(Background, above) In this close-up image of Jupiter, taken by the Voyager 2 spacecraft, the moon Io is clearly visible. One of the most peculiar moons in the solar system, and certainly the most active, Io's multihued surface is caused by various chemical compounds deposited by currently active volcanos.

(Inset A) Color-enhanced photo of Io taken by Voyager 1 with a remarkable resolution of 7 km. The red color results largely from sulfur compounds. The white areas are probably covered with sulfur dioxide frost. And the smaller dark areas are "lava lakes," possibly composed of liquid sulfur.

(Inset B) This region on Io, known as Pele, shows the largest of its volcanos. The field of view is only several hundred km across.

(Inset C) At top left, a plume from an erupting volcano, known as Prometheus, is clearly seen at the limb of Io against the blackness of space.

(Inset D) The absence of impact craters on Io demonstrates the youth of its surface deposits. A variety of lava flows can be seen at several places on Io, like this one stretching for several hundred km from the large volcano Ra Patera.

LEARNING GOALS

Studying this chapter will enable you to:

Specify the ways in which Jupiter differs from the terrestrial planets in its physical and orbital properties.

Discuss the processes responsible for the appearance of Jupiter's atmosphere.

Describe Jupiter's internal structure and composition, and explain how these are inferred from external measurements.

Summarize the characteristics of Jupiter's magnetosphere.

Discuss the orbital properties of the Galilean moons of Jupiter, and describe the appearance and physical properties of each.

Explain how tidal forces can produce enormous internal stresses in a jovian moon, and discuss the results of those stresses.

Beyond the orbit of Mars, the solar system is very different from our own backyard. The outer solar system presents us with a totally unfamiliar environment--huge gas balls, peculiar moons, ringlike structures, and a wide variety of physical and chemical phenomena, many of which are still only poorly understood. Although the jovian planets--Jupiter, Saturn, Uranus, and Neptune--differ from one another in many ways, we will find that they have much in common, too. As with the terrestrial planets, we will learn from their differences as well as from their similarities. Our study of these alien places begins with the jovian planet closest to Earth--Jupiter, the largest planet in the solar system and a model for the other jovian worlds.

ORBITAL PROPERTIES

Back Named after the most powerful god of the Roman pantheon, Jupiter is by far the largest planet in the solar system. Ancient astronomers could not have known the planet's true size, but their choice of names was very apt.

Jupiter is the third-brightest object in the night sky (after the Moon and Venus), making it very easy to locate and study. Its sidereal orbital period is 11.9 Earth years; the orbital semi-major axis is 5.20 A.U. (778 million km), with an eccentricity of 0.05. As in the case of Mars, Jupiter is brightest when it is near opposition. Its closest approach to Earth is about 3.95 A.U., when opposition happens to occur near perihelion. Jupiter's angular size is greatest then--the planet can be up to 50´´ across, and a lot of detail can be discerned through even a small telescope.

Figure 11.1 shows two of the best views of Jupiter ever obtained from Earth--one from the ground and one from space. Figure 11.1(a) shows a ground-based photograph of the planet, and Figure 11.1(b) shows a Hubble Space Telescope image taken during the opposition of December 1990. Notice the alternating light and dark bands that cross the planet parallel to its equator and the prominent dark spot at the lower right of Figure 11.1(a).

Jupiter Rotation

Figure 11.1 (a) Photograph of Jupiter made with a large ground-based telescope, showing several of its moons. (b) A Hubble Space Telescope image of Jupiter, in true color. Features as small as a few hundred kilometers across are resolved.

MASS AND RADIUS

Unlike any of the terrestrial planets, Jupiter has many moons, with a wide range of sizes and properties. The four largest are visible from Earth with a small telescope (or even with the naked eye). They are known as the Galilean moons, after Galileo Galilei, who first observed them early in the seventeenth century. Astronomers have been able to study the motion of the Galilean moons for quite some time. Consequently, Jupiter's mass has long been known. It is 1.9 × 1027 kg, or 318 Earth masses. Jupiter has more than twice the mass of all the other planets combined.

