Venus atmosphere

Scientists believe that the sulfur in Venus atmosphere came from volcanic eruptions. Earth has experienced its fair share of volcanic eruptions, too. However, the sulfur from early eruptions on Earth was incorporated into solid sulfur compounds. Indeed, sulfur is an important element found in many of the compounds that make up Earths crust. 

The sulfur dioxide in Venus atmosphere is turned into sulfuric acid by two different chemical reactions. In the first reaction, the sulfur dioxide reacts with oxygen to form sulfur trioxide ... 

The oxygen that reacts with the sulfur dioxide comes from water (H20) that is also present in Venus atmosphere. When the sun s high-energy ultraviolet (UV) rays hit a water molecule, it dissociates (breaks down) into hydrogen and oxygen—the elements that make up water. 

Almost 80% of the sunlight that hits Venus is reflected back into space by the thick clouds surrounding the planet before it ever reaches the surface. Even so, temperatures at the surface of Venus are much hotter than those on Earth. However, this is not because Venus is closer to the Sun than the Earth. Scientists believe that the difference in the temperatures of the two planets is due to a runaway greenhouse effect caused by the large amount of sulfur dioxide in Venus atmosphere. 

This is the same reaction that occurs in Venus atmosphere. Here on Earth, however, this reaction can be slow. To speed it up, the reaction is catalyzed. Remember, a catalyst is a chemical that speeds up a chemical reaction without taking part in the reaction itself. When chemicals are mixed together, they will only react with one another if their particles (atoms and molecules) collide. If the reactants never come into contact, a chemical reaction will not take place. 

Hoffman JH, Hodges RR, McElroy MB, Donahue TM, Kolpin M (1979) Composition and structure of the Venus atmosphere results from Pioneer Venus. Science 205 49-52 Hoffman PE, Kaufman AJ, Halverson GP, Schrag DP (1998) Neoproterozoic snowball earth. Science 281 1342-1346... 

Several Soviet Venera and Vega spacecraft landed on the surface of Venus in the early 1980s, and survived for a few minutes before succumbing to the stifling heat. X-ray fluorescence chemical analyses for a number of major elements in surface samples were reported. Chemical and isotopic analyses of the Venus atmosphere were made by Pioneer Venus, Venera, and other orbiters. 

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). 

VIII-2.1 The Earth s Atmosphere, 330 VIII-2.2 Atmospheric Air Pollution, 332 VIII-2.3 Photochemical Air Pollution in the Troposphere, 332 VIFI-2.4 Air Pollution in the Stratosphere, 340 VI11-3 Photochemistry of the Atmospheres of Other Planets, 352 VH1—3.1 Photochemistry of the Mars Atmosphere, 352 VII 1—3,2 Photochemistry of the Venus Atmosphere. 356 VIIl-3.3 Photochemistry of the Jovian Atmosphere, 357... 

Hydroxyl radicals are produced by the photolysis of HzO, which is present to an extent of 0.2% in the Mars and Venus atmosphere. Besides water, HC1, a minor constituent (6 x 10-7 mixing ratio) in the Venus atmosphere, may provide additional H atoms [McElroy et al. (678)]. 

The temperature profile of the Venus atmosphere is shown in Fig. V111— 13. The surface temperature and pressure have recently been determined by space probes to be 747 + 20°K and 88 15 bars, respectively. 

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. 

Because of the very low concentrations of 02 observed in the Venus alnn, phere, the proposed CIO cycle may be important in the Venus atmosplu n However, very little information is available on the rate constants involvin( CIO and the CIO cycle remains a hypothesis. Abundances of H202, m.i 03 in the Venus atmosphere must be exceedingly small. McElroy ct, il (678) considered the photolysis of HC1 as another source of H atoms m addition to the photolysis of H20... 

The photochemistry of the Mars and Venus atmospheres may be Ixih, understood if the minor constituents, such as H202, 03, and H2, can I.. measured and if the rate constants involving H02 and ClOO radicals >.m be measured more accurately in the laboratory. McElroy et al. (678) bclm, that CO and H20 are converted to C02 and H2 in the hot region neai tin surface... 

