12.3 Saturn’s Interior and Magnetosphere


Figure 12.8 Saturn’s Interior Saturn’s internal structure, as deduced from Voyager 1 and 2 observations and computer modeling.
Figure 12.8 depicts Saturn’s internal structure (compare with Figure 11.10 for Jupiter). This picture has been pieced together by planetary scientists using the same tools—Voyager observations and theoretical modeling—that they used to infer Jupiter’s inner workings. Saturn has the same basic internal parts as Jupiter, but their relative proportions are somewhat different: Saturn’s metallic hydrogen layer is thinner, and its core is larger. Because of its lower mass, Saturn has a less extreme core temperature, density, and pressure than Jupiter. The central pressure is around a tenth of Jupiter’s—not too different from the pressure at the center of Earth.

Infrared measurements indicate that Saturn’s 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 Jupiter’s excess energy—that the planet has a large reservoir of heat left over from its formation—doesn’t work for Saturn. Saturn is smaller than Jupiter and so must have cooled more rapidly—rapidly 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 Saturn’s apparent helium deficit. At the temperatures and high pressures found in Jupiter’s interior, liquid helium dissolves in liquid hydrogen. In Saturn, where the internal temperature is lower, the helium doesn’t 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 Earth’s atmosphere to form a mist. The amount of helium condensation was greatest in the planet’s cool outer layers, where the mist turned to rain about 2 billion years ago. A light shower of liquid helium has been falling through Saturn’s 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 Saturn’s atmosphere—much 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 planet’s gravitational field compresses it and heats it up. The gravitational energy thus released is the source of Saturn’s 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 Saturn’s 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.


Figure 12.9 Aurora on Saturn An ultraviolet camera aboard the Hubble Space Telescope recorded this image of an remarkably symmetrical aurora on Saturn during a solar storm in 1998. (NASA)
Saturn’s electrically conducting interior and rapid rotation produce a strong magnetic field and an extensive magnetosphere. Probably because of the considerably smaller mass of Saturn’s metallic hydrogen zone, the planet’s basic magnetic field strength is only about 1/20 that of Jupiter, or about 1000 times greater than that of Earth. The magnetic field at Saturn’s cloud tops (roughly 10 Earth radii from the planet’s center) is approximately the same as at Earth’s surface. Voyager measurements indicate that, unlike Jupiter’s and Earth’s magnetic axes, which are slightly tilted, Saturn’s magnetic field is not inclined with respect to its rotation axis. Saturn’s magnetic field, like Jupiter’s, is oriented opposite that of Earth—that is, an Earth compass needle would point toward Saturn’s south pole rather than its north. Figure 12.9 shows an aurora on Saturn, imaged in 1998 by the Hubble Space Telescope.

Saturn’s magnetosphere extends about 1 million km toward the Sun and is large enough to contain the planet’s ring system and the innermost 16 small moons. Saturn’s 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 Saturn’s magnetosphere, the details of its structure are different from those of Jupiter’s 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 Earth’s ionosphere (they lie in the AM band) and were not detected until the Voyager craft approached the planet.


Concept Check

Where did Saturn’s atmospheric helium go?