9UNP0521SD_David_Grinspoon_WordPress
 
- You might wonder what the connection is between what David is going to talk about. We heard the impact and now David's going to talk about planetary climates. David. 
- Thanks. Ordinarily, I go by the title of Curator of Astrobiology at the Denver Museum of Nature and Science, but this thing works. Tonight, I'll be the Science Dude. 
(laughter)
 And we just heard about perhaps one of the ultimate catastrophes that happened in the history of Earth, although, of course, catastrophes are also opportunities and we wouldn't be here without the extinction event that killed the dinosaurs and led to the rise of the mammals. I'm going to talk in a more general sense about climate catastrophes in the solar system and how we can learn a little bit about some useful knowledge about climate change on Earth by studying the other planets. 

Why do we explore the planets? Are we just blasting money up into space? Maybe sometimes it seems that way, and everybody who works in the field of space exploration is used to confronting this question of, "Why are you guys spending all that money up there when we've got all these problems down here?" You know, it's a fair question. I think those of us who get public money to work on the esoteric seeming questions, we should be confronted with this question. But the fact is, there are a number of reasons why we explore the planets, and granted, some of them are things like, "Well, it's fun." It really is, by the way, kids. 
(laughter)
 And we're naturally curious. Human beings are curious and we can't help but explore, it's part of our nature. 

But also there are other reasons why we explore. I mean, I think, not to get too whoo-whoo on you, but it is kind of a spiritual quest. It connects us with the wider universe and it give us meaning. But also, there are some very practical reasons for exploring the planets, especially these days. It's very important for us to understand how planets work, because we need to understand how our own planet works so that we can survive, and that's becoming increasingly clear. So yeah, it's fun and we have curiosity, but there are also practical reasons and it's ultimately about survival. We are at the point now where our civilization is capable, clearly, of changing the earth and so it's incumbent upon us to understand what that means, and understand how planets work. You can't understand how planets work by just staying on one of them. You have to compare them and get a larger perspective. And that is, perhaps, these days, the most compelling reason to keep exploring. 

There's this supposed Chinese proverb. I think it's actually, I was looking it up, I don't think it's actually a real Chinese proverb, but it's a supposed Chinese proverb, but it's still a good one. "May you live in interesting times." And it's actually supposed to be a curse. But I want to briefly mention two current developments in science, which make the times we live in very interesting. One of them is the imminent discovery of new Earths. We are clearly on the edge of discovering planets which are at least Earth-like in many respects. And as you probably know, the last decade or so has been a revolution in terms of discovering that, in fact, there are many planets out there. 15 years ago, the only planets we knew about were the, depending on who you listened to, the eight or nine or thousand, in our own solar system. But now we know that in fact almost all the planets, as we long suspected, most of the planets, of course, are out there. 

And when you look up at the stars tonight or some night that's clear, almost all of those stars have planets around them, we now know. We didn't know that, we suspected it, but we have the technology now to discover planets around other stars and we're just on the edge. It's easier to discover big ones, and it's harder to discover small ones, so we've discovered lots of other Jupiters and we're just on the edge of discovering the new Earths and in fact there's a spacecraft that's up now called Kepler that's going to tell us how many Earths there are out there. And it's really going to be announced any day now. The spacecraft is up there, it's working, it takes a while to get this data and sort it out. And the way Kepler works, it stares at 150,000 stars very intently and watches for the slight dimming as planets cross in between us and those stars. It's very, very sensitive to the slight dimming of light when a planet orbits in front of that star. It can notice that and if it happens rhythmically, periodically, then we say, ah, there's a planet orbiting that star. And we are just on the edge, certainly any year now, possibly even any day now, we are going to know something a lot more about how many Earths there are out there. And Kepler, this is just a diagram. 

It stares in one particular direction off into space. It doesn't cover most of space, but that's just a random direction and presumably it will give us a representative sample and we will soon know how many Earth-like planets there are up orbiting the stars. But what do we mean when we say "Earth-like"? How Earth-like are they really? What Kepler will tell us is if there are planets the same size as Earth orbiting stars that are similar to our Sun. But are they really Earth-like? Well, we don't know because that involves a lot more that's going to be hard to observe. But ultimately that really depends on how their atmospheres have evolved. And so it becomes interesting to us not just thinking about the atmospheres on the planets in our own solar system but how they represent, possibly, the atmospheres of these Earth-like planets out there all around the galaxy. But it's going to be very hard for us any time in the foreseeable future to really examine those atmospheres close up. 

