Measurements made by the Venera and Pioneer Venus spacecraft have allowed us to paint a fairly detailed picture of Venus's atmosphere. 
(Sec. 6.6) Figure 9.17 shows the run of temperature and pressure with height. Compare this figure with Figure 7.4, which gives similar information for Earth. The atmosphere of Venus is about 90 times more massive than Earth's, and it extends to a much greater height above the surface. On Earth, 90 percent of the atmosphere lies within about 10 km of sea level. On Venus the corresponding (90 percent) level is found at an altitude of 50 km instead. The surface temperature and pressure of Venus's atmosphere are much greater than Earth's. However, the temperature drops more rapidly with altitude, and the upper atmosphere of Venus is actually colder than our own.
Figure 9.17 The structure of the atmosphere of Venus, as determined by U.S. and Soviet probes. (One bar is the atmospheric pressure at sea level on Earth.)
Venus's troposphere extends up to an altitude of nearly 100 km. The reflective clouds that block our view of the surface lie between 50 and 70 km above the surface. Data from the Pioneer Venus multiprobe indicate that the clouds may actually be separated into three distinct layers within that altitude range. Below the clouds, extending down to an altitude of some 30 km, is a layer of haze. Below 30 km, the air is clear. Above the clouds, a high-speed "jet stream" blows from west to east at about 300—400 km/h, fastest at the equator and slowest at the poles. This high-altitude flow is responsible for the rapidly moving cloud patterns seen in ultraviolet light. Figure 9.18 shows a sequence of three ultraviolet images of Venus in which the variations in the cloud patterns can be seen. Note the characteristic V-shaped appearance of the clouds—a consequence of the fact that, despite their slightly lower speeds, the winds near the poles have a shorter distance to travel in circling the planet and so are always forging ahead of winds at the equator. Near the surface, the dense atmosphere moves more sluggishly—indeed, the fluid flow bears more resemblance to Earth's oceans than to its air. Surface wind speeds are typically less than 2 m/s (roughly 4 mph).
Figure 9.18 Three ultraviolet views of Venus, taken by the Pioneer Venus orbiter, showing the changing cloud patterns in the planet's upper atmosphere. The wind flow is in the direction opposite the "V" in the clouds. Notice the motion of the dark region marked by the arrow. Venus's retrograde rotation means that north is at the bottom of these images, and west is to the right. The time difference between the left and right photographs is about 20 hours.
As we saw earlier, observations from Earth revealed the presence of carbon dioxide in Venus's atmosphere but were inconclusive about other possible constituents. We now know that carbon dioxide is in fact the dominant component of the atmosphere, accounting for 96.5 percent of it by volume. Almost all of the remaining 3.5 percent is nitrogen. Trace amounts of other gases, such as water vapor, carbon monoxide, sulfur dioxide, and argon, are also present. This composition is clearly radically different from Earth's atmosphere. The absence of oxygen is perhaps not surprising, given the absence of life (recall our discussion of Earth's atmosphere in Chapter 7). (Sec. 7.2) However, there is no sign of the water vapor that we might expect to find if a volume of water equivalent to Earth's oceans had evaporated. If Venus started off with Earthlike composition, something has happened to its water—it is now a very dry planet.
For a long time, the chemical makeup of the reflective cloud layer surrounding Venus was unknown. At first scientists assumed the clouds were water vapor or ice, as on Earth, but the reflectivity of the clouds at different wavelengths didn't match that of water ice. Later infrared observations carried out in the 1970s showed that the clouds (or at least the top layer of clouds) are actually composed of sulfuric acid, created by reactions between water and sulfur dioxide. Sulfur dioxide is an excellent absorber of ultraviolet radiation and could be responsible for many of the cloud patterns seen in ultraviolet light. Spacecraft observations confirmed the presence of these compounds in the atmosphere. They also indicated that there may be particles of sulfur suspended in and near the cloud layers, which may account for Venus's characteristic yellowish hue.
Given the distance of Venus from the Sun, the planet was not expected to be such a pressure cooker. Calculations based on Venus's orbit and reflectivity indicated a temperature not much different from Earth's, and early measurements of the cloud temperatures seemed to concur. Certainly, scientists reasoned, Venus could be no hotter than the sunward side of Mercury, and it should probably be much cooler. This reasoning was obviously seriously in error.
Why is Venus's atmosphere so hot? And if, as we believe, Venus started off like Earth, why is it now so different? The answer to the first question is fairly easy: given the present composition of its atmosphere, Venus is hot because of the greenhouse effect. Recall from our discussion in Chapter 7 that "greenhouse gases" in Earth's atmosphere, particularly water vapor and carbon dioxide, serve to trap heat from the Sun. (Sec. 7.2) By inhibiting the escape of infrared radiation reradiated from Earth's surface, these gases serve to increase the planet's equilibrium temperature, in much the same way as an extra blanket keeps you warm on a cold night. Continuing the analogy a little further, the more blankets you place on the bed, the warmer you will become. Similarly, the more greenhouse gases there are in the atmosphere, the hotter the surface will be.
