Jupiter and Saturn
The Sun-Like Planets
The rapid rotation of Jupiter and Saturn, coupled with their composition of low-density materials, argues that their internal structures are more fluid than solid. Another significant factor is that Jupiter and Saturn give off more heat than they receive from the sun. In the case of Jupiter it is about 1.5 to 2 times the amount from the sun, and for Saturn it is between 2 and 3 times the amount. Hence Jupiter and Saturn have internal sources of heat. It is ex· tremely unlikely that the heat source is anything as exotic as that in the sun and the stars; Jupiter and Saturn are not small stars. But it is fair to say that they are more like the sun than like the earth, and they are clearly an intermediate type of body. The internal heat source is more likely the conversion of gravitational potential energy into thermal energy as the two planets contracted during their formation and after. In fact it is likely that they are still contracting but very slowly.
Both Jupiter and Saturn have dense cores of rocky and icy materials- rather than compressed hydrogen and helium. The core is about 4 percent of the mass of Jupiter and 25 percent for Saturn, with temperatures in the range of 20,000 to 30,000 K and densities ranging from 2 to 20 grams per cubic centimeter. Surrounding the core is a layer ex· isting under a pressure in excess of 3 million times the earth's atmospheric pressure. In it hydrogen and helium behave more like liquid metals than solids. The upper boundary of the metallic-liquid zone is rather abrupt, giving way to a molecular·liquid mantle of hydrogen and helium. Through both the metallic- and molecular-liquid zones, which are 96 and 75 percent, respectively, of the mass of Jupiter and Saturn, the temperature and density decrease. The molecular· liquid mantles gradually change to molecular gases, which are then the atmospheres of the two planets.