So in a sense, when we examine the atmospheres of Earth and Mars and Venus, sort of the Earth-like planets in our own neighborhood, we are looking at representative planets that might tell us what may be happening on all those billions of planets out there that we are just on the edge of discovering among the stars. So that's one of these two revolutions I wanted to mention. The other one, and I will try to connect the two, is the possible, we've got the imminent discovery of new Earths, and the other one is the possible imminent destruction of the old Earth. Not to be too much of a downer, but I'm not breaking the news to you. You've heard about this. We are doing things to our planet, if you will, the old Earth, that are possibly not too safe for our descendants and so this is another reason why we really want to understand how planets work. And of course, I'm talking about AGW, anthropogenic global warming, the human effects on the atmosphere that are to some degree warming the Earth and threatening other species as well as potentially our own ability to maintain our civilization on our planet. Each of these revolutions causes us to question what we know about how the Earth functions and evolves. And each one gives new urgency to our efforts to explore and understand the other Earth-like planets in our own solar system and in particular to understand what leads a planet to habitability, that is, I mean the ability of a planet to support life. We're interested, of course, in this as an intellectual, even, if you will, spiritual question. 

Are we alone in the universe? Are there other planets out there that can support life? We want to know that because we're curious. It's important to who we are and how we fit into the universe. But also, this question of habitability is very important right now for our own Earth. What are the limits to how a planet can change and still support a biosphere? And of course, there are clues to that in not just the ancient history of our own planet, but the current states and histories of the other planets in our solar system as well. We've all heard a lot about climate models and these projections of possible environments of the Earth into the future. What's the temperature going to be, how much is the sea level going to rise, how much is the atmosphere going to change? Well, how can we really know the answer to these questions? 

We don't have a crystal ball. How can we see into the future? Well, we don't have a crystal ball, but we have these tools called climate models. What are climate models? Well, they basically, they are sets of equations in computers where we break the atmosphere into a grid, depending on how big your computer is and how sophisticated your model is, maybe 30, 50, or 100 layers deep. And then lots of different little bins of latitude and longitude. You're basically breaking the atmosphere into these cubes and then modeling their behavior, all the different things that could happen. The radiation, how they get heated and cooled, how they move around and how each cell affects its neighboring cells. And you put as much physics, as much information as you can into this model with this grid and you basically hit the button and say "go" and you watch how the atmosphere behaves. And if you've got all the physics right and you have all the equations right, then you can accurately model and predict and project into the future what's going to happen to the atmosphere. 

Well, we've got a lot of really smart physicists and a lot of really smart atmospheric scientists, but how do we know we're doing it right? We've got a lot depending on this now. In a sense, we are all banking on the climate modelers knowing what they're doing, right? But how do we know they know what they're doing? It's a really tough problem. And it's not the kind of science where you can do an experiment, where you've got a beaker sitting on the laboratory and you say, "I'm going to predict that if I pour this chemical in it's going to turn blue." And then if it does, you say, I was right. You can't really do that with climate change. By the time we can test whether we were right or not, it might be too late. So what do we do? 

Well, one thing we can do is we can ask the other planets. Because if our models are right, they should work for the other planets and those are very different situations. It's not like a minor change on the Earth. It's like, let's take a really different situation, let's put in the same physics and if the model works, we should get the right answer. So this is, in a general sense, how the other planets are very useful in increasing our self-knowledge, in a sense, or our own planetary knowledge about the Earth. So here's a modern family portrait of the planets. Notice how Pluto keeps

- hey, Pluto, get out of there! Hey! 
(laughter)

 Oh well. I'm not going to argue, fine. If it makes you feel better, you can stay, Pluto, for now. But anyways, this is the modern family portrait of the solar system. This is emblematic of our times. If you had lived a couple generations ago, we wouldn't know what any of these places looked like, other than just points of light up in the sky or fuzzy images in telescopes. Maybe a couple of generations from now, this'll be old hat. But this is kind of new information for us, and what we can do is compare and contrast and we're starting to really learn the histories of these planets and see how our own Earth fits in this larger set of family histories. And in particular, when thinking about climate change, it's very useful to think of the comparison between Venus and Earth and Mars because these are the three nearby examples of rocky planets with thick atmospheres. All the planets are useful for exploring in different ways, but these three, Venus, Earth, Mars, are a very convenient trio because this is the local neighborhood and all three of them are sort of variations on the same kind of planet. 

