Carbon dioxide Venus atmosphere

The greenhouse effect is obviously demonstrated by our neighboring planets. Venus has an atmosphere of almost pure COj. In addition it is covered by white clouds. Its surface temperature is about 430°C. Thus Hquid water cannot be present on its surface. If present in earHer periods, also then with high contents of carbon dioxide, the radiation would have increased the surface temperature to levels at which water has vaporized. Water vapor also is an effective greenhouse gas, so the temperature increased by the combined action of water and carbon dioxide. Venus is thus a nightmare scenario of the danger with the greenhouse effect. 

The atmosphere of Venus is chiefly carbon dioxide in a concentration much higher than that found on Earth. Surprisingly, no evidence has been found for carbon monoxide, though ultraviolet light decomposes CO2 to form CO. The atmosphere of Mars is thought to be largely nitrogen (around °8%) and some carbon dioxide. 

FIGURE 9.15 A radar imago of the surface of Venus. Although the rocks are very hot, the partial pressure of carbon dioxide in the atmosphere is so great that carbonates may be abundant. 

Water vapour makes a sizeable contribution, and probably the largest, to radiation trapping and as the temperature increases the water vapour concentration increases. Temperature rises as a result of increased water vapour concentration and hence a mechanism for a positive feedback in the greenhouse effect that might lead to a runaway greenhouse effect. When the vapour pressure for water reaches saturation, condensation occurs and water rains out of the atmosphere this is what happens on Earth and Mars. On Venus, however, the water vapour pressure never saturates and no precipitation occurs and the global warming continues to increase. Thus Venus suffers from extreme temperatures produced by both its proximity to the Sun and the presence of water vapour and carbon dioxide in its atmosphere. 

This co-evolution of life, atmospheric carbon dioxide and oxygen levels, and a relatively moderate climate (compared with other planets) make Earth unique. Earth has far less atmospheric carbon dioxide than Mars and Venus, neighboring planets that were formed at about the same time. The atmospheres of both Mars and Venus are made of more than 95% carbon dioxide. On these planets, there are no photosynthesizing life forms to alter the levels of atmospheric carbon dioxide or to produce oxygen. 

Venus atmosphere consists mainly of CO2 of high density. It is perhaps the least well understood atmosphere, because the existing laboratory studies of collision-induced absorption in carbon dioxide and the theoretical analyses attempted have revealed some unexpected complexity. Some of the problems mentioned have to do with the strong ternary components observed furthermore, the pair interaction is strongly anisotropic and the anisotropy has never been accounted for. More work is required for a better understanding (Tipping 1985). 

The recent advances in modem technology continue to open new opportunities for the observation of chemical reactions on shorter and shorter time scales, at higher and higher quantum numbers, in larger and larger molecules, as well as in complex media, in particular, of biological relevance. As an example of open questions, the most rapid reactions of atmospheric molecules like carbon dioxide, ozone, and water, which occur on a time scale of just a few femtoseconds, still remain to be explored. Another example is the photochemistry of the atmospheres of nearby planets like Mars and Venus or of the giant planets and their satellites, which can help us to understand better the climatic evolution of our own planet. 

The atmospheric composition of Venus is similar to that of Mars (see Table VIII—3). Carbon dioxide is the main constituent. The CO mixing ratio is about 5 x 10"5, but the Oz mixing ratio is less than 10 6. Minor constituents that are present in the Venus atmosphere but not in the Martian atmosphere are HC1 and HF in mixing ratios of 6 x 10 7 and 1.5 x 10 9, respectively. 

The photochemical processes of triatomic molecules have been extensively studied in recent years, particularly those of water, carbon dioxide, nitrous oxide, nitrogen dioxide, ozone, and sulfur dioxide, as they are important minor constituents of the earth s atmosphere. (Probably more than 200 papers on ozone photolysis alone have been published in the last decade.) Carbon dioxide is the major component of the Mars and Venus atmospheres. The primary photofragments produced and their subsequent reactions are well understood for the above-mentioned six triatomic molecules as the photodissociation involves only two bonds to be ruptured and two fragments formed in various electronic states. The photochemical processes of these six molecules are discussed in detail in the following sections. They illustrate how the knowledge of primary products and their subsequent reactions have aided in interpreting the results obtained by the traditional end product analysis and quantum yield measurements. 

Oxygen is the most abundant element in the Earth s crust and accounts for 23 % of the mass of the atmosphere. In fact, Earth is the only planet in the solar system with an oxidizing atmosphere. On Mars, oxygen provides only 0.15% of the atmospheric mass and in the atmospheres of the outer planets, oxygen is essentially nonexistent. In the hot atmosphere of Venus, the oxygen has reacted and is present mainly as carbon dioxide. In that form, and as certain other gaseous oxides, it contributes to the warming of the planet (Box 15.1). 

The necessary starting point for any study of the chemistry of a planetary atmosphere is the dissociation of molecules, which results from the absorption of solar ultraviolet radiation. This atmospheric chemistry must take into account not only the general characteristics of the atmosphere (constitution), but also its particular chemical constituents (composition). The absorption of solar radiation can be attributed to carbon dioxide (C02) for Mars and Venus, to molecular oxygen (02) for the Earth, and to methane (CH4) and ammonia (NH3) for Jupiter and the outer planets. 

