Jupiter is visually dominated by two features: one is a series of ever-changing atmospheric bands arranged parallel to the equator; the other is an oval atmospheric blob called the Great Red Spot, or often just the "Red Spot." The cloud bands, clearly visible in Figure 11.1, display many colors—pale yellows, light blues, deep browns, drab tans, and vivid reds among others. Shown in more detail in Figure 11.3, a close-up photograph taken as Voyager 1 sped past in 1979, the Red Spot is the largest of many features associated with Jupiter's weather. It seems to be an Earth-sized hurricane that has persisted for hundreds of years.
Figure 11.3 Voyager 1 took this photograph of Jupiter's Red Spot (upper right) from a distance of about 100,000 km. The resolution is about 100 km. Note the complex turbulence patterns to the left of both the Red Spot and the smaller white oval vortex below it. (For scale, planet Earth is about the size of the white oval.)
Spectroscopic studies of sunlight reflected from Jupiter gave astronomers their first look at the planet's atmospheric composition. Radio, infrared, and ultraviolet observations later provided more details. The most abundant gas is molecular hydrogen (86.1 percent by number), followed by helium (13.8 percent). Together these two gases make up over 99 percent of Jupiter's atmosphere. Small amounts of atmospheric methane, ammonia, and water vapor are also found.
Unlike the gravitational pull of the terrestrial planets, the gravity of the much more massive jovian planets is strong enough to have retained even hydrogen. More Precisely 8-1 Little, if any, of Jupiter's original atmosphere has escaped.
Researchers generally accept that hydrogen and helium also make up the bulk of the planet's interior. This belief is based not on direct evidence of the interior—until the collision of a comet with Jupiter, described in Section 14.2, there was virtually none—but largely on theoretical studies of the internal structure of the planet, such as we have already seen in the discussions of Jupiter's density and shape.
Astronomers generally describe Jupiter's banded appearance—and, to a lesser extent, the appearance of the other jovian worlds as well—as a series of bright zones and dark belts crossing the planet. The zones and belts vary in both latitude and intensity during the year, but the general pattern remains. These variations are not seasonal in nature—Jupiter has no seasons—but instead appear to be the result of dynamic motion in the planet's atmosphere. The light-colored zones lie above upward-moving convective currents in Jupiter's atmosphere. The dark belts are regions representing the other part of the convection cycle, where material is generally sinking downward, as illustrated schematically in Figure 11.4.
Figure 11.4 The colored bands in Jupiter's atmosphere are associated with vertical convective motion. Upwelling warm gas results in the lighter-colored zones; the darker bands lie atop lower-pressure regions where cooler gas is sinking back down into the atmosphere. As on Earth, surface winds tend to blow from high- to low-pressure regions. Jupiter's rotation channels these winds into an east—west flow pattern, as indicated.
Because of the upwelling material below them, the zones are regions of high pressure; the belts, conversely, are low-pressure regions. Thus, belts and zones are Jupiter's equivalents of the familiar high- and low-pressure systems that cause our weather on Earth. However, a major difference between Jupiter and Earth is that Jupiter's rapid rotation has caused 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 between the two, the zones lie slightly higher in the atmosphere than the belts. The associated temperature differences (recall that temperature increases with depth) and the resulting differences in chemical reactions are the basic reasons for their different colors.
Underlying the bands is an apparently very stable pattern of eastward and westward wind flow, referred to as Jupiter's zonal flow. This zonal flow is evident in Figure 11.5, which shows the wind speed at different planetary latitudes measured relative to the rotation of the planet's interior (determined from studies of Jupiter's magnetic field). As mentioned earlier, the equatorial regions of the atmosphere rotate faster than the planet, with an average flow speed of some 85 m/s, or about 300 km/h, in the easterly direction. The speed of this equatorial flow is quite similar to that of the jet stream on Earth. At higher latitudes, there are alternating regions of westward and eastward flow, roughly symmetric about the equator, with the flow speed generally diminishing toward the poles.
Figure 11.5 The wind speed in Jupiter's atmosphere, measured relative to the planet's internal rotation rate. The alternations in wind direction are associated with the atmospheric band structure.
