2017 LPL Evening Lectures
The Dark Side of the Moon by Jeffrey Andrews-Hanna - September 6, 2017
07 Sep 2017
Chapter 1: What inspired the discussion about the dark side of the moon?
Without further ado, tonight's speaker is Dr. Jeff Andrews Hanna. Jeff got his bachelor's at Cornell University in Ithaca, New York, and his PhD at one of my favorite universities, since it's my PhD alma mater as well, Washington University in St. Louis. And tonight, Jeff is going to be talking about the dark side of the moon, or do you believe in gravity, right?
Sure. All right. Thanks, Tim, for the introduction.
Thank you.
Thank you. So I frame my talk around this idea of the dark side of the moon, which is something that comes up a lot in pop culture. We see this quite a bit beginning with, of course, Pink Floyd's famous album, The Dark Side of the Moon, in which I'm told that they were referring to lunacy rather than lunar science. It's a little bit of a subtle distinction, if you ask me.
But since then, the dark side of the moon comes up a lot in popular culture. oftentimes symbolizing the mysterious and the unknown. Recently we saw this in the latest Transformers movie, Dark of the Moon, where the dark side of the moon, this unknown, unexplored place where the Transformers had a secret military base and they're waiting to take over the Earth.
Because anything could be there on the dark side of the moon. I mean, really, who knows? People love conspiracies, especially conspiracies involving NASA. So I'm going to show you this secret image I'm not supposed to show you, revealing the dark side of the Moon. This is a very grainy image. And the reason why it's grainy is because all conspiracy theories have to revolve around a grainy image.
And also because this image of the Moon was taken from Mars. And this is an actual image of the Moon and the Earth. And in this image that NASA doesn't want you to see, because they've been hiding the fact for decades, that not only does the moon have a dark side, but the Earth has a dark side too. That's right, I don't want anyone to panic, but as I speak and as you sit here and listen,
We in Tucson are hurtling towards the dark side of the Earth at a speed of about 1,000 miles per hour. And literally, if you could look out the window, you would see the landscape being plunged into darkness. And it would stay that way until about 6 a.m.
tomorrow morning when your alarm goes off, you get up, you walk the dog, you make breakfast, you go to work or school or whatever, and you repeat every 24 hours. Because, well, of course the moon has a dark side, and of course the Earth has a dark side, but Really, it's the night side, not the dark side.
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Chapter 2: How does pop culture influence our understanding of the moon?
There's not a secret Transformers base on the dark side of the Earth. It's just nighttime there, and we'll be there shortly. And the same is true of the moon. So the moon has a far side that's always facing away and a near side that's always facing the Earth. And I think in popular culture, people confuse the idea of the far side of the moon with the dark side.
But in reality, because the moon always faces the same side at the earth, an observer on the moon would actually, it would always see, they'd always see the earth staying in the same place in the sky, but they'd see the sun rising and setting as the moon orbits the earth. And it would rise and set on about a 28 day cycle instead of our 24 hour cycle on the earth.
So if this is a little disappointing that the moon doesn't have a permanently dark side that's vast and mysterious, to make it even worse, the night side of the moon isn't really all that dark. This is a picture from the most recent solar eclipse that a friend of mine took. He went up to where it was a total eclipse.
And if you could take your eyes away from the beautiful solar corona to the dark hole in the middle, that's the moon covering up the sun. And specifically, that's the night side of the moon covering up the sun. And if you look closely, even in the lighting in this room, you can actually see details on the night side of the moon. If I stretch the contrast more,
That looks just like a full moon because it's the same face of the moon that we see normally. But what's lighting it up if the sun is behind the moon? This is the moon illuminated in earth shine because just like a full moon really lights up the earth, a full earth really lights up the moon. And in fact, you don't have to wait for a solar eclipse to see this.
If you go out in a week or so and look at a crescent moon, if you look carefully, you'll notice you can actually see the unilluminated hemisphere of the moon, the night side of the moon, you can see that lit up in earth shine.
