Jupiter has at least 16 moons. The four largest—the Galilean satellites—are each comparable in size to Earth's Moon. (Sec. 2.5) Moving outward from Jupiter, the four are named Io, Europa, Ganymede, and Callisto, after the mythical attendants of the Roman god Jupiter. They move in nearly circular orbits about their parent planet. When the Voyager 1 spacecraft passed close to the Galilean moons in 1979, it sent some remarkably detailed photographs back to Earth, allowing planetary scientists to discern fine surface detail on each. More recently, in the late 1990s, the Galileo mission expanded still further our knowledge of these small but complex worlds. We will consider the Galilean satellites in more detail in a moment.

Within the orbit of Io lie four small satellites, all but one discovered by Voyager cameras. The largest of the four, Amalthea, is less than 300 km across and is irregularly shaped. E. E. Barnard discovered it in 1892. It orbits at a distance of 181,000 km from Jupiter's center—only 110,000 km above the cloud tops. Its rotation, like that of most of Jupiter's satellites, is synchronous with its orbit because of Jupiter's strong tidal field. Amalthea rotates once per orbit period, every 11.7 hours.

Beyond the Galilean moons lie eight more small satellites, all discovered in the twentieth century but before the Voyager missions. They fall into two groups of four moons each. The moons in the inner group move in eccentric, inclined orbits, about 11 million km from the planet. The outer four moons lie about 22 million km from Jupiter. Their orbits too are fairly eccentric, but retrograde, moving in a sense opposite all the other moons' orbit (and Jupiter's rotation). It is very likely that each group represents a single body that was captured by Jupiter's strong gravitational field long after the planet and its larger moons originally formed. Both bodies subsequently broke up, either during or after the capture process, resulting in the two families of similar orbits we see today. The masses, and hence the densities, of these small worlds are unknown. However, their appearance and sizes suggest compositions more like asteroids than their larger Galilean companions. Table 11.1 presents the general properties of Jupiter's moons.

THE GALILEAN MOONS AS A MODEL OF THE INNER SOLAR SYSTEM

If we think of Jupiter's moon system as a scaled-down solar system, then the Galilean moons correspond to the terrestrial planets. Their orbits are direct (that is, in the same sense as Jupiter's rotation), roughly circular, and lie close to Jupiter's equatorial plane. They range in size from slightly smaller than Earth's Moon (Europa) to slightly larger than Mercury (Ganymede). Figure 11.13 compares the appearances and sizes of the four Galilean satellites. Figure 11.14 shows two of the moons photographed against the background of their parent planet.

Figure 11.13 The Voyager 1 spacecraft photographed each of the four Galilean moons of Jupiter. Shown here to scale, as they would appear from a distance of about 1 million km, they are, clockwise from upper left, Io, Europa, Callisto, and Ganymede.

Figure 11.14 Voyager 1 took this photo of Jupiter with ruddy Io on the left and pearl-like Europa toward the right. Note the scale of objects here: Both Io and Europa are comparable in size to our Moon, and the Red Spot (seen here to the left bottom) is roughly twice as big as Earth. (See also the chapter-opening image)

The parallel with the inner solar system continues with the realization that the moons' densities decrease with increasing distance from Jupiter. Based largely on detailed measurements made by Galileo of the moons' gravitational fields, researchers have built up fairly detailed pictures of the moons' compositions and internal structures (Figure 11.15). The innermost two Galilean moons, Io and Europa, have thick rocky mantles, possibly similar to the crusts of the terrestrial planets, surrounding iron/iron sulfide cores. Io's core accounts for about half that moon's total radius. Europa has a thin water/ice outer shell about 150 km thick. The two outer moons, Ganymede and Callisto, are clearly deficient in rocky materials. Lighter materials, such as water ice, may account for as much as half of their total mass. Ganymede appears to have a relatively small metallic core topped by a rocky mantle and a thick icy outer shell. Callisto seems to be a largely undifferentiated mixture of rock and ice.

Figure 11.15 Cutaway diagrams showing the interior structure of the four Galilean satellites. Moving outward from Io to Callisto, the moons' densities steadily decrease as the composition shifts from rocky mantles and metallic cores in Io and Europa, to a thick icy crust and smaller core in Ganymede, to an almost uniform rock—ice mix in Callisto.

Many astronomers think that the formation of Jupiter and the Galilean satellites may in fact have mimicked on a small scale the formation of the Sun and the inner planets. For that reason, studies of the Galilean moon system may provide us with valuable insight into the processes that created our own world. We will return to this parallel in Chapter 15. So interested are mission planners in learning more about the Galilean moon system that the already highly successful Galileo mission has been extended for two more years, to concentrate first on Europa, then on Jupiter's atmosphere and magnetosphere, and finally on Io. The two moons will be studied at resolutions as fine as a few meters during extremely close passages by the spacecraft.

