The most stable equilibrium chemical form that different elements exist in depends upon the temperature and on the abundance of other molecules and atoms with which reactions may occur. Similarly, the equilibrium ortho:para ratio of hydrogen is a function of temperature. The rate at which equilibrium is reached depends upon the temperature itself, density, and sometimes the presence of catalyzing aerosols. Hence, in rapidly overturning atmospheres the composition of air which has been rapidly uplifted from warmer, denser levels may be partially "quenched" at its deeper equilibrium composition. Measurement of the abundances of so-called "disequilibrium" species thus provides information on the upwelling rates, and hence on vertical circulation, and is a very important diagnostic tool in understanding the circulation of the giant planet atmospheres.
The molecular forms of carbon and nitrogen are good examples of disequilibrium species in giant planet atmospheres. The chemical form of nitrogen and carbon that is observed depends upon the following equilibrium reactions:
Above about 1,000 K, the right-hand side of these reactions dominates, whereas at lower temperatures the left-hand side dominates. Hence, in the observable, cool parts of the giant planet atmospheres we do not expect to see much CO or N2 unless vertical transport is particularly vigorous. This conclusion seems to be true for all of the giant planets with the notable exception of Neptune, where there is some evidence that small quantities of both molecules are present in the observable atmosphere, suggesting rapid upwelling, which is consistent with Neptune's observed high internal heat source.
Carbon dioxide may also appear in disequilibrium in the upper troposphere and stratosphere through the reaction although carbon dioxide has not been detected in the lower tropospheres of any of the giant planets.
Germane, arsine, and phosphine
The abundances of phosphine, arsine, and germane in the atmospheres of Jupiter and Saturn are higher than would be expected if the atmospheres were in chemical equilibrium. These gases are produced by equilibrium chemistry at p ~ 1 kbar, T ~ 1,000 K, and convert slowly at the cold temperatures found in the upper parts of these atmospheres to the chemical forms given below, with a timescale of the order of 100 days. The chemistry here is highly complicated and there are lots of intermediate steps. The reader is referred to, for example, Atreya (1986) for more
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