11.5 The Moons of Jupiter

At last count, Jupiter has at least 28 satellites. The four largest—the Galilean moons—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 features 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. Table 11.1 presents some general properties of Jupiter’s moon system.

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.

The latest addition to Jupiter’s family of satellites is a group of twelve small moons spotted in 2000 by researchers using dedicated large telescopes and specially designed instruments and software to scan large areas of the sky for very faint objects. All the new moons are very small—probably less than 5 or 10 km across—and their masses are unknown. Their orbits have not yet been accurately determined, but are all moderately eccentric, with semimajor axes between 10 and 25 million km, and are inclined at 15–30º to Jupiter’s equatorial plane. Most of the orbits are retrograde. In short, the properties of these newcomers seem quite consistent with those of the other eight outlying satellites just described. Very likely they have the same origin—these moons are space "junk" captured by Jupiter, probably long ago.

THE GALILEAN MOONS AS A MODEL OF THE INNER SOLAR SYSTEM

Jupiter’s Galilean moons have several interesting parallels with 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 11.14 is a Voyager 1 image of Io and Europa, with Jupiter providing a spectacular backdrop. Figure 11.15 shows the four Galilean moons to scale.

Figure 11.16 Galilean Moon Interiors 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 and ice mix in Callisto. Both Ganymede and Europa are thought to have layers of liquid water beneath their icy surfaces.

The similarity to the inner solar system continues with the fact that the moons’ densities decrease with increasing distance from Jupiter. (Sec. 6.4) 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 11.16). 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 water/ice outer shell between 100 and 200 km thick. The two outer moons, Ganymede and Callisto, are clearly deficient in rocky materials. Lighter materials, such as water and 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.

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 were mission planners in learning more about the Galilean moon system that the already highly successful Galileo mission was extended for two more years to allow for even more detailed study, particularly of Europa. (In fact, the renamed Galileo Europa Mission has so far lasted well into 2001.) The Galilean moons were 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, all four Galilean satellites are locked into states of synchronous rotation by Jupiter’s strong tidal field, 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. (Sec. 8.4) 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. (Discovery 6-1) This may be the result of a complex, but poorly understood, three-body resonance in the Galilean moon system, something not found among the terrestrial worlds.

IO: THE MOST ACTIVE MOON

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 Moon, but there the resemblance ends. Shown in Figure 11.17, 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 four months later.

By the time Galileo arrived in 1995, several of the volcanoes observed by Voyager had subsided. However, many new ones were seen—in fact, Galileo found that Io’s surface features can change significantly in as little as a few weeks. In Figure 11.18, one volcano is seen ejecting matter to an altitude of over 100 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. According to Galileo’s instruments, lava temperatures generally range from 650 to 900 K, the higher end of the range implying that at least some of the volcanism is similar to that found on Earth. However, temperatures as high as 2000 K—far hotter than any earthly volcano—have been measured at some locations.

The orange color immediately surrounding the volcanoes 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 11.17 and 11.18 are volcanoes.) Its surface is exceptionally smooth, mostly varying in altitude by less than about 1 km, although some volcanoes are several kilometers high. The smoothness is apparently the result of molten matter that constantly fills in any "dents and cracks." This remarkable moon has the youngest surface of any known object in the solar system. Io also has a thin, temporary atmosphere made up primarily of sulfur dioxide, presumably the result of gases ejected by volcanic activity.

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 11.19; see also Figure 11.13), 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. In fact, neutral atoms have also been observed in the Io plasma torus.) 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.

What causes such astounding volcanic activity on Io? That moon is far 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 exerts 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. 8.4) 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.

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. 8.4) 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 (100 m) 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. 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: LIQUID WATER LOCKED IN ICE

Europa (Figure 11.20) is a very different world from Io. Lying outside Io’s orbit, 671,000 km (9.4 Jupiter radii) from Jupiter, Europa showed relatively few craters on its surface in images taken by Voyager, suggesting geologic youth—perhaps just a few million years. Recent activity has erased any scars of ancient meteoritic impacts. The dark areas are rocky deposits that may have come from the moon’s interior, or may have been swept up by Europa as it moved in its orbit. Europa’s surface also displays a vast network of lines crisscrossing bright, clear fields of water ice. Some of these linear "bands," or fractures, extend halfway around the satellite and resemble in some ways the pressure ridges that develop in ice floes on Earth’s polar oceans.