Kolodner, M.A., and Steffes, P.G. 1998. The microwave absorption and abundance of sulfuric acid vapor in the Venus atmosphere based on new laboratory measurements. Icarus 132 151-169. 

One important point should be emphasized here. This is the paucity of spacecraft data on the chemical composition and thermal structure of Venus lower atmosphere below —22 km altitude (von Zahn et al., 1983). About 80% of Venus atmospheric mass is below this altitude. Furthermore, altitudes of 0-12 km span the region where the atmosphere is interacting with the surface. However, with three exceptions we have no data on the chemical composition of Venus nearsurface atmosphere. First is the older measurements of CO2 and N2 from crude chemical experiments on the Venera 4-6 landers. Second, the water-vapor profile measured by the Pioneer Venus large probe neutral mass spectrometer. Third, the measurements of water-vapor and gaseous sulfur by spectrophotometer experiments on the Venera II-I4 landers. The gas chromatograph and mass spectrometer experiments on... 

The chemical composition of Venus atmosphere is described below. This discussion is based on sources listed in Table 3, Fegley and Treiman (1992), and Warneck (1988). 

As shown in Table 3, Venus atmosphere is dominantly CO2 (96.5%) and N2 (3.5%), with smaller amounts of SO2, H2O, CO, OCS, HCl, HF, the noble gases, and reactive species such as SO that are produced photochemically. The abundances of CO2, N2, the noble gases, HCl, and HF are apparently constant throughout most of Venus atmosphere, but other gases such as SO2,... 

Carbon monoxide is the second most abundant carbon-bearing gas in Venus atmosphere. The CO abundance in Venus lower atmosphere is altitude dependent and decreases toward the surface as follows 45 lOppmv (—64 km), 30 18 ppmv (42 km), 20 3 ppmv (22 km), and 17 1 ppmv (12 km). This gradient is consistent with photochemical production of CO from CO2 in Venus upper atmosphere and CO consumption by thermochemical reactions with sulfur gases in Venus lower atmosphere and with minerals at its surface. Carbon monoxide is also photo-oxidized back to CO2 via catalytic cycles that are described in Section 1.19.3.3. 

Volcanic outgassing is plausibly the major source of HCl and HF in Venus atmosphere. Thermochemical equilibrium calculations suggest that formation of chlorine- and fluorine-bearing minerals are important sinks for these two gases. Observations by Connes et al. (1967) and Bezard et al. (1990) give the same HCl and... 

Venus atmosphere is so dry that Earth-based and spacecraft measurements of the water-vapor abundance are extremely difficult. Historically, many of the in situ water-vapor measurements gave values much higher than the actual water-vapor content. However, reliable values are now available from several sources including the Pioneer Venus mass spectrometer, spectrophotometer experiments on Venera 11-14, Earth-based FTIR spectroscopy of Venus lower atmosphere on the nightside, and IR observations during the Galileo and Cassini flybys of Venus. 

Venus upper atmosphere is even drier than the lower atmosphere, and the average water-vapor mixing ratio above the clouds is only a few ppmv. The very low H2O mixing ratios were hard to explain until it was realized that Venus clouds are 75% sulfuric acid, which is a powerful drying agent. When dissolved in the acid, most of the water reacts with H2SO4 to form hydronium (HaO ) and bisulfate (HSO4) ions. As a result, the concentrations of free H2O in the acid solution and in the vapor over the acid are extremely low. The partial pressure of water at Venus cloud tops is lower than that over water ice at the same temperature. Thus, the clouds are responsible for the extreme dryness of Venus upper atmosphere, and play an important role in the photochemical stability of Venus atmosphere (see Section 1.19.3.3). 

Table 4 summarizes the data on the isotopic composition of Venus atmosphere. Aside from the noble gases, the most important difference between Venus and Earth is the high D/H ratio, which is —150 times greater than the D/H ratio of 1.558 X 10 in standard mean ocean water (SMOW). The high D/H ratio strongly suggests. 

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