A Study in Contradictions.
Some of you may know a piece of music called ``The Birth of Venus'' from Respighi's `Three Botticelli Portraits.' Most of you, I am sure, will have seen the famous painting which inspired that piece of music. It shows Venus, the goddess of beauty, standing on a scallop shell and emerging from the sea. The interesting thing is that mythologically Venus has long been associated with love and beauty, but in fact recent studies have shown that this planet is almost literally hellish at the surface, and completely inhospitable to any imaginable lifeform.
Chapters 10 and 11 of the textbook provide a detailed discussion of Venus (and the other terrestrial planets), and you should read them thoroughly. In this section of the web notes, however, I want to focus my attention on one particular aspect (but will expect you to be generally conversant with much of the other material presented in the text). I want you to develop a thorough understanding of what has caused the runaway greenhouse effect on Venus, a subject which is closely allied to the familiar present-day problem of global warming here on planet Earth. Venus provides us with a salutary example, and a warning!
Before launching into the topic of the greenhouse effect, however, I will take the time to present some more general discussion, including a bit of interesting history not found in the text. Venus has had a considerable significance over the last few centuries, for several reasons worth knowing about.
The Appearance of Venus: Its Importance.As you know already, Venus takes on phases ranging from full to thin crescents -- an effect only seen with the aid of a telescope. This range demonstrated to Galileo that Venus orbits the sun, and proved the correctness of the Copernican system. For this reason, it can rightly be said that Venus played a critical role in refining our understanding of the nature of the Solar System. Neither Galileo nor the many astronomers who were to follow him for centuries thereafter were able to discern any features on Venus because it is covered with thick cloud. The clouds, by the way, are very reflective, which is part of the reason that Venus looks so bright (its proximity to the sun and to the Earth are also important, of course). The clouds of Venus have what is called a very high albedo, which is merely another word for reflectivity. By contrast, the moon reflects only a few percent of the light which lands on it, and indeed its surface is comparable to the dark volcanic sand one finds on the beaches of some tropical islands. As a consequence of its albedo and proximity, Venus is the third-brightest object we regularly see in the sky, with only the sun and moon exceeding it in brilliance. The thickness of the clouds made it essentially impossible for astronomers to see any structure on the surface of Venus and to determine how the planet might be rotating, an interesting question which is explored in the text. But it also conjured up a lot of speculative ideas about what it might be like "below the clouds." A fairly common theme of about fifty years ago was that Venus might be like the very early Earth - warm because of its proximity to the sun, moist and muggy underneath the clouds, and probably swampy and wet. It was not uncommon in science fiction stories of the time to have astronauts discover dinosaurs plunging around in the Venusian landscape. My use of the adjective `Venusian' reminds me that the technically correct word is `Venereal', and indeed you can see how this word applies to what we now call STDs (sexually transmitted diseases): Venus is the mythological goddess of love and beauty. The word `venerated' (loved, or admired) has the same origin. The Russian probes which first successfully landed on the surface of Venus were called the Venera probes, but in general the adjective `venereal' is not a word we use in astronomy.
Transits of Venus: Captain Cook.Although Venus was difficult to study in detail, it served one interesting purpose in early attempts to find the distance of the sun. Recall that, following the work of Copernicus, people had a good `scale model' of the solar system, but did not know any actual distances precisely. Edmund Halley, of the famous Halley's comet, pointed out that transits of Venus could be used for the purpose of figuring out the true size of the solar system - or, to be specific, or measuring how far the Venus itself is from the Earth (after which all other distances can be worked out). (By the way, we now measure the distances very straightforwardly by using radar signals which we bounce off the planets. We can easily measure the precise length of time it takes for the radar signal to go out to Venus and return, and thus deduce its distance. Venus is especially useful in that it is the closest planet to us, and its rocky surface means that we get a nice healthy radar "echo" from it.) What is a transit of Venus? Well, remember that the orbit of Venus is smaller than that of the Earth, and if these two planets were in precisely the same plane we would see Venus pass between us and the sun quite regularly. Because it is far away, and because of its small size, it would not cover the face of the sun the way the moon eclipses the sun. Instead, we would see a small black spot, the sihouette of Venus, move slowly from one side of the sun to the other in an event we call a transit. In fact, transits are rather rare because the orbital plane of Venus is not precisely the same as that of the Earth. Consequently, transits occur only about once every century (although we are fortunate enough to be enjoying the opportunity now: we saw one transit earlier in 2004 and will enjoy another one in the year 2012 A.D.). Edmund Halley realized that the duration of a transit (how long it takes Venus to cross the face of the sun) would depend on where you were looking from. The reason, of course, is that people in different locations on the Earth will see the transit from slightly different perspectives. This is shown diagrammatically in the following pair of figures (which are drawn with much exaggerated proportions in order to make the effect clear.) A more realistic idea of the appearance of a transit is nicely demonstrated in the next figure, one which demonstrates what could be seen during a transit of the planet Mercury across the face of the sun in November 1999. (The principle is the same as for Venus). As that figure shows, an observer in Sydney, Australia, would see Mercury skim the very edge