Transcription
So I'm supposed to introduce myself. Hello, I'm Dave. You should please call me Dave, because Dr. Brain, our professor Brain, just sounds really spooky. So let's keep it first name, please. I'm an assistant professor at the University of Colorado. So I had a really long commute this morning from, you know, a couple miles away or so, and I'm in the same department that Fran is in: astrophysics and planetary science. I consider myself a planetary space physicist. I'm interested in magnetic fields and plasmas around unmagnetized planets: Venus, Mars, the moon. I would say that most of the community knows me as a Mars person, but I also have a Venus component and a Mars component to my research.
So this, I think—yes, Nick? Okay, yeah. The talk is posted on the schedule now, and Ken just said that she thinks that the talk may be posted where all the other talks are as well. That may have happened as well. Okay. So this is going to be an incredibly exciting talk for all of you, mostly because it's the last talk, I guess, of the summer school. But I'll also try and make it exciting in its own right and keep things pretty big picture, tying in a lot of the concepts that you've been talking about for the past several days, all to the service of understanding how the atmospheres of planets might evolve when they're embedded in a space environment and subject to photons and particles coming from a star.
So this question right here—we're going to come back to at the very end if there's time permitting—but this type of question about a planet named Carl Franny on right here, is is what I want the talk to be geared around: the qualities of the planet, the characteristics of the planet, the characteristics of the star. How do all of these things get mixed together in a soup to help determine what the climate of a planet is? Is it habitable or not? Is the atmosphere a static object, or is it something that has evolved incredibly over the history of that planet? Okay, I'm not going to ask you to answer this or participate right now, but every time you see a blue box, that means you have to talk first to each other, then to me. The magnetic fields affect planet surfaces. So at your tables right now, take 30 seconds, talk to each other, and then I'm going to cold call on three people. Go. You have 25 seconds left. Ten seconds left. Five, four, three, two, one. You're done. Okay. Why? Yes. The answer is yes. Why? I'm glad we're having this little chat. Great. So we see magnetization of rocks, the magnetization of rocks, for example, in the mid-Atlantic, where there's a sleep seafloor spreading Center, records the history of the dipole of Earth. So you would say that that is a an example of how magnetic fields affect planet surfaces. Okay. Any other—I said I'd call on three, but but that was the answer I was looking for. Anybody else have something different? Yeah. So can you think of an example in our solar system where this might might be true? Are you a planetary scientist-ish? Okay. Do you—can you think of a member of the National Academy of Sciences sitting very close to me who might know an object in the solar system? Oh, Ganymede, a moon of Jupiter, for example, has different colorations that depend upon latitude, and the colorations seem to be correlated with the locations of open and closed field lines on that body, and so they're—the magnetic field is is likely shielding particles from the surface in some areas and focusing particles toward the surface and other areas. Then that result is differences in color. Yeah. The lunar example, lunar albedo swirls underneath the magnetic fields. There. Anything else? Yeah. Okay. So your answer is tying to my next question, so let's go right there. The magnetic fields affect atmospheres. Now you have 15 seconds. Go. This is a gimme. Okay. You have five seconds left. Alright, times up. I didn't hear very much discussion there. That means you all know the answers already. Why? You have an example. How is Mars an example? Okay. So Mars lost its atmosphere because it doesn't have a big magnetic field to protect it from the solar wind. Other answers? Yes. Okay. So the escape rates at Earth and Mars are not that different from each other, so maybe the presence or absence of a magnetic field doesn't make a difference. The magnetic fields affect atmospheres. Yes. So this paper is by Way et al. It compares Mars Express data—I think—FAST data from Earth or something like that. So we're going to get to both of these ideas after the break. The magnetic fields make a difference at all? Do disturbed periods enhance atmospheric loss or not? But there—I'm looking for something much simpler. To: magnetic fields affect atmospheres. Yeah. Aurora. Tada. Okay. Yes, of course they do. The magnetic fields affect climate. 30 seconds. Go. Go. If 15 seconds left. Five, four, three, two, one. What you get? Yes. Magnetic fields do affect climate. You have an example. If they affect the atmosphere, they must affect climate. All right. Now let's go. We are needed. You guys—you guys—I don't know where the group started, end over here, but somebody in—okay. What's that? Magnetic field reversals might do something. You don't know. Okay. Maunder Minimum seems to have been correlated with climate. It's certainly correlated with changes in the star, maybe correlated with magnetic fields as well. I talked to almost everybody now, haven't I? Yeah. Oh my goodness. That's a whole separate lecture on on accuracy of science depicted in movies, especially *The Core*. Let's move on. Okay. Here's my answer to the last question: We don't know. Okay. This is a big question mark, and this is a big issue facing many of the communities that you connect with, is because people often assume the answer to this question. Most of the people I encounter assume that the answer to this question is yes. There are some people that you'll encounter that assume that the answer to this last question is no. But regardless, however strongly someone asserts their answer to this question, the real answer is we don't know whether magnetic fields influence climate. Yeah, I'm okay. Let's talk about what climate is, and we'll do that in just a minute because I'm still stuck on the first slide. Okay. We'll come back to this question again at the end.