They're rocky and they have thick atmospheres. So I'm going to focus for the rest of my little time that I have here tonight on the comparison between these three planets and what we know about climate on these worlds and how that illuminates in general the question of climate change. These are landscape pictures of Venus, Earth, and Mars. A typical place on Earth, you think about it, Earth is something like 70% ocean, so if you landed on a random place on Earth it would look like this. A somewhat typical place on Mars. This is from the Viking I lander. This was in fact our first landscape picture of Mars and I think it's still one of the most beautiful ones. You can see Mars is a dry, desolate planet with rocks and sand dunes and an ancient dry desert world. And this is one of the few landscape pictures we have of Venus. This is from one of the Russian Venera landers and the camera's sort of pointing down, so you're mostly seeing the foreground, but you're seeing a little bit of sky up here in the corner. 

It's very hard to land something on Venus because it's 900 degrees there, everywhere, all the time. And that has to do with the main topic I want to drive home tonight, which is that planets have very different environments that are the result of their atmospheres and climate is something that is a signature of a planet's total environment that has to do with not just the physical size of the planet and where it is in relation to the sun, but the atmosphere. It's the atmosphere that determines the climate. And the interesting thing about Venus, Earth, and Mars, is that as far as we can tell, Venus and Mars both used to look like this. The evidence that we have, which we're still trying to improve upon and that's why we keep exploring, but the evidence we have suggests that all three of these planets were more Earth-like. They probably all had liquid water on their surfaces. And then something happened to the climates of Mars and Venus, two very different things happened to them, which I'll get to in a second, but they both diverged from these Earth-like conditions. So climate can change. A stable climate is not something we should take for granted. Venus and Mars both have atmospheres of almost all CO2, carbon dioxide. 

And you know about carbon dioxide because it's a trace gas in Earth's atmosphere. Carbon dioxide in Earth's atmosphere is about 300 parts per million, and even at that level, we're worried about it, not because carbon dioxide is inherently bad, but because it's increasing because we're driving our cars and running our factories and burning coal and all that stuff. As you know, you've heard about this, it's not news to you, it's a greenhouse gas and as the carbon dioxide of Earth's atmosphere increases, it's becoming warmer. Well, Venus is a planet where the atmosphere is essentially all carbon dioxide. It's over 96% carbon dioxide. And it's really, really, really hot there. 900 degrees everywhere all the time. Now, it's not just that it's all carbon dioxide, but the atmosphere of Venus is very, very thick. It's almost 100 times as thick as Earth's atmosphere. So it's not just that the air there is mostly CO2, which is a greenhouse gas. 

There's a lot of air. Really efficiently traps heat. The interesting thing is that Mars is also almost all CO2 in its atmosphere, more than 95% carbon dioxide. But Mars is quite cold. Not just because it's farther from the Sun than Earth, but also the atmosphere of Mars is very, very thin. It's in contrast to the atmosphere of Venus, which is 100 times the atmospheric pressure of Earth. The atmospheric pressure of Mars is only about 1% of the atmospheric pressure of Earth. So you'd have to multiply it by 100 times even to get to one Earth atmosphere of pressure. So Mars has a very thin CO2 atmosphere and so consequently it does not have much of a greenhouse effect and consequently is it not only really, really cold there, it averages about -70 or -80 degrees Fahrenheit, but the temperature fluctuates widely there with the seasons and even between day and night because the atmosphere is so thin it just can't hold any heat. So Mars is sort of closer to being an airless body. 

Earth is somewhere in between, and Venus has a really thick, solid greenhouse of almost pure CO2. So that's the current states of these atmospheres. And then the next question we ask is, how did they come to be so different when the geological and other evidence we have tells us that apparently they were all much more similar when they were young? Well, one thing we do to try to answer this question is to look at the deep history of the Earth. The long term climate history of the Earth. And when we do that, we encounter something that we call the "faint young Sun paradox." What do I mean by that? Well, the Sun, we know, was less bright when it was young and when the planets formed. As a star like the Sun evolves, it slowly gets hotter over its lifetime, and it was significantly, about 30% or 40% dimmer when the Earth formed. And this is kind of a complicated looking graph, but let me lead you through it. 

