Photochemistry

Abstract

The first published account of the importance of photochemistry in the Jovian atmosphere is that of Wildt (1937), who surmised the possibilities for the recycling of CH₄ and NH₃ following their photodecomposition. Numerous minor constituents are produced in a planetary atmosphere, predominantly by photochemical processes, subsequent to the absorption of solar ultraviolet photons by various atmospheric molecules. The knowledge of the distribution of minor constituents can be important in answering many key questions concerning atmospheric transport, meteorology, atmospheric stability, physics of the interior, and atmospheric coupling to the exterior. On all the major planets, CH₄ undergoes photochemical transformation. On Uranus and Neptune, it is expected to form a cloud as well, whereas its products, C₂H₂, C₂H₆, and polyacetylenes, are likely to condense in the form of tropospheric/stratospheric hazes. On Jupiter and Saturn, NH₃ forms a visible cloud. Its saturated vapor abundance above the clouds, however, is large enough for it to be photolyzed as well. For all practical purposes, the NH₃ photochemistry is both spectroscopically and physically separated from the CH₄ photochemistry. This is because CH₄ is photolyzed below 1600 Å (mostly by the Lyman-α photons which have a large flux), and NH₃ between 1600 Å and 2300 Å. Furthermore, gaseous NH₃ abundance on Jupiter and Saturn is negligible above atmospheric pressure levels of∼0.1 bar.