So here's our approach for the time that we have together. The title that I was given: What was Climates of Terrestrial Planets? But I'm aware that you're in a—he'll aware that you're in a heliophysics summer school right now. So for the first part of our time together, I'm gonna try and get from a description of the climates as they exist today and how they might have evolved to the heliophysics connection, which we all, I think, are pretty aware of, but I'm gonna try and paint that picture for you, and a lot of it might be review. Then we're gonna have a break, and I'm gonna go pee, I'm sure, because I had all sorts of caffeine before I got up here. And then we're gonna come back, and I'm gonna try and get from heliophysics back to the climates again, with this big question mark still—that we don't really know whether magnetic fields influence climate. Okay. Part one: Climates. What are the climates of the terrestrial planets like today? Here's a table. It seems like a good idea to start with a table summarizing the climates. The moon and Mercury have been kicked out of the conversation of terrestrial planets because they don't have significant atmospheres; they have bound exospheres. Titan is something you could consider adding to this chart as well, even though it's in the outer solar system, it behaves in many ways like a terrestrial planet, certainly in the interaction of the space environment. But we're gonna stick to Venus, Earth, and Mars. And for this slide, I just want to focus on the first four rows. Venus is hot. Earth is great, especially in Boulder. And Mars is too cold. Earth is only about 15 degrees above the freezing point of water. If we didn't have greenhouse gases in our atmosphere, the temperature on Earth would be about 15 degrees below the freezing point of water. So thank goodness for greenhouse warming in the case of Earth. Venus maybe isn't so appreciative of all the greenhouse warming that it's getting. Okay. Pressures on the three planets as well. Earth is one bar, by definition, thankfully. Somebody somewhere decided to call that one to save us at least some math somewhere along the line. Venus has a huge, thick atmosphere. Mars has a very, very thin atmosphere. And this is, of course, related to the amount of greenhouse warming that the three atmospheres receive. Next: composition. Venus and Mars are both CO2 atmospheres—about 95 percent CO2. The second most common constituent is nitrogen. Earth almost certainly started the same way as Venus and Mars—a 95 percent CO2 atmosphere—but Earth has oceans; it has liquid water. And it turns out that the CO2 in the atmosphere reacts with minerals, and the 95 percent CO2 that our atmosphere started with is now trapped at the bottom of the oceans in carbonate rock. It can be re-released into the atmosphere because we have plate tectonics; that carbonate rock can be transported along, subducted, melted, and the CO2 can be re-released in a CO2 cycle. But for now, our CO2—at—fixed CO2 atmosphere—the CO2 has been sucked out, leaving nitrogen as the most common constituent. Oxygen came late in Earth's history, somewhere between 2.4 and 1.7 billion years ago. It started being produced biologically and then built up in the atmosphere after it could no longer efficiently react with surface rocks. And so oxygen is the second most common constituent. So Earth's atmospheric composition is very different from that of Venus or Mars, but they started out the same way. The atmosphere evolved. The last thing I'll tell you attention to here is water content in the atmosphere. Water is, of course, important for life. All three atmospheres have water in them. The Venus atmosphere is incredibly dry—dry as a bone—20 parts per million or so. Earth, relatively wet—10,000 parts per million. And Mars somewhere in between—a 210 parts per million. What you might not be aware of, however, is that this 210 parts per million for Mars is pretty much the carrying capacity of that atmosphere; it's the relative humidity at Mars is near 100 percent. Okay. So I can't support much more. Hey, the rest here is just sort of for your interest, but these things are the things that I'm kind of—kind of—focus on for the heliophysics connection. There's precipitation, places, and we could talk about circulation; we could talk about dynamics and seasonal variations as well, but let's just focus on sort of these basic properties. Okay. That was one slide. We're done with climates of the terrestrial planets and and sort of how they exist today. Now let's go to how they change and whether they've changed.