Jupiter is such a large planet that many celestial mechanicians--those researchers concerned with the motions of interacting cosmic objects--regard our solar system as containing only two important objects--the Sun and Jupiter. To be sure, in this age of sophisticated and precise spacecraft navigation, the gravitational influence of all the planets must be considered, but in the broadest sense, our solar system is a two-object system with a lot of debris. As massive as Jupiter is, though, it is still some 1000 times less massive than the Sun. This makes studies of Jupiter all the more important, for here we have an object intermediate in size between the Sun and the terrestrial planets.

Knowing Jupiter's distance and angular size, we can easily determine its radius. It is 71,400 km, or 11.2 Earth radii. More dramatically stated, more than 1400 Earths would be needed to equal the volume of Jupiter. From the size and mass, we derive an average density of 1300 kg/m3 (1.3 g/cm3) for the planet. Here (as if we needed it) is yet another indicator that Jupiter is radically different from the terrestrial worlds. It is clear that, whatever Jupiter's composition, it is not made up of the same material as the inner planets (recall from Chapter 7 that the Earth's average density is 5500 kg/m3). Studies of the planet's internal structure indicate that Jupiter must be composed primarily of hydrogen and helium. The enormous pressures in the planet's interior greatly compress these light gases, producing the average density we observe--very high for hydrogen, but still considerably lower than that of the terrestrial planets.

Figure 11.2 All spinning objects tend to develop an equatorial bulge because rotation causes matter to push outward against the inward-pulling gravity. The size of the bulge depends on the mechanical strength of the matter and the rate of rotation. The inward-pointing arrows denote gravity, the outward arrows the "push" due to rotation.

ROTATION RATE

As with other planets, we can attempt to determine Jupiter's rotation rate simply by timing a surface feature as it moves around the planet. However, in the case of Jupiter (and, indeed, all the gaseous outer planets), there is a catch--Jupiter has no solid surface. All we see are cloud features in the planet's atmosphere. Unlike Earth's atmosphere, different parts of Jupiter's atmosphere, with no solid surface to "tie them down," move independently. Visual observations and Doppler-shifted spectral lines prove that the equatorial zones rotate a little faster (9h50m period) than the higher latitudes (9h56m period). Thus, Jupiter exhibits differential rotation--the rotation rate is not constant from one location to another. Differential rotation is not possible in solid objects like the terrestrial planets, but it is normal for fluid bodies such as Jupiter.

Observations of Jupiter's magnetosphere provide a more meaningful measurement of the rotation period. The planet's magnetic field is strong and emits radiation at radio wavelengths. Careful studies show a periodicity of 9h56m in this radio emission. Scientists assume that this measurement matches the rotation of the planet's interior, where the magnetic field arises. We see that the planet's interior rotates at the same rate as the clouds at its poles. The equatorial zones rotate more rapidly.

A rotation period of 9h56m is fast for such a large object. In fact, Jupiter has the fastest rotation rate of any planet in the solar system, and this rapid spin has altered Jupiter's shape. As illustrated in Figure 11.2, a spinning object tends to develop a bulge around its midsection. The more loosely the object's matter is bound together, or the faster it spins, the larger the bulge becomes. In objects like Jupiter, which are made up of gas or loosely packed matter, high spin rates can produce a quite pronounced bulge. Jupiter's equatorial radius (71,400 km) exceeds its polar radius (66,800 km) by about 6.5 percent.*

*Earth also bulges slightly at the equator because of rotation. However, our planet is much more rigid than Jupiter, and the effect is much smaller--the equatorial diameter is only about 40 km larger than the distance from pole to pole, a tiny difference compared with the Earth's full diameter of nearly 13,000 km. Relative to its overall dimensions, Earth is smoother and more spherical than a billiard ball.

But there is more to the story of Jupiter's shape. Jupiter's observed equatorial bulge also tells us something very important about the planet's deep interior. Careful calculations indicate that Jupiter would be more flattened than it actually is if its core were composed of hydrogen and helium alone. To account for the planet's observed shape, we must assume that Jupiter has a small, dense, probably rocky core, between 10 and 20 times the mass of the Earth. This is one of the few pieces of data we have on Jupiter's internal structure.