Not all the properties of the Galilean moons find analogs in the inner solar system, however. For example, because of Jupiter's tidal effect, all four Galilean satellites are in states of synchronous rotation, so they all keep one face permanently pointing toward their parent planet. By contrast, of the terrestrial planets, only Mercury is strongly influenced by the Sun's tidal force, and even its orbit is not synchronous. And, of course, the Jupiter system has no analogs of the jovian planets. Finally, inspection of Table 11.1 shows a remarkable coincidence in the orbit periods of the three inner Galilean moons: their periods are almost exactly in the ratio 1:2:4—a kind of "Bode's law" for Jupiter. Interlude 6-1 This is most probably the result of a complex, but poorly understood, three-body resonance in the Galilean moon system, something not found among the terrestrial worlds.

IO

Io, the densest of the Galilean moons, is the most geologically active object in the entire solar system. Its mass and radius are fairly similar to those of Earth's own Moon, but there the resemblance ends. Shown in Figure 11.16, Io's surface is a collage of reds, yellows, and blackish browns—resembling a giant pizza in the minds of some startled Voyager scientists. As the spacecraft glided past Io an outstanding discovery was made: Io has active volcanoes! Voyager 1 photographed eight erupting volcanoes, and six were still erupting when Voyager 2 passed by 4 months later. In Figure 11.17, one volcano is seen ejecting matter to an altitude of over 200 km. The gases are spewed forth at speeds up to 2 km/s, quite unlike the (relatively) sluggish ooze that emanates from Earth's insides.

Figure 11.16 Jupiter's innermost moon, Io, is quite different in character from the other three Galilean satellites. Its surface is kept smooth and brightly colored by the moon's constant volcanism. The resolution of the photograph in (a) is about 7 km. In the more detailed image (b), features as small as 2 km across can be seen.

Figure 11.17 One of Io's volcanoes was caught in the act of erupting while the Voyager 1 spacecraft flew past this fascinating moon. Surface features here are resolved to within a few kilometers. The volcano's umbrella-like profile shows clearly against the darkness of space. The plume measures about 100 km high and 300 km across.

The orange color immediately surrounding the volcano most likely results from sulfur compounds in the ejected material. In stark contrast to the other Galilean moons, Io's surface is neither cratered nor streaked. (The circular features visible in Figures 11.16 and 11.17 are volcanoes.) Its surface is exceptionally smooth, apparently the result of molten matter that constantly fills in any "dents and cracks." Accordingly, we can conclude that this remarkable moon has the youngest surface of any known object in the solar system. Of further significance, Io also has a thin, temporary atmosphere made up primarily of sulfur dioxide, presumably the result of gases ejected by volcanic activity. By the time Galileo arrived, several of the volcanoes observed by Voyager had subsided; however, many new ones were seen.

Io's volcanism has a major effect on Jupiter's magnetosphere. All the Galilean moons orbit within the magnetosphere and play some part in modifying its properties, but Io's influence is particularly marked. Although many of the charged particles in Jupiter's magnetosphere come from the solar wind, there is strong evidence that Io's volcanism is the primary source of heavy ions in the inner regions. Jupiter's magnetic field continually sweeps past Io, gathering up the particles its volcanoes spew into space and accelerating them to high speed. The result is the Io plasma torus (Figure 11.18; see also Figure 11.12), a doughnut-shaped region of energetic heavy ions that follows Io's orbital track, completely encircling Jupiter. (A plasma is a gas that has been heated to such high temperatures that all its atoms are ionized.) The plasma torus is quite easily detectable from Earth, but before Voyager its origin was unclear. Galileo made detailed studies of the plasma's dynamic and rapidly varying magnetic field. Spectroscopic analysis shows that sulfur is indeed one of the torus's major constituents, strongly implicating Io's volcanoes as its source. As a hazard to spacecraft—manned or unmanned—the plasma torus is formidable. The radiation levels there are lethal.

Figure 11.18 The Io plasma torus is the result of material being ejected from Io's volcanoes and swept up by Jupiter's rapidly rotating magnetic field. Spectroscopic analysis indicates that the torus is composed primarily of sodium and sulfur atoms.