of the sun, taking about 52 minutes to do so, whereas an observer in St Louis, Missouri, would see Mercury a little farther in towards the centre of the sun -- although still rather close to the edge -- in a transit of about an hour's duration. The longer transit time is because, from the point of view of someone in St Louis, a wider part of the sun is being crossed. What Halley showed, in his analysis of the Venus transit, is that the difference in the times taken could be used to determine the distance between the Earth and Venus, provided we knew how far apart the independent observers are on the face of the Earth. (This is again an example of Once we know the distance of Venus, we know all the distances in the solar system because we already have a perfect scale model, from Kepler and Copernicus. It was in an attempt to do exactly this that Captain Cook sailed to the southern Pacific Ocean, eventually making the observations from a small peninsula in Tahiti. (Later in the trip he was killed by the natives of the Sandwich Islands, later renamed the Hawaiian Islands. Astronomers visiting Hawaii now are generally greeted more hospitably.) It is an interesting piece of history that there was sufficient astronomical research interest, much of it stemming from the Admiralty's interest in navigation, to mount this expedition from Great Britain. As it happens, however, the transit measurements did not solve the problem. The difficulty was deciding exactly when the transit started and ended. In observing the transit, we have to decide the exact instant it begins so that we can start our clock, but instead of a nice clear entry, we get a very blurry image. (Imagine an NHL goal judge trying to see if a sliding puck crosses the goal line. Now imagine him doing this if he had very blurry vision, or if he was looking through a fog!) This is because the thick atmosphere of Venus scatters and smears out the light to such an extent that we see what came to be called the `black drop' effect -- the inability of oberservers to define very precisely the moment the transit began or ended. Unfortunately, this made it impossible to use the measurements to determine the solar distance much more precisely than they already knew it. Still, it was a clever idea.
The Rotation of Venus: Cook That Chicken.As noted, the rotation of Venus is impossible to figure out from simple eyeball observations, or even from photographs. First of all, Venus is completely shrouded in thick clouds, a situation which is made worse by the fact that its proximity to the sun means that we can often only observe it at low altitudes in the twilight sky. (Compare this to Jupiter: At regular intervals, it stands nice and high in the dark night sky, and on its surface we can see conspicuous features such as its famous Red Spot. These features can be seen to go around and around the planet once every ten hours or so.) It was only with the development of radar that we were able to determine the rotation rate of Venus, using the Doppler shift. Obviously, if a planet like Venus is spinning, that means that at any moment one side of it is coming towards you while the opposite side is moving away, rather like a merry-go-round (unless you happen to be looking down on it from high above its North or South Pole). Using radar signals in much the same way as the police do, astronomers study the reflected radio radiation from the different parts of Venus -- the approaching side and the receding side -- to work out how rapidly it is spinning. These studies yielded a surprising and puzzling answer. It turns out that Venus is rotating amazingly slowly, once every 243 days. (It orbits the sun every 225 days, so it takes longer to spin once on its axis than it does to go completely around the sun!) Moreover, the rotation is retrograde: that is, Venus is spinning slowly backwards as it orbits the sun. The reason for this odd behaviour is not known. It may be that a large chunk of material ran into Venus late in its formative stages and just happened to give it this out-of-the-ordinary spin; but there is some suggestion that gravitational interactions with the Earth are responsible. You may think the slow rotation rate inconsequential, but at first glance there seems to be a strange inconsistency in the behaviour of Venus. Ordinarily, you would expect a slowly rotating planet to have great extremes of temperature on its two faces. Think of the Earth, for example. When it is nighttime here, the temperature drops appreciably because of the loss of the sun's radiant energy. (As I write these words, the Weather Channel is telling me that tomorrow's high temperature is likely to be about -2 degrees, with an overnight low of about -13.) How much worse would it be if the Earth were to slow down in its spin, so that each day and night lasted a month, say? How cold might a winter's night be then? But Venus, which spins so very slowly, is very hot all over its surface, even on the side which is facing away from the sun. Why? In class, I asked you to imagine cooking a chicken on the spit of a barbeque, a job you could do in several ways. Here are two extreme approaches. The first would be to face the chicken one way, and to roast the side nearest the fire for (say) half an hour. Then you would turn the chicken around to cook the second half (during which time the first half of it would be cooling off). This would produce very uneven cooking. Alternatively, and more sensibly, you could arrange to have the chicken turning fairly rapidly and continuously so that all parts of it are cooked at a more-or-less steady temperature. Venus is a bit like the chicken cooked in the first of these styles, with one side facing the heat for a long time. Remarkably, however, Venus is uniformly hot despite the slow rotation. How can this be? The fundamental answer is that the very thick atmosphere of Venus, and the circulation of air within it, spreads the heat over the planet's face. As you know, this happens on the Earth as well. Fortunately for life on Earth, the heat which is so abundantly provided in the tropical regions is redistributed by ocean currents and air currents, and the final temperature distribution is not the same as it would be if the planet were completely airless. For a real contrast, however, read the discussion of the planet Mercury on page 275. Since Mercury has no atmosphere to redistribute the heat, and is moreover slowly rotating, it displays enormous extremes of temperature on its sunlit and dark sides.
