By looking at the geology on Mars, we have some idea that perhaps there was some sort of large scale climate change that happened on the planet. What could cause such a thing? Again, one way to explore such a question is to look at the Earth. And think if analogous processes happened on the Earth that might be happening on Mars. What causes large scale climate change on Earth? One of the major climate variations over the past hundreds of millions of years has been the cycle of glaciation. Degalciation, glaciation, deglaciation. These cycles are now called Milankovitch cycles. I can never spell it right, but I think that's it. Milankovitch cycles, and they're caused by the change in the orbit of the Earth. As it goes around the sun, there are a couple of things that matter. Imagine that the Sun is here in the center again. The Earth is going around. And normally, we think of the Earth as being in a pretty circular orbit because it is in a pretty circular orbit. But it does have some eccentricities. And so, I'm going to draw a greatly exaggerated, eccentric orbit here. And what's interesting is, of course, the, the Earth's eccentricity doesn't stay the same all the time. All of the planets are interacting gravitationally. On average, they stay in the same places, but over different periods of time, the tugs from Jupiter, from Saturn. From from the other planets are always tugging along at the other ones, and so they're wobbling around back and forth in what are called Secular long- Term Variations in the orbits. On average they stay the same, but over these times they, they can move a little bit. So the eccentricity of the Earth goes from its relatively circular state like it is now to moderately eccentric. The other thing that happens is the North Pole of the Earth precesses. We know that the North Pole of the Earth is tilted by about. 23.5 degrees. And that's 23.5 degrees compared to the plane that it goes around the Sun. So, Sun is over here. Which that means, is that at some points during the year, this North Pole, we'll just talk about the North Pole to make it easier. The North Pole is pointing away from the Sun, we call this winter. At some points, on the other side, the North Pole is pointing more toward the Sun. We call those, at least if we lived in the Northern Hemisphere,we call those summer. But you can imagine something else interesting happening,. Which is that what if we had a very extreme eccentricity like this and now summer, north polar summer occurred here, because the tilt is in this direction. That means that north polar winter occurs here because the tilt is in this direction. What is that mean? Well, also the Earth is closer at this stage and summer would be even more extreme. Winter would be even more extreme. Currently we're almost the opposite. The time that we're closest to the sun is close to the peak of north polar winter and in fact because the Earth's eccentricity is so small. It's not even noticeable. But the important thing here is that the position where this angle is pointing, the position where the pole is pointing precesses. That means that it moves around like a, like a top that's spinning and doing this as it's being pulled around. The Earth's axis is doing the same thing. It is circling around the, the straight up and down point. Which is right here. It's still always that 23.5 degrees but sometimes it points one way. Sometimes it points the other way and it sweeps around. Which is why of course the thing that we call the North Star right now over the course of tens of 1,000 of years that North Star, the star will still be there in the sky but the Earth's rotation axis won't happen to point directly towards that star. And we won't have the North Star. Just like right now we really don't have a South Star. Sometimes we don't have a South Star. Sometimes we don't have a North Star, but this will precess around, which means that, if again, if we had this extreme eccentricity, sometimes you'll have extreme summers, cold winters. Sometimes it will be the other way around. The obliquity will point in this direction and you'll have milder summers and milder winters and everything will sort of even out. You can argue and it had been argued time and time again that this can't matter very much because your hot summers are canceled out by your cold winters. Your mild seasons are all the same. But, the Earth's climate is not linear. You can't say, just because a summer is hotter and a winter is colder, it all turns out to be the same. Colder winters cause ice to form, ice reflects more sunlight, it gets colder. The warmer summers can melt some of that, but eventually, you can, these cycles will lead into the ice ages. And you can see this, where we look at real data from the orbital elements of the Earth, the orbit that the Earth is going around the Sun. And, and a couple things that I'll describe, here on the bottom, in a minute. So you see this is, well here's that obliquity. I said it was 23.5 and it's constant. It's mostly constant, it wobbles around a little bit, due to the influence of all the planets. But we can call this, essentially. A constant value not doing anything. The eccentricity, this is time now, 0. The eccentricity, you can see right now, happens to be quite small, but it can get as high as 5% over here, or back down to 0. So, we're in, we're in a low eccentricity phase right now, for a couple hundred thousand years in either direction. The interesting one that just changes is that this is the precession of the obliquity, the, the way it's circling right here. And here we're plotting sine of this angle, so sine is going between minus 1 and 1 in a very smooth fashion. That just means the obliquity is moving around like this. And from that, you can calculate the average daily insulation, how much power is reaching the surface at the canonical place to do it is 65 degrees North. 