Alright. So what could you do to change the planet's climate? Suppose you were trying to terraform a planet, or you had extreme superpowers of some kind. This time you have 60 seconds to come up with as many ideas as possible for changing the climate of a planet. And before you do that—climate—I forgot to define it. Climate is the long-term average of weather. So climate is the state of the atmosphere over long periods. Then need to define the term long. Long, in this sense, is sort of decade or beyond timescales, anywhere from decades to billions of years. So what could you do to change a planet's climate? You have 60 seconds. Go. Anything you want. 20 seconds left. Oh, no cheating. No cheating. Five, four, three, two, one. All right. Alright. Someone from this column right here. Anyone? Okay. Okay. This column, second, next column. Oh boy, you guys are a total disadvantage in the next column. How would you do that? All right. I asked for that one. Okay. What's that? Yeah. All right. Last column. Last column. Change sides of the planet. What does that do? Good. Okay. Other answers. Let's go here and then there. Okay. A good—what else? All right. Good. Change the gravity. Okay. Okay. So not just distance, but change orbital parameters. Good. Oh, that's so sad. Okay. Boy, sort of asked for emancipation I guess as a planet. Yeah. Okay. Yeah. Inclination. And then all the way in the back. Okay. All right. So now if you have another answer, then tough luck because I'm moving on. Okay. Good. So this was just to get you thinking. I think almost every answer here—I'm trying to figure out if a nuclear bomb fits into the paradigm I set up. Maybe it can. That's right. You need to sterilize and start over. So I think—I think that most of what you said—all of these are great—and I think most of what you said can fall into one of four categories, and those four categories I think fall relatively obviously out of two equations right here, two simple equations that you would get as an undergrad taking a planetary atmospheres course or planetary astronomy course or atmospheric science course. So one is for at the radiation balance in planetary atmospheres. The amount of radiation coming in that a planet receives depends upon how much light is coming from the star—the solar constant—divided by the distance away from that star squared of the planet. You have to do 1 minus albedo because the planet doesn't necessarily absorb all light coming from the star; it reflects some away. You could paint the polar cap black, for example, and change the albedo of the planet. And then there's a cross-sectional area of the planet—πr². You're a big cross-sectional obstacle, and some light is missing you, and some light is hitting you. Okay. That's absorbed and re-radiated by the planet, and this is an equal sign here for terrestrial planets because the amount of radiation coming from the interior is a very small fraction of the radiation that's being returned from the incident sunlight. So you're radiating a σT effective to the fourth—this sort of mushy concept of effect temperature for planetary atmospheres—and you're radiating over the entire surface area of the planet. Or for a slow rate of slow rotator, you might need to change that four to a two or something like that because you would only be radiating away from the warm side of the planet. Okay. This effective temperature, I said, is a mushy concept, and that effective temperature is the radiation coming from some distance above the surface, maybe at the surface itself. But if you want to relate effective temperature to surface temperature, you do so through the optical depth of the atmosphere, at least in the simplest of all possible approximations, which I'm sure you've all been through in some graduate or undergraduate course where you assume an atmosphere of a planet or a star is made up of slabs that radiate and communicate back and forth between each other. The effect of temperature and surface temperature are then related by optical depth, which is how much stuff you have in the atmosphere. Okay. That's the math. Interpretation-wise, it's these four things: solar output, s; planetary albedo, a; greenhouse gas content, or what the atmosphere is made of, or how much of it is there, τ; planetary orbital elements, a d², distance from the star, but also encompassed in this is obliquity of the planet—how much it's tilted—eccentricity of the orbit, which isn't really captured very well in the equations as written here, but that's all tied up together. So these four things can influence the surface temperature of a planet, the climate of a planet, how the atmosphere works over long periods. I'll just give you two examples right now. First of all, solar output. You've all heard of the faint young Sun. Based on observations of Sun-type stars, here we are today, and this is age of our star from young to old, and this is solar luminosity. And so long