This is time, so this is now, because it's billions of years before the present, so this is now, this is a billion years ago, 2 billion years ago, 3, 4, 4.5 billion years ago when the planets formed. And this curve is solar luminosity, in other words, how bright the sun was. And this is now, it's 1, this is the present day brightness of the Sun, and how much energy it's been putting out. And if you go back in time, the sun is much, much less bright, 4.5 billion years ago. And given that, we can calculate how hot the Earth should have been. And these two curves, this is the actual surface temperature of the Earth now, right here, TS is surface temperature. And TE is equilibrium temperature. This is how hot the Earth would be without any atmosphere. So if you took the Earth today and took the atmosphere away, it would be below the freezing point of water, the temperature. And that would be bad. 

We probably wouldn't be here. But this difference, between the equilibrium temperature and the actual surface temperature, this is the greenhouse effect. This is the amount by which the atmosphere heats up the Earth today and it gets us into this nice, healthy temperature range above the freezing point of water where we're comfortable. So that's fine, but the weird thing is, given this dimmer Sun in the past, if you go back in time and you calculate how hot the Earth was in time, you come to a point early in the history of the Earth when the surface temperature you calculate was below the freezing point of water. The Earth was completely frozen over. This is given the current amount of greenhouse effect projected into the past. Well, okay, that's what the physics tells us, but the problem is the geology. If you look at the history of the Earth, you can find rocks from back at this time which tell us that the Earth was not frozen over. You find sedimentary rocks that were made by flowing water moving sediments around. So that's why it's called the "faint young Sun paradox." 

Because just the physics tells us the Earth should have been frozen over, but the geological record tells us it wasn't, that the climate was more steady. So something must be wrong with these calculations. When the Earth was young, the Sun was much less bright, so why wasn't the early Earth frozen over? Well, the greenhouse effect must have been stronger then. And that tells us that in some way, the atmosphere evolved. This calculation is for if we have today's atmosphere in the past, the Earth would have been frozen over. So something changed. Was there more carbon dioxide? Maybe, or maybe there was some other gas. But clearly, things were not just constant the way they were because the Earth wasn't frozen over and it should have been with today's atmosphere. 

Well, in order to understand why the Earth's climate has been so constant, even with this changing Sun and with other kinds of changes, we need to start really thinking about the evolution of climate. And an important thing to understand when we're thinking about how climate changes is the concept of feedback. It's positive and negative feedbacks. These are situations where, feedback is an element of a physical system where something either magnifies or diminishes because of the way different parts of the system interact. A really good example of a negative feedback is a thermostat. For instance, in this room, presumably, I hope there's a thermostat. So when the temperature's cool, it exceeds a certain limit and the cooling is switched on. And the temperature begins to fall. And when the temperature drops below this limit, the cooling switches off again and a steady room temperature is maintained so there's an element in the system which is sensing another element in the system and leading to a behavior which stabilizes the system. That is negative feedback. 

And there are examples in climate of negative feedbacks and you could also have a positive feedback. If you wired your thermostat wrong, so that every time it got below a certain temperature the cooler switched on instead of off, then it would just keep getting colder and colder and colder. If you wired it wrong, that would be a positive feedback, that would be unpleasant. Now, there are examples of negative and positive feedbacks in climate. So an example of a negative feedback is clouds. If you heat up the surface of a planet, you get more water vapor because it evaporates. That makes more clouds. Clouds block sunlight, they reflect sunlight and that cools things down. So a hotter surface leads to a physical effect which creates a cooler surface. That is a negative feedback. 

It stabilizes the system. It tends to keep the system within a certain temperature range. Now, an example of a positive feedback is ice feedback. So if you have a hotter surface, it tends to melt ice. Less ice means there's less sunlight reflecting surface. Water absorbs more sunlight and reflects less sunlight than ice and that means you will absorb more sunlight and things will get even hotter and the ice will just keep melting. So ice feedback is a positive feedback, in fact, something like this might be happening to Earth today and it's something we're concerned about. So feedbacks can work in both directions, and that's one of the tricky things about understanding climate change and modeling climate change, is putting in all of these feedbacks correctly because they lead to behavior that we call nonlinear behavior and that tends to make the equations kind of complex and unpredictable. So why has Earth's climate been so stable over all those billions of years, even with the warming Sun? Well, there is a climate thermostat that operates over the long term on Earth. 