65 North, you might think, why North versus South? The Northern Hemisphere happens to have most of the landmass, and so it has where most of the climate feedbacks happen there. Okay, so when you look very carefully you see that the insulation of that location is high, it's low, it's high, it's low. You have these short term variations. You have these bigger term variations like this and if you look at things like temperature records from ice cores, which are here. You look at things like temperature records found by looking at. The oxygen isotopic ratio of, of plankton of the, of the shells of plankton that are found in the ocean and you find them in sediments so you know how old they are you can see that both of these record a record of. Hot, then cold, then hot. Or going backwards, warm right now, but it used to be cold, warm back then, used to be cold, warm, used to be cold. And also, with big cycles in between. These big cycles in between are the ice ages that we actually know. This is a hundred thousand years here. Our ice ages, our last ice age ended right here, at 10,000 years. And we've been relatively constant ever since then. And there were periods of ice advance, and retreat in through there. And, and then, as it gradually warmed up, then cooled back down again. And you can see that it matches pretty well with these data here. It's not, it doesn't, the plot doesn't look exactly the same. But over time people have found that, that these are a good match, and, and that these cycles that are, that are clearly seen in ice ages over the past million years have a very clear cause and effect with this, this insulation. Okay, that's great. What about Mars? The answer is that all of this really subtle stuff, doesn't actually matter very much for Mars because of one very important thing and that's this. This is that obliquity, that 23.5 that it, that. The Earth currently is right now and the Earth barely moves and this band hardly makes a difference. One of the reasons why it's difficult to take the earth and change its tilt, 'because the earth actually has this Moon around it which is a pretty large Moon compared to Moon's in the solar system. And that orbital angular momentum of the two together is what you have to shift if you wanted to move. The obliquity of the Earth a little bit. You have to eart, move the Earth and this very, this very large lever arm of the moon. If you didn't have that Moon, it'd be really easy to turn the Earth around with different torques, and in fact, Mars does not have a Moon like that. And Mars, the obliquity of Mars is allowed to go crazy. In fact, here's a simulation of what Mars would have been like for the past 80 million years from a paper from 1993. How do you do such a thing? Well, to get the details, you actually have to resort to numerical simulation, where you, where you really do simulate the positions of all the planets. And the Earth and the Moon and the spin of Mars as its happening and you see something like this. Right now Mars is very similar to the Earth in its obliquity down here in the 23 degree range and that's, that's how it is now but in the past. In the recent past it jumped up pretty big its been up here as high as 45 degrees it goes down as low as 10 degrees, those are huge variations in the obliquity. This matters much more than those tiny variations that we have on the Earth that cause those very large effects such as ice ages. How does this work? Well, let's think about it for a minute. Let's have Mars have zero obliquity. That means that there's no summer. There's no winter. It's, it's exactly the same all the time. And what would you get? Well, you'd probably get polar caps up here of some sort, you'd get polar caps down here of some sort. You'd never get very cold winters, although you'd never get, the warmer summers up here. So you get something that's, kind of a moderate climate. Let's change it a little bit, now. Let's make it more like that 20 degrees that it is now. You have something like in the summertime, your ice caps can start to disappear. But in the winter, you grow very large ice caps. And those ice caps can grow enough, maybe get covered by dust, insulate them, that they can become a permanent on the North Pole, permanent on the South Pole. And in fact, that's the way Mars is today, as I'll show you in a minute. Imagine, though, that you have something like this crazy 45 degree obliquity. In this case you, you could have polar caps, certainly in the wintertime you get these polar caps that extend quite a large way. In summertime it's hard to evaporate them all, so maybe they evaporate some but then come the next winter they get bigger every year. You end up with a planet that doesn't have thick polar caps. At the North and South Pole as it does now. But it has polar caps that it can extend a good way down to the equator. And just to prove it's true I show you a NASA illustration. You have these polar caps going all the way to about something like 30 degrees. There's the Hellas base in there that you might recognize. It's a little bit smaller up here. A little bit smaller up here because the elevations down here are a little lower, so it's a little bit warmer. But, you might have had a planet that looked something like that. Pretty extreme. It is going to lead to those