should have pointed it out in an earlier slide, but the effect here is, it’s when the Moon is in the Earth’s magnetotail, it is shielded from the solar wind, but then the Moon is then being hit by the magnetospheric ions, which are in the Earth’s magnetosphere.

And then finally, just micrometeor bombardment hitting the Moon from all directions at all times, okay. So that’s going on on a sporadic fashion, and then there are well known. When we have a meteor shower here on the Earth, say the Perseids in August, you got a meteor shower going on at the Moon, okay. And then finally, the one that we start out with, we know we do get a atmosphere from the volcanic outgassing. Okay, now that’s not really going on very often now, but there are occasionally evidence of gas coming out from the Moon still. Okay, so that’s how you create an atmosphere, how do you lose an atmosphere? You can lose an atmosphere very easily. Like I said, something gets hot, the particles get more energetic, they pick up speed, and then it’s sort of like the graph on the lower left. Here’s like driving on the highway. Most of the cars are driving at one speed, but you always have those cars that are zooming ahead, and if they zoom fast enough, they escape the Moon. Okay, now that process can be helped along by radiation pressure from the Sun literally blowing the gas particles off, away from the Moon once they’ve escaped the surface.

Okay, and so you actually have an example on the right there. A model showing you in the panel, the second panel down that says model, what you needed to create the data that was obtained in the first panel on the right, which is to say, if you look in the opposite direction of the Moon at new moon, then you see that there is an enhancement in the sky of sodium emission. So it turns out that sodium is being blown off the Moon, basically like a comet tail that goes behind the Moon in the anti-solar direction. And then again, just mentioning that if your particle becomes ionized, then you can have it swept away by the solar wind. So what does that leave us? Okay, so what that leaves us is the Moon has what we said, the surface-bounded exosphere, just a very thin atmosphere, source is both solar wind and material coming from the regolith. And I’ve listed the six most common elements or molecules in the atmosphere. Again, over a huge range of temperatures, because it gets minus 270 degrees when you get nighttime there for two weeks, then it goes up to 250 degrees during the daytime. And in the end, what this atmosphere on the daytime size, is around a 100,000 to 1,000,000 particles per cubic centimeter, right. So a cubic centimeter is a little over 3/8 of an inch, and that compares to the atmospheric pressure in this room, where you got 20 quadrillion particles per each cubic centimeter. So you can count the differences is about 14 or 15 extra zeros on that.

Okay, so this is all good. So to explain a little bit about what I and my colleagues have done, we need to understand a little bit about how we took the data, as well as a little bit about anytime you’re looking at the Moon, what you’re looking at. Okay, the Moon is a very bright object, right, and it’s reflected sunlight. So you have the Sun illuminating the Moon. If you have an atmosphere, that atmosphere itself can emit some light. You then are looking at the Moon, but that light has to go then through the Earth’s atmosphere. So anytime you’re looking at something with respect to the Moon, you’ve got to be able to disentangle those three effects. What comes from the Sun, what comes from the Moon, and what is the Earth doing to your observation. A typical– This is a typical spectrum of what the sunlight looks like, it’s the rainbow, okay. If you look in high resolution, you see a bunch of dark lines.

Those dark lines are letting you know what the composition of the Sun is, it lets you know what the temperature of the Sun is, it lets you know about the magnetic field of the Sun, it lets you know about the velocity. A bunch of information that you can determine by analyzing all those little dark lines. Okay, so that’s the background that you have to look at any light that comes from the Moon. So the way people have gone about it is say, “Jeez, that’s an awful lot of light that comes from the Sun. “I don’t wanna look at that. “Let me look at a very select region of stuff “that might be coming from stuff that could be there, “or I know that is there, somebody else has found it. ” So a favorite spot then is to look at that very small segment in that box, and those two very dark lines are created by sodium gas. And so if you look at a profile, then this is what the solar spectrum looks like. This is the intensity as you’re going through this very small, yellow region of color, and those very deep lines are the signature of sodium atoms. Okay, so people doing that.