It's what we call the carbonate silicate cycle. And it works like this. It has to do with the natural cycle of CO2 in the atmosphere. Volcanoes are always putting out CO2 no matter what. Sometimes it's a little more, sometimes it's a little less, but they don't really respond to the climate, there's just always a volcanic source of CO2. So why doesn't CO2 just build up in the atmosphere since it's always being pumped in by volcanoes? Well, it's also being removed from the atmosphere by what we call weathering reactions. That is, carbon dioxide is dissolved in rainwater and ultimately it goes through some chemical reactions and it ends up being deposited as carbonate rocks. Ultimately, those carbonate rocks get subducted, they get dragged inside the Earth by the plate tectonics, by the churning of the Earth and they end up, the carbon gets broken down and ultimately pumped out again as volcanoes. That's the cycle of CO2 in about 500,000 years or so, half a million years. 

The CO2 gets cycled through the whole Earth. So the reason why it's a negative feedback or a thermostat is because volcanoes are always pumping out CO2, but this removal of CO2 by these chemical reactions depends on the climate. When the climate is hot, that weathering speeds up. The CO2 is removed faster because there's more rainfall and rainfall is what's washing the CO2 out of the air and dissolving it. When the climate is cold, that removal of CO2 by weathering reactions slows down. And when the Earth gets frozen over, which it has a few times in its history, it stops completely. Because when it's just ice here, you're not having rainfall and these chemical reactions stop. So when that happens, when the Earth gets cold or frozen over, this removal part of the cycle isn't happening and the CO2 builds up in the atmosphere. And that eventually makes it get hotter again. So that is why it's a thermostat. 

If it's hotter, the system removes CO2 and cools it down. If it's colder, the system builds up CO2 and heats it up. So over the long term, Earth's climate is stable. Now, this isn't to say, hey, don't worry about climate change, no problem. Because this is something that corrects itself after about a million years. After a million years, we could all be dead. The Earth might be just fine, but this doesn't help us with our short term climate problems, but it's a reason why over the long term, the Earth's climate has been stable. Now these same processes occurred early on Venus and Mars. You had volcanism, you probably had weathering and liquid water. But this climate thermostat appears to have broken on Venus and Mars early on. 

And it broke in different ways on each of those planets and that's what I'm going to talk about next. This climate thermostat that's worked so well over the entire history of Earth seems to have broken on Venus and Mars. What happened? Well, on Venus, Venus lost its liquid water early on in a process called runaway greenhouse that I'll describe in a second. And with no liquid water, then you didn't have this silicate weathering and didn't have any way to remove the CO2 and all the CO2 ends up in the atmosphere and it just gets stuck. It's like that thermostat that's wired wrong and there's no way for it to cool off. On Mars, most of the volcanism stopped early for reasons I'll describe in a minute, so this part of the thermostat turned off. There wasn't any more CO2 coming out so there was no source of CO2 and the CO2 ultimately was lost to space, for reasons that I'll also describe. So this part of the thermostat broke on Mars, this part of the thermostat broke on Venus and only on Earth has this system apparently been running smoothly for 4.5 billion years. Now just a little bit of comparative planetology. 

Looking at the bulk properties of Venus versus Earth, in some ways Venus is strikingly similar to Earth. It's called Earth's twin or sometimes Earth's evil twin for a reason. Look at this. Look at how similar the two planets are in diameter and in mass. These are nerdy units. These are times 10 to the 24th kilograms, but it doesn't really matter what the unit is, the point is, Venus is about 5 and Earth is about 6, so Venus is slightly larger in mass than Earth and almost exactly the same diameter, but if you look at the entire range of planets in the solar system, there are no other two planets that are anything like this similar to one another in terms of mass and size. They really are twins. There's no other pair of planets that we know of that are this similar. And yet, these numbers in red show how strikingly different the environments of these planets are. Venus has, as I said, almost 100 times the atmospheric pressure of Earth. 