The Surface of Venus.As noted, the rotation of Venus was first worked out from the analysis of radar signals bounced off its surface from the great Arecibo radio telescope in Puerto Rico. This also provided very precise measurements of the distance to Venus, as well as the first radar images of Venus -- that is, topographic maps, showing geological features of moderately large size. (There is a limit to how much detail we can detect from so far away.) Even better and more detailed radar images have since been provided by instruments on spacecraft which were put into orbit around Venus so that they could carry out long-term studies using `side-looking' radar. One of these topographic images is shown on page 285 of your text. Venus is just about as big and massive as the Earth, and should have the same difficulty as the Earth does in radiating away its internal heat to space. On this basis alone, one would expect quite active geology on Venus. It is reassuring, therefore, to note that the surface of Venus has uplifted continent-size regions which are presumably the result of tectonic activity similar to that seen on the Earth. This is consistent with our expectations. On the other hand, the similarities are not as pronounced as all that! The continental plates on the Earth stand out in sharp relief, with mountain ranges providing evidence of great activity at the plate margins. Venus has much less delineation, and it is a little worrying that the geological activity on Venus seems to have produced so little large-scale structure. If we are to trust our general geophysical understanding, it would certainly be important to understand exactly why Venus has undergone so much less tectonic activity than the Earth. Planetologists have considered and speculated over a variety of possible explanations. One possibility, for instance, is that Venus has less `water of hydration' in its rocks, since it formed in a part of the solar system where it was quite hot in the solar nebula. The lowered abundance of water may have made the rocks in the upper mantle stiffer and less easily moved about by tectonic forces, so no pronounced continental drift could occur. Finally, I should point out that much of what we know about the surface rocks, temperatures, pressures and so on is attributable to some beautiful space experiments by the former Soviet Union, experiments in which robust automated probes were landed right onto the rocky surface of Venus. Because of the appalling pressures and temperatures, compounded by the corrosiveness of the atmosphere (which contains hydrofluoric, sulphuric, and hydrochloric acid), the probes did not survive for long -- no more than a few hours! -- but the scientific returns were very valuable. A picture of the surface under the footpad of one of the landers is shown on page 287.
An Introduction to the Greenhouse Effect.If you thought about it in very simple terms, you might expect Venus to be somewhat hotter than the Earth, since it is nearer the Sun than we are. On the other hand, it has a very reflective cloud cover, with the result that much of the incoming solar radiation is merely reflected into space rather than reaching and warming the surface. (This great reflectivity, coupled with is proximity to us and the sun, explains why Venus looks so bright in the night sky.) If only a fraction of the incoming solar energy makes it to the planetary surface, you might expect Venus to be only a little warmer than we are. Indeed, you might remember that science fiction writers assumed that it would be warm enough to support a flourishing population of dinosaurs! But until the 1960s, no one knew for certain. The surprising discovery was that the surface of Venus is appallingly hot - hot enough, indeed, to melt lead! This is far hotter than it would be if Venus were a simple chunk of airless rock, and is hard to understand in simple terms. What is going on? Let's backtrack just for a minute. Suppose you could strip all the atmosphere away from Venus and leave it as a bare Earth-sized chunk of rock -- what would it be like? In that case, incoming radiation from the sun, most of which is visible light, would be absorbed and would heat up the surface rocks. The rock would in turn reradiate its thermal energy at (invisible) infrared wavelengths, and at some quite modest temperature a balance would be struck, with the outflowing infrared radiation carrying away just as much energy as is carried in by the visible photons from the sun. How is it that the atmosphere of Venus changes the balance so dramatically? Why is it so hot? The answer is that there is a `runaway greenhouse effect.' But before discussing that further, I must tell you something relevant about planetary atmospheres in general.
