Saturn is much less colorful than Jupiter. Figure 12.2 shows yellowish and tan cloud belts that parallel the equator, but these regions display less atmospheric structure than do the belts on Jupiter. No obvious large "spots" or "ovals" adorn Saturn's cloud decks. Bands and storms do exist, but the color changes that distinguish them on Jupiter are largely absent on Saturn.
Figure 12.2 Saturn as seen by the Hubble Space Telescope in December 1994. At the time, a rare storm was visible near the planet's equator. The bland colors are approximately truethat is, as the human eye sees things. The insert shows the northern polar region at an earlier time.
Astronomers first observed methane in the spectrum of sunlight reflected from Saturn in the 1930s, about the same time that it was discovered on Jupiter. However, it was not until the early 1960s, when more sensitive observations became possible, that ammonia was finally detected. In Saturn's cold upper atmosphere, most ammonia is in the solid or liquid form, with relatively little of it present as a gas to absorb sunlight and create spectral lines. Astronomers finally made the first accurate determinations of the hydrogen and helium content in the late 1960s. These Earth-based measurements were later confirmed with the arrival of the Pioneer and Voyager spacecraft in the 1970s.
Saturn's atmosphere consists of molecular hydrogen (92.4 percent), helium (7.4 percent), methane (0.2 percent), and ammonia (0.02 percent). As on Jupiter, hydrogen and helium dominatethese most abundant elements never escaped from Saturn's atmosphere because of the planet's large mass and low temperature (see More Precisely 8-1). However, the fraction of helium on Saturn is far less than is observed on Jupiter (where, as we saw, helium accounts for nearly 14 percent of the atmosphere) or in the Sun. It is extremely unlikely that the processes that created the outer planets preferentially stripped Saturn of nearly half its helium or that the missing helium somehow escaped from the planet while the lighter hydrogen remained behind. Instead, astronomers believe that at some time in Saturn's past the heavier helium began to sink toward the center of the planet, reducing its abundance in the outer layers and leaving them relatively hydrogen-rich. We will return to the reasons for this differentiation and its consequences in a moment.
Figure 12.3 illustrates Saturn's atmospheric structure (compare with the corresponding diagram for Jupiter, Figure 11.6). In many respects, Saturn's atmosphere is quite similar to Jupiter's, except that the temperature is a little lower because of its greater distance from the Sun and because its clouds are somewhat thicker. Since Saturn, like Jupiter, lacks a solid surface, we take the top of the troposphere as our reference level and set it to 0 km. The top of the visible clouds lies about 50 km below this level. As on Jupiter, the clouds are arranged in three distinct layers, composed (in order of increasing depth) of ammonia, ammonium hydrosulfide, and water ice. Above the clouds lies a layer of haze formed by the action of sunlight on Saturn's upper atmosphere.
Figure 12.3 The vertical structure of Saturn's atmosphere. As with Jupiter, there are several cloud layers, but Saturn's weaker gravity results in thicker clouds and a more uniform appearance.
The total thickness of the three cloud layers in Saturn's atmosphere is roughly 200 km, compared with about 80 km on Jupiter, and each layer is itself somewhat thicker than its counterpart on Jupiter. The reason for this difference is Saturn's weaker gravity. At the haze level, Jupiter's gravitational field is nearly two and a half times stronger than Saturn's, so Jupiter's atmosphere is pulled much more powerfully toward the center of the planet. Thus Jupiter's atmosphere is compressed more than Saturn's, and the clouds are squeezed more closely together. The colors of Saturn's cloud layers, as well as the planet's overall butterscotch hue, are due to the same basic cloud chemistry as on Jupiter. However, because Saturn's clouds are thicker, there are few holes and gaps in the top layer, so we rarely glimpse the more colorful levels below. Instead, we see only different levels in the topmost layer, which accounts for Saturn's rather uniform appearance.
Saturn has atmospheric wind patterns that are in many ways reminiscent of those on Jupiter. There is an overall eastwest zonal flow, which is apparently quite stable. Computer-enhanced images of the planet that bring out more cloud contrast (see Figure 12.4) clearly show the existence of bands, oval storm systems, and turbulent flow patterns looking very much like those seen on Jupiter. Scientists believe that Saturn's bands and storms have essentially the same cause as does Jupiter's weather. Ultimately, the large-scale flows and small-scale storm systems are powered by convective motion in Saturn's interior and the planet's rapid rotation.
Figure 12.4 We see more structure in Saturn's cloud cover when computer processing and artificial color are used to enhance the image contrast, as in these Voyager 1 images of the entire gas ball and of a smaller, magnified piece of it.
The zonal flow on Saturn is considerably faster than on Jupiter and shows fewer eastwest alternations, as can be seen from Figure 12.5 (compare with Figure 11.5). The equatorial eastward jet stream, which reaches a speed of about 400 km/h on Jupiter, moves at a brisk 1500 km/h on Saturn, and extends to much higher latitudes. Not until latitudes 40° north and south of the equator are the first westward flows found. Latitude 40° north also marks the strongest bands on Saturn and the most obvious ovals and turbulent eddies. Astronomers still do not fully understand the reasons for the differences between Jupiter's and Saturn's flow patterns.
Figure 12.5 Winds on Saturn reach speeds even greater than those on Jupiter. As on Jupiter, the visible bands appear to be associated with variations in wind speed.
In September 1990, amateur astronomers detected a large white spot in Saturn's southern hemisphere, just below the equator. In November of that year, when the Hubble Space Telescope imaged the phenomenon in more detail, the spot had developed into a band of clouds completely encircling the planet's equator. Some of these images are shown in Figure 12.6. Astronomers believe that the white coloration arose from crystals of ammonia ice formed when an upwelling plume of warm gas penetrated the cool upper cloud layers. Because the crystals were freshly formed, they had not yet been affected by the chemical reactions that color the planet's other clouds.
Figure 12.6 Circulating and evolving cloud systems on Saturn, imaged by the Hubble Space Telescope.
Such spots are relatively rare on Saturn. The previous one visible from Earth appeared in 1933, but it was much smaller than the 1990 system and much shorter lived, lasting for only a few weeks. Another storm system is visible in Figure 12.2, taken in 1994. The turbulent flow patterns seen around the 1990 white spot have many similarities to the flow around Jupiter's Great Red Spot. Scientists speculate that these white spots represent long-lived weather systems on Saturn and hope that routine observations of such temporary atmospheric phenomena on the outer worlds will enable them to gain greater insight into the dynamics of planetary atmospheres.