12.3 Saturns Interior and Magnetosphere
Infrared measurements indicate that Saturns surface (that is, cloud-top) temperature is 97 K, substantially higher than the temperature at which Saturn would reradiate all the energy it receives from the Sun. In fact, Saturn radiates almost three times more energy than it absorbs. Thus Saturn, like Jupiter, has an internal energy source. (Sec. 11.3) But the explanation behind Jupiters excess energythat the planet has a large reservoir of heat left over from its formationdoesnt work for Saturn. Saturn is smaller than Jupiter and so must have cooled more rapidlyrapidly enough that its original supply of energy was used up long ago. What, then, is happening inside Saturn to produce this extra heat?
The explanation for this strange phenomenon also explains the mystery of Saturns apparent helium deficit. At the temperatures and high pressures found in Jupiters interior, liquid helium dissolves in liquid hydrogen. In Saturn, where the internal temperature is lower, the helium doesnt dissolve so easily and tends to form droplets instead. The phenomenon is familiar to cooks, who know that it is generally much easier to dissolve ingredients in hot liquids than in cold ones. Saturn probably started out with a fairly uniform solution of helium dissolved in hydrogen, but the helium tended to condense out of the surrounding hydrogen, much as water vapor condenses out of Earths atmosphere to form a mist. The amount of helium condensation was greatest in the planets cool outer layers, where the mist turned to rain about 2 billion years ago. A light shower of liquid helium has been falling through Saturns interior ever since. This helium precipitation is responsible for depleting the outer layers of their helium content.
So we can account for the unusually low abundance of helium in Saturns atmospheremuch of it has rained down to lower levels. But what about the excess heating? The answer is simple: As the helium sinks toward the center, the planets gravitational field compresses it and heats it up. The gravitational energy thus released is the source of Saturns internal heat. In the distant future the helium rain will stop, and Saturn will cool until its outermost layers radiate only as much energy as they receive from the Sun. When that happens, the temperature at Saturns cloud tops will be 74 K. As Jupiter cools, it too may someday experience helium precipitation in its interior, which will cause its surface temperature to rise once again.
Saturns magnetosphere extends about 1 million km toward the Sun and is large enough to contain the planets ring system and the innermost 16 small moons. Saturns largest moon, Titan, orbits about 1.2 million km from the planet, so it is sometimes found just inside the outer magnetosphere and sometimes just outside, depending on the intensity of the solar wind (which tends to push the sunward side of the magnetosphere closer to the planet). Because no major moons lie deep within Saturns magnetosphere, the details of its structure are different from those of Jupiters magnetosphere. For example, there is no equivalent of the Io plasma torus. (Sec. 11.5) Like Jupiter, Saturn emits radio waves, but as luck would have it, they are reflected from Earths ionosphere (they lie in the AM band) and were not detected until the Voyager craft approached the planet.
Where did Saturns atmospheric helium go?