Giant planets atmospheres

Isotopic abundances for hydrogen have been measured in giant planet atmospheres, as shown in Figure 14.11. The D/H ratios in Jupiter and Saturn are similar to those in the Sun, but lower than those in the Earth s oceans or in comets. D/H ratios in Uranus and Neptune... 

Chemical equilibrium abundances for many C-, N-, and 0-bearing gases for T = 500-2500K and P = 10" -1000 bars for a solar composition gas can be found in 54). Although these calculations were applied to brown dwarfs and giant planet atmospheres, the pressure conditions also include those appropriate for O-rich photospheres and their inner CSEs (10" - 10" bars). 

The very stable molecules CO (observed) and N2 (not observable) are the major C-, 0-, and N-bearing gases throughout the entire CSE, as expected from thermochemical equilibrium. Under the low total pressures in the CSE, conversions of CO to CH4 or N2 to NH3 as the major C- or N-bearing gases does not occur. Even if pressure conditions were favorable, these reactions would not reach equilibrium because they are kinetically inhibited (these conversions are quenched even in the much denser giant planet atmospheres (e.g., 54). This does not mean that CH4 or NH3 should be absent from the CSE it only means that their abundances are likely less than that expected from thermochemical equilibrium. In 0-rich CSE, most oxygen is evenly distributed between CO and H2O, but CO2, produced by the rapid water gas reaction (CO + H2O = H2 + CO2) is also an abundant gas (54) and has been observed. 

The second most abundant constituent of the atmospheres of the giant planets, helium, does not react chemically and carmot condense even at the cold tropopauses of Uranus and Neptime. For a long time helium was expected to be uniformly mixed in all giant planet atmospheres. Smoluchowski (1967), Salpeter (1973), Hubbard ... 

Brown, T.M. Transmission spectra as diagnostics of extrasolar giant planet atmospheres. Astrophys. J. 553, 1006-1026 (2001)... 

The giant planets possess low surface temperatures and have atmospheres that extend several thousand miles. The markings on Jupiter, the largest planet, consist of cloud formations composed of methane containing a small amount of ammonia. The atmosphere of Jupiter absorbs the extreme red and infrared portions of the spectrum. These absorptions correspond to the absorption spectra of ammonia and methane, suggesting the presence of these gases in Jupiter s... 

In contrast to the terrestrial planets, the giant planets are massive enough to have captured and retained nebular gases directly. However, concentrations of argon, krypton, and xenon measured in Jupiter s atmosphere by the Galileo spacecraft are 2.5 times solar, which may imply that its atmosphere preferentially lost hydrogen and helium over the age of the solar system. 

Estimated compositions ofthe giant planets are given in Table 14.3, normalized to the solar composition. The relative proportions of rock and volatiles are estimated from mean densities, the rock compositions are assumed to be chondritic, and the ratios of hydrogen to helium are derived from spectroscopic or spacecraft measurements of atmosphere compositions. 

Table 14.3 Abundances of major elements and molecules in the atmospheres of the giant planets, relative to solar abundances (Lunine, )...

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. 

West, R.A. 1999. Atmospheres of the giant planets. Encyclopedia of the Solar System. Academic Press, New York. 

Dust affects the structure of early planetary atmospheres and is probably an important factor controlling the rate at which giant planets grow. Long after the planets are fully formed, dust continues to be produced by collisions between asteroids in regions where giant-planet perturbations prevented the growth of planets. 

What is the relationship between the atmospheric circulation patterns and the deep circulation Fluid planets are in many ways more complex than the solid planets, in that the atmospheric circulation patterns have some relationship—greater or lesser—to deep circulations. How the coupling occurs in the giant planets of our solar system, and the strength of the coupling, remain unresolved. 

Hubbard W. B. (1989) Structure and composition of giant planet interiors. In Origin and Evolution of Planetary and Satellite Atmospheres (eds. S. K. Atreya, J. B. Pollack, and M. S. Matthews). University of Arizona Press, Tucson, pp. 539—563. 

Models of planetary evolution assume that at the time of planetary formation the solar system had a single universal and well-mixed composition from which aU parts of the solar system were derived (see Podosek, 1978). Information as to the elemental and isotopic characteristics of this primordial composition is presently available from the Sun, meteorites, and the atmospheres of the giant planets (Wider, 2002). In the case of the Sun, distinction is usually made between the present-day composition, which is available via spectral analysis of the solar atmosphere and capture of the solar wind, either directly in space or by using metallic foU targets, and the proto-Sun (the composition at the time of planetary accretion) whereby the lunar regolith and/or meteorites are utilized as archives of ancient solar wind. As discussed below, the distinction is only really important for helium due to production of He by deuterium burning. 

A problem with adopting the solar wind He/" He ratio as representative of the solar nebula is the production of He from deuterium very early in solar system history consequently, the solar wind value (—4.4 X 10 ) is too high by a factor between —2.5 and —3 relative to the proto-Sun (Geiss and Reeves, 1972). To circumvent this difficulty, recourse has been made to analyzing the giant planets whose atmospheres are expected to reflect proto-solar values (Wieler, 2002). Jupiter is the only giant planet whose atmospheric He/" He ratio has been determined (Mahaffy et al., 1998). 

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