Sulfur dioxide, Venus’ atmosphere

An element is a substance that cannot be broken down into simpler substances by ordinary chemical means. A chemical compound is a substance made up of two or more elements that have been chemically bonded together. Scientists believe that solid sulfur compounds do not exist on Venus like they do on Earth because, at about 900° Fahrenheit (480° Celsius), the surface temperature on Venus is too hot for them to form in the first place. This temperature is well above the melting point of sulfur (235°F [ 113°C]). Therefore, instead of being incorporated into rocks, the sulfur on Venus continues to float around in the atmosphere in the form of the chemical compound sulfur dioxide (S02). 

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. 

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. 

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. 

One of the most intriguing hits of data about vulcanism on Venus has been reported by Larry Esposito, at the University of Colorado s Laboratory for Atmospheric and Space Physics. Using data obtained from the Hubble Space Telescope, Esposito found that the abundance of sulfur dioxide at the top of the Venusian cloud layer in 1995 was about 20 times less than it had been when measured during the 1978 visit by the Pioneer Venus Orbiter spacecraft. He compared these results with some of the earliest measurements of sulfur dioxide made on the planet dating to the early 1970s. At that point, the abundance... 

Contrast the ozone cycle with the atmospheric cycles that predominate on other planets Venus has a sulfuric acid cycle from its interior sulfur dioxide volcanoes, while Mars has a peroxide cycle from solar radiation shearing the oxygens off carbon dioxide. The Martian atmosphere has a little water in it, and even a trace of ozone built from these oxygen atoms, but its reactions are uncontrolled and out of kilter, making dangerous peroxide H Oj molecules. Their chemical cycles are destructive, but ours is protective. 

Sulfuric acid is produced in the upper atmosphere of Venus by the Sun s photochemical action on carbon dioxide, sulfur dioxide, and water vapor. Ultraviolet photons of wavelengths less than 169 nm can photodissociate carbon dioxide into carbon monoxide and atomic oxygen. Atomic oxygen is highly reactive. When it reacts with sulfur dioxide, a trace component of the Venusian atmosphere, the result is sulfur trioxide, which can combine with water vapor, another trace component of Venus s atmosphere, to yield sulfuric acid. In the upper, cooler portions of Venus s atmosphere, sulfuric acid exists as a liquid, and thick sulfuric acid clouds completely obscure the planet s surface when viewed from above. The main cloud layer extends from 45-70 km above the planet s surface, with thinner hazes extending as low as 30 km and as high as 90 km above the surface. The permanent Venusian clouds produce a concentrated acid rain, as the clouds in the atmosphere of Earth produce water rain. 

Venus has the densest atmosphere of all the terrestrial planets. The principal atmospheric constituents are carbon dioxide and nitrogen (N2) their mixing ratios are approximately 96.5% and 3.5%, respectively, below, 100-km altitude. Trace constituents below 100-km altitude are in the range of 0.1%. The Venus atmosphere is covered with thick clouds composed primarily of sulfuric acid and contaminants, making the surface invisible from above. The total pressure at the bottom of the cloud layer ( 47 km) is approximately 1.3 atm. Water is highly depleted throughout the atmosphere. The mean physical structure of the atmosphere (pressure and temperature profile) is reasonably well known from the data returned by a number of space probes. The surface pressure and temperature (on a mean surface) are approximately 94 atm and 737 K, respectively. 

Signatures of water vapor are scattered throughout the terrestrial spectrum shown in Fig. 6.2.1 below 500 cm and between 1300 cm and 2000 cm the atmosphere is nearly opaque at low latitudes. Water vapor features also appear in the spectra of Mars and Venus, but are reduced in strength. The Venus spectrum also shows the broad absorption features of liquid sulfuric acid (H2SO4) clouds near 900 cm (Fig. 6.2.2) and possibly of sulfur dioxide (SO2) gas near 1360 cm (Fig. 6.2.3). In the Earth spectrum the strong vs-band of ozone (O3) at 1042 cm and the V4-band of methane (CH4) at 1306 cm can easily be identified. 

Venus -- has a slow rotation when compared to Earth. Venus and Uranus rotate in opposite directions from the other planets. This opposite rotation is called retrograde rotation. The surface of Venus is not visible due to the extensive cloud cover. The atmosphere is composed mostly of carbon dioxide. Sulfuric acid droplet in the dense cloud cover gives Venus a yellow appearance. Venus has a greater greenhouse effect than observed on Earth. The dense clouds Combined with carbon dioxide traps heat. Venus was named after the Roman goddess of love. 

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