Then on the left here, you see this all going up, there in that very streak light, is away from the limb of the Moon, but all that multicolored stuff you see at the bottom again is a scattered light from the Sun, but by concentrating on a very small part of sunlight, they were able to isolate the fact that they could see emission from sodium atoms. So this happened a little over 30 years ago, and it’s illustrated a little better here. A few years after that, somebody actually blocked out the Moon, used something called the coronagraph, blocked out the Moon, looked at that very small region of the yellow part of the sunlight, and took a picture. And now you can see the size of the sodium cloud around the Moon. It’s several times the size of the Moon. It’s very thin, but it’s big. And you’re seeing the size because the Moon is losing this sodium, right. I mentioned earlier that there’s a sodium tail being blown away from the Moon. Okay, so this was sort of an introduction to get myself and my colleagues motivated, as I mentioned, the interest in the Moon from about 13 years ago, and then ultimately we went down to Kitt Peak National Observatory near Tucson, and used the McMath-Pierce, a solar telescope, to make our own observations to try and study the atmosphere of the Moon. And this is a unique structure, no other telescope building looks like this, okay.

It’s a seven on its side, and I’ve got several pictures that I’m just gonna go through, ’cause I wanna highlight how unique this is. 60% of this structure is actually underground, it’s a three mirror system. There’s one mirror up at the very top, you see one, and then you see two at the very bottom, and then bouncing light up to mirror three, and then into your observing room. And I’m just gonna show you some pictures, and this effort here will show you a lot of students who have helped out over the years. They’ve been very helpful. And it’s just been my most fun as a NASA scientist is being able to do observing. And this telescope is just totally unique to do that. And the view from inside is just this tunnel, and you’ve got these mirrors to bounce light around, and that bounces it around into this room. And the room basically is a laboratory. And so we set up this instrument, and we’re gonna look at even a smaller part of the yellow spectrum than I showed you earlier, because we really wanna isolate the sodium emissions.

And I’m just showing you different pictures in here. The thing I wanna point out is all the equipment you’re looking at is older than any one of those students in that picture. [audience laughing] Okay, it was the 1960s control panel, the latest upgrade was 1983. And I think the oldest person there was born in ’89, but I don’t wanna identify her. [audience chuckling] The way that this worked for us is the reason we like this telescope. It was designed to look at the Sun, and it was designed to create a very big image of the Sun. And since the Moon is about the same angular size as the Sun, this is what the image you got to see of the Sun. And so what we would do is we would look right off the edge of the Moon and see if we could detect the sodium emission from the Moon. This was not at all clear to us that we could do this, the Moon is a very, very bright object. And we were therefore very pleasantly surprised.

So, this is just one of those deep features that I showed you a few slides ago from sodium. And so what you’re looking at there is you see we’re interested in the bright bumps in the middle. Everything else is either from the sunlight, reflected sunlight, or it’s from the Earth’s atmosphere. So in the top one you see the deep dip, that’s the solar Fraunhofer line, and you see where the atmospheric absorption features are marked in blue, and then you got this peak of emission of sodium on top, potassium at the bottom. And what we care about is how bright is that peak and how wide is it. That was our objective and from that, we’re gonna try to figure out what’s going on. So we would look at this and we would go off from the surface of the Moon at different locations, to try and map out this distribution. You saw what the big picture look like in that earlier picture, when I showed you the Moon blocked out with an occulting disk, and you saw how big the cloud is. Here we’re actually trying to measure the physical properties of the sodium. In this particular case, what’s the temperature of that gas, and what are its philosophies, what are its kinematics? But what you notice, we don’t really need to worry about the details, right, but what you notice is that the sodium emission is much more extended than the potassium emission.

Now, we already knew that sodium was brighter than potassium, other people had shown that, but here, we’re making it very clear how much more extended the sodium is than the potassium. And then if we take that a little further, what you see, the details don’t matter other than potassium is on the right, sodium is on the left, and you see that the structure of the width of the line is very different between the two gases. Okay, the sodium on the left, top left, has a much more symmetric looking set of dots, so whatever that width of that line is doing and whatever it means, it doesn’t seem to care if you’re looking at it before full moon, the waxing phase, or after full moon, the waning phase, whereas potassium, the state of the atoms are very different in the waxing phase versus the waning phase. And if you look at the bottom set of two things, and if you look at the intensity, you’ll see that that where both of them decrease towards full moon, the decrease in potassium is much more severe, as well as something seems to be going on around full moon with sodium that we can’t see in potassium. Basically, we couldn’t detect potassium at full moon at all. Okay, I have hit my highlights of what I’m trying to talk about, so I thank you. And for those of you who are interested in green flashes just as a atmospheric phenomenon, I have a picture of one, which we took from the solar telescope. When I was down there, we saw green flashes routinely. I mean I’ve literally seen probably a 1,000 green flashes. Okay, anyways, thank you.

[audience applauding]