Internal Structure of Jovian Planets
Larger masses and the fact that the Jovian planets contain far more easily vaporized materials than the terrestrial planets do mean that the resulting internal structures for the Jovian planets do not resemble those of the terrestrial. Jupiter and Saturn are the only planets in the solar system composed primarily of hydrogen and helium (as is the sun): Only hydrogen and helium could give Jupiter and Saturn their mean densities of 1.31 and 0.69 grams per cubic centimeter, respectively, for the temperatures and pressures that characterize each planet. On the other hand, the mean densities of Uranus and Neptune are about twice that of Saturn even though their masses are much smaller. Thus their compositions are probably dominated by oxygen, carbon, and nitrogen, the third, fourth, and fifth most abundant elements in the solar system, even though their atmospheres contain significant amounts of hydrogen (and possibly helium).
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.
Uranus and Neptune
Simillar But Different
Like Jupiter and Saturn, Uranus and Neptune have a three-layered structure, but unlike the solar·system giants, each layer is of quite different chemical com· position. The core of each planet is probably a rocky (iron and silicates primarily) and icy (methane, ammonia, and water principally) material. For Uranus the pressure of overlying layers may not be sufficient to make the core solid, but it remains a thick viscous liquid with convective motions in it. On the other hand, Neptune's greater mean density suggests that its core is solid,
Surrounding the core of each planet is a liquid mantle of water, methane, and ammonia, in which t,here may be some convective motions (for Neptune but not for Uranus). Finally, each planet has a thick crust of hydrogen and helium that is compressed by gravity into a very dense gas. The crusts gradually give way to low·density atmospheres. Thus like Jupiter and Saturn, these planets have no solid surface sur· rounded by a thin atmosphere as the terrestrial planets have.
Calculated models for the interiors of both planets suggest that their central temperatures are on the order of 7000 K. Since Jupiter and Saturn emit more radiant energy than they receive from the sun, is it not likely that the same is true for Uranus and Neptune? Yes, one might well expect that to be the situation for both; yet it is not true for Uranus and is true only for Neptune, which radiates about twice as much heat as it receives from the sun. Why this difference in what should be reasonably similar bodies? We really do not know.
Larger masses and the fact that the Jovian planets contain far more easily vaporized materials than the terrestrial planets do mean that the resulting internal structures for the Jovian planets do not resemble those of the terrestrial. Jupiter and Saturn are the only planets in the solar system composed primarily of hydrogen and helium (as is the sun): Only hydrogen and helium could give Jupiter and Saturn their mean densities of 1.31 and 0.69 grams per cubic centimeter, respectively, for the temperatures and pressures that characterize each planet. On the other hand, the mean densities of Uranus and Neptune are about twice that of Saturn even though their masses are much smaller. Thus their compositions are probably dominated by oxygen, carbon, and nitrogen, the third, fourth, and fifth most abundant elements in the solar system, even though their atmospheres contain significant amounts of hydrogen (and possibly helium).
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.
Uranus and Neptune
Simillar But Different
Like Jupiter and Saturn, Uranus and Neptune have a three-layered structure, but unlike the solar·system giants, each layer is of quite different chemical com· position. The core of each planet is probably a rocky (iron and silicates primarily) and icy (methane, ammonia, and water principally) material. For Uranus the pressure of overlying layers may not be sufficient to make the core solid, but it remains a thick viscous liquid with convective motions in it. On the other hand, Neptune's greater mean density suggests that its core is solid,
Surrounding the core of each planet is a liquid mantle of water, methane, and ammonia, in which t,here may be some convective motions (for Neptune but not for Uranus). Finally, each planet has a thick crust of hydrogen and helium that is compressed by gravity into a very dense gas. The crusts gradually give way to low·density atmospheres. Thus like Jupiter and Saturn, these planets have no solid surface sur· rounded by a thin atmosphere as the terrestrial planets have.
Calculated models for the interiors of both planets suggest that their central temperatures are on the order of 7000 K. Since Jupiter and Saturn emit more radiant energy than they receive from the sun, is it not likely that the same is true for Uranus and Neptune? Yes, one might well expect that to be the situation for both; yet it is not true for Uranus and is true only for Neptune, which radiates about twice as much heat as it receives from the sun. Why this difference in what should be reasonably similar bodies? We really do not know.