So I would argue there really isn't a dark side of the moon, at least not in the sense of a permanently dark unknown side of the moon, unless of course you think about it this way. Light that we see, visible light, only penetrates about 1 1,000th of a millimeter into the lunar surface, about a micrometer.
What that means is that all of our cameras and our eyes from Earth are only seeing the very, very, very topmost surface of the moon. That leaves the entire vast interior of the moon unseen, at least unseen in the traditional sense. So pictures are only skin deep. If there's a dark side of the moon, it's the inside of the moon.
But unlike the sort of metaphorical dark side of the moon, the inside of the moon is not a complete unknown. We can say things about it. So we have a few different ways that we can probe the inside of the moon. One approach is using radar. So radio waves can propagate through rock to distances of a kilometer or so.
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Chapter 3: What is the difference between the dark side and far side of the moon?
And the other thing is its orbit altitude. It orbited incredibly low to the surface of the moon. This is a map of the minimum altitude of the spacecraft as a function of location on the lunar surface. The orbit was changing now and then, so you see a lot of variability. But what you'll notice is that a lot of the moon is covered in these deep shades of blue orbits less than about eight kilometers.
Now, if anybody pays attention to satellite orbits, that is incredibly low. That is much lower than any satellite has ever orbited another body before. For comparison, The Mars Reconnaissance Orbiter, an orbiter of Mars from which we have gravity data from Mars, orbited at about 300 kilometers above the surface.
GOCE, which is a real cutting edge gravity mission to the Earth, orbited at an altitude of about 220 kilometers. 747 cruising altitude, somewhere around 13 kilometers, about 40,000 feet. GRAIL, getting much of the moon's surface, about eight kilometers, that's lower than a typical airliner is flying.
So picture next time you're flying in an airplane, just look out the window and imagine looking down below you and seeing two spacecraft going zipping by at a speed fast enough that you probably wouldn't see them zip by. And that's what GRAIL was doing. We can't do this on the Earth because the Earth has this pesky, annoying atmosphere that just ruins all of our gravity observations.
On the Moon, without an atmosphere, we can orbit as low as we want, and eventually, those two satellites orbited at zero kilometer altitude, and they just smacked into the lunar surface, and that was the end of the mission. It was planned, not an accident.
But because of that low orbit altitude, we've got better gravity data for the moon than we have for any other body in the solar system, including Earth. We have a better global gravity model for the moon than Earth. And that to me is pretty impressive. This is a map of the gravity field of the moon.
But before I talk about this and talk about what we learn about the moon from this, I want to give you just kind of like a five slide summary of the moon. So, introducing the Moon, a few things everyone should know about the Moon.
One is that we think the Moon formed in a giant impact about four and a half billion years ago, when a giant proto-planet slammed into the Earth as the Earth was forming, launched material out into space. That material formed our Moon. The Moon began hot, at least the outer parts were molten early in its evolution, something we call a magma ocean.
And this was the state of the Moon until it started to solidify, and eventually this magma ocean solidified to make, similar to what we have on the Earth, a low density crust and a higher density mantle. And we need to know that to understand the gravity data we see. When we look at the surface of the moon, we see craters. We see craters, craters everywhere.
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Chapter 4: How do scientists study the interior of the moon?
Nowadays, we could write computer codes to do the same thing, but a slide projector and a basketball Probably wasn't a basketball, but a sphere does the same thing. When they did that and walked around to the side of their projected moon, they saw all of these arcuate structures surrounding a point just out of view.
And they said, that looks a lot like what we see in these things we're interpreting in impact basins on the near side. They named this the oriental basin. We now know that oriental is the youngest, freshest, best preserved impact basin. If you want to understand impact basins, oriental is your go-to basin. And that was discovered right here.
When they looked at this, they said, all of these scarps, that looks kind of like a fault scar, a cliff that has a fault underneath it extending down into the earth. So they interpreted there to be faults underlying these scarps penetrating deep into the lunar interior. When they looked, they saw dark patches in some of those rings. And they said, that looks like lava.
