Introduction to the Outer Planets—Jupiter
Beyond Mars lies a vast gap in the Solar System that is occupied only by a swarm of relatively small rocks. These rocks, more appropriately called planetoids or asteroids , will be discussed later in this book. For now, we will turn our attention to the five known outer planets: Jupiter, Saturn, Uranus, Neptune , and Pluto .
Jupiter is the Roman name for the Greek god Zeus , who was, according to legend, the most powerful of all the gods. If size and mass translate into power, then Jupiter is the most powerful of the planets.
Jupiter has power in tangible ways. Its gravitational pull is 2.5 times as strong as that of Earth. You would have to be in good physical condition to stand up for long on Jupiter without fainting, if it had a surface and if you could get down to it. However, you would never make it down. The atmosphere would blow your landing vessel out of control and ultimately crush it, but you would be dead or near death from radiation sickness before then. The barrage of high-speed subatomic particles commanded by Jupiter’s immense magnetic field would be lethal to astronauts who ventured near the giant planet.
The Year And The Day
Jupiter orbits the Sun at a distance of 5.2 astronomical units (AU). This means that its orbital radius is 520 percent that of Earth (Fig. 7-1). This is about 778 million kilometers (484 million miles). The best viewing of Jupiter is done when the planet is at opposition. Jupiter only receives 3.7 percent as much sunlight per unit area as Earth. However, Jupiter reflects sunlight well. This fact and its immense size mean that Jupiter is usually the second brightest planet in the sky after Venus. Only Mars, when at a favorable opposition, outshines Jupiter at its opposition.
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 distance from us varies. In general, the greater the angle between Jupiter and the Sun, the brighter Jupiter appears as seen from Earth. Jupiter orbits the Sun much more slowly than Earth; it takes nearly 12 of our years to revolve once around the Sun with respect to the distant stars. Jupiter reaches an opposition approximately once every 13 Earth months.
Jupiter rotates rapidly on its axis. The complete Jovian day, midnight to midnight, lasts for 9 hours and 51 minutes Earth time. We would need to adopt a different method of time measurement if we were to visit this planet. Most likely we would divide the day into 10 hours and sleep every other night, bedding down before sunset and getting up after sunrise. However, as we have already seen, a colony on Jupiter will never exist; it is doubtful that there will even be permanent space stations in near orbit.
The volume of Jupiter is 1,300 times that of Earth and is greater than the combined volumes of all the other planets. At the equator, Jupiter measures 143,000 kilometers (89,000 miles) in diameter. This is more than 11 times the diameter of Earth (Fig. 7-2). The pole-to-pole diameter of Jupiter is somewhat less than the equatorial diameter. This difference is easy to see, even in a small telescope. The oblateness (flattening) of the planet is the result of its rapid spin and its largely liquid and gaseous composition.
Early astronomers were able to deduce the mass of Jupiter. First, they had to know how far away the planet was at the time of observation. This was determined by parallax among the distant stars and double-checked by measuring the time it takes Jupiter to make one complete revolution around the Sun. Once the distance to Jupiter was known, the orbital radius of one of its moons was determined by reverse triangulation. Then the period of revolution of that moon was measured; from this, the mass of Jupiter was calculated according to straightforward physics laws. The result was surprising: Jupiter is much less dense than the Earth. Whatever Jupiter is made of, it is nothing like our planet. The only elements light enough to explain Jupiter’s low density are hydrogen and helium. These two elements make up almost all the planet.
Nowadays astronomers believe that Jupiter has a rocky, molten core several times the volume of Earth. This is surrounded by hydrogen under so much pressure that it acts like a metallic liquid; if we could get a sample of it, it would look like elemental mercury. This metallic hydrogen is a good conductor of electricity. Because of this, and because the planet is spinning rapidly, enormous electric currents are induced. A powerful magnetic field is generated by the current, and this field, called the magnetosphere of Jupiter, extends millions of kilometers beyond the visible sphere. High-speed solar particles, called the solar wind , squash the magnetic lines of flux on the Sunward side of Jupiter and stretch the lines of flux on the side of Jupiter opposite the Sun (Fig. 7-3). The magnetic field has effects that extend far beyond Jupiter’s orbit.
Above the metallic hydrogen layer there is syrupy hydrogen-helium mixture that gradually becomes a liquid and finally thins out until it is gaseous. It is believed that there is no defined surface. In the upper 1,000 kilometers (about 600 miles) of Jupiter’s visible atmosphere, various elements in small amounts produce the clouds we see through our telescopes and that were so vividly rendered by the Pioneer and Voyager space probes.
Atmosphere And Weather
The plane of Jupiter’s equator is tilted 3.1 degrees relative to the plane of its orbit, so there are in effect no seasons on Jupiter. However, this is more than made up for by the violent and turbulent winds that roar around the planet in light-colored zones and dark-colored belts . At the boundaries between the zones and belts, the largest of which can be seen though a small telescope, eddies occur, similar to the high- and low-pressure systems we have here on Earth. The typical wind speeds on Jupiter, however, are many times greater than those we consider normal on our planet. In the extreme, Jovian winds exceed 400 kilometers (250 miles) per hour, comparable with the gales inside the most severe Earthly tornadoes.
The light-colored zones on Jupiter are the tops of the highest clouds, reflecting sunlight in the same way as the tops of thunderstorms or hurricanes reflect sunlight back into space from Earth’s atmosphere. The dark belts are gaps in the high clouds, but further down, Jupiter is cloudy everywhere.
Imagine what it would be like to ride a hardy hybrid space/air shuttle-craft down into Jupiter in the middle of one of the dark belts. You would see massive white clouds above and on either side, rising like walls, boiling and shredding as the winds blew them around and ripped wisps of cloud off into the clear. You would see lightning strokes hundreds of kilometers long, and you would hear interminable peals of thunder. As you continued your descent, the sky would get red, then brown, and finally would deepen to black, punctuated by flashes of lightning, each flash followed by thunderclaps so loud that the whole vessel would shudder. Then your instruments would go crazy, indicating that the ship was tumbling, swooping, and diving as updrafts and downdrafts tossed it around like a falling snowflake. At that moment you would decide that it was time to return to the main ship!
The Great Red Spot
Jupiter’s face is pockmarked by oval-shaped disturbances. The most prominent of these is known as the Great Red Spot and has been watched by astronomers for centuries. It spins outward, which happens to be counterclockwise because the spot is in the southern hemisphere of Jupiter. (If it were in the northern hemisphere, it would spin clockwise).
- ntil 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 certain 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 irregularities in the adjacent cloud bands betray that it is still there. It always returns to its full red glory sooner or later. The Voyager photographs convinced almost everyone that the Great Red Spot is a revolving high-pressure 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 semipermanent 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.
Practice problems of this concept can be found at: The Outer Planets Practice Problems
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