Galileo Mission to Jupiter Part I, Probe Separation

Galileo Mission to Jupiter Part II, Bus Arrival

Galileo Mission to Jupiter Part III, Probe on Final Approach

Galileo Mission to Jupiter Part IV, Atmospheric Re-entry

Galileo Mission to Jupiter Part V, Parachute Deployment

Galileo at Jupiter, the Main Findings

Back Jupiter is visually dominated by two features, one a series of ever-changing atmospheric bands arranged parallel to the equator and the other an oval atmospheric blob called the Great Red Spot, or often just the "Red Spot." The cloud bands, seen clearly in Figure 11.1, display many colors--pale yellows, light blues, deep browns, drab tans, and vivid reds among others. Scientists believe that chemical compounds in Jupiter's atmosphere create these different colors, but the detailed chemistry is still not completely understood. The Red Spot (shown in more detail in Figure 11.3) is one of many features associated with Jupiter's weather. It seems to be an Earth-sized hurricane that has persisted for hundreds of years.

Figure 11.3 Voyager 1 took this photograph of Jupiter's Red Spot (upper right) from a distance of about 100,000 kilometers. The resolution is about 100 km. Note the complex turbulence patterns to the left of both the Red Spot and the smaller white oval vortex below it. (For scale, planet Earth is about the size of the white oval.)

ATMOSPHERIC COMPOSITION

Spectroscopic studies of sunlight reflected from Jupiter gave scientists their first look at the planet's atmospheric composition. Radio, infrared, and ultraviolet observations later provided more details. The most abundant gas is molecular hydrogen (86.1 percent by number), followed by helium (13.8 percent). Together they make up over 99 percent of Jupiter's atmosphere. Scientists generally accept that these two gases also make up the bulk of the planet's interior. This belief is based not on direct evidence of the interior (until the recent collision of a comet with Jupiter, described in Chapter 14, there was virtually none), but largely on theoretical studies of the internal structure of the planet, such as we have already seen in the discussion of Jupiter's shape. Small amounts of atmospheric methane, ammonia, and water vapor are also found.

Unlike the gravitational pull of the terrestrial planets, the gravity of the larger jovian planets is strong enough to have retained even hydrogen. Little, if any, of Jupiter's original atmosphere has escaped. Because of the great abundance of hydrogen, all of the common elements other than helium (in particular, carbon, nitrogen, and oxygen) are chemically combined with it.

ATMOSPHERIC STRUCTURE AND COLOR

None of the atmospheric gases just listed can, by itself, account for Jupiter's observed coloration. For example, frozen ammonia and water vapor would simply produce white clouds, not the many colors actually seen. We now believe that Jupiter's clouds are arranged in several layers and are the product of complex and continuous chemical processes occurring in the planet's turbulent atmosphere. The various visible clouds lie at different levels--specifically, the white ammonia clouds generally overlie the more brightly colored layers, whose composition we will discuss in a moment. Above the clouds themselves there is a thin, faint layer of haze, created by chemical reactions similar to those that cause smog on the Earth. When we observe Jupiter's many colors, we are actually looking down to many different depths in the planet's atmosphere.

Figure 11.4 shows a diagram of Jupiter's atmosphere. The planet lacks a solid surface to use as a reference level for measuring altitude, so by convention scientists take the top of the troposphere to lie at 0 km. As on other planets, the weather on Jupiter is the result of convection in the troposphere, so the colored clouds, which are associated with planetary weather systems, all lie at negative altitudes in the diagram. The haze layer lies at the upper edge of Jupiter's troposphere, at an altitude of zero. The temperature at this level is about 110 K. Above the troposphere, as on Earth, the temperature rises as the atmosphere absorbs solar ultraviolet light. Below the haze layer, at a depth of about 40 km (shown as -40 km in Figure 11.4), lie white wispy clouds made up of ammonia ice. At these cloud tops, the temperature is approximately 125-150 K. It increases quite rapidly with increasing depth.