That looks like volcanic deposits. We think that maybe that lava flowed up through those faults and erupted on the surface. At this time, in 1962, that was speculation based on some fuzzy Earth-based images. Now, 2017, we've got the data we can use to actually test their ideas. So this is the Oriental Basin as I typically think of it. This is a topography map of Oriental.
You can see the Central Depression and these beautiful bullseye ring pattern around it. All of these red lines of mountains with scarps along the edges of it, that's what they were seeing in their images projected onto that sphere. Now we can take the topography and look at the Bouguer gravity looking into the subsurface of Oriental.
What they were trying to interpret from their images, we can now image with gravity data. So here's the gravity map of Oriental. What I can then do is take that Bouguer gravity, this is topography on the left, and I'm taking that gravity to model what the crust-mantle interface would be.
So if we've got a low density crust on top of a high density mantle, and that's the source of the gravity anomalies, we can use the gravity anomalies to figure out what that boundary between the crust and the mantle is doing. And here's a cross section through that. What we see is that, well, within the center of the basin, the mantle's been lifted upwards, and the crust is thinner.
Well, that's what we expect in a big impact. It should excavate out the crust. When we look outside the central region, in the surface, we see these wiggles that correspond to those scarps that you can see in the images. In the subsurface, we see the same wiggles and they're parallel to what's going on at the surface, but shifted over by a little bit.
This is telling us that there are indeed faults originating at those rings and that those faults go all the way through the lunar crust down into the lunar mantle. Now, while I've been using gravity data to try to understand the structure of the basin, a colleague of mine, Brandon Johnson at Brown, has been using models to investigate the impact process.
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Chapter 5: What role does gravity play in understanding the moon's structure?
what we see is a square. And the problem is that impact craters, generally speaking, aren't square. They generally don't have straight sides. They typically don't have corners. As an example, you could argue, oh, but when things get big, all bets are off, right? But this is a giant impact basin on Mars called the Borealis Basin. This is topography and kind of a model of crustal thickness.
And it's round. It's elliptical, just like most basins are, so most basins are actually more elliptical than circular, but they've got a pretty consistent size and shape. Prossal Arm is quite a bit smaller than that, and it's a giant rectangular square.
So this really doesn't match what we expected to see in looking for an impact basin, and so we want to look for other hypotheses to try to explain it. Now, this is still a big puzzle. What we can do with gravity data, we can try to model the subsurface structure. This area, as I said, has been flooded with a lot of lava. Lava igneous rocks tend to be more dense.
And so if we want to look at the structure, we want to look at both the thickness of that low-density crust as well as the thickness of the lava. And we have to take both of those into account. But here in this proselytum region, again, we've got this crazy square-type pattern.
If we look at a few of these border structures in detail and look at cross-section through those, so this is the gravity data. This is a cross-section through the structure as we model it based on the gravity data.
And those long linear segments we see bordering this region, if we look at a cross-section through those, what we see is that the crust has gotten thinner, the mantle's been lifted up, and the lava, the mare there, has become thicker.
This cross section here, well, if we just look at the crustal part, that thin crust in a narrow zone, that's what you expect when the crust is rifting, when you've got extension stretching and thinning out the crust, and then these volcanic eruptions then flooded the whole area in basalt and made these dark maria that we see today. And that's what we're interpreting this as.
This is an image of the East African Rift, one of the most famous rift valleys on the Earth. That is what we think is happening on the moon. Ancient, very ancient rifting was stretching apart the crust, causing these enormous tectonic patterns and this giant square pattern. And then the whole region got flooded with lava.
Now, this is a topographic map of the region around the East African Rift Valley called the Afar Triple Junction, the Red Sea, the Gulf of Aden, the East African Rift Valley here. As a geophysicist, as a theoretician, I have the privilege of being able to just take this whole area and flood the whole continent with lava. It's something that we can do in theory, but not in practice.
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