The Structure of Atmospheres.Have you ever wondered why it is cooler near the top of a mountain than lower down? Why is there so much snow even on Mount Everest and on Mauna Kea, mountains which are near the equator where the climate is generally warmer? After all, as you climb up, you seem to be getting a little closer to the sun. Should you not be getting warmer? Amazingly, the fundamental reason is that the lower parts of the Earth's atmosphere are heated from underneath, by the hot ground, even though the fundamental source of heat is the Sun itself. (One point I must emphasize is that it has absolutely nothing to do with the internal heat of the planet, and indeed the situation would be essentially unchanged even if the interior of the Earth were to cool off completely.) Once you develop an understanding of how this can happen, you will have a perfect comprehension of the greenhouse effect and why it has proven so catastrophic on Venus. To appreciate the discussion which follows, you should consider pages 294-307 of the text, a section within which the Earth's atmosphere is discussed in some detail. Look in particular at the figure on page 304, a figure which shows how the temperature varies with altitude in the Earth's atmosphere. The figure confirms what the snow-capped mountains told you: it is indeed cooler high in the Earth's atmosphere than at sea level. (Note, though, that the detailed temperature profile is rather complex.) Let us consider what is happening. The first point to note is that the Earth's atmosphere is largely transparent to visible light, which is why we can see a long way through it and out to the sun and stars. Some of the visible light enters our eyes and provides us with information, but what happens to the rest of it? Well, if the visible light hitting the ground was simply reflected, it would then go streaming right back out through the atmosphere and disappear into space. (Some of the visible light does exactly this, of course, which is why astronauts in the Space Shuttle see the Earth by reflected sunlight and can readily discern the oceans and continents. If you were on the Moon, you would see the familiar shapes of Africa and the Americas thanks to the sunlighted reflected from the surface of the Earth.) But of course not all of the visible light bounces off. Some of it is absorbed by the Earth's surface. For example, a black asphalt road absorbs most of the light landing on it, whereas a shiny field of white snow reflects most of it. Since the incoming light carries energy, which cannot simply vanish -- remember the Conservation of Energy! -- the light which is absorbed heats up the surface material, whether it be water, desert sands, rocks, or anything else. (If you doubt this, put your bare hand onto the roof of a black car on a sunny summer day and listen to the sizzle!) Once warmed up, these various materials and objects radiate an increased amount of infrared radiation. Of course, even when baked by the summer sun, the rocks never get hot enough to radiate visible light in the way that an incandescent lamp or the sun does. (You probably know that some rocks, like the molten lavas from a volcano, do emit visible light, but they are raised to high temperatures in other ways. But in most cases the visible light you see from the everyday things around you, including the rocks, is merely reflected sunlight. ) As we will see in what follows, the critical point in the greenhouse effect is that the infrared radiation cannot readily get out to space because the atmosphere is not transparent to infrared radiation. The energy is trapped. But what is it that makes the atmosphere opaque to infrared radiation? On the Earth, the most important constituent is water vapour - not simply in the clouds, but rather the water vapour molecules which are widely distributed throughout the atmosphere. On Venus, the big culprit is carbon dioxide.
Other Warm Parts of the Atmosphere.Before considering the greenhouse effect in more detail, let's turn our attention to other parts of the Earth's atmosphere. Look again at the figure on page 304 of your text. It shows that, as we climb up into the atmosphere, the air gets progressively cooler. This is because we are farther away from the warm surface layers which trap much of the re-radiated infrared. But you can see that considerably higher up in the atmosphere there are a couple of regions where the temperature rises again - one in the stratosphere, and one very high up indeed. An input of energy is required to make the particles move around quickly. Can we identify the sources of energy? What makes these two regions special? Consider them in turn: the ozone layer. In this region, which is in the stratosphere, ozone is relatively abundant. (Ozone is a molecule which consists of three atoms of oxygen, unlike the more familiar two-atom version.) Ozone molecules readily absorb ultraviolet radiation from the sun, with the energy of the photons being used up in breaking apart the molecules and causing the particles to move around at high speed. The benefit for us, of course, is that the dangerous ultraviolet photons (which can cause skin cancers and do other damage) do not reach the ground. Some of our human pollutants, such as aerosols from spray cans and the refrigerants used in air conditioners, are thought to be depleting the ozone layer, which is cause for serious concern. You have no doubt heard about the growing 'hole' in the ozone layer, first discovered more than a decade ago. the ionosphere (also known as the thermosphere). Here, at the very outer parts of the Earth's atmosphere, atoms and molecules are being hit by X-rays, by very energetic ultraviolet light, and by fast-moving particles from the sun (in the `solar wind') and from outer space (the `cosmic rays', which are mostly electrons). These fast-moving partices have enough energy to knock electrons right off atoms, so that the outer part of the atmosphere is an ionized gas. As luck would have it, such a gas reflects many kinds of radio waves. This is useful for `ham' radio operators because their messages can bounce repeatedly off the ionosphere and the ground and thus make their way around the globe to remote locations. The atmospheric particles at these altitudes are fast-moving, thanks to the energetic kicks they get from incoming electrons and other cosmic rays, so they have an especially high `temperature.' For this reason, the region is often referred to as the thermosphere, to indicate the temperature. It is also called the exosphere, since it is from this zone that some of the particles escape ('exit') the Earth. The important point to note about these two regions is that they are heated directly by the sun, with energy flowing straight in from outside and being absorbed right on the spot. In this way, they are quite distinct from what is happening near sea level, in the troposphere, where the heating is accomplished from the ground up by radiation which has been 'processed' (rather than being in the form in which the sun emits it). Finally, I should add -- since many people are mistaken about this -- that the ozone layer and the question of its depletion are completely independent of the issue of the greenhouse effect (and global warming). These are quite two independent (but worrying) ecological and environmental problems!