Jupiter and Uranus are outer planets composed mainly of gases. Jupiter s atmosphere contains reddish-brown clouds of ammonia. Uranus has an atmosphere made up mainly of hydrogen and helium with clouds of water vapor. This combination looks greenish to an outside observer. In addition, Mars has an atmosphere that is 95% carbon dioxide, and Venus has a permanent cloud cover of sulfur dioxide that appears pale yellow to an observer. Mercury has no permanent atmosphere. Saturn has 1 km thick dust and ice rings that orbit the planet. The eight planets in our solar system are diverse, each having different chemical compositions within and surrounding the planets. Out Earth is by far the friendliest planet for human existence. 

Two important applications of radiation to determine molecular structure—X-ray crystallography and magnetic resonance—were discussed in Chapters 3 and 5. In this chapter we will discuss a variety of other techniques. Microwave absorption usually forces molecules to rotate more rapidly, and the frequencies of these absorptions provide a direct measure of bond distances. Individual bonds in a molecule can vibrate, as discussed classically in Chapter 3. Here we will do the quantum description, which explains why the greenhouse effect, which overheats the atmosphere of Venus and may be starting to affect the Earth s climate, is a direct result of infrared radiation inducing vibrations in molecules such as carbon dioxide. 

The properties of supercritical fluids are generally different from those of regular fluids. For example, supercritical water is relatively nonpolar and acidic. Further, the properties of a supercritical fluid, such as its density and viscosity, change with changing pressure and temperature, dramatically as the critical point is approached. Thus, carbon dioxide is not listed in Table 6.1 because it has no liquid phase at terran atmospheric pressure. Carbon dioxide has a critical temperature of 304.2 K and pressure of 73.8 atm, however. It is therefore a supercritical fluid above that pressure, and may even exist as a potential biosolvent for rocky planets having the approximate mass of Earth (or Venus). 

The significance of this particular solubility is spectacularly demonstrated by comparing the earth with its sister planet, Venus. The atmospheric pressure at the surface of Venus is nearly a hundred times greater than at the surface of the earth, and the Cytherean atmosphere itself is more than 96 percent carbon dioxide. The earth s atmosphere would be similar if the oceans had not dissolved the carbon dioxide and precipitated the excess in the form of limestone. One can scarcely begin to imagine the tons of Indiana limestone resting on our shoulders if the earth, like Venus, had no oceans. 

On Venus, however, the two compounds experienced different conditions. Since Venus is closer to the Sun, temperatures are higher than on Earth, preventing the formation of liquid water and allowing the escape of outgassed water vapor into space. In the absence of water vapor, the conversion of carbon dioxide to carbonates is much more difficult, if not impossible. As a result, carbon dioxide outgassed from the planet s surface simply remained in the atmosphere, accumulating to its modern-day very high levels. 

High temperatures trapped a great deal of carbon dioxide in Venus s atmosphere the carbon dioxide, in turn, trapped heat in the atmosphere. As is well known, carbon dioxide is an important greenhouse gas, capable of trapping infrared radiation (heat) released from a planet s surface. The primary difference in atmospheric temperatures between Venus and Earth is the very large amount of heat trapped in Venus s atmosphere by the large concentrations of carbon dioxide. Earth s atmosphere, by contrast, contains relatively small amounts of heat trapped by its correspondingly small concentrations of carbon dioxide. 

Shulze-Makuch and Irwin also suggest that the presence of microbes may explain the unexpectedly low concentration of carbon monoxide in the Venusian atmosphere. One would normally expect a greater abundance of this gas, they say, because lightning and solar radiation tend to break down carbon dioxide into carbon monoxide and other products perhaps microbes have evolved a mechanism for using carbon monoxide as a raw material in their metabolism. Shulze-Makuch and Irwin are hoping that ESA s Venus Express mission will provide additional data that will help resolve the question of whether even the simplest form of life can exist on Venus. 

The Urey reaction is significant on Earth because it is thought to be one way in which carbon dioxide is removed from the atmosphere thus, it may influence climate change. It may be that a similar reaction takes place on Venus, likewise controlling the concentration of carbon dioxide in the Venusian atmosphere. As yet, however, there are no data to suggest that carbonates exist in abundance on the planet s surface. 

This reaction can take place, however, only if the concentration of carbon monoxide is about twice that of carbon dioxide. This condition is clearly not possible in an atmosphere that is nearly pure C02. Thus it seems unlikely or impossible that pure iron exists on Venus s surface. 

The atmosphere of Venus contains carbon dioxide and nitrogen gases. At the planet s surface, the temperature is about 730 K, the total atmospheric pressure is 98 atm, and the partial pressure of carbon dioxide is 94 atm. If scientists wanted to collect 10.0 moles of gas from the surface of Venus, what volume of gas should they collect ... 

As time evolved and the energy provided by the sun increased, the gradual removal of carbon dioxide from the atmosphere became critical to avoid a runaway greenhouse effect (with extremely high surface temperatures) as observed on Venus. This removal of CO2 was accomplished by weathering of calcium silicate (CaSiOs) minerals by acidic C02-rich rainwater, leading to the formation of limestone (CaC03). 

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