ORIGIN, STRUCTURE, COMPOSITION, AND WEATHER Like the other planets, Jupiter probably began to form when dust and ice particles
in a primeval nebula around the early Sun condensed and coalesced due to collisions. Jupiter was in a perfect location for growing large: far enough from the Sun that solids could condense, and close enough that there was plenty of material to accrete. Once Jupiter's "embryo" (including rocky material now in its core, several times as massive as the Earth) became large enough, its gravity pulled together a surrounding envelope of gas. Although Jupiter was not large enough to begin nuclear burning, the compression of its own gravity generated a tremendous amount of heat when the planet formed. Even now, 4.6 billion years later, Jupiter radiates at least 60% more energy than it receives from the Sun. Jupiter is primarily composed of
hydrogen and helium, in similar proportion to their abundance in the Sun and the solar nebula. There is no solid surface under the
atmosphere, only a gradual transition to liquid. About one-fourth of the way in, pressures and temperatures are so high (two million times the pressure at the Earth's surface and 10,000¡ C) that the liquid hydrogen becomes metallicÑthat is, the molecules are stripped of their outer electrons. Astronomical observations have suggested a slight depletion in atmospheric helium, compared with solar abundance, which was attributed to precipitation of helium in the metallic hydrogen, raining down toward the core. However, measurements made by the Galileo space probe within Jupiter found no depletion of the hydrogen, at least at its entry location. Jupiter's atmosphere also contains trace amounts of water, ammonia, methane, and other carbon compounds. Astronomers theorize that three cloud layers exist, each separated vertically by about 30 km (19 mi). The lowest are made of water ice or droplets, the next are crystals of an ammonia/hydrogen sulfide compound, and the highest are ammonia ice. Of the observed clouds the blue ones are warmest and therefore lowest. Browns, whites, and reds lie sequentially higher. These shades are believed to be caused by chemical disequilibrium, which allows sulfur, phosphorus, and organic compounds to color the clouds. This disequilibrium may be due to impact by charged particles, rapid vertical motion through changing temperature levels, or lightning. The winds on Jupiter move in jets parallel to the equator. The speedsÑboth eastward and westward, and varying with latitudeÑare tens to a hundred meters per second relative to the rotating interior. The latitudes of the zonal jets correlate well with positions of broad, alternating bands of whitish and orange to brown clouds. The color differences may be due to gas rising in some bands and descending in others. Eddies and storms form and dissipate, some lasting a few days and others much longer. Some get caught between regions of different wind speeds and are sheared apart. Enduring eddies, such as larger white spots and the Earth-size Great Red Spot, survive by rolling like ball bearings between zones. The two Voyager spacecraft that flew past Jupiter in 1979 observed lightning and auroras there. Further information was provided in July 1994, when the fragmented comet or asteroid Shoemaker-Levy plowed into the planet and produced collisions observed by Earth telescopes. In December 1995 a probe from the spacecraft Galileo sent back data for 57 minutes as it plunged into the planet. It returned complete data as it descended to a depth of 200 km (125 mi), then fell silent at a depth of 600 km (373 mi). For the next few years the Galileo orbiter continued to monitor Jupiter and its moons with an array of instruments, including a high-resolution camera. The orbiter discovered strong, localized, convective storms. Lightning hundreds of times stronger than on Earth is so common and frequent that it appears in snapshots of the night side of the planet. A wide range in waterpor abundance was observed in the upper atmosphere. The probe found little water, indicating that it entered at an unusually dry site. Similarly, the probe entry site was quite clear of clouds, although clouds are known to dominate the appearance of the planet. The probe did detect the ammonia cloud layer at some distance from the entry site. Orbiter images confirm that clouds are variable and patchy in places, so that the probe entered by chance at an unusually clear spot. The internal energy of Jupiter plays an important role in Jupiter's atmosphere. The Galileo probe discovered wind speeds of hundreds of kilometers per hour well below the cloud levels, where little solar energy is deposited. The energy source must be the residual internal heat of Jupiter. Moreover, the probe found higher temperatures than expected in the upper atmosphere, another indication of the importance of internal heat, which must be brought up by the active convection.