The Meaning of Temperature.It is also worth emphasising that the thermosphere and the stratospheric regions are not 'hot' in the way that you might think! If you were to step out of a spacecraft into the near-vacuum 100 kilometers above the Earth's surface, you would not feel comfortably warm. In physics, we use the word temperature in a variety of ways, one of which is to describe (and to quantify) how rapidly the atoms and molcules are jiggling about within in a solid or liquid, or how rapidly they are freely racing around in a gas. But a hot gas may be so diffuse that it contains only a few atoms per cubic centimeter, and the total energy represented by its rapidly-moving particles would be rather little even though each of them is absolutely tearing about. If you were outside a spacecraft 100 km up, fast-moving particles would run into you only occasionally. In any event, your perception of temperature on Earth is conditioned mostly by the fact that you are immersed in a sea of infrared radiation flowing from all the warm objects around you - the warm walls of the buildings, the atmosphere around you, the Earth beneath your feet, and so on. In part, this explains why you may feel cool indoors on a winter day even if the thermostat is set to a comfortable level: the house may be filled with the warm air produced by the furnace, but the structure of the building (the interiors of the walls, the roof, and so on) will be cooled by the outside air and will be radiating less infrared energy than it does in the summer. When an object is in equilibrium , it has reached a temperature which is in balance in the sense that any heat flowing onto it from the outside, or generated within it, is matched by the amount being radiated away to the surrounding space. This is the situation for Venus, which is outrageously hot but not getting any hotter; we have to understand why.
Consider a Fireplace.Imagine a fireplace of the sort I have in my own house, one in which there is a set of glass doors which can be folded into the 'closed' position in front of the fire (to prevent sparks coming out, for example). When you light the fire, keeping the glass doors open, you can see the flames by the visible light they give off, and you can feel the radiant heat (which is infrared radiation) with your outstretched hands. What happens when you then close the glass doors? (I invite you to try this experiment.) The answer is the following: Provided the glass doors are not covered with soot, you will still be able to see the flames because the glass is transparent to visible light. The infrared radiation, on the other hand, does not pass readily through the glass. When the glass doors are first closed, your outstretched hands will notice an abrupt drop in the radiant heat of the fire. Why does the infrared radiation not pass through the glass? There are two possibilities. One is that the infrared radiation is simply reflected by the glass, and returns towards the fire itself. In that case, the glass doors will undergo no temperature change as time passes -- they are not absorbing any significant energy. The other possibility is that the glass absorbs the infrared radiation being emitted by the fire. This means, or course, that the glass is absorbing energy, which inevitably implies that the glass will heat up, perhaps quite a lot (depending on the intensity of the fire). It is easy to confirm that the latter is the case. (If you doubt me, try touching the panes of glass after a few minutes. I guarantee that you will burn your fingers!) Of course, this does not go on indefinitely, with the glass eventually getting so hot that it melts. Instead, the now-hot glass itself begins to emit a growing amount of infrared radiation. This radiation flows out in all directions - some towards the fire, some out into the room, and so on - but eventually a balance is reached when the radiation which is being absorbed by the glass is exactly compensated by the amount emitted by the glass itself in the outward direction. An equilibrium has been reached (until the fire consumes all the fuel and dies away; then things cool back down to room temperature).
