7.4 Jupiter’s Atmosphere

Just as Earth has been our yardstick for the terrestrial worlds, Jupiter will be our guide to the outer planets. We therefore begin our study of jovian atmospheres with Jupiter itself.

Overall Appearance and Composition

Visually, Jupiter is dominated by two atmospheric features (both clearly visible in Figure 7.1); a series of ever-changing atmospheric cloud bands arranged parallel to the equator and an oval atmospheric blob called the Great Red Spot. The cloud bands display many colors—pale yellows, light blues, deep browns, drab tans, and vivid reds, among others. The Red Spot is one of many features associated with Jupiter’s weather. This egg-shaped region, the long diameter of which is approximately twice Earth’s diameter, seems to be a hurricane that has persisted for hundreds of years.

The most abundant gas in Jupiter’s atmosphere is molecular hydrogen (roughly 86 percent of all molecules), followed by helium (nearly 14 percent). Small amounts of atmospheric methane, ammonia, and water vapor are also found. None of these gases can, by itself, account for Jupiter’s observed coloration. For example, frozen ammonia and water vapor would produce white clouds, not the many colors we see. Scientists believe that complex chemical processes occurring in Jupiter’s turbulent atmosphere are responsible for these colors, although the details of the chemistry are not fully understood. The trace elements sulfur and phosphorus may play important roles in influencing the cloud colors—particularly the reds, browns, and yellows. The energy that powers the reactions comes in many forms: the planet’s own internal heat, solar ultraviolet radiation, aurorae in the planet’s magnetosphere, and lightning discharges within the clouds.

Figure 7.7 Jupiter’s Convection The colored bands in Jupiter’s atmosphere are associated with vertical convective motion. Upwelling warm gas results in the lighter-colored zones. The darker belts lie atop lower-pressure regions, where cooler gas is sinking back down into the atmosphere. As on Earth, surface winds tend to blow in the direction from high-pressure regions to low-pressure regions. Jupiter’s rotation channels these winds into an east–west flow pattern, as indicated by the three arrows drawn atop the belts and zones.

Astronomers describe the banded structure of Jupiter’s atmosphere as consisting of a series of lighter-colored zones and darker belts crossing the planet. The zones and belts vary in both latitude and intensity during the year, but the general pattern is always present. These variations appear to be the result of convective motion in the planet’s atmosphere. The zones lie above upward-moving convective currents in Jupiter’s atmosphere. The belts are the downward part of the cycle, where material is generally sinking, as illustrated in Figure 7.7.

Because of the upwelling material below them, the zones are regions of high pressure. The belts, conversely, are low-pressure regions. Thus, the belts and zones are the planet’s equivalents of the familiar high- and low-pressure systems that cause weather on Earth. A major difference is that Jupiter’s rapid rotation causes these systems to wrap all the way around the planet, instead of forming localized circulating storms as on our own world. Because of the pressure difference, the zones lie slightly higher in the atmosphere than the belts. Cloud chemistry is very sensitive to temperature, and the temperature difference between the two levels is the main reason for the different colors of the belts and zones.

Underlying the bands is an apparently very stable pattern of eastward and westward wind flow called Jupiter’s zonal flow. The equatorial regions of Jupiter’s atmosphere rotate faster than the planet as a whole, with an average flow speed of about 300 km/h in the easterly direction, somewhat similar to the jet stream on Earth. At higher latitudes, there are alternating regions of westward and eastward flow, the alternations corresponding to the pattern of belts and zones. The flow speed diminishes toward the poles. Near the poles, where the zonal flow disappears, the band structure vanishes also.

Atmospheric Structure

Planetary scientists believe that Jupiter’s clouds are arranged in several layers, with white ammonia clouds generally overlying the colored layers, whose composition we will discuss in a moment. Above the ammonia clouds lies a thin, faint layer of haze created by chemical reactions similar to those that cause smog on Earth. When we observe Jupiter’s colors, we are looking down to many depths in the planet’s atmosphere.

Figure 7.8 Jupiter’s Atmosphere The vertical structure of Jupiter’s atmosphere contains clouds that are arranged in three main layers, each with quite different colors and chemistry. The colors we see in photographs of the planet depend on the cloud cover. The white regions are the tops of the upper ammonia clouds. The yellows, reds, and browns are associated with the second cloud layer, which is composed of ammonium hydrosulfide ice. The lowest (bluish) cloud layer is water ice, however, the overlying layers are sufficiently thick that this level is not seen in visible light.

