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FIGURE 15.10 The four Galilean moons pictured by Voyager 1. Clockwise from upper left, Io, Europa, Ganymede, and Callisto. Io and
Europa are about the size of Earth’s Moon; Ganymede and Callisto are larger than Mercury.

particles. Some rings are composed of particles large enough to
EXAMPLE 15.2
be mea sured in meters, while others are composed of particles
An interesting question exists with regard to whether liquid water that are dust-sized (Figure 15.12). Saturn is about 9.5 AU from
could exist on Europa, which is one of Jupiter’s moons composed of the Sun, but its system of rings is easily spotted with a good pair
water ice. Consider that a 20 cm thick area of ice with a surface area of binoculars. Saturn also has the lowest average density of any
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of 3.5 × 10 cm exists on the surface of Jupiter’s moon. Assume the of the planets, about 0.7 times the density of water.
solar energy received at this moon is the same as that for Jupiter, the ice
absorbs all of the solar energy, and its temperature is at −220°C. Could The surface of Saturn, like Jupiter’s surface, has bright and
the energy received raise the temperature of the ice to its melting point? dark bands that circle the planet parallel to the equator. Saturn
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(Answer: 1.0 × 10 cal received from the Sun is greater than 7.0 × 10 also has a smaller version of Jupiter’s Great Red Spot, but in gen-
cal needed to raise the temperature of the ice. The energy received could eral, the bands and spot are not as highly contrasted or brightly
raise the temperature of the ice to its melting point) colored as they are on Jupiter.
The international Cassini-Huygens mission entered orbit
around Saturn on July 1, 2004, after a 3.5 billion km (2.2 bil-
lion mi), 7 year voyage from Earth. Establishing orbit was the
SATURN first step of a 4 year study of Saturn and its rings and moons.
Saturn is slightly smaller and substantially less massive than So far, the mission has found that Saturn’s largest moon, Ti-
Jupiter and has similar features (see Figure 15.8) to those of tan, has a surface shaped by rock fracturing, winds, and ero-
Jupiter, but it is readily identified by its unique, beautiful system sion. Among the new discoveries is a 1,500 km (930 mi) long
of rings. Saturn’s rings consist of thousands of narrow bands of river of liquid methane. Titan’s atmosphere is rich in organic

15-11 CHAPTER 15 The Solar System 387