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.
