On the basis of Jupiter's distance from the Sun, astronomers had expected to find the temperature of the cloud tops to be around 105 K. At that temperature, they reasoned, Jupiter would radiate back into space exactly the same amount of energy as it received from the Sun. When radio and infrared observations were first made of the planet, however, astronomers found that its blackbody spectrum corresponded to a temperature of 125 K instead. Subsequent measurements, including those made by Voyager and Galileo, have verified that finding. Although a difference of 20 K may seem small, recall from Chapter 5 that the energy emitted by a planet grows as the fourth power of the surface temperature (in Jupiter's case, the temperature of the cloud tops). (Sec. 3.4) A planet at 125 K therefore radiates (125/105)⁴, or about twice as much energy as a 105 K planet. Put another way, Jupiter actually emits about twice as much energy as it receives from the Sun. Thus, unlike any of the terrestrial planets, Jupiter must have its own internal heat source.

What is responsible for Jupiter's extra energy? It is not the decay of radioactive elements within the planet—that must be occurring, but not nearly at the rate necessary to produce the temperature we record. Nor is it the process that generates energy in the Sun, nuclear fusion—the temperature in Jupiter's interior, high as it is, is far too low for that (see Interlude 11-1). Instead, astronomers theorize that the source of Jupiter's excess energy is the slow escape of gravitational energy released during the planet's formation. As the planet took shape, some of its gravitational energy was converted into heat in the interior. That heat is still slowly leaking out through the planet's heavy atmospheric blanket, resulting in the excess emission we observe. Despite the huge amounts of energy involved—Jupiter's emits about 4 10¹⁷ watts more energy than it receives from the Sun—the loss is quite slight compared with the planet's total energy. A simple calculation indicates that the average temperature of the interior of Jupiter falls by only about a millionth of a kelvin per year.

Jupiter's clouds, with their complex chemistry, are probably less than 200 km thick. Below them, the temperature and pressure steadily increase as the atmosphere becomes the "interior" of the planet. Much of our knowledge of Jupiter's interior comes from theoretical modeling. Planetary scientists use all available bulk data on the planet—mass, radius, composition, rotation, temperature, and so on—to construct a model of the interior that agrees with observations. Our statements about Jupiter's interior are, then, really statements about the model that best fits the facts. However, because the interior consists largely of hydrogen and helium—two simple gases whose physics we think we understand well—we can be fairly confident that Jupiter's internal structure is now understood.

Both the temperature and the density of Jupiter's atmosphere increase with depth below the cloud cover. However, no "surface" of any kind exists anywhere inside. Instead, Jupiter's atmosphere just becomes denser and denser, because of the pressure of the overlying layers. At a depth of a few thousand kilometers, the gas makes a gradual transition into the liquid state (see Figure 11.10). By a depth of about 20,000 km, the pressure is about 3 million times greater than atmospheric pressure on Earth. Under those conditions, the hot liquid hydrogen is compressed so much that it undergoes another transition, this time to a "metallic" state with properties in many ways similar to those of a liquid metal. Of particular importance for Jupiter's magnetic field (see Section 11.4) is that this metallic hydrogen is an excellent conductor of electricity.

Figure 11.10 Jupiter's internal structure, as deduced from Voyager measurements and theoretical modeling. The outer radius represents the top of the cloud layers, some 70,000 km from the planet's center. The density and temperature increase with depth, and the atmosphere gradually liquefies at a depth of a few thousand kilometers. Below a depth of 20,000 km, the hydrogen behaves like a liquid metal. At the center of the planet lies a large rocky core, somewhat terrestrial in composition but much larger than any of the inner planets. Although the values are very uncertain, the temperature and pressure at the center are probably about 40,000 K and 50 million (Earth) atmospheres, respectively.

As mentioned earlier, Jupiter's observed flattening requires that there be a relatively small, dense core at its center, containing perhaps 15 times the mass of Earth. The core's exact composition is unknown, but planetary scientists think that it contains much denser materials than the rest of the planet. Present best estimates indicate that it consists of "rocky" materials, similar to those found in the terrestrial worlds. (Note that the term rocky here refers to the chemical composition of the core, not to its physical state. At the high temperatures and pressures found deep in the jovian interiors, the core material bears little resemblance to rocks found on Earth's surface.) In fact, it now appears that all four jovian planets contain large rocky cores, and that the formation of such a large "terrestrial" planetary core is a necessary stage in the process of building up a gas giant.

Because of the enormous pressure at the center of Jupiter—approximately 50 million times that on Earth's surface, or 10 times that at Earth's center—the core must be compressed to a very high density (perhaps twice the core density of Earth). It is probably not much more than 20,000 km in diameter, and the central temperature may be as high as 40,000 K.