ago, the Sun was 30 percent fainter or something like that. This is the s in this energy balance equation up here. So you're changing the amount of energy incident upon the planet; you're changing what happens in the system itself. And the solid line is what you were supposed to be looking at. Planetary orbital elements. From Mars, adding a moon was a great answer because Earth's moon stabilizes our obliquity. We only rock back and forth by a degree or two. Our tilt is relatively stabilized by the presence of our moon. Mars lacks a big moon, and therefore the tilt of Mars—even though it's almost identical to Earth's tilt today—we're 23 and 1/2 degrees, Mars is 25 degrees—that's a difference between this and this right there. Okay. But Mars wobbles back and forth in its obliquity over huge ranges. It gets down almost—it gets up almost to zero degrees, and it'll go all the way down to 60 degrees. Since what's rocking back and forth and back and forth, its eccentricity is changing, and that affects the amount of sunlight incident upon different parts of the planet. And so at high obliquity periods, you could have a Mars that looks something like that. Yeah. If we lost our moon, what would happen to the climate right away? Immediately, I suspect immediately you'd notice very little change, very little difference, because the absence of our moon isn't directly affecting any of these admittedly oversimplified equations right here. What the moon does affect over long terms—and and you know the scale here is in millions of years—what the moon will—will—absence of our moon will allow is more wobble. They're—our obliquity should change, start changing more drastically as a result. Yeah. This right here is a model result. Okay. So this is a computer simulation, and it's a chaotic system. At some point, it goes unstable on that calculation, so you can only predict a certain distance in the future and a certain distance backward before you lose all confidence in the calculation. Yeah. It's the tilt of the—yeah—of the spin axis of the planet with respect to its orbital plane. Yep. So there's precession as well, not shown in this plot. Yes. The tilt itself changes with respect to the orbital plane, so it's not just a wobbling of a top always at 25 degrees, but it's doing this back and forth as well. Yeah. Good. All right. I like the amount of discussion. This is good. Okay. Briefly, and three slides, I'm going to give you some evidence the climate has changed on the on the three terrestrial planets. Okay. One is this boring-looking plot, and altitude—I—I don't care about that axis so much. What I care about is the x-axis here. This is the D/H ratio on Venus—deuterium to hydrogen ratio. Deuterium and hydrogen are, of course, isotopes of each other, and it's that ratio on Venus compared to the same ratio on Earth. And the D/H ratio on Venus is very large. Okay. At least a hundred, maybe 200, 250, and it's an altitude-dependent measurement, so there's a lot of wiggle room there, but it's very large compared to Earth. And this has taken as evidence that Venus's atmosphere and climate has changed a lot—that hydrogen has been removed from the atmosphere. Why are isotopes like this taken as evidence? And the answer comes a little bit from the last talk where we discussed the idea of diffusive equilibrium. When you get above the homopause, above the turbopause, above the region of the atmosphere that—where everything is well mixed—when you get above that region, every species has its own individual scale height—scale heights written over there, kT/mg. So species of different mass have different scale height, and so their density profiles fall off differently with altitude. What this means is—this is scientifically inaccurate to say, but gives you a mental picture—light stuff is floating, and heavy stuff is sinky. So if something is stripping away atmosphere from the top, the light stuff goes away first and leaves the atmosphere enriched in the heavy stuff. Deuterium should be enriched relative to hydrogen, even though they existed in very different amounts in the atmosphere. When you compare the deuterium-hydrogen ratio on Venus to that on Earth, you see that this stripping away process has been much more active on Venus than it has been on Earth. Okay. This is evidence for climate change on Venus and also an indication of where the water went. Venus is bone-dry. Maybe the hydrogen was removed away to space. Okay. It's hard to get any evidence for climate change from the surface of Venus itself, and that's because the entire surface of the planet has been resurfaced within the last 500 million years. People argue about whether that was one global resurfacing event or it's been gradual resurfacing over time, but the paucity of craters on the Venus surface—the absence of impact craters—there—very few—that tells us that the surface is young, and therefore any evidence of past climate beyond 500 million years ago, even if we could see it, even if we could get through the atmosphere or land and rove around and stuff like that, we don't have a very long climate record from the surface itself. We're stuck with these atmospheric indications. Mars, on the other hand, there are three different lines of evidence that things changed: geomorphologic. Basically, we can take pretty pictures of the surface, and we can tell that it used to be different. So for the next five hours, I'm—
