Jovian atmosphere and clouds. The most fundamental aspect in understanding a planet's atmosphere is the vertical temperature structure. Chemical constituents of Jupiter's and Saturn's atmospheres include primarily hydrogen, presumably helium, some methane, and some ammonia. In composition these atmospheres more closely resemble that of the sun than those of the terrestrial planets. Traces of other molecular compounds have also been identified in Jupiter. Some of these compounds may be solids in the colder atmosphere of Satu rn, and that is why they have not been identified. Low-lying clouds of waterice crystals, mid-level clouds of ammonium hydrosulfide, and high-altitude clouds of ammonia are the most conspicuous physical feature of the atmospheres of Jupiter and Saturn. Large-scale motions in the atmospheres of the two planets are responsible for light- and dark-colored bands of clouds, vigorous eastward and westward winds, turbulent motions, and the formation of circulation cells. Especially notable is the Great Red Spot in the atmosphere of Jupiter.
Little is known about the atmospheres of Uranus and Neptune, which contain comparatively more oxygen, nitrogen, carbon, silicon, and iron than do Jupiter and Saturn. Although similar to each other in some respects, there are differences with respect to each other and to Jupiter and Saturn. Neptune's atmosphere possesses a variable haze that is apparently not present in that of Uranus.
Internal structure of the Jovian planets. Because of the greater mass of the Jovian planets and a composition dominated by hydrogen, helium, and other light elements, the internal structure of these planets differs from that of the terrestrial planets. Arou nd possibly a small, dense core of rocky and icy materials, Jupiter and Saturn have metallic-liquid and molecularliquid layers for an internal structure overlain by extensive gaseous layers that are the atmosphere. The internal structures of Uranus and Neptune are different from those of Jupiter and Saturn because of their smaller masses and different compositions. Jupiter, Saturn, and Neptune radiate more energy than they receive from the sun. The source of energy is probably the slow contraction of these planets.
Magnetospheres about the Jovian planets. Jupiter is the strongest emitter of radio radiation in the solar system with the exception of the sun. This is the result of the magnetosphere, which is quite extensive. Saturn's magnetic field also defines a magnetospheric zone in which it can control the motion of subatomic particles. Whether or not magnetospheres exist about Uranus and Neptune will not be known with certainty until Voyager 2 reaches them in 1986 and 1989.
Ring systems of the Jovian planets. Ring systems are formed of countless individual particles in orbit about the planet and shaped by interactions of the particles with each other and by the gravitational influence of satellites. Saturn's ring system is more extensive than that of Jupiter or Uranus. Rings of Saturn are composed primarily of water-ice or icecoated rocky particles. The three major rings are composed of thousands of narrow ringlets. Saturn's ring system is far more complex than had been anticipated prior to Voyager. The ring systems of Jupiter and Neptune are fainter than Saturn's and are probably composed of silicates or carbon-rich material. Their structure appears to be quite different from that of Saturn.
Satellites of Jupiter and Saturn. Compared to the terrestrial planets, Jupiter and Saturn have many satellites; Jupiter has at least 16 and Saturn, 17. Jupiter's Galilean satellites resemble in many respects the terrestrial planets and have apparently been shaped b similar forces. Active volcanoes have been found on 10, although they do not much resemble those on earth. 10 and Europa, with size, density, and mas comparable to that of the moon, are probably a rock\ silicate-rich material similar in structure to that of the moon. While Ganymede and Callisto are as large as Mercury, they have a low mean density and are probably composed of a mixtu re of rocky and icy materials. Saturn's satellite system contains the only satellite with an atmosphere: Titan is a rocky and icy mixture with an atmosphere dominated by nitrogen. Saturn also has four interesting, intermediate-sized satellites:
Rhea, Iapetus, Dione, and Tethys.
Comets. Comets have four principal parts: nucleus, coma (head), hydrogen cloud, and tail. The two types of tail point away from the sun. ComeK,ere an icy conglomerate of frozen water, methane, ammonia, carbon dioxide, and some small particulate matter and dust grains.