Before Galileo’s arrival, some researchers had theorized that Europa might be completely covered by an ocean of liquid water whose top is frozen at the low temperatures prevailing 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 strongly support the former idea. Figure 11.20(d) is a Galileo image of this weird moon, showing what look like "icebergs"—flat chunks of ice that have been broken apart, moved several kilometers, and reassembled, perhaps by the action of water currents below. Mission scientists estimate that Europa’s surface ice may be several kilometers thick and that there may be a 100-km-deep liquid ocean below it.

Other detailed images of the surface support this hypothesis. Figure 11.21(a) shows a region where Europa’s icy crust appears to have been pulled apart and new material has filled in the gaps between the separating ice sheets. Elsewhere on the surface, Galileo found what appeared to be the icy equivalent of lava flows on Earth—regions where water apparently "erupted" through the surface and flowed for many kilometers before solidifying. The "puddle" shown in Figure 11.21(b) strongly suggests local flooding of the terrain. The scarcity of impact craters on Europa implies that the processes responsible for these features did not stop long ago. Rather, they must be ongoing.

Further evidence comes from studies of Europa’s magnetic field. Magnetic measurements made by Galileo on repeated flybys of the moon revealed that Europa has a weak magnetic field that constantly changes strength and direction. This finding is entirely consistent with the idea that the field is generated by the action of Jupiter’s magnetism on a shell of electrically conducting fluid about 100 km below Europa’s surface—in other words, the salty liquid water layer suggested by the surface observations. These results convinced quite a few skeptical scientists of the reality of Europa’s ocean.

The likelihood that Europa has an extensive layer of liquid water below its surface ice 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). Europa may well contain more liquid water than exists on our entire planet! Of course, the existence of water does not necessarily imply the emergence of life. Europa, even in its liquid ocean, is still a hostile environment compared with Earth. Nevertheless, the possibility, even a remote one, of life on Europa was an important motivating factor in the decision to extend the Galileo mission for two (now three) more years. Astronomers now eagerly await the launch of NASA’s proposed Europa Orbiter mission in 2003.

GANYMEDE AND CALLISTO: FRATERNAL TWINS

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 11.22, 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 11.22 is called Galileo Regio.

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 11.23) 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 11.23c), some regions believed to have been smooth, and hence young, are now seen to be heavily splintered by fractures and thus probably very old.

In 1996, Galileo detected a weak magnetosphere surrounding Ganymede, making it the first moon in the solar system on which a magnetic field had been observed, and implying that Ganymede has a modest iron-rich core. The moon’s magnetic field is about one percent that of Earth. In December 2000, the magnetometer team reported fluctuations in the field strength similar to those near Europa, suggesting that Ganymede too may have liquid or "slushy" water under its surface. Recent observations of surface formations similar to those attributed to flowing water "lava" on Europa appear to support this view.

Callisto, shown in Figure 11.24, 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 11.24. 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. Yet even on this frozen world there are hints—as yet unconfirmed—from Galileo’s magnetometers that there might be a thin layer of water, or more likely slush, deep below the surface.

Ganymede’s internal differentiation indicates that the moon was largely molten at some time in the past, yet Callisto is undifferentiated, and hence apparently never melted. Researchers are uncertain why two such similar bodies should have evolved so differently. Complicating things further is Ganymede’s magnetic field and possible subsurface liquid water, which suggest that the moon’s interior may still be relatively warm. If that is so, then Ganymede’s heating and differentiation must have happened relatively recently—less than a billion years ago, based on recent estimates of how rapidly the moon’s heat escapes into space. Scientists have no clear explanation for how this could have occurred. Heating by meteoritic bombardment ended too early, and radioactivity probably could not have provided enough energy at this late time. (Sec. 7.3) Some astronomers speculate that interactions among the inner moons, possibly related to the 1:2:4 near-resonance mentioned earlier, may have been responsible. These interactions might have caused Ganymede’s orbit to change significantly about one billion years ago, and prior tidal heating by Jupiter could have helped melt the moon’s interior.

Concept Check

What is the ultimate source of all the activity observed on Jupiter’s Galilean satellites?

Why are scientists so interested in the existence of liquid water on Europa and Ganymede?