Figure 11.4 The vertical structure of Jupiter's atmosphere. Jupiter's clouds are arranged in three main layers, each with quite different colors and chemistry. The colors we see in photographs of the planet depend on the cloud cover. The white regions are the tops of the upper ammonia clouds. The yellows, reds, and browns are associated with the second cloud layer, which is composed of ammonium hydrosulfide ice. The lowest cloud layer is water ice and bluish in color. However, the overlying layers are sufficiently thick that this level is not seen in visible light.

A few tens of kilometers below the ammonia clouds, the temperature is a little warmer--over 200 K--and the clouds are probably made up mostly of droplets or crystals of ammonium hydrosulfide, produced by reactions between ammonia and hydrogen sulfide in the planet's atmosphere. However, instead of being white (the color of ammonium hydrosulfide on Earth), these clouds are tawny. This is the level at which atmospheric chemistry begins to play its part in determining Jupiter's appearance. Many planetary scientists believe that molecules containing the element sulfur, and perhaps even sulfur itself, play an important role in influencing the cloud colors--particularly the reds, browns, and yellows, all colors associated with sulfur or sulfur compounds. It is also possible that compounds containing the element phosphorus contribute to the coloration. At deeper levels in the atmosphere, the ammonium hydrosulfide clouds give way to clouds of water ice or water vapor. This lowest cloud layer, which is not seen in visible-light images of Jupiter, lies some 80 km below the top of the troposphere.

Deciphering the detailed causes of Jupiter's distinctive colors is a difficult task. The cloud chemistry is complex, and it is very sensitive to small changes in atmospheric conditions, such as pressure and temperature, as well as to chemical composition. The atmosphere is in incessant, churning motion, causing these conditions to change from place to place and from hour to hour. In addition, the energy that powers the reactions comes in many different forms: the planet's own interior heat, solar ultraviolet radiation, aurorae in the planet's magnetosphere, and lightning discharges within the clouds themselves. All these factors combine to keep the complete explanation of Jupiter's appearance beyond our present grasp.

ATMOSPHERIC BANDS

Astronomers generally describe Jupiter's banded appearance--and, to a lesser extent, the appearance of the other jovian worlds as well--as a series of bright zones and dark belts crossing the planet. The zones and belts vary in both latitude and intensity during the year, but the general pattern remains. These variations are not seasonal in nature--Jupiter has no seasons--but instead appear to be the result of dynamic motion in the planet's atmosphere. The light-colored zones lie above upward-moving convective currents in Jupiter's atmosphere. The dark belts are caused by the other part of the convection cycle, representing regions where material is generally sinking downward, as illustrated schematically in Figure 11.5.

Figure 11.5 The colored bands in Jupiter's atmosphere are associated with vertical convective motion. Upwelling warm gas results in the lighter-colored zones; the darker bands lie atop lower-pressure regions where cooler gas is sinking back down into the atmosphere. As on Earth, surface winds tend to blow from high- to low-pressure regions. Jupiter's rotation channels these winds into an east­west flow pattern, as indicated.

Because of the upwelling material below them, the zones are regions of high pressure; the belts, conversely, are low-pressure regions. Thus, the belts and zones are the planet's equivalents of the familiar high- and low-pressure systems that cause our weather on Earth. A major difference is that Jupiter's rapid rotation has caused these systems to wrap all the way around the planet, instead of forming localized circulating storms, as on our own world. Because of the pressure difference, the zones lie slightly higher in the atmosphere than the belts. The temperature difference between the two (recall that the temperature decreases with altitude), and the associated changes in chemical reactions, is the basic reason for their different colors.

Underlying the bands is an apparently very stable pattern of eastward and westward wind flow, often referred to as Jupiter's zonal flow. This zonal flow is evident in Figure 11.6, which shows the wind speed at different planetary latitudes measured relative to the rotation of the planet's interior (determined from studies of Jupiter's magnetic field). As we have already seen, the equatorial regions of the atmosphere rotate faster than the planet, with an average flow speed of some 85 m/s, or about 300 km/h, in the easterly direction. The speed of this equatorial flow is quite similar to that of the jet stream on Earth. At higher latitudes, there are alternating regions of westward and eastward flow, roughly symmetric about the equator, with the flow speed generally diminishing toward the poles.