Going to go through that evidence with pictures from the surface. And actually, I'm just gonna show you one because this one should be enough to convince you that Mars changed. When you look at a place on Mars called Wario Valles, from high elevations up here to low elevations down here, you see that there's there are these channels incised into the surface. These are dried-up riverbeds on the surface of Mars. In order to have rivers on the surface of Mars, you need to have temperatures above the freezing point of water, and those temperatures don't exist on Mars today. This tells you that Mars changed. You need tens of thousands, thousands, tens of thousands, hundreds of thousands of years to carve something like this, especially a branched river pattern on the surface of the planet. This tells you that Mars was a very different place at the time that these were created. The time that these were created was well past the era of the late heavy bombardment in the early solar system, and so these conditions on Mars persisted well past the early impact stage. Okay, was it water?
Well, you can also take a spectral information from the Martian surface and do some geochemistry; basically, what is the surface made of? Anywhere you see rainbow colors in this image of the surface, there are things called phyllosilicates. Phyllosilicate is Latin for "I love silicate," apparently. And you can tell I'm not a geologist because I just said that. But phyllosilicates are clays. Clays form in the presence of liquid water. Hey, this was water that carved the Martian surface, and there are all sorts of arguments why it had to be water, but this is one piece of evidence; it suggests that it had to be water and not something else. Finally, the isotope ratios that we just discussed, not just D to H, but argon, carbon, nitrogen, oxygen—all of these are enriched in the heavy isotope. And being enriched in the heavy isotope is indication that the light, floaty stuff was somehow removed, and therefore this also suggests that there was climate change on Mars.
Earth, and I'm way out of my depth here, so I'm gonna go very quickly, but there are tons of lines of evidence that Earth's climate has changed over time. You can use ice cores, and various aspects from the ice cores—trapped gas and bubbles tell you something about atmospheric composition on Earth as a function of time. Isotopes tell you something about ocean temperatures as a function of time, especially oxygen isotopes. Even pollen trapped in ice cores tell you something about what kind of vegetation was growing at different times in Earth's history. This is for relatively recent climate change; the oldest ice cores we have only go back about 800,000 years, and I think interpretations become very cautious after about 200,000 years or so. You can use tree rings, and also something analogous in coral, and the spacing of the tree rings tell you whether it was a good growing year or a bad growing year for that tree, and therefore you can infer something about climate conditions going back in time. But if you really want to go far back in time, then you have to use rocks, sediment that you can date. So you can use fossils and pollen trapped in those rocks, the composition of the rocks, mineral minerals that are or are not present to tell you something about temperature. Layering of rocks can tell you something about big sudden changes in climate, big shifts and climate when you see sort of a discontinuity in in layers, and also even texture of rocks tells you something about the environment in which they were created, and there you can use that environment to tell you something about what conditions were like as a function of time. So Earth's climate has also not been static; it changes back and forth with time. Okay, of those four ways to change climate, climate has clearly changed; we've discussed ways that you can change climate. Of those four ways, I'm going to focus mostly on one right now for the rest of our time, and that's change in greenhouse gas abundance, change in surface pressure of the planet, change in how much stuff is there. Okay, because I'm gonna argue that this is the one that is most directly related to heliophysics, and that's why you're all in this room right now. Okay, so how do you change the amount of gas in an atmosphere? There are about four or five different ways, several of which you mentioned. So let's start from the left and move towards the right.
