Spectroscopic studies of sunlight reflected from Uranus's and Neptune's dense clouds indicate that the two planets' outer atmospheres (the parts we actually measure spectroscopically) are similar to those of Jupiter and Saturn. The most abundant element is molecular hydrogen (84 percent), followed by helium (about 14 percent) and methane, which is more abundant on Neptune (about 3 percent) than on Uranus (2 percent). Ammonia, which plays such an important role in the Jupiter and Saturn systems, is not present in any significant quantity in the outermost jovian worlds.
The abundances of gaseous ammonia and methane vary systematically among the jovian planets. Jupiter has much more gaseous ammonia than methane, but moving outward from the Sun, we find that the more distant planets have steadily decreasing amounts of ammonia and relatively greater amounts of methane. The reason for this variation is temperature. Ammonia gas freezes into ammonia ice crystals at about 70 K. This is cooler than the cloud-top temperatures of Jupiter and Saturn but warmer than those of Uranus (58 K) and Neptune (59 K). Thus, the outermost jovian planets have little or no gaseous ammonia in their atmospheres, so their spectra (which record atmospheric gases only) show just traces of ammonia.
The increasing amounts of methane are largely responsible for the outer jovian planets' blue coloration. Methane absorbs long-wavelength red light quite efficiently, so sunlight reflected from the planets' atmospheres is deficient in red and yellow photons and appears blue-green or blue. As the concentration of methane increases, the reflected light should appear bluer. This is just the trend observed: Uranus, with less methane, looks blue-green, while Neptune, with more, looks distinctly blue.
Voyager 2 detected only a few features in Uranus's atmosphere, and even these became visible only after extensive computer enhancement (see Figure 13.7). Uranus apparently lacks any significant internal heat source, and because of the planet's low surface temperature, its clouds are found only at low-lying, warmer levels in the atmosphere. The absence of high-level clouds means that we must look deep into the planet's atmosphere to see any structure, so the bands and spots that characterize flow patterns on the other jovian worlds are largely "washed out" on Uranus by intervening stratospheric haze.
Figure 13.7 (a) This Voyager view of Uranus approximates the planet's true color but shows little else. Parts (b), (c), and (d) are Hubble Space Telescope photographs made at roughly 4-hour intervals, showing the motion of a pair of bright clouds (labeled A and B) in the planet's southern hemisphere. (The numbers at the top give the time of each photo.)
From computer-processed images astronomers have learned that Uranus's atmospheric clouds and flow patterns move around the planet in the same sense as the planet's rotation, with wind speeds in the range 200—500; km/h. Tracking these clouds allowed the measurement of the differential rotation mentioned earlier. Despite the odd angle at which sunlight is currently striking the surface (recall that it is just after midsummer in the northern hemisphere), the planet's rapid rotation still channels the wind flow into bands reminiscent of those found on Jupiter and Saturn. Even though the predominant wind flow is in the east—west direction, Uranus's atmosphere seems to be quite efficient at transporting energy from the heated north to the unheated southern hemisphere. Although the south is currently in total darkness, the temperature there is only a few kelvins less than in the north.
Neptune's clouds and band structure are much more easily seen. Although it lies at a greater distance from the Sun, Neptune's upper atmosphere is actually slightly warmer than that of Uranus. Like Jupiter and Saturn, but unlike Uranus, Neptune has an internal energy source—in fact, it radiates 2.7 times more heat than it receives from the Sun. The cause of this heating is still uncertain. Some scientists have suggested that Neptune's excess methane has helped "insulate" the planet, tending to maintain its initially high internal temperature. If that is so, then the source of Neptune's internal heat is the same as Jupiter's—it is energy left over from the planet's formation. 
(Sec. 11.3) The combination of extra heat and less haze may be responsible for the greater visibility of Neptune's atmospheric features (see Figure 13.8), as its cloud layers lie at higher levels in the atmosphere than do those of Uranus.
Figure 13.8 (a) Close-up views, taken by Voyager 2 in 1989, of the Great Dark Spot of Neptune, a large storm system in the planet's atmosphere, possibly similar in structure to Jupiter's Great Red Spot. Resolution in the photo on the right is about 50 km; the entire dark spot is roughly the size of planet Earth. (b) These three Hubble Space Telescope views of Neptune were taken about 10 days apart in 1994, when the planet was some 4.5 billion km from Earth. The aqua color is caused by absorption of red light by methane; the cloud features (mostly methane ice crystals) are tinted pink here because they were imaged in the infrared but are really white in visible light. Neptune apparently has a remarkably dynamic atmosphere that changes over just a few days. Notice that the Great Dark Spot has now disappeared.
Neptune sports several storm systems similar in appearance to those seen on Jupiter (and assumed to be produced and sustained by the same basic processes). The largest such storm, known simply as the Great Dark Spot, is shown in Figure 13.8(a). Discovered by Voyager 2 in 1989, the Spot was about the size of Earth, was located near the planet's equator, and exhibited many of the same general characteristics as the Great Red Spot on Jupiter. The flow around it was counterclockwise, as with the Red Spot, and there appeared to be turbulence where the winds associated with the Great Dark Spot interacted with the zonal flow to its north and south. The flow around this and other dark spots may drive updrafts to high altitudes, where methane crystallizes out of the atmosphere to form the high-lying cirrus clouds. Astronomers did not have long to study the Dark Spot's properties, however. As shown in Figure 13.8(b), when the Hubble Space Telescope viewed Neptune in 1994, the Spot had disappeared.