Figure 11.6 The wind speed in Jupiter's atmosphere, measured relative to the planet's internal rotation rate. The alternations in wind direction are associated with the atmospheric band structure.

As Figure 11.6 shows, the belts and zones are closely related to Jupiter's zonal flow pattern. However, closer inspection shows that the simplified picture presented in Figure 11.5, with wind direction alternating between adjacent bands, as Jupiter's rotation deflects surface winds into eastward or westward streams, is really too crude to explain the actual flow. Scientists now believe that the interaction between convective motion in Jupiter's atmosphere and the planet's rapid rotation channels the largest eddies into the observed zonal pattern, but that smaller eddies cause irregularities in the flow. Near the poles, where the zonal flow disappears, the band structure vanishes also.

THE WEATHER ON JUPITER

In addition to the zonal flow pattern, Jupiter has many "small-scale" weather patterns. The Great Red Spot, shown in Figure 11.3--a close-up photograph taken as the Voyager 1 spacecraft glided past in 1979--is a prime example. The Great Red Spot was first reported by the British scientist Robert Hooke in the midseventeenth century, so we can be reasonably sure that it has existed in one form or another for more than 300 years, and it might well be much older. Voyager observations show the spot to be a region of swirling, circulating winds, rather like a whirlpool or a terrestrial hurricane--a persistent and vast atmospheric storm. The size of the Spot varies, although it averages about twice the diameter of Earth. Its present dimensions are roughly 25,000 km by 15,000 km. It rotates around Jupiter at a rate similar to the planet's interior, perhaps suggesting that its roots lie far below the atmosphere.

The origin of the Spot's red color is unknown, as is its source of energy, although it is generally supposed that the Spot is somehow sustained by Jupiter's large-scale atmospheric motion. Repeated observations show that the gas flow around the Spot is counterclockwise, with a period of about six days. Turbulent eddies form and drift away from its edge. The Spot's center, however, remains quite tranquil in appearance, like the eye of a hurricane on Earth. The zonal motion north of the Spot is westward, while that to the south is eastward (see Figure 11.7), supporting the idea that the Spot is confined and powered by the zonal flow. However, the details of how this occurs are still a matter of conjecture. Computer simulations of the complex fluid dynamics of Jupiter's atmosphere are only now beginning to hint at answers.

Figure 11.7 These Voyager 2 close-up views of the Red Spot, taken four hours apart, show clearly the turbulent flow around its edges. The general direction of motion of the gas north of (above) the Spot is westward (to the left), while gas south of the Spot flows east. The Spot itself rotates counterclockwise, suggesting that it is being "rolled" between the two oppositely directed flows. The colors have been exaggerated somewhat to enhance the contrast.

Actually, storms, which as a rule are much smaller than the Red Spot, may be quite common on Jupiter. Spacecraft photographs of the dark side of the planet reveal both auroral activity and bright flashes resembling lightning. The Voyager mission discovered many smaller light- and dark-colored spots that are also apparently circulating storm systems. Note the several white ovals in Figures 11.3 and 11.7, south of the Red Spot. Like the Red Spot, they rotate counterclockwise. Their high cloud tops give them their color. These particular white ovals are known to be at least 40 years old. Figure 11.8 shows a brown oval, a "hole" in the clouds that allows us to look down into the lower atmosphere. For unknown reasons, brown ovals appear only in latitudes around 20°N. Although not as long-lived as the Red Spot, these systems can persist for many years or even decades.

Figure 11.8 A brown oval in Jupiter's northern hemisphere. Its color comes from the fact that it is actually a break in the upper cloud layer, allowing us to see deeper in. The oval's length is approximately equal to the diameter of the Earth.

