Atmospheres of Jupiter and Saturn
There are many aspects of a planet's atmosphere that astronomers want to know about, such as chemical composition, temperature, density, cloud composition, winds, and how these change with height, position over the surface, and time. Such detail is not now available for the earth's atmosphere much less for the atmospheres of the other planets. Probably the most fundamental aspect when trying to understand a planet's atmosphere is the vertical temperature structure for jupiter and Saturn. On the way up through jupiter's and Saturn's tropospheres the measured temperature profile first declines and then increases into the stratosphere, where photons from the sun can be directly absorbed.
The first constituents of Jupiter's atmosphere to be identified were methane and ammonia in the 1930s. Some 30 years later the most abundant element, hydrogen, was identified and estimated to be 1000 times more prevalent than methane and ammonia. From these identifications estimates for the hydrogen, carbon, nitrogen, and oxygen abundances indicate that jupiter's chemical composition (and similarly for Saturn) is more like that of the sun rather than like that of the terrestrial planets. In the 1970s and 80s, primarily through infrared observations, several additional molecules were found to be minute constituents of Jupiter's atmosphere. Many of these molecules are probably also present in Saturn's atmosphere, but Saturn is colder than Jupiter, so that some of these compounds probably freeze, forming solid crystals; thus they are not in a gaseous state capable of being observed spectroscopically.
Helium, the second most abundant element in the composition of the sun and presumably in Jupiter and Saturn, is not directly observable by spectroscopic means. From data from the Pioneer and Voyager missions indirect determinations of the helium abundance have been made. The values derived, 10 percent for Jupiter and 6 percent for Saturn, are consistent with the solar-composition hypothesis.
The most conspicuous aspect of Jupiter's and Saturn's atmospheres in visible light is their clouds. Knowing something about the vertical temperature profile and the chemical composition of the atmosphere has given astronomers clues to the basic constituents forming the clouds. For Jupiter and Saturn there appear to be three distinct cloud layers, as we have tried to show in Figure 9.3. The lowest is formed from water-ice crystals or possibly liquid drops, the next from ammonium hydrosulfide crystals (NH4SH), and the highest from ammonia crystals (NH3). The middle one can also be thought of as a compound of the more elementary molecules ammonia and hydrogen sulfide. All the molecules forming the basic cloud particles should lead to white particles; so other molecules are responsible for coloring the clouds, which are red, yellow, brown, blue, and white. The most likely coioring agent is the element sulfur, which forms a variety of colored particles depending upon its molecular structure. This has not been confirmed.
Infrared images of jupiter and Saturn show that the
cloud color correlates with altitude. Seen from the outside the blue clouds lie at the deepest levels in the atmosphere and are visible only through holes in the upper clouds. Brown clouds are the next highest, above which lie the white clouds; and finally the red clouds are the top layer. Compared to jupiter the greater spread in altitude for the clouds i Saturn's atmosphere results from the smaller mass of Saturn, whose gravity is not so effective in compressing the atmosphere as is the more massive jupiter.