The average surface temperature of Earth is a nice, cool 68 degrees. The average surface temperature of Venus is a nice, warm, not so nice warm 890 degrees. And then these are also nerdy units, but the point of this number is that Earth has 100,000 times as much water as Venus, which is really strange, because Earth and Venus seem to be made out of basically the same stuff, and yet something happened to all the water on Venus. What happened? Well, we think that Venus had what we call a runaway greenhouse effect. Basically we think they started out as identical twins, but Venus was born on the wrong side of the tracks. Those tracks in this case being the inner edge of the habitable zone, or the distance from the Sun where a planet can maintain its water. Well, why did Venus lose its water? Well, the runaway greenhouse works like this. If you're like Earth but you've got too much sun, then there's a powerful positive feedback. 

I've got Jimi Hendrix here to represent feedback. 
(laughter)
 If you have a higher surface temperature, that leads to evaporation. And then you've got more water vapor in the atmosphere. More water vapor, water is a very powerful greenhouse gas, water vapor. So if you've got more water vapor in the atmosphere, that leads to what we call more infrared opacity, or just more ability to block the radiation from the planet and that leads the surface temperature to be higher, which leads to more evaporation, and uh-oh, more surface heating and if you get a planet with water hot enough, it's just going to run away. There's no stopping this. It's a runaway greenhouse. And ultimately, you get a steam atmosphere and then when you've got that much water up in the atmosphere, it's very vulnerable to being lost to space. Basically what happens is that the water vapor gets broken up by sunlight into hydrogen and oxygen. 

The hydrogen, being a very light gas, sort of streams off into space. They hydrogen escapes, and the result after billions of years of this is that today Venus has only 1/100,000 times Earth's water inventory. In other words, Venus has almost no water compared to Earth. And this in a nutshell is how we understand this strange divergence in conditions between Venus and Earth. That's sort of the cartoon version. We're still trying to work on the details. Venus is a hard place to explore because it's so hot. It's hard to send spacecraft there, and even we send orbiters there, but Venus is completely shrouded in clouds, and so we're studying it, but there are aspects of it that are very challenging to study. What we have learned is that Venus has a very dynamic climate. It seems to have ongoing volcanism, volcanoes on the surface. 

We're still trying to confirm that, but we've got some tantalizing evidence recently telling us that we think Venus does have active volcanoes. And those volcanoes put out gases, which make the clouds. And the clouds are blocking radiation and the radiation is also affecting things on the surface. So what you have is this complex cycle of gases and clouds and volcanoes on the surface, and it's really, it's turning out to be a very difficult place to understand. But in exciting ways, because it has this complexity of its behavior and this ongoing evolution that in a way mirrors the Earth in complexity, and we don't know of too many other places like that that have ongoing geology that's affecting the climate. And so you've heard of the expression Earth system science, which is how people have come to think of Earth science, where you can't just study the oceans or the clouds or the icecaps or the geology, but you sort of try to understand the whole system. Well, now we're trying to do Venus system science where we've learned that in order to understand any part of this system and understand the climate, we have to study the surface, the atmosphere, the interior, the clouds, and sort of all of these pieces of the system we study. We have to try to figure out the interactions. Well, how do we do that? Well, we have spacecraft that are helping us crack these mysteries. 

And there's a spacecraft that's at Venus right now called Venus Express, and here's some nice images from one of the cameras on board that spacecraft. And Venus Express is a European Space Agency orbiter that's now been at Venus for several years and it's really sort of our first weather satellite at Venus and is teaching us a lot about the climate of that planet. And in addition now there's another spacecraft, a Japanese spacecraft called Akatsuki, which is on its way to Venus. It was successfully launched and will get there this winter and so we will have two spacecraft at Venus and in addition, we have a lot of plans and proposals we're hoping for future missions and there are actually several proposals now for new American missions to Venus. We want to send orbiters, we want to send balloons, we want to send landers, and we've got lots of ambitious plans. All we need is funding. 
(laughter)
 So if there's one take-home lesson from this lecture, write your congressperson. 
(laughs)
 No, no, no. 

But anyways, we've got some exciting ongoing exploration and we've got big plans for future missions and it's all in the service of trying to crack this mystery, trying to understand the climate system of Venus and how it compares to the climate system of Earth. And now let me talk just a little bit, I'm going to switch gears and talk about Mars for a second. On Mars, there's lots of evidence for ancient surface water. Mars is a cold, dry desert now, but we see evidence for dried up rivers. We see evidence for layered sediments. This could be some place in Utah where sediments were laid down in ancient seas. We see evidence even for river deltas. Here's a river delta, and you can see this river. I'm going to show you now. This is sort of like the Mississippi delta, where a river has flowed out and laid these sediments and then I'm going to show you a close-up of this. 