What causes such astounding volcanic activity on Io? Surely that moon is too small to have geological activity like Earth. Io should be long dead, like our own Moon. At one time, some scientists suggested that Jupiter's magnetosphere might be the culprit—perhaps the (then-unknown) processes creating the plasma torus were somehow also stressing the moon. We now know that this is not the case. The real source of Io's energy is gravity—Jupiter's gravity. Io orbits very close to Jupiter—only 422,000 km, or 5.9 Jupiter radii, from the center of the planet. As a result, Jupiter's huge gravitational field produces strong tidal forces on the moon. If Io were the only satellite in the Jupiter system, it would long ago have come into a state of synchronous rotation with the planet, just like our own Moon, for the reasons discussed in Chapter 8. (Sec. 8.3) In that case, Io would move in a perfectly circular orbit, with one face permanently turned toward Jupiter. The tidal bulge would be stationary with respect to the moon, and there would be no internal stresses and hence no volcanism.

But Io is not alone. As it orbits it is constantly tugged by the gravity of its nearest large neighbor, Europa. These tugs are small and not enough to cause any great tidal effect, but they are sufficient to make Io's orbit slightly noncircular, preventing the moon from settling into a precisely synchronous state. The reason for this effect is exactly the same as in the case of Mercury, as discussed in Chapter 8. (Sec. 8.3) In a noncircular orbit, the moon's speed varies from place to place as it revolves around its planet, but its rate of rotation on its axis remains constant. Thus it cannot keep one face always turned toward Jupiter. Instead, as seen from Jupiter, Io rocks or "wobbles" slightly from side to side as it moves. The large tidal bulge, however, always points directly toward Jupiter, so it moves back and forth across Io's surface as the moon wobbles. These conflicting forces result in enormous tidal stresses that continually flex and squeeze Io's interior.

Just as repeated back-and-forth bending of a piece of wire can produce heat through friction, the ever-changing distortion of Io's interior constantly energizes the moon. This generation of large amounts of heat within Io ultimately causes huge jets of gas and molten rock to squirt out of the surface. Galileo's sensors indicated extremely high temperatures in the outflowing material. It is likely that much of Io's interior is soft or molten, with only a relatively thin solid crust overlying it. In fact, Io's volcanoes are probably more like geysers on Earth, but the term volcano has stuck. Researchers estimate that the total amount of heat generated within Io as a result of tidal flexing is about 100 million megawatts. This phenomenon makes Io one of the most fascinating objects in our solar system.

EUROPA

Europa (Figure 11.19) is a very different world from Io. Lying outside Io's orbit, 671,000 km (9.4 Jupiter radii) from Jupiter, it has relatively few craters on its surface, suggesting geologic youth, perhaps just a few million years. Recent activity must have erased the scars of ancient meteoritic impacts. Europa's surface displays a vast network of lines crisscrossing bright, clear fields of water ice. Some of these linear "bands," or fractures, appear to extend halfway around the satellite and resemble in some ways the pressure ridges that develop in ice floes on Earth's polar oceans.

Figure 11.19 The second Galilean moon is Europa. Its icy surface is only lightly cratered, indicating that some ongoing process must be obliterating impact craters soon after they are formed. The origin of the cracks crisscrossing the surface is uncertain. The resolution of the Voyager 2 mosaic in (a) is about 5 km. The two images below it (b and c) display even finer detail. (d) At 20-m resolution—the width of a typical house—this image from the Galileo spacecraft shows a smooth yet tangled surface resembling the huge ice flows that cover Earth's polar regions.

Before Galileo's arrival, some researchers had theorized that Europa is completely covered by an ocean of liquid water whose top is frozen at the low temperatures that prevail so far from the Sun. In this view, the cracks in the surface are attributed to the tidal influence of Jupiter and the gravitational pulls of the other Galilean satellites, although these forces are considerably weaker than those powering Io's violent volcanic activity. However, other planetary scientists had contended that Europa's fractured surface was instead related to some form of tectonic activity, one involving ice rather than rock. High-resolution Galileo observations now appear to support the former idea. Figure 11.19(d) is a Galileo image of this weird moon, showing what look like "icebergs —flat chunks of ice that have been broken apart and reassembled, perhaps by the action of water currents below. Mission scientists speculate that Europa's ice may be several kilometers thick and that there may be a 100-km-deep ocean below it.

If Europa does have an ocean of liquid water below its surface ice, it opens up many interesting avenues of speculation about the possible development of life there. In the rest of the solar system, only Earth has liquid water on or near its surface, and most scientists agree that water played a key role in the appearance of life here (see Chapter 28). However, bear in mind that the existence of water does not necessarily imply the emergence of life. Europa is a very hostile environment compared with Earth. The surface temperature on Europa is just 130 K, and the atmospheric pressure is only a billionth the pressure on our planet. Nevertheless, the possibility, however remote, of life on Europa was an important motivating factor in the decision to extend the Galileo mission for two more years.