features that we saw? Is it going to lead to streams in through here? Remember, most of those, the, the value networks were, Milwaukeean. They were closer to the eq, equator. No. What's it going to lead to well one thing that it might lead to though that we could possibly still see and in fact I think we can still see is interesting layers happening at the poles the preserve poles now because you have. These climate fluctuations. You should have times when the ice caps are building up, times when the ice caps are retreating. You have dust in the atmosphere that can sometimes cover these different layers, and if you could look carefully at this poles of exists now. You might be able to see some sort of evidence of these past climates. Let's take a look. Here's a classic view of the north polar ice cap in the summertime and, while a few lectures back we debated whether or not these were water ice or CO2 ice, it's now known that the, the CO2 ice, the one that you can see from the Earth, advancing across the surface and retreating in the summertime. That indeed was CO2 that was coming out of the atmosphere. But the, the permanent polar caps, these big mounds that you can see here, are mounds of water ice with the CO2 covering. And they have all sorts of interesting features. There are these big canyons that are in there, smaller cracks that through which you can see these these, these more dusty features. They're dunes across the surface of it. Let's zoom in and see what you see when you look in some of these cracks. Okay, here's the image we just saw. We're going to zoom into here. And look at this, here are some of these cracks in detail. And now we're going to zoom into this crack and see what we see in really, really super high resolution. And here's the edge of the crack. You can tell the edge of the crack along through here. And now it's starting to go down, and you're looking through layers. Almost reminds you a little bit about that, that Grand Canyon picture we saw in the other lecture. Look at these layers. Layer, layer, layer these are going down, down, down through the cracks. And then some of the layers are extending out like this, again just kind of like the Grand Canyon. Look down here, tons and tons of little layers. What are these layers? Well, it's something like a cycle of. Ice forming, dust covering the ice, some of the ice evaporating, more ice forming, dust covering the ice. And you can see it. This is a case that must be for a polar cap that's growing, or has been growing for a while. It's certainly not one where the polar cap disappears. Totally every summer, has built up this big mound. What can you do with this? Can you use these sort of images, these sort of measurements of the features across here to say something about what the martian climate has been doing? I think the answer to that question is a resounding, maybe. Here, here's some measurements from a paper in 2002, where. Indeed they looked right through one of those, those chasms and looked at the bright versus dark layers, made some assumption that the bright layers meant that a certain amount of ice was coming down, the dark layers meant that there were, there was atmospheric deposition of, of dust. And tried to match it with what was known about the insulation, the amount of sunlight that was coming in, basically the same idea that we just saw for what you do on the Earth. And, here's what you see. This is looking at the insulation, now, calculated over the past million years, that's what this red dashed line is, and comparing it to these ice layers. Why a million years? Yeah. That's a good question. You have to make an assumption about how long this layer has been around. How, how much time each one of those individual layers takes to create. And you sort of do that by taking the data, looking at the insulation, trying to see if you can get a match and asking yourselves whether this looks any good or not. Well, you have peak here like this. That's kind of cool. Another peak kind of here like this. Peaks kind of here, look at that double peak, kind of. Triple peak, kind of, maybe. Peak here, peak here, peak here. It's kind of good. I don't think that anybody would argue, in particular even the authors of this paper I don't think they would argue that this is the answer that they had found out what these layers are from and that this is the right time period, but it's not a bad match. And, eventually, perhaps, with more data on these, these polar layer deposits, as they're called, and, with a better understanding of, of how the climate responds to these, we'll have a better idea of how to read these. Beautiful records of something that have preserved at the poles. Something is probably a record of the last million years. Couple of millions of years. And remember we're, in the end what we're interested in when we're trying to understand the climate history, the long term climate history, when perhaps there might have been actual water on the surface. A million years is not a very long time period. A million years is still almost yesterday on the martian time scale Amazonian time period. That whole big 3 billion year long Amazonian time period where basically nothing happened. If we're really going to understand what Mars used to be like, not just how it fluctuates. Even extreme fluctuations. If we really understand how, what Mars used to be like, we're going to have to think of ways in which Mars could have changed on a billions of year timescale, rather than a millions of year timescale. We'll continue to explore that in the next couple of lectures.