There's a meander in this river. You can even see where this river spilled its banks and wandered. And this tells us that there was a sustained period of liquid flow. This wasn't just a temporary flood that had this kind of meandering behavior. So there's lots of evidence from orbit of an ancient climate on Mars that was warmer and wetter. And in fact, when we go to the surface of Mars, which we've now done beautifully with the Mars exploration rovers, which are still functioning there many, many years after the end of the supposed 90-day mission, we can actually go up to the rocks and scratch and sniff and ask the rocks questions. And the rocks answer us, and they say, "Go home." 
(laughter)
 Sorry. What they actually say is, there was water here once. 

These rocks are pay dirt. These particular rocks here are sulfate minerals that when we go up with those instruments and we knock and say, "Hello, tell us your story" and we look at the chemical composition and we look with a microscope and understand the structure, these rocks very clearly were laid down in ancient pools of acidic, sulfur-rich water long ago on Mars. And we even see some sections, some very deep sections of these sediments telling us that there were long periods of time where Mars was water rich. So that's interesting, but it creates a puzzle. What happened to Mars? Mars was clearly once much wetter and warmer than it is today, so what was so different? Well, when Mars was young, it seems to have had a lot more air. And therefore a stronger greenhouse. So where did it go, what happened? Well, for one thing, Mars lost its ability to replenish its air because the volcanoes died out young. 

There's a lot of volcanoes on Mars, but some basic physics tells us that small objects cool off faster than large objects. You know this if you've ever taken potatoes out of the oven and after a few minutes the small ones are cool and the big ones are still hot. It's what we call the surface-to-volume ratio. That's the nerdy term. But basically, the interior of these small potatoes has more access to the exterior. It's easier for them to radiate away their energy, whereas the bigger ones, the interior can't cool off as easily. The same thing is true of planets. And Venus and Earth are both fairly large for rocky planets and they're still hot on the inside. They still have active geology. Mars is substantially smaller and it lost its internal heat early and its volcanoes died. 

And so it lost its source for CO2. The other thing, Mars is sort of doubly damned because since it's so small, it also has smaller gravity, weaker gravity, which makes it harder for it to hang on to its atmosphere. And there's something we've learned called impact erosion, where the Martian atmosphere was blasted off into space in large impact events. The kind of events that Jan talked about were very common when the planets were young. They were getting hit all the time. We know this because we look at the surface of Mars and it's covered with big craters, especially the southern hemisphere, which is older. And you can count the craters on Mars, and we have some idea of when they all hit. And if you model that and you look at when the craters occur over time and you model the effects of that on the atmosphere, each one of those big impacts blows a little bit of air out into space. And over time, the models show us that most of the atmosphere of Mars was lost very early on. So the ancient greenhouse was much stronger and we believe that helps us explain why Mars was once warmer and wetter than it is today. 

So both Venus and Mars experienced radical climate change from more Earth-like conditions to their current uninhabitable states. I'm going to wrap up now and Bill Nye's going to come up and talk to you more specifically about climate change on Earth. I want to leave you with this question. Is there another Earth out there? Well, it's probably true that soon Kepler will announce the discovery of planets that are similar to Earth, and it's going to be very exciting. There are going to planets, I mean, in science we don't know anything until we have proof because this is science, it's not faith, but I can tell you, as sure as I am about any future scientific result, I pretty much know, I'd put money on it, that very soon we're going to know about other Earth-sized planets because things are going very rapidly in that direction. But is there another Earth out there? I would say no, because one thing we've learned by studying Venus, Earth, and Mars is that planets are all individuals. They have complex histories and there will be planets that have some of the same qualities of Earth, but we're not going to find another Earth. We're not going to find another planet that's just like ours. 

So is there another Earth out there, no. See? 
[laughter]

Anyways, in conclusion, planetary climate is complex. And it's uncertain. Radical climate change does happen to planets, don't let this happen to your planet. Earth has been remarkably stable, but that is no guarantee of future performance. We need all the help we can get, including data from other planets. Earth is unique, but planetary observations and modeling give us a reality check on our assumptions about climate modeling, and they provide indispensable perspective on the uniqueness, on the deep past, and on the long term fate of Earth. And in memory of Jack Horpenheimer, I will say, I will end with the words, "Keep looking up." 

Thank you. 
(applause)