As Figure 11.5 shows, the belts and zones are closely related to Jupiter's zonal flow pattern. However, closer inspection shows that the simplified picture presented in Figure 11.4, with wind direction alternating between adjacent bands as Jupiter's rotation deflects surface winds into eastward or westward streams, is really too crude to describe the actual flow. Scientists now believe that the interaction between convective motion in Jupiter's atmosphere and the planet's rapid rotation channels the largest eddies into the observed zonal pattern, but that smaller eddies tend to cause irregularities in the flow. Near the poles, where the zonal flow disappears, the band structure vanishes also.
None of the atmospheric gases listed earlier can, by itself, account for Jupiter's observed coloration. For example, frozen ammonia and water vapor would simply produce white clouds, not the many colors actually seen. Scientists believe that the cloud colors are the result of complex chemical processes occurring in the planet's turbulent upper atmosphere, although the details are still not fully understood. When we observe Jupiter's colors, we are actually looking down to many different depths in the planet's atmosphere.
Based on the best available data and mathematical models, Figure 11.6 is a cross-sectional diagram of Jupiter's atmosphere. Since the planet lacks a solid surface to use as a reference level for measuring altitude, the top of the troposphere is conventionally taken to lie at 0 km. As on all planets, weather on Jupiter is the result of convection in the troposphere, so the clouds, which are associated with planetary weather systems, all lie at negative altitudes in the diagram. Just above the troposphere lies a thin, faint layer of haze created by photochemical reactions (that is, reactions involving sunlight) similar to those that cause smog on Earth. The temperature at this level is about 110 K; it increases with altitude as the atmosphere absorbs solar ultraviolet radiation.
Figure 11.6 The vertical structure of Jupiter's atmosphere. Jupiter's clouds 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 cloud layer is water ice and bluish in color. However, the overlying layers are sufficiently thick that this level is not seen in visible light.
Jupiter's clouds are arranged in three main layers. Below the haze, at a depth of about 40 km (shown as —40 km in Figure 11.6), lies a layer of white wispy clouds made up of ammonia ice. The temperature here is approximately 125—150 K; it increases quite rapidly with increasing depth. A few tens of kilometers below the ammonia clouds, the temperature is a little warmer—over 200 K—and the clouds are probably made up mostly of droplets or crystals of ammonium hydrosulfide, produced by reactions between ammonia and hydrogen sulfide in the planet's atmosphere. At deeper levels in the atmosphere, the ammonium hydrosulfide clouds give way to clouds of water ice or water vapor. This lowest cloud layer, which is not seen in visible-light images of Jupiter, lies some 80 km below the top of the troposphere.
Instead of being white (the color of ammonium hydrosulfide on Earth), Jupiter's middle cloud layer is tawny in color. This is the level at which atmospheric chemistry begins to play a role in determining Jupiter's appearance. Many planetary scientists believe that molecules containing the element sulfur, and perhaps even sulfur itself, are important in influencing the cloud colors—particularly the reds, browns, and yellows, all colors associated with sulfur or sulfur compounds. It is also possible that compounds containing the element phosphorus contribute to the coloration.
Deciphering the detailed causes of Jupiter's distinctive colors is a difficult task, however. The cloud chemistry is complex and very sensitive to small changes in atmospheric conditions, such as pressure and temperature, as well as to chemical composition. The atmosphere is in incessant, churning motion, causing these conditions to change from place to place and from hour to hour. In addition, the energy that powers the reactions comes in many different forms: the planet's own interior heat, solar ultraviolet radiation, aurorae in the planet's magnetosphere, and lightning discharges within the clouds themselves. All these factors combine to keep a complete explanation of Jupiter's appearance beyond our present grasp.
This description of Jupiter's atmosphere, which was based largely on Voyager data, was put to the test in December 1995 when the Galileo atmospheric probe arrived at the planet. The probe survived for about an hour before being crushed by atmospheric pressure at an altitude of —150 km (that is, right at the bottom of Figure 11.6). Initially, the data from the probe appeared to contradict many of the facts we have just reported! Preliminary analysis of the data indicated that Jupiter was much windier, hotter, and drier than expected, and severely depleted in helium. Furthermore, no clear evidence of the three-layered cloud structure depicted in Figure 11.6 was found.