Figure 7.8 is a diagram of Jupiter’s atmosphere, based on observations and computer models. Because the planet lacks a solid surface to use as a reference level for measuring altitude, scientists conventionally take the top of the troposphere (the turbulent region containing the clouds we see) to lie at 0 km. With this level as the zero point, the troposphere’s colored cloud layers all lie at negative altitudes in the diagram. As on other planets, weather on Jupiter is the result of convection in the troposphere. The haze layer lies at the upper edge of Jupiter’s troposphere. The temperature at this level is about 110 K. Above the troposphere, as on Earth, the temperature rises as the atmosphere absorbs solar ultraviolet light.

Below the haze layer, at an altitude of -30 km, lie white, wispy clouds of ammonia ice. The temperature at this level is 125–150 K. A few tens of kilometers below the ammonia clouds, the temperature is a little warmer—above 200 K—and the clouds are made up mostly of droplets or crystals of ammonium hydrosulfide, produced by reactions between ammonia and hydrogen sulfide. At deeper levels, the ammonium hydrosulfide clouds give way to clouds of water ice or water vapor. The top of this lowest cloud layer, which is not seen in visible-light images of Jupiter, lies some
80 km below the top of the troposphere.

In December 1995, the Galileo atmospheric probe arrived at Jupiter. The probe survived for about an hour before being crushed by atmospheric pressure at an altitude of -150 km (that is, just at the bottom of Figure 7.8). While initially the data from the probe appeared not to support the picture just presented, most of the discrepancies were the result of improperly calibrated instruments. In addition, the probe’s entry location (Figure 7.9) was in Jupiter’s equatorial zone and, as luck would have it, coincided with an atypical hole almost devoid of upper-level clouds, and with abnormally low water content. Galileo’s revised findings on wind speed, temperature, and composition were in good agreement with the description presented above.

Figure 7.9 Galileo’s Entry Site The arrow on this Hubble Space Telescope image shows where the Galileo atmospheric probe plunged into Jupiter’s cloud deck on December 7, 1995. The entry location was in Jupiter’s equatorial zone and apparently almost devoid of upper-level clouds. Until its demise, the probe took numerous meteorological measurements, transmitting those signals to the orbiting mother ship, which then relayed them to Earth. (NASA)

Weather on Jupiter

In addition to the large-scale zonal flow, Jupiter has many smaller-scale weather patterns. The Great Red Spot (Figure 7.10) is a prime example. Observational records indicate that it has existed continuously in one form or another for more than 300 years, and it may well be much older. Voyager observations showed the Spot to be a region of swirling, circulating winds, like a whirlpool or a terrestrial hurricane, a persistent and vast atmospheric storm. The size of the Spot varies, its length averaging about 25,000 km. It rotates around Jupiter at a rate similar to that of the planet’s interior, suggesting that its roots lie far below the atmosphere.

Astronomers think that the Spot is somehow sustained by Jupiter’s large-scale atmospheric motion. The zonal motion north of the Spot is westward, while that to the south is eastward (Figure 7.10), supporting the idea that the Spot is confined and powered by the zonal flow. Turbulent eddies form and drift away from the spot’s edge. The center, however, remains tranquil in appearance, like the eye of a hurricane on Earth.

The Voyager mission discovered many smaller spots that are also apparently circulating storm systems. Examples of these smaller systems are the white ovals seen in many images of Jupiter (see, for example, the one just south of the Great Red Spot in Figure 7.10). Their high cloud tops give these regions their white color. The white oval in Figure 7.10 is known to be at least 40 years old.

Figure 7.11 shows a brown oval—a hole in the overlying clouds that allows us to look down into the lower atmosphere. For unknown reasons, brown ovals appear only in latitudes around 20° north. Although not as long-lived as the Red Spot, they can persist for many years.

Although we cannot explain their formation, we can at least offer a partial explanation for the longevity of storm systems on Jupiter. On Earth a large storm, such as a hurricane, forms over the ocean and may survive for many days, but it dies quickly once it encounters land because the land disrupts the energy supplied and flow patterns that sustain the storm. Jupiter has no continents, so once a storm is established and has reached a size at which other storm systems cannot destroy it, apparently little affects it. The larger the system, the longer its lifetime.

Concept Check

List some similarities and differences between Jupiter’s belts and zones and weather systems on Earth.