Atmosphere of Saturn

Studies of the reactions of many atmospherically important atomic and free radical species were described in Section 9 this Section deals primarily with important molecular species. A brief review of the progress achieved recently in the field of atmospheric chemistry has been provided by Cox, " with emphasis on the reactions of O3 and important H-, N-, C-, halogen-, and S-containing species. Waynehas reviewed extraterrestrial atmospheric photochemistry and Strobel " has reviewed the photochemistries of the atmospheres of Jupiter, Saturn, and Titan. Kaye and Strobeldescribed a 1-dimensional photochemical model of PHj chemistry in the atmosphere of Saturn. A study of the photochemical reactions of H2O and CO in the Earth s primitive atmosphere has been presented by Bar-Nun and Chang. " They concluded that even if the primitive atmosphere initially contained no H2 and contained carbon only in the form of CO and CO2, photochemical processes would have enriched the environment with a variety of organic compounds. 

The N( D) + H2 reaction has been less studied than 0( D) + H2. However atomic nitrogen is of fundainental int( r( st in combustion and astrophysical and atmospheric chemistry. For instance, reactions involving this species with simple hydrocarbons play a role in the atmosphere of Saturn s moon Titan. The N( D) - -H2 reaction is perhaj)s a better prototype of an insertion reaction than 0( D)- -H2. Here, there is no abstraction mechanism due to an excited PES [34]. 

In general, methane and ethane will produce temperature inversions in the upper atmosphere of an outer-solar-system body if they exist in a region v/hero the pressure is low enough that the far-infrared pressure-induced opacity is low, but where their density is sufficiently high to absorb sunlight in near-infrared bands and produce sufficient opacity to emit thermally in the middle-infrared bands. The variety of properties exhibited by the upper atmospheres of Saturn, Uranus, and Heptune indicates radical differences in the atmospheric circulation of these bodies, rio satisfactory explanation of these phenomena has been advanced. 

Samuelson, R. E. (1985). Clouds and aerosols of Titan s atmosphere. In The Atmospheres of Saturn and Titan, pp. 99-107. European Space Agency SP-241. 

The water clouds are found deepest. Generally the cloud layers in the atmosphere of Saturn are deeper than in Jupiter s atmosphere because of the lower temperature on Saturn. 

At the bottom of its atmosphere at a temperature of -23°C there is a layer of water ice clouds (they extend for about 10 km). The ammonium hydrosulfide clouds extend for about 50 km and the temperature drops to -93°C. Ammonia ice clouds extend from about 80 km to regions where the temperature is - 153°C. Near the top of Saturn s atmosphere between 200 km to 270 km the atmosphere consists mainly of hydrogen and helium. The strongest known wind systems occur in the atmosphere of Saturn (up to 500 m/s). 

Chemical constraints on the water abundance in the deep atmosphere of Saturn were given by Visscher and Channon, 2005 [346]. Saturn is known to develop the largest scale convective storms in the Solar System. Water storms that also occur in the atmosphere of Jupiter may develop velocities up to 150 m/s (Hueso and Sanchez-Lavega, 2004 [167]). 

Visscher, C., Fegley, B.J. Chemical constraints on the water and total oxygen abundances in the deep atmosphere of Saturn. Astrophys. J. 623, 1221-1227 (2005)... 

Diacetylene (HC=C—C=CH) has been identified as a component of the hydrocarbon rich atmospheres of Uranus Neptune and Pluto It is also present m the atmospheres of Titan and Triton satellites of Saturn and Neptune respectively... 

Water can be found, in all three aggregate states, almost everywhere in the universe as ice in the liquid phase on the satellites of the outer solar system, including Saturn s rings and in the gaseous state in the atmospheres of Venus, Mars and Jupiter and in comets (as can be shown, for example, from the IR spectra of Halley s comet). The OH radical has been known for many years as the photodissociation product of water. 

Methane is a major component in the atmospheres of Jupiter ( KM), Saturn ( ... 

The next most likely possibility is cometary delivery of the atmosphere but again there are some problems with the isotope ratios, this time with D/H. The cometary D/H ratios measured in methane from Halley are 31 3 x 10-5 and 29 10 x 10-5 in Hayuatake and 33 8 x 10-5 in Hale-Bopp, whereas methane measurements from Earth of the Titan atmosphere suggest a methane D/H ratio of 10 5 x 10-5, which is considerably smaller than the ratio in the comets. The methane at least in Titan s atmosphere is not exclusively from cometary sources. Degassing of the rocks from which Titan was formed could be a useful source of methane, especially as the subnebula temperature around Saturn (100 K) is somewhat cooler than that around Jupiter. This would allow volatiles to be more easily trapped on Titan and contribute to the formation of a denser atmosphere. This mechanism would, however, apply to all of Saturn s moons equally and this is not the case. 

Alkanes are often found in natural systems. They are the main constituents in the atmospheres of the planets Jupiter, Saturn, Uranus, and Neptune. Methane is also thought to have been a major component of the atmosphere of the early Earth. Natural gas and oil are primarily made of alkanes. 

Aspects of the chemical composition of the atmospheres of Jupiter, Saturn, Uranus, and Neptune were measured by the Voyager and Galileo spacecraft in the 1980s and 1990s,... 

Hydrogen isotopic compositions, expressed as molar D/H ratios, of solar system bodies. The relatively low D/H values in the atmospheres of Jupiter and Saturn are similar to those in the early Sun, whereas D/H ratios for Uranus and Neptune are intermediate between the Jupiter-Saturn values and those of comets and chondrites. The Earth s oceans have D/H shown by the horizontal line. Mars values are from SNC meteorites. Modified from Righter et al. (2006) and Lunine (2004). 

Trafton has shown in 1964 that the opacity in the far infrared of the atmospheres of the outer planets is due to the rototranslational band of H2-H2 and H2-He pairs [393], It is now clear that collision-induced absorption plays a major role in the thermal balance and atmospheric structure of the major planets. The Voyager emission spectra of Jupiter and Saturn show dark fringes in the vicinity of the So(0) and So(l) lines of H2, Fig. 7.3, which are due to collision-induced absorption in the upper,... 

The Earth s atmosphere is composed primarily of non-polar molecules like N2 and O2, especially at greater altitudes where the H2O concentrations are small. One would therefore expect collision-induced contributions to the absorption of the Earth s atmosphere from N2-N2, N2-O2 and O2-O2 pairs. The induced rototranslational absorption of nitrogen has not been detected in the Earth s atmosphere, presumably because of strong interference by water absorption bands, but absorption in the various induced vibrational bands is well established (Tipping 1985). Titan (the large moon of Saturn) has a nitrogen atmosphere, somewhat like the Earth methane is also present. Collision-induced absorption by N2-N2 and N2-CH4 is important in the far infrared. 

A. R. W. McKellar. Experimental verification of hydrogen dimers in the atmospheres of Jupiter and Saturn from Voyager IRIS far-infrared spectra. Astrophys. J., 326 L75, 1988. 

Several applications of IR spectroscopy to astrophysics have been made. Small amounts of methane in the earth s atmosphere have been detected by the observation of weak IR absorption lines in solar radiation that has passed through the earth s atmosphere. Intense IR absorption bands of CH4 have been found in the spectra of the atmospheres of Jupiter, Saturn, Uranus, and Neptune. Bands of ammonia have been observed for Jupiter and Saturn bands of C02 have been observed in the Venusian spectrum and bands of H20 have been observed in the Martian spectrum. 

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