12-2 Unlike the terrestrial planets, Jupiter and Saturn exhibit differential rotation
Observations of features like the Great Red Spot and smaller storms allow astronomers to determine how rapidly Jupiter and Saturn rotate. At its equator, Jupiter completes a full rotation in only 9 hours, 50 minutes, and 28 seconds, making it not only the largest and most massive planet in the solar system but also the one with the fastest rotation. However, Jupiter rotates in a strikingly different way from Earth, the Moon, Mercury, Venus, or Mars.
Differential Rotation
If Jupiter were a solid body like a terrestrial planet (or, for that matter, a billiard ball), all parts of Jupiter’s surface would rotate through one complete circle in this same amount of time (Figure 12-3a). But by watching features in Jupiter’s cloud cover, Gian Domenico Cassini discovered in 1690 that the polar regions of the planet rotate a little more slowly than do the equatorial regions. (You may recall this Italian astronomer from Section 11-2 as the gifted observer who first determined Mars’s rate of rotation.) Near the poles, the rotation period of Jupiter’s atmosphere is about 5 minutes longer than at the equator (about 9 hours, 55 minutes, and 41 seconds). Saturn, too, has a longer rotation period near its poles, by about 25 minutes (10 hours, 39 minutes, and 24 seconds near the poles compared to 10 hours, 13 minutes, and 59 seconds at the equator).
ANALOGY
you can see this kind of rotation, called differential rotation, in the kitchen. As you stir the water in a pot, different parts of the liquid take different amounts of time to make one rotation around the center of the pot (Figure 12-3b). Differential rotation shows that neither Jupiter nor Saturn can be solid throughout their volumes: They must be at least partially fluid, like water in a pot.
CONCEPT CHECK 12-1
Will Jupiter’s moons of rock and ice, such as Ganymede, exhibit differential rotation?
The Compositions of Jupiter and Saturn
Spacecraft observations were needed to determine the compositions of Jupiter and Saturn
If Jupiter and Saturn have partially fluid interiors, they cannot be made of the rocky materials that constitute the terrestrial planets. An important clue to the compositions of Jupiter and Saturn are their average densities, which are only 1326 kg/m3 for Jupiter and 687 kg/m3 for Saturn. (By comparison, Earth’s average density is 5515 kg/m3.) To explain these low average densities, Rupert Wildt of the University of Göttingen in Germany suggested in the 1930s that Jupiter and Saturn are composed mostly of hydrogen and helium atoms—the two lightest elements in the universe—held together by their mutual gravitational attraction to form a planet. Wildt was motivated in part by his observations of prominent absorption lines of methane and ammonia in Jupiter’s spectrum. (We saw in Section 7-3 how spectroscopy plays an important role in understanding the planets.) A molecule of methane (CH4) contains four hydrogen atoms, and a molecule of ammonia (NH3) contains three. The presence of these hydrogen-rich molecules was strong, but indirect, evidence of abundant hydrogen in Jupiter’s atmosphere.
Direct evidence for hydrogen and helium in Jupiter’s atmosphere, however, was slow in coming. The problem was that neither gas produces prominent spectral lines in the visible sunlight reflected from the planet. To show the presence of these elements conclusively, astronomers had to look for spectral lines in the ultraviolet part of the spectrum. These lines are very difficult to measure from Earth, because almost no ultraviolet light penetrates our atmosphere (see Figure 6-25). Astronomers first detected the weak spectral lines of hydrogen molecules in Jupiter’s spectrum in 1960. The presence of helium on Jupiter and Saturn was finally confirmed in the 1970s and 1980s, when spacecraft first flew past these planets and measured their hydrogen spectra in detail.
Today we know that the chemical composition of Jupiter’s atmosphere is 86.2% hydrogen molecules (H2), 13.6% helium atoms, and 0.2% methane, ammonia, water vapor, and other gases. The percentages in terms of mass are somewhat different because a helium atom is twice as massive as a hydrogen molecule. Hence, by mass, Jupiter’s atmosphere is approximately 75% hydrogen, 24% helium, and 1% other substances, quite similar to that of the Sun. We will see evidence in Section 12-6 that Jupiter has a large rocky core made of heavier elements. It is estimated that the breakdown by mass of the planet as a whole (atmosphere plus interior) is approximately 71% hydrogen, 24% helium, and 5% all heavier elements.
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Saturn’s Missing Helium
Like Jupiter, Saturn is thought to have a large rocky core. But unlike Jupiter, data from Earth-based telescopes and spacecraft show that the atmosphere of Saturn has a serious helium deficiency: Its chemical composition is 96.3% hydrogen molecules, 3.3% helium, and 0.4% other substances (by mass, 92% hydrogen, 6% helium, and 2% other substances). Saturn’s atmosphere is a puzzle because Jupiter and Saturn are thought to have formed in similar ways from the gases of the solar nebula (see Section 8-4), and so both planets (and the Sun) should have essentially the same abundances of hydrogen and helium. So where did Saturn’s helium go?
The explanation may be simply that Saturn is smaller than Jupiter, and as a result Saturn probably cooled more rapidly. (We saw in Section 7-6 why a small world cools down faster than a large one.) This cooling would have triggered a process analogous to the way rain develops here on Earth. When the air is cool enough, humidity in Earth’s atmosphere condenses into raindrops that fall to the ground. On Saturn, however, it is droplets of liquid helium that condense within the planet’s cold, hydrogen-rich outer layers. In this scenario, helium is deficient in Saturn’s upper atmosphere simply because it has fallen deeper into the planet. By contrast, Jupiter’s helium has not rained out because its upper atmosphere is warmer and the helium does not form droplets.
ANALOGY
An analogy to helium “rainfall” within Saturn is what happens when you try to sweeten tea by adding sugar. If the tea is cold, the sugar does not dissolve well and tends to sink to the bottom of the glass even if you stir the tea with a spoon. But if the tea is hot, the sugar dissolves with only a little stirring. In the same way, it is thought that the descending helium droplets once again dissolve in hydrogen once they reach the warmer depths of Saturns interior.
In this scenario, Jupiter and Saturn both have about the same overall chemical composition. But Saturn’s smaller mass, less than a third that of Jupiter, results in less gravitational force tending to compress its hydrogen and helium. This lack of compression explains why Saturn’s density is only about half that of Jupiter, and is in fact the lowest of any planet in the solar system.
CAUTION!
Because Jupiter and Saturn are almost entirely hydrogen and helium, it would be impossible to land a spacecraft on either planet. An astronaut foolish enough to try would notice the hydrogen and helium around the spacecraft becoming denser, the temperature rising, and the pressure increasing as the spacecraft descended. But the hydrogen and helium would never solidify into a surface on which the spacecraft could touch down. Long before reaching the planets rocky core, the pressure of the hydrogen and helium would reach such unimaginably high levels that any spacecraft, even one made of the strongest known materials, would be crushed.
CONCEPT CHECK 12-2
If Jupiter and Saturn formed with nearly the same chemical composition, why might Jupiter be observed to have more helium than Saturn?