GANYMEDE AND CALLISTO

The two outermost Galilean moons are Ganymede (at 1.1 million km, or 15 planetary radii, from the center of Jupiter) and Callisto (at 1.9 million km, or 26 Jupiter radii). The density of each is only about 2000 kg/m³, suggesting that they harbor substantial amounts of ice throughout and are not just covered by thin icy or snowy surfaces. Ganymede, shown in Figure 11.20, is the largest moon in the solar system, exceeding not only Earth's Moon but also the planets Mercury and Pluto in size. It has many impact craters on its surface and patterns of dark and light markings that are reminiscent of the highlands and maria on Earth's own Moon. In fact, Ganymede's history has many parallels with that of the Moon (with water ice replacing lunar rock). The large, dark region clearly visible in Figure 11.20 is called Galileo Regio.

Figure 11.20 Jupiter's largest moon, Ganymede, is also the largest satellite in the solar system. The dark regions on the surface are the oldest and probably represent the original icy crust of the moon. The largest dark region visible in the Voyager 2 image in (a) is called Galileo Regio. It spans some 320 km. The lighter, younger regions are the result of flooding and freezing that occurred within a billion years or so of Ganymede's formation. The light-colored spots are recent impact craters. The resolution of the detailed image in (b) is about 3 km.

As with the inner planets, we can estimate ages on Ganymede by counting craters. We learn that the darker regions, like Galileo Regio, are the oldest parts of Ganymede's surface. These regions are the original icy surface of the moon, just as the ancient highlands on our own Moon are its original crust. The surface darkens with age as micrometeorite dust slowly covers it. The light-colored parts of Ganymede are much less heavily cratered, so they must be younger. They are Ganymede's "maria" and probably formed in a manner similar to the way that maria on the Moon were created. Intense meteoritic bombardment caused liquid water—Ganymede's counterpart to our own Moon's molten lava—to upwell from the interior and flood the impacting regions before solidifying.

Not all of Ganymede's surface features follow the lunar analogy. Ganymede has a system of grooves and ridges (shown in Figure 11.21) that may have resulted from crustal tectonic motion, much as Earth's surface undergoes mountain building and faulting at plate boundaries. Ganymede's large size indicates that its original radioactivity probably helped heat and differentiate its interior, after which the moon cooled and the crust cracked. Ganymede seems to have had some early plate tectonic activity, but the process stopped about 3 billion years ago when the cooling crust became too thick. The Galileo data suggest that the surface of Ganymede may be older than was previously thought. With the improved resolution of that spacecraft's images (Figure 11.21c), some regions believed to have been smooth, and hence young, are now seen to be heavily splintered by fractures and thus probably very old. Galileo also detected a weak magnetosphere surrounding Ganymede, making it the first moon in the solar system on which a magnetic field has been observed.

Figure 11.21 (a) "Grooved terrain" on Ganymede may have been caused by a process similar to plate tectonics on Earth. (b) The resolution of this detailed Voyager 2 image is about 3 km. (c) The image on the left was taken by Voyager 2 in 1979, that on the right by Galileo in 1996. The 55-by-35-km area shown here reveals a multitude of ever-smaller ridges, valleys, and craters, right down to the resolution limit of Galileo's camera (about 100 m, the length of a football field).

Callisto, shown in Figure 11.22, is in many ways similar in appearance to Ganymede, although it has more craters and fewer fault lines. Its most obvious feature is a huge series of concentric ridges surrounding each of two large basins. The larger of the two, on Callisto's Jupiter-facing side, is named Valhalla and measures some 3000 km across. It is clearly visible in Figure 11.22. The ridges resemble the ripples made as a stone hits water, but on Callisto they probably resulted from a cataclysmic impact with an asteroid or comet. The upthrust ice was partially melted, but it resolidified quickly, before the ripples had a chance to subside. Today, both the ridges and the rest of the crust are frigid ice and show no obvious signs of geological activity (such as the grooved terrain on Ganymede). Apparently, Callisto froze before plate tectonic or other activity could start. The density of impact craters on the Valhalla basin indicates that it formed long ago, perhaps 4 billion years in the past.

Figure 11.22 Callisto, the outermost Galilean moon of Jupiter, is similar to Ganymede in composition but is more heavily cratered. The large series of concentric ridges visible on the left of the image is known as Valhalla. Extending nearly 1500 km from the basin center, they formed when "ripples" from a large meteoritic impact froze before they could disperse completely. The resolution in this Voyager 2 image is around 10 km.