We cannot explain their formation, but we can offer at least a partial explanation for the longevity of these storm systems on Jupiter. On Earth a large storm, such as a hurricane, forms over the ocean and may survive for many days, but it dies quickly once it encounters land. The Earth's continental landmasses disrupt the flow patterns that sustain the storm. Jupiter has no continents, so once a storm is established and has reached a size at which other storm systems cannot destroy it, apparently little affects it. The larger the system, the longer its lifetime.

Back On the basis of Jupiter's distance from the Sun, astronomers had expected to find that the temperature of the cloud tops was around 105 K. At that temperature, they reasoned, Jupiter would radiate back into space exactly the same amount of energy as it received from the Sun. When radio and infrared observations were first made of the planet, however, astronomers found that its Planck spectrum corresponded to a temperature of 125 K instead. Subsequent measurements, including those of Voyager, as we have just seen, have verified that finding. Although a difference of 20 K may seem small, recall from Chapter 5 that the energy emitted by a planet grows as the fourth power of the surface temperature (in Jupiter's case, the temperature of the cloud tops). A planet at 125 K radiates (125/105)4, or about twice as much energy as a 105 K planet. Put another way, Jupiter actually emits about twice as much energy as it receives from the Sun. Thus, unlike any of the terrestrial planets, Jupiter must have its own internal heat source.

What is responsible for Jupiter's extra energy? It is not the decay of radioactive elements within the planet--that must be occurring, but not at nearly the rate necessary to produce the temperature we record. Nor is it the process that generates energy in the Sun, nuclear fusion--the temperature in Jupiter's interior, high as it is, is far too low for that. (See Interlude 11-1.) Instead, astronomers theorize that the source of Jupiter's excess energy is the slow escape of gravitational energy released during the planet's formation. As the planet took shape, some of its energy was converted into heat in the interior. That heat is still slowly leaking out through the planet's heavy atmospheric blanket, resulting in the excess emission we observe. Despite the huge amounts of energy involved--Jupiter's energy emission is about 4 1017 watts more than it receives from the Sun--the energy loss is quite slight compared with the planet's total energy. A simple calculation indicates that the average temperature of the interior of Jupiter falls by only about a millionth of a kelvin per year.

Jupiter's clouds, with their complex chemistry, are probably less than 200 km thick. Below them, the temperature and pressure steadily increase, as the atmosphere becomes the "interior" of the planet. Much of our knowledge of Jupiter's interior comes from theoretical modeling. Planetary scientists use all available bulk data on the planet--mass, radius, composition, rotation, temperature, and so on--to construct a model of the interior that agrees with observations. Our statements about Jupiter's interior are, then, really statements about the model that best fits the facts. However, because the interior consists largely of hydrogen and helium--two simple gases whose physics we think we understand well--we can be fairly confident that Jupiter's internal structure is now understood.

Figure 11.9 shows that both the temperature and the density of Jupiter's atmosphere increase with depth below the cloud cover. However, no "surface" of any kind exists anywhere inside. Instead, Jupiter's atmosphere just becomes denser and denser, because of the pressure of the overlying layers. At a depth of a few thousand kilometers, the gas makes a gradual transition into the liquid state. By a depth of about 20,000 km, the pressure is about 3 million times greater than atmospheric pressure on Earth. Under those conditions, the hot liquid hydrogen is compressed so much that it undergoes another transition, this time to a "metallic" state, with properties in many ways similar to a liquid metal. Of particular importance for Jupiter's magnetic field, this metallic hydrogen is an excellent conductor of electricity.

Figure 11.9 Jupiter's internal structure, as deduced from Voyager measurements and theoretical modeling. The outer radius represents the top of the cloud layers, some 70,000 km from the planet's center. The density and temperature increase with depth, and the atmosphere gradually liquefies over the outermost few thousand kilometers. Below a depth of 20,000 km, the hydrogen behaves like a liquid metal. At the center of the planet lies a large rocky core, somewhat terrestrial in composition but much larger than any of the inner planets. Although very uncertain, the temperature and pressure at the center are probably about 40,000 K and 50 Mbar, respectively. (One bar is the pressure of Earth's atmosphere at sea level. One Mbar is one million bars.)