Most of these discrepancies are now known to have been the result of improperly calibrated instruments; Galileo's revised findings on wind speed, temperature, and composition are in good agreement with the picture just presented. In addition, the probe's entry location was in Jupiter's equatorial zone and, as luck would have it, coincided with an atypical hole almost devoid of upper-level clouds (see Figure 11.7). The warm temperature readings—425 K at 150 km depth, a little higher than indicated by Figure 11.6—are consistent with the probe's having entered a relative clearing in Jupiter's cloud decks, where convective heat can more readily rise (and thus be detected). The low water content may also be normal for the hot, windy regions near Jupiter's equator.
Figure 11.7 The arrow on this Hubble 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 was apparently almost devoid of upper-level clouds. Until its demise, the probe took numerous meteorological measurements, transmitting those signals to the mother ship overhead, which then relayed them to Earth.
Most experts were somewhat surprised by the depth to which Jupiter's winds continued; Galileo's probe measured high wind speeds throughout its descent into the clouds and not just at the cloud tops, implying that heat deep within the planet, rather than sunlight, drives Jupiter's weather patterns. Finally, complex organic molecules were sought but not found. Some simple carbon-based molecules, such as ethane (C2H6), were detected by one of the onboard spectrometers, but nothing suggesting prebiotic compounds or bacteria floating in the atmosphere. That same instrument also detected traces of phosphine (PH3), which may be a key coloring agent for Jupiter's clouds.
In addition to the zonal flow pattern, Jupiter has many "small-scale" weather patterns. The Great Red Spot (Figure 11.3) is a prime example. The Great Red Spot was first reported by the British scientist Robert Hooke in the mid-seventeenth century, and we can be reasonably sure that it has existed continuously, in one form or another, for over 300 years. It may well be much older. Voyager observations showed the spot to be a region of swirling, circulating winds, rather like a whirlpool or a terrestrial hurricane—a persistent and vast atmospheric storm. The size of the Spot varies, although it averages about twice the diameter of Earth. Its present dimensions are roughly 25,000 km by 15,000 km. It rotates around Jupiter at a rate similar to that of the planet's interior, perhaps suggesting that its roots lie far below the atmosphere.
The origin of the Spot's red color is unknown, as is its source of energy, although it is generally supposed that the Spot is somehow sustained by Jupiter's large-scale atmospheric motion. Repeated observations show that the gas flow around the Spot is counterclockwise, with a period of about 6 days. Turbulent eddies form and drift away from its edge. The Spot's center, however, remains quite tranquil in appearance, like the eye of a hurricane on Earth. The zonal motion north of the Spot is westward, whereas that to the south is eastward (see Figure 11.8), supporting the idea that the Spot is confined and powered by the zonal flow. However, the details of how this occurs are still a matter of conjecture. Computer simulations of the complex fluid dynamics of Jupiter's atmosphere are only now beginning to hint at answers.
Figure 11.8 These Voyager 2 close-up views of the Red Spot, taken 4 hours apart, show clearly the turbulent flow around its edges. The general direction of motion of the gas north of (above) the Spot is westward (to the left), whereas gas south of the Spot flows east. The Spot itself rotates counterclockwise, suggesting that it is being "rolled" between the two oppositely directed flows. The colors have been exaggerated somewhat to enhance the contrast.
Storms, which as a rule are much smaller than the Red Spot, may be quite common on Jupiter. Spacecraft photographs of the dark side of the planet reveal both auroral activity and bright flashes resembling lightning. The Voyager mission discovered many smaller light- and dark-colored spots that are also apparently circulating storm systems. Note the several white ovals in Figures 11.3 and 11.8, south of the Red Spot. Like the Red Spot, they rotate counterclockwise. Their high cloud tops give them their color. These particular white ovals are known to be at least 40 years old. Figure 11.9 shows a brown oval, a "hole" in the clouds that allows us to look down into the lower atmosphere. For unknown reasons, brown ovals appear only in latitudes around 20° N. Although not as long-lived as the Red Spot, these systems can persist for many years or even decades.
Figure 11.9 A brown oval in Jupiter's northern hemisphere. Its color comes from the fact that it is actually a break in the upper cloud layer, allowing us to see deeper into the atmosphere. The oval's length is approximately equal to the diameter of Earth.
We cannot explain their formation, but we can offer at least 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. Earth's continental landmasses disrupt the flow patterns that sustain the storm. Jupiter has no continents, so once a storm becomes established and reaches a size at which other storm systems cannot destroy it, apparently little affects it. The larger the system, the longer its lifetime.
