12-9 Saturn’s rings are composed of numerous icy fragments, while Jupiter’s rings are made of small rocky particles

Astronomers have long known that Saturn’s rings could not possibly be solid sheets of matter. In 1857, the Scottish physicist James Clerk Maxwell (whom we last encountered in Section 5-2) proved theoretically that if the rings were solid, differences in Saturn’s gravitational pull on different parts of the rings would cause the rings to tear apart. He concluded that Saturn’s rings are made of “an indefinite number of unconnected particles.”

Ring Particles

In 1895, James Keeler at the Allegheny Observatory in Pittsburgh became the first to confirm by observation that the rings are not rigid. He made this conclusion by photographing the spectrum of sunlight reflected from Saturn’s rings. As the rings orbit Saturn, the spectral lines from the side approaching us are blueshifted by the Doppler effect (recall Section 5-9, especially Figure 5-26). At the same time, spectral lines from the receding side of the rings are redshifted. Keeler noted that the size of the wavelength shift increased inward across the rings—the closer to the planet, the greater the shift. This variation in Doppler shift proved that the inner portions of Saturn’s rings are moving around the planet more rapidly than the outer portions. Indeed, the orbital speeds across the rings are in complete agreement with Kepler’s third law: The square of the orbital period about Saturn at any place in the rings is proportional to the cube of the distance from Saturn’s center (see Section 4-7 and Box 4-4). This result is exactly what would be expected if the rings consisted of numerous tiny “moonlets,” or ring particles, each individually circling Saturn.

Saturn’s rings are quite bright; they reflect 80% of the sunlight that falls on them. (By comparison, Saturn itself reflects 46% of incoming sunlight.) Astronomers therefore long suspected that the ring particles are made of ice and ice-coated rock. This hunch was confirmed in the 1970s, when the American astronomers Gerard P. Kuiper and Carl Pilcher identified absorption features of frozen water in the rings’ near-infrared spectrum. The Voyager and Cassini spacecraft have made even more detailed infrared measurements that indicate the temperature of the rings ranges from −180°C (−290°F) in the sunshine to less than −200°C (−330°F) in Saturn’s shadow. Water-ice is in no danger of melting or evaporating at these temperatures.

To determine the sizes of the particles that make up Saturn’s rings, astronomers analyzed the radio signals received from a spacecraft as it passed behind the rings. How easily radio waves can travel through the rings depends on the relationship between the wavelength and the particle size. The results show that most of the particles range in size from pebble-sized fragments about 1 cm in diameter to chunks about 5 m across, the size of large boulders. Most abundant are snowball-sized particles about 10 cm in diameter. Recall that Mercury could fit inside the Cassini division, so seeing even the largest boulder-sized ring particles is not possible in images such as Figure 12-16.

The Roche Limit

All of Saturn’s material may be ancient debris that failed to accrete (fall together) into satellites. The total amount of material in the rings is quite small. If Saturn’s entire ring system were compressed together to make a satellite, it would be no more than 100 km (60 mi) in diameter. But, in fact, the ring particles are so close to Saturn that they will never be able to form moons.

To see why, imagine a collection of small particles orbiting a planet. Gravitational attraction between neighboring particles tends to pull the particles together. However, because the various particles are at differing distances from the parent planet, they also experience different amounts of gravitational pull from the planet. This difference in gravitational pull is a tidal force that tends to keep the particles separated. (We discussed tidal forces in detail in Section 4-8. You may want to review that section, and in particular Figure 4-26.)

The closer a pair of particles is to the planet, the greater the tidal force that tries to pull the pair apart. At a certain distance from the planet’s center, called the Roche limit, the disruptive tidal force is just as strong as the gravitational force between the particles. (The concept of this limit was developed in the mid-1800s by the French mathematician Edouard Roche.) Inside the Roche limit, the tidal force overwhelms the gravitational pull between neighboring particles, and these particles cannot accrete to form a larger body. Instead, they tend to spread out into a ring around the planet. (The Cosmic Connections figure depicts this process.) Indeed, most of Saturn’s system of rings visible in Figure 12-16 lies within the planet’s Roche limit.

All large planetary satellites are found outside their planet’s Roche limit. If any large satellite were to come inside its planet’s Roche limit, the planet’s tidal forces would cause the satellite to break up into fragments. We will see in Chapter 14 that such a catastrophic tidal disruption may be the eventual fate of Neptune’s large satellite, Triton.

Jupiter’s Ring and the Roche Limit

When the Voyager 1 spacecraft flew past Jupiter in 1979, it trained its cameras not just on the planet but also on the space around the planet’s equator. It discovered that Jupiter, too, has a system of rings that lies within its Roche limit (Figure 12-18). These rings differ from Saturn’s in two important ways. First, Jupiter’s rings are composed of tiny particles of rock with an average size of only about 1 μm (= 0.001 m = 10⁻⁶ m) and that reflect less than 5% of the sunlight that falls on them. Second, there is very little material in the rings of Jupiter, less than 1/100,000 (10⁻⁵) the amount of material in Saturn’s rings. As a result, Jupiter’s rings are extremely faint, which explains why their presence was first revealed by a spacecraft rather than an Earth-based telescope. The ring particles are thought to originate from meteorite impacts on Jupiter’s four small, inner satellites, two of which are visible in Figure 12-18.

Figure 12-18: R I V U X G
Jupiter’s Main Ring This Galileo image shows Jupiter’s main ring almost edge-on. The ring’s outer radius, about 129,000 km from the center of Jupiter, is close to the orbit of Adrastea, the second closest (after Metis) of Jupiter’s moons. Not shown is an even larger and even more tenuous pair of “gossamer rings,” one of which extends out to 181,000 km and the other out to 222,000 km.

(NASA/JPL; Cornell University)

We will see in Chapter 14 that the rings of Uranus and Neptune are also made of many individual particles orbiting inside each planet’s Roche limit. Like the rings of Jupiter, these rings are quite dim and difficult to see from Earth.