Volcanism, outgassing from the interior adds new atmospheric particles. This is one of the two main ways in which we have gases in our atmosphere today. Any primordial atmosphere that we formed with, we believe was lost very quickly, and what we have today are secondary atmospheres that came largely from the interior of the planets and that have subsequently evolved with time. So outgassing is a source; the bigger the planet, the more heat you have inside, the longer you can expect outgassing to be an important process. Impacts—I don't hopefully that cartoon right there suggests impact to you—and after a while it started not looking at all like impact to me anymore, but I was done with PowerPoint at that time. Impacts giveth and impacts taketh away; both, you can crash big things into atmospheres, and I mean what really what could be more fun than that, just crashing big things into atmospheres and into planets. When you crash into atmospheres, you can splash all sorts of atmosphere away. If you're a large enough impactor, you can remove all of the atmosphere above the tangent plane to the point of impact. I threw fluid process, he's just pushing it out through a vapor plume. But impactors, especially comets but also asteroids, are made of stuff, volatile that exists in atmospheres today. So not only do you splash atmospheric particles out, but you can bring atmospheric particles in as well. You can exchange atmospheric particles back and forth with the surface; you can make polar caps grow and shrink. You can also do things a little more permanently by maybe turning Mars red, rusting the entire planet—whew, oxidation processes. But most surface exchange, and you can also trap particles basically in pores in the surface, and then it can migrate down as well and also be re-released. Most surface exchange processes are reversible; you can add or remove particles through surface exchange. Here's where heliophysics comes in; these are loss mechanisms. So now let's skip this one and go over here to the right. Hydrodynamic escape is believed to be the way that we got rid of our early atmospheres. The early atmospheres were likely very hot, and that heat caused the particles to shake around and and to give the light particles in the atmosphere enough energy to escape away from the planet thermally. Those are the red ones right here, escaping thermally. Heavy stuff may have not had enough energy to escape thermally, but there was so much light stuff leaving that it got stuck in that flow, entrained in that flow, and escaped hydrodynamically from the planet. Okay, this is blow-off, sometimes called blow-off, and it's believed to have been important for all three terrestrial planets in the first 100 million years or so, and the way we got rid of our primordial atmospheres. Now we go back to something called escape to space, and this is a very loose term for a whole suite of processes; in my mind it's about six processes, but it depends upon who you ask. These are ways of getting particles from the atmosphere into space. Jeans escape would be one process, and we're going to go through the six that I have, but Jeans escape is one. So they can escape as neutral particles, basically on ballistic trajectories that are not gravitationally bound to the planet, or as this simulation right here suggests, they can also escape as charged particles. And this is not a simulation as well; you can see the particle turns around and go back; it goes backwards at some point. This is PowerPoint art constructed while having a beer. There we go. Okay, so now let's go through the escape processes, and that'll take us to a break. What do I want to say? Let's find out. Oh, okay, it's your turn again. So what requirements must be met for an atmospheric particle to escape the planet? There's more than one. Okay, you have 60 seconds again. Ready, set, go. Thirty seconds. Five, four, three, two, one [Music]. Okay, I haven't heard from your row yet. What what do you what needs to happen for an atmospheric particle to escape? Okay, good. Okay, good. All right, what's up? And can you say it aloud? Er, sorry, I'm stuck. Temperature, temperature, temperature. Okay, okay. What else? Yeah, yeah. I mean, giving a particle escape velocity and having it go down that doesn't really accomplish the objective right there. Yeah, what else? Yes, please. And what else? I can make you you guys want a thumb wrestle to determine. Good. All right, so wait, back to me, back to me. I'm the king of the what's happening. Velocity. Okay, we have the concept of escape