Jupiter and Saturn Atmospheres
The alternating light and dark cloud bands that parallel the equator of Jupiter and Saturn are constantly undergoing changes in color and intensity on a small scale within the bands. Apparently this is because of the formation or dissolution of clouds of differing chemical compositions at different altitudes and latitudes. There are large-scale patterns, such as the bands themselves and the Great Red Spot on Jupiter, that last for years and sometimes centuries. This complex behavior involves the dynamics of the atmosphere for both planets.
The dominant observable motions in the atmospheres are alternating eastward (direction of rotation) and westward winds that correlate with the colored bands. As shown in Figure 9.4, Jupiter has five or six eastward- and westward-moving wind streams in each hemisphere, while Saturn has fewer but stronger wind streams. Those winds are measured relative to each planet's rotation. In the case of the earth there is only one low-latitude westward wind stream, known as the "trade winds," and one mid latitude eastward-moving
jet stream. Jupiter and Saturn also have some vertical streaming.
Evidence suggests that these eastward- and westward-moving winds have been constant in latitude and velocity for the last 80 or 90 years. However, the cloud bands with which they correlate are changing, with small eddies between the wind streams sheared apart in 1 or 2 days. eddy currents are deviations in what are otherwise alternating streams flowing east or west in the atmosphere. Where the steady winds have velocities up to 100 meters per second or so, the eddy velocities are a few tens of meters per second.
The cloud motions on a small scale are by no means orderly. Voyager scientists were unprepared for the diversity and sometimes high state of turbulence in the cloud motions as photographed by the spacecraft. Surprisingly, photographs failed to reveal cloud features smaller than about 100 kilometers across. Narrow bands appear to coalesce and widen, while wide bands break apart. Material seems to be transferring between bands.
Conspicuous in the southern hemisphere of Jupiter is the oval Great Red Spot, which has varied both in size and intensity since its telescopic discovery three centuries ago. It measures about 14,000 by 40,000 kilometers. The sense of circulation of gas in the spot and other ovals is almost always counterclockwise in the southern hemisphere and clockwise in the northern. This indicates that they are highpressure cells.
Great Red Spot and circulate around it in a week or so i the interior is relatively calm by comparison. In addition to the Great Red Spot, smaller white and darkcolored ovals are also apparently circulation cells.
Saturn also has circulation cells in its atmosphere.
It is not known whether the eddies and ovals on both Jupiter and Saturn extend as deep into their respective planets as do the wind streams. The long-term persistance of the winds and the short life for eddies and ovals are possibly related to the mass of material involved in the phenomena. Thus the winds probably extend deep into the planet, while the shorter-lived eddies are relatively shallow structures. However, this is still quite speculative.
The winds on the earth draw their energy from unequal heating by the sun between the equator and the poles, and in general the temperature decreases poleward at almost all levels of the atmosphere. On Jupiter the temperature difference between equator and pole is no more than 3 K, while for the much shorter distance of the earth it is more like 30 K. Even though the sun heats the equatorial region of Jupiter and Saturn more than it does the polar regions, just as it does for the earth, some mechanism must transport heat from the interior of the planet into the polar regions, reducing the temperature difference.