Back to Venus...Before considering the complexity of the thick Venusian atmosphere, let us first see if we can understand the behaviour of a simpler body like the moon -- an airless lump of rock bathed in sunlight. Let us, for instance, imagine the moon emerging from a lunar eclipse during which it has been in the Earth's shadow for a couple of hours. It has cooled off somewhat as a result, but is now moving back into full sunlight. What will happen, and what sort of equilibrium temperature will be reached? As the moon emerges from the Earth's shadow, some of the visible sunlight now reaching the lunar surface is simply reflected, which is why we can again see it in the night sky. These reflected photons carry off all the energy they arrived since they merely rebound with no change of wavelength (or energy). But not all the light is reflected. Some of it is absorbed by the lunar rocks, heating them to some extent, just as our asphalt city streets get hot under the summer sun. The newly-warmed rocks begin to glow in the infrared (but never get hot enough to emit visible light). Nothing prevents the infrared photons escaping to space, since there is no atmosphere (and certainly no glass doors!). A new equilibrium is reached when the total energy being absorbed from the sun in the form of visible photons is exactly equal to the total energy being radiated away to space in the form of infrared photons. In fact, measurements confirm that the sunlit lunar surface is at exactly the moderate temperature you would expect according to this sort of reasoning. The presence of a thick atmosphere makes a big difference. This is perhaps most easily understood if we consider how things would slowly change soon after Venus first formed, when the sun first started to shine on it. (Alternatively, you can imagine building a `new Venus' somewhere out in the cold depths of space, and then bringing it promptly into the inner Solar System so that it is being bathed in sunlight for the first time in its history.) In the very earliest days, the atmosphere of the newly-formed Venus was not as hot as it is now. Some of the visible light which penetrated the atmosphere was simply reflected and escaped back into space. (The clouds of Venus are quite thick, but a surprising amount of visible sunlight still gets through. To appreciate this, think about the Earth! Even on a heavily overcast day, it is still much brighter than at night. This tells us that quite a lot of sunlight gets through the clouds.) Some of the visible light, however, was simply absorbed by the surface rocks, heating them to some extent. The rocks started to give off infrared radiation. The outflowing infrared radiation was absorbed by the atmosphere, which acts like the glass doors of the fireplace in this analogy. This effect heated the atmosphere more and more, until it is now in equilibrium, but at a very high temperature. "In equilibrium" means that enough radiation is escaping the top of the hot atmosphere out into space to make up for the amount being absorbed lower down, just as our glass fireplace doors reached an equilibrium at a high temperature which could burn your fingers. Please note that by now, billions of years after the formation of Venus, the situation is stable: Venus is not continuing to grow hotter. But the price is that the Venusian atmosphere is very hot, just as the fireplace doors themselves get painfully hot to the touch. Unfortunately this makes Venus quite inhospitable for life. Warning: The analogy to the fireplace breaks down in one important respect. The fire within the fireplace generates heat through chemical reactions, so that the heat source itself is completely contained behind the glass doors. By contrast, all of the energy now lodged in the hot atmosphere of Venus came originally from the sun itself -- that is, the heat source is exterior to Venus.
...and Back to the Earth.Perhaps now you understand why it is warmest near the surface of the Earth, and cooler higher up in the atmosphere. For any particular part of the atmosphere is to be warm, it must absorb energy. Most of the energy arriving at the Earth is in the form of visible light from the sun, and of course the Earth's atmosphere is transparent to visible light - which is why we can see the stars! The atmosphere itself cannot absorb much energy at those wavelengths. Unlike the atmosphere, the surface rocks are not transparent; neither are they perfectly reflecting. (Even a field of white snow and ice absorbs some of the incident energy. Darker areas, of course, absorb a fair bit of the visible light, and heat up significantly as a consequence. At its new, higher temperature, the surface of the Earth then emits an enhanced amount of infrared radiation. The atmosphere absorbs this energy, and is warmed from beneath. The effect is most important at low altitudes because that is where the atmosphere is thickest and absorbs the most infrared. Just to emphasise the point again: the Earth's atmosphere is warmest at the bottom simply because it is heated from the bottom, even though the heat source is the remote sun! The radiation coming off the earth itself is what the atmosphere absorbs, although of course it is really `repackaged sunlight' in the last analysis. (There are, of course, some additional complications - for instance, the warm air rises, and carries some of the heat to higher altitudes.)
The Critical Point.Many people, including a disappointingly large number of students in this course, think that the greenhouse effect is caused when visible light from the sun enters the atmosphere of Venus, reaches the rocks, is reflected from them, but then cannot get back out. This makes no sense! If the atmosphere is transparent for a photon on its way in, that photon, if merely reflected from the surface, will get back out - the atmosphere will still be transparent to it. The critical point is, and must be, that the light changes from a visible photon (emitted by the sun) to an infrared photon (emitted by the hot rocks). To help remember this, consider the following analogy. You may walk through a narrow door into a restaurant, but then eat so much that you are transformed (changed in girth) and cannot get back out. If you are merely reflected - driven away by quick inspection of an expensive menu, for instance - you will pass through the door as easily as when you came in. If you (or the photon) are to be captured, something has to change.
In Your Own Home.I used to wonder how we could afford to have big glass windows in houses and buildings. Doesn't heat just pour out through them? If you walk past a house at night, the light inside comes streaming out, so that you can see everything that happens (unless the curtains are drawn). Doesn't the radiant heat (the infrared radiation) do the same? You now know that it does not. The glass windows, so transparent to visible light, are not transparent to the infrared radiation emitted by the warm walls, furniture, air, and people inside the house. The heat is kept in by the glass to a surprisingly large extent. (But it is true that windows are important sources of heat loss in the winter, mostly because of bad seals and cracks around the joints, so that warm air can leak out and cold winds can come in.)
