JUPITER - The Fifth Planet
Fifth planet from the sun, jupiter is the largest and most massive of the planets in the solar system. In our night sky it glows with a bright, steady yellow light, outshining.the stars. The mean diameter of jupiter is about 11 times greater than the earth's, and jupiter is more than 1000 times larger in volume than the earth. However, jupiter's mass is barely more than 300 times that of the earth even though it exceeds the combined masses of all the other bodies orbiting the sun. Thus its mean density is about one-fourth that of the earth. Because its axis is tilted only 3° from the perpendicular to its orbital plane, the planet has little seasonal change.
Not all portions of the visible layers of jupiter, which appear as alternating dark and light bands parallel to the equator, rotate in unison. The equatorial region completes its rotation several minutes sooner than adjacent higher latitudes. This phenomenon is known as differential rotation and is possible in fluid media, such as gases. It is not something one expects a solid body, like the surface of a terrestrial planet, to do. Figure 6.7b shows the consequences of differential rotation to the outer layers. jupiter's rapid 10-hour rotation and low density combine to flatten the planet about 6 percent in its polar diameter. This is again more characteristic of a fluid body that will readily deform than of a solid body that does not easily flow. The dark-band structure is composed of reddish and brown shades with irregular patches of gray, blue, and white clouds. The light zones are primarily yellow in color. The entire band structure is constantly undergoing changes in color and intensity. Clearly what we are viewing are clouds in jupiter's atmosphere and not a solid surface such as the terrestrial planets have. Most striking of all the atmospheric features is the Great Red Spot, which has been observed for at least 300 years. It is immense, being about four times the size of the earth.
In the early days of radio astronomy jupiter was foilnd to be an intense source of radio radiation. If this radiation were just part of the planet's thermal radiation, then jupiter would have to be extraordinarily hot. Since it is not, the radiation must be due to nonthermal processes, such as free electrons spiraling about magnetic lines of force.