velocity up here, and now we've incorporated the idea of mass, and there's some argument. So how could we resolve this energy? So let's change escape velocity to have energy greater than escape energy, and then mass gets folded in as a result. Good. Anything else? Yes. Whoo. Yeah, it's charged and it has escaped energy, but it's magnetized. If it's tightly bound to that field line, if it's in that flux tube, it's just going to move right back again, and you're gonna give me another one now. Anything else? No. Okay, good. We're moving on. I'm glad I didn't see any hands right there, and it's not because I didn't look up, although I didn't look up. Okay, yeah, that last one we're gonna come back to, but the other things that were said that we said we're going to handle right now. Okay, has to have escaped energy. Escape energy, you know, in simple terms, is derived as a balance between kinetic energy of the particle and the gravitational potential energy. When you do that, you get escape velocity; that in no way depends upon the mass of the particle; that cancels out. Velocity doesn't depend upon mass, but the energy of the particle does. Okay, so for Venus, Earth—not only that, you're escaping usually from high altitude, so it's not just the radius of the planet. If you want to be very precise about it, you want to add the altitude from which the particle is escaping. So with escape velocity is an altitude-dependent concept. So for Venus, Earth, and Mars, escape velocities are roughly 10 kilometers per second for Venus on Earth, and that's because Venus and Earth are almost the same size; Venus is 95% the size of Earth. Mars is about half the size of Earth; has a much smaller escape velocity; it's easier to escape from Mars. Okay, lower escape velocity. Let's translate that to energy for protons and for oxygen. Escape energy for Venus and Earth is about half an Evi for for hydrogen and about 10 eb for oxygen. Mars, those numbers go way down by a factor of about 5; it's easier to get stuff away from Mars, but you your ability to escape does depend upon your mass; the energy required for you to escape does depend upon your mass. You also have to be going upward. Good catch on that one. And no collisions as well, or very few. So this is the concept of exobase, which is a very mushy concept, and depending upon who you talk to in the community, they'll tell you the exobase is dead, that it's not a relevant concept, but it's a useful teaching tool nonetheless. Exobase is considered to be the region in the atmosphere where collisions become unimportant, and you could define that any number of ways, but most people say that the scale height of the particle is roughly equal to the mean free path of the particle between collisions; a 1 over the density times the collisional cross section. This makes sense; for example, if you keep everything about an atmosphere the same but increase its temperature—temperature is just a measure of average kinetic energy—if you increase the temperature of an atmosphere, you increase collisions, and therefore the atmosphere becomes more collisional; the exobase must move up as a result. If temperature goes up, that means density must have gone down; your exobase region has moved to a region of lower density, higher. So this makes sense. Okay, there are reservoirs for escape that we've talked about and that you've discussed in another talk, so I'm not going to dwell here, but there are three swimming pools from which particles can be drawn as they're removed from a planet through interactions with the space environment. Thermosphere and exosphere are both neutral regions, and exosphere is just where you become collisionless and where particles are on ballistic trajectories, some of which can escape from the planet, some of which can't. Thermosphere, the temperature increases with altitude, and at some point you pass the homopause and you move into a region of diffuse equilibrium. Hey, what's important to me to note is that the ionosphere is often considered as a distinct region of the atmosphere, but it's not spatially distinct at all; it overlaps the thermosphere and the exosphere both on all three planets; it's just charged particles. Another thing to keep in mind is that the density of the ionosphere is always much less than the density of neutrals; that becomes less and less true as you go up to very high altitudes, but at that point you no longer really care about it anymore. Alright, let's stop here and take a break, and then we'll talk about escape processes and implications.