The same effect naturally occurs on Venus, whose dense atmosphere is made up almost entirely of a primary greenhouse gas, carbon dioxide. As illustrated schematically in Figure 9.19, the thick carbon dioxide blanket absorbs nearly 99 percent of all the infrared radiation released from the surface of Venus, and it is the immediate cause of the planet's sweltering 730 K surface temperature. Furthermore, the temperature is nearly as high at the poles as at the equator, and there is not much difference between the temperatures on the day and night sides. The circulation of the atmosphere spreads energy very efficiently around the planet, making it impossible to escape the blazing heat, even during the planet's 2-month-long night.
Figure 9.19 Because Venus's atmosphere is much thicker and denser than Earth's, a much smaller fraction of the infrared radiation leaving the planet's surface actually escapes into space. The result is a much stronger greenhouse effect than on Earth and a correspondingly hotter planet. The outgoing infrared radiation is not absorbed at a single point in the atmosphere; instead, absorption occurs at all atmospheric levels. (The arrows are meant to indicate only that absorption occurs, not that it occurs at one specific level.)
But why is Venus's atmosphere so different from Earth's? Why is there so much carbon dioxide in the atmosphere of Venus, and why is the atmosphere so dense? To address these questions, we must consider the processes that created the atmospheres of the terrestrial planets and then determined their evolution. In fact, we can turn the question around and ask instead, "Why is there so little carbon dioxide in Earth's atmosphere compared with that of Venus"
We believe that Earth's atmosphere has evolved greatly since it first appeared. Our planet's secondary atmosphere was outgassed from the interior by volcanic activity 4 billion years ago. (Sec. 7.2) Since then it has been reprocessed, in part by living organisms, into its present form. On Venus, the initial stages probably took place in more or less the same way, so that at some time in the past, Venus might well have had an atmosphere similar to the primitive secondary atmosphere on Earth, containing water, carbon dioxide, sulfur dioxide, and nitrogen-rich compounds. What happened on Venus to cause such a major divergence from subsequent events on our own planet?
On Earth, nitrogen was released into the air by the action of sunlight on the chemical compounds containing it. Meanwhile, the water condensed into oceans, and much of the carbon dioxide and sulfur dioxide eventually became dissolved in them. Most of the remaining carbon dioxide combined with surface rocks. Thus, much of the secondary outgassed atmosphere quickly became part of the surface of the planet. If all the dissolved or chemically combined carbon dioxide were released back into Earth's present-day atmosphere, its new composition would be 98 percent carbon dioxide and 2 percent nitrogen, and it would have a pressure about 70 times its current value. In other words, apart from the presence of oxygen (which appeared on Earth only after the development of life) and water (the absence of which on Venus will be explained shortly), Earth's atmosphere would be a lot like that of Venus! The real difference between Earth and Venus, then, is that Venus's greenhouse gases never left the atmosphere the way they did on Earth.
When Venus's secondary atmosphere appeared, the temperature was higher than on Earth, simply because Venus is closer to the Sun. However, the Sun was probably somewhat dimmer then (see Chapter 22)—perhaps only half its present brightness—so there is some uncertainty as to exactly how much hotter than Earth Venus actually was. If the temperature was already so high that no oceans condensed, the outgassed water vapor and carbon dioxide would have remained in the atmosphere, and the full greenhouse effect would have gone into operation immediately. If oceans did form and most of the greenhouse gases left the atmosphere, the temperature must still have been sufficiently high that a process known as the runaway greenhouse effect came into play.
To understand the runaway greenhouse effect, imagine that we took Earth from its present orbit and placed it in Venus's orbit, some 30 percent closer to the Sun. At that distance from the Sun, the amount of sunlight striking Earth's surface would be about twice its present level, so the planet would warm up. More water would evaporate from the oceans, leading to an increase in atmospheric water vapor. At the same time, the ability of both the oceans and surface rocks to hold carbon dioxide would diminish, allowing more carbon dioxide to enter the atmosphere. As a result, the greenhouse heating would increase, and the planet would warm still further, leading to a further increase in atmospheric greenhouse gases, and so on. Once started, the process would "run away," eventually leading to the complete evaporation of the oceans, restoring all the original greenhouse gases to the atmosphere. Although the details are quite complex, basically the same thing would have happened on Venus long ago, ultimately leading to the planetary inferno we see today.
The greenhouse effect on Venus was even more extreme in the past, when the atmosphere also contained water vapor. By intensifying the blanketing effect of the carbon dioxide, the water vapor helped the surface of Venus reach temperatures perhaps twice as hot as at present. At those high temperatures, the water vapor was able to rise high into the planet's upper atmosphere—so high that it was broken up by solar ultraviolet radiation into its components, hydrogen and oxygen. The light hydrogen rapidly escaped, the reactive oxygen quickly combined with other atmospheric gases, and all water on Venus was lost forever.
Although it is highly unlikely that global warming will ever send Earth down the path taken by Venus, this episode highlights the relative fragility of the planetary environment. No one knows how close to the Sun Earth could have formed before a runaway greenhouse effect would have occurred. But in comparing our planet with Venus, we have come to understand that there is an orbital limit, presumably between 0.7 and 1.0 A.U., inside of which Earth would have suffered a similar catastrophic runaway. We must consider this "greenhouse limit" when assessing the likelihood that planets harboring life formed elsewhere in the Galaxy.