Some Everyday Examples.The greenhouse effect is all around us; the Earth experiences it just as Venus does. Because of the presence of carbon dioxide and water vapour, the temperature on the Earth is somewhat higher than it would be otherwise. These gases prevent all the surface heat flowing out into space, and in fact without this modest greenhouse effect the Earth's surface would be colder by some tens of degrees and correspondingly less hospitable. To that extent, then, we welcome a greenhouse effect on the Earth. There are many other examples of the greenhouse effect, including the one that gave it its name: a greenhouse. Visible light from the sun enters the greenhouse and is absorbed by the pots, plants and soil. These are warmed, and radiate infrared radiation which cannot get back out through the glass. (In a greenhouse, the layer of glass traps much more of the infrared radiation than does the water vapour in the air in the building.) a car in the sunlight . You leave your car on a sunny day and come back to find it blazing hot inside (even if you left the windows open a bit); the dog has heat stroke. The reason is the same - the visible light is absorbed by the seats and dashboard, and the infrared radiation cannot get out through the glass. People often assume that an open window will solve the problem, but unless there is a strong breeze or the window is wide open this usually does not help. the desert at night. The desert exists because you are in a climatologically dry part of the world, which means that there is generally very little water vapour distributed in the atmosphere above you. The sun heats the ground in the day, but the infrared radiation can simply stream out at night. Thus it gets surprisingly cold. In a sense, then, the desert represents the opposite of a greenhouse effect. citrus plantations in Florida. When owners of orange plantations in Florida hear that a heavy frost is expected, they use "fog machines" to surround the trees with moist fog at night. The point of this is that the water vapour in the fog prevents the ground heat from radiating away, and the orchard stays warmer and avoids the killing frost. cold clear winter nights. You may have noticed that the very coldest winter nights seem to be those when the sky is crystal clear. Cloudy nights are often warmer, but in fact the clouds have little to do with it. The presence of clouds is a sign of relatively high humidity, which means that there is water vapour distributed throughout the atmosphere (not just in the clouds). That causes a greenhouse effect, preventing the day's accumulated heat from escaping. By contrast, a night of crystal clarity may be the sign of a very dry atmosphere, with little or no greenhouse effect. (This effect is exacerbated by the fact that cold air can hold less water vapour in it anyway, so the onset of really cold weather automatically brings dry air and reduces the protective greenhouse blanket.)
The Fundamental Cause of Venus's Greenhouse Effect.The `greenhouse effect' on Venus is not caused by a layer of glass, of course, as in the fireplace analogy, but neither is it caused by the layer of thick clouds, as many of you probably think. It is caused almost entirely by carbon dioxide gas, which is the major constituent of the Venusian atmophere. (Look at the numbers in the textbook.) Since that is not what the clouds are made of, they have nothing to do with it. (The clouds consist mostly of droplets of sulphuric acid.) By contrast, the carbon dioxide is widely distributed in the Venusian atmosphere, so there is no particularly identifiable layer which is the culprit. It is the cumulative result of the deep, dense atmosphere as a whole -- definitely not the clouds! Indeed, the clouds somewhat lessen the greenhouse problem, because they reflect quite a fair fraction of the incoming visible light back out to space before it ever reaches the surface of Venus. If you were to clear the clouds away, the greenhouse effect would probably be much worse, since so much more energy would penetrate right to the ground. Venus is uncomfortable for several reasons. One is clearly the temperature, and a second is that the atmosphere contains corrosive chemicals like hydrochloric and hydrofluoric acid, plus sulphuric acid droplets in the clouds. But a third point is that the atmosphere is very thick. At the surface of Venus, the atmospheric pressure is about 90 times that at the surface of the Earth. If you were there, you would feel an external pressure on your body equivalent to what you would experience about 1 km below the surface of the sea. Any skin diver or submariner knows what that implies.
