Jupiter’s face is pockmarked by oval-shaped disturbances. The most promi- nent of these is known as the Great Red Spot and has been watched by astronomers for centuries. It spins outward, which happens to be counter- clockwise because the spot is in the southern hemisphere of Jupiter. (If it were in the northern hemisphere, it would spin clockwise).
Until the Great Red Spot was observed by space probes close up, astronomers were not sure what it was. Some people thought the spot was a solid object, floating on a liquid sea and poking up through the clouds.
Others thought it was caused by a volcano beneath the clouds; this theory was lent support because of the reddish hue. (Astronomers are still not cer- tain why the spot is red.) It has been known to fade to dusky gray from time to time, and once in a while it seems almost to vanish, although the irregu- larities in the adjacent cloud bands betray that it is still there. It always returns to its full red glory sooner or later. The Voyagerphotographs con- vinced almost everyone that the Great Red Spot is a revolving high-pres- sure weather system. It is not alone. Smaller systems dot the face of Jupiter.
How can a weather disturbance stay active for so long? To answer this, we have only to look at our own Earth. The Azores-Bermuda high, which dominates the weather over the North Atlantic Ocean, is present almost all the time. It is not visually obvious, as is Jupiter’s spot, but the high is no less permanent. In fact, the Azores-Bermuda high, which carries tropical storms into the Caribbean in summer and temperate storms into Europe in autumn, has been around longer than the spot. There are other semiperma- nent systems on Earth too. A low-pressure center exists in the far North Pacific, just south of the Aleutian Islands. It grows powerful every fall and winter, hurling storm after storm at the North American coastline.
Sometimes it seems almost to disappear, although motions of nearby clouds and the jet streams betray that it is still there. It always swells to full force once the brief Arctic summer ends.
Certain climate phenomena persist because they feed on heat from the Sun (and in the case of Jupiter, from inside the planet as well); once they get going and get large enough, they form positive-feedback systems.
Unless some tremendous outside force intervenes, such systems just keep on swirling around. If there were no land masses on Earth, hurricanes might persist for years, decades, or even centuries, traveling around and around the planet, because there would be nothing to break them up.
Saturn
Saturn is one of the most familiar planets because of its appearance. It is surrounded by a system of rings that can be seen though a small amateur telescope. It is the only planet with rings substantial enough to be seen eas-
ily. However, this planet is unique in other ways. It is the least dense of the planets. Its specific gravity(density relative to the density of water) is only 0.7. This means that Saturn is only 70 percent as massive as it would be if it were made entirely of water. The planet would float if it could be placed in a large enough, deep enough lake. It has the greatest amount of oblate- ness of any planet. The axis of Saturn’s magnetic field corresponds almost precisely with its rotational axis, a fact that has befuddled scientists in their attempts to explain the dynamics of planetary magnetism.
In mythology, Saturn is the Roman god of agriculture. The name also refers to the father of the Greek god Zeus. Because Zeus and Jupiter are the same entity, Saturn might well be attached in mythology with an impor- tance equal to or greater than that of Jupiter. Jupiter is Saturn’s mythical son; without Saturn, Jupiter would never have been born, or would have turned out much different. (Of course, this is only according to the ancient myths; we know better than to believe that those tales are true.)
Before telescopes revealed the ring system, the name Saturnwas asso- ciated with old age and dullness. If it were not for the rings, Saturn would indeed be a somewhat less interesting version of Jupiter, at least from an observational point of view.
THE YEAR AND THE DAY
Saturn orbits the Sun at a distance of 9.54 AU (Fig. 7-4). It orbital radius is about 1,430 million kilometers (888 million miles). The best viewing of Saturn is done when the planet is at opposition. Saturn is almost twice as far away from the Sun as is Jupiter, and the ringed planet receives only 1.1 percent as much sunlight per unit area as Earth. Saturn reflects sunlight well, and this is enhanced by the ring system. Saturn looks similar to Jupiter with the unaided eye but is somewhat dimmer, comparing favorably with Mars most of the time. Saturn is easy to relocate once you have found it on any given night.
Saturn, like Jupiter, does not pass through phases; it always appears full or almost full. Its brilliance in the sky, as we see it, changes because its dis- tance from us varies. In general, the greater the angle between Saturn and the Sun, the brighter Saturn appears as seen from Earth. The brilliance of Saturn is also affected by the angle at which the rings are presented to us. If the rings are edge-on, the planet looks dimmer at a given distance from us than if the rings are seen from above or below. Saturn takes 291Ⲑ2Earth years to make a complete revolution around the Sun with respect to the distant
stars. Thus Saturn reaches an opposition approximately once every 121Ⲑ2 Earth months.
Saturn, like Jupiter, rotates rapidly on its axis. The complete day, midnight to midnight, lasts for about 10 hours and 40 minutes Earth time, as determined by observations of the magnetic field. The planet’s upper clouds rotate slightly faster than this at latitudes near the equator. Near the poles, the atmosphere appears to rotate at about the same speed as the planet’s magnetic field.