A Warning for the Earth.How is it that Venus suffered this fate and the Earth escaped it? The first thought is that Venus is simply closer to the sun, so that the total energy it receives makes the critical difference. In fact, this is indeed part of the answer, although the very reflective nature of the clouds of Venus somewhat moderates that effect. But this is not the principal point. The next obvious thought is that perhaps Venus is anomalously rich in carbon dioxide for some reason. That is not the case. On Earth, we have just about as much carbon dioxide in total as does Venus. The critical difference, however, is that the Earth's carbon dioxide is almost all trapped in the Earth itself, rather than free in the atmosphere. If this were not so, the Earth itself might suffer a catastrophic `runaway greenhouse effect.' In large measure, we have to thank the prompt appearance of life on Earth for this happy circumstance. The early life forms used carbon dioxide in doing things like making corals and sea shells (leaving fossil remnants which, hundreds of millions of years later, gave us Kingston, the Limestone City). There are also complex geochemical reactions which lead to the capture of carbon dioxide in crustal minerals, and with one thing and another we have an atmosphere quite unlike that of Venus. The problem is that we are tinkering with that balance. We are burning fossil fuels at a prodigious rate, releasing carbon monoxide and dioxide into the atmosphere; and we are clearing forests which act as `sinks' for the carbon dioxide. (The latter point is in fact not as important as you might think. I deplore the wholesale clearing of the rain forests and other wooded areas, but mostly for other reasons. Their contribution to the carbon dioxide balance is not nearly as important as are the great oceans, for instance.) Measurements over recent decades show that there have been detectable increases both in the mean atmospheric temperature and in the proportion of carbon dioxide in the atmosphere. Could this lead to an uncontrollable runaway? Are we in peril of turning into another Venus? Such a fate might be a long time in the future, but it is a question we should address. Among the foreseeable short-term consequences would be the desertification (i.e. drying out) of lots of areas of land, and a large-scale melting of ice caps and glaciers with a consequent rise in mean sea levels. Cities like London and New York might well be flooded. In the very long term, the entire Earth might be made uninhabitable. These consequences are by no means clear, however. It may be that there is a natural regulatory system: higher temperatures would lead to the partial melting of ice caps, which would put more water vapour into the atmosphere. This would slightly increase the greenhouse effect, since water vapour is a greenhouse gas, but would also lead to the formation of more clouds and a rise in the planet's overall reflectivity. Thus the total visible sunlight reaching the surface and being absorbed might be reduced or little changed. It would be naively optimistic not to consider the worse-case scenario, however, and try to forestall such a future. Mostly we think of global warming and the greenhouse effect from the point of view of how catastrophically human activity and intervention may be changing our ecosphere and environment. But changes in our environment could also result from external effects of various kinds over which we have no control and cannot be blamed. Suppose, for instance, that the sun itself were to fade somewhat in brightness (which it seems unlikely to do). Wouldn't that be very harmful for life on Earth? Surprisingly, this would not necessarily be the case. It has been suggested, for instance, that certain kinds of aquatic plants would grow more vigorously in slightly cooler conditions, with the result that lakes, ponds, and even ocean bays would quickly become covered with dark green growth. These darker areas would absorb more sunlight than the highly reflective water, and a larger fraction of the sun's heat would be retained, perhaps to such an extent that the net temperature change would be rather small, even if the sun temporarily underwent a fairly significant dimming. Such a 'self-regulating' Earth sounds almost as though it 'knows enough' to cushion itself against potentially harmful changes! Although they did not say exactly that, the first well-developed speculations along these lines can be found in the writings of James Lovelock and Lynn Margulis, in what has come to be known as the Gaia Hypothesis. (Gaia is an ancient god of the Earth.) Related speculations by subsequent authors become almost mystical, with invocations of a nearly sentient living, breathing Earth. But there may be no need for that; it is entirely possible that natural processes might have the same effect with no consciousness or volition whatever. Indeed, the very fact that life on life on Earth has persisted over billions of years may be attributable to just this sort of process. The writings of Lovelock are certainly worth exploring. There are a number of interesting web sites at which you can read much more about the Gaia Hypothesis. I suggest that you do a simple web search on that topic -- you may be astonished by the range of resources that unearths for you. Previous chapter:Next chapter
0: Physics 015: The Course Notes, Fall 2004 1: Opening Remarks: Setting the Scene. 2: The Science of Astronomy: 3: The Importance of Scale: A First Conservation Law. 4: The Dominance of Gravity. 5: Looking Up: 6: The Seasons: 7: The Spin of the Earth: Another Conservation Law. 8: The Earth: Shape, Size, and State of Rotation. 9: The Moon: Shape, Size, Nature. 10: The Relative Distances and Sizes of the Sun and Moon: 11: Further Considerations: Planets and Stars. 12: The Moving Earth: 13: Stellar Parallax: The Astronomical Chicken 14: Greek Cosmology: 15: Stonehenge: 16: The Pyramids: 17: Copernicus Suggests a Heliocentric Cosmology: 18: Tycho Brahe, the Master Observer: 19: Kepler the Mystic. 20: Galileo Provides the Proof: 21: Light: Introductory Remarks. 22: Light as a Wave: 23: Light as Particles. 24: Full Spectrum of Light: 25: Interpreting the Emitted Light: 26: Kirchhoff's Laws and Stellar Spectra. 27: Understanding Kirchhoff's Laws. 28: The Doppler Effect: 29: Astronomical Telescopes: 30: The Great Observatories: 31: Making the Most of Optical Astronomy: 32: Adaptive Optics: Beating the Sky. 33: Radio Astronomy: 34: Observing at Other Wavelengths: 35: Isaac Newton's Physics: 36: Newtonian Gravity Explains It All: 37: Weight: 38: The Success of Newtonian Gravity: 39: The Ultimate Failure of Newtonian Gravity: 40: Tsunamis and Tides: 41: The Organization of the Solar System: 42: Solar System Formation: 43: The Age of the Solar System: 44: Planetary Structure: The Earth. 45: Solar System Leftovers: 46: The Vulnerability of the Earth: 47: Venus: 48: Mars: 49: The Search for Martian Life: 50: Physics 015 - Parallel Readings.
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(Sunday, 18 August, 2019.)