As we have already mentioned, Jupiter's observed flattening requires that there be a small, dense core at its center, containing perhaps 15 times the mass of the Earth. The core's exact composition is unknown, but planetary scientists think that it contains heavier materials than the rest of the planet. Present best estimates indicate that it consists of "rocky" materials, similar to those found in the terrestrial worlds. In fact, it now appears that all of the jovian planets contain large rocky cores and that the formation of such a large "terrestrial" planetary core is a necessary stage in the process of building up a gas giant. Because of the high pressure at the center of Jupiter--approximately 50 million times that on Earth's surface, or 10 times that at its center--the core must be compressed to quite high densities (perhaps twice the core density of the Earth). It is probably not much more than 20,000 km in diameter, and the central temperature may be as high as 40,000 K.

Back For decades, ground-based radio telescopes monitored radiation leaking from Jupiter's magnetosphere, but only when the Pioneer and Voyager spacecraft reconnoitered the planet in the mid-1970s did astronomers realize the full extent of the planet's magnetic field. Jupiter, as it turns out, is surrounded by a vast sea of energetic charged particles, mostly electrons and protons, somewhat similar to Earth's Van Allen belts but much larger. The radio radiation detected on Earth is emitted when these particles are accelerated to very high speeds--close to the speed of light--by Jupiter's powerful magnetic field. This radiation is several thousand times more intense than that produced by Earth's magnetic field, and represents a potentially serious hazard for manned and unmanned space vehicles alike. Sensitive electronic equipment (not to mention even more sensitive human bodies) would require special protective shielding to operate for long in this hostile environment.

Direct spacecraft measurements show Jupiter's magnetosphere to be almost 30 million km across, roughly a million times more voluminous than Earth's magnetosphere, and far larger than the entire Sun. As in the case of Earth, the size of Jupiter's magnetosphere is determined by the interaction between the planet's magnetic field and the solar wind. Outside, the solar wind particles flow freely away from the Sun, past the planet. Inside, their motions are governed by the planetary magnetic field. Jupiter's magnetosphere has a long tail extending away from the Sun at least as far as Saturn's orbit (over 4 A.U. farther out from the Sun), as sketched in Figure 11.10. On the sunward side, the magnetopause--the boundary of Jupiter's magnetic influence on the solar wind--lies about 3 million km from the planet. The outer magnetosphere appears to be quite unstable, sometimes deflating in response to "gusts" in the solar wind, then growing back. In the inner magnetosphere, Jupiter's rapid rotation has forced most of the charged particles into a flat current sheet, lying on the planet's magnetic equator. The portion of the magnetosphere close to Jupiter is sketched in Figure 11.11. Notice that the planet's magnetic axis is not exactly aligned with its rotation axis, but is inclined to it at an angle of approximately 10°.

Figure 11.10 The Pioneer 10 spacecraft did not detect any solar particles while moving behind Jupiter. Accordingly, as sketched here, Jupiter's magnetosphere apparently extends beyond the orbit of Saturn.

Figure 11.11 Jupiter's inner magnetosphere is characterized by a flat current sheet, consisting of charged particles squeezed into the magnetic equatorial plane by the planet's rapid rotation. The plasma torus is a ring of charged particles associated with the moon Io; it is discussed on p.246.

Ground-based and spaceborne observations of the radiation emitted from Jupiter's magnetosphere imply that the intrinsic strength of the planet's magnetic field is nearly 20,000 times greater than Earth's. The existence of such a strong field further supports our theoretical model of the interior; the conducting liquid interior that is thought to make up most of the planet should combine with Jupiter's rapid rotation to produce a large dynamo effect and a strong magnetic field, just as observed.

Io Cutaway

Galileo Flyby of Io

Oxygen on Europa, Ozone on Ganymede, Iron inside Io

Back Jupiter has at least 16 moons. In many ways, the entire Jupiter system resembles a miniature solar system. Its four largest moons--the Galilean satellites--are each comparable in size to Earth's Moon. 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 moon and greatly expanding our knowledge of these small, distant 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 to all the other moons (and to 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, 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). The parallel with the inner solar system continues with the realization that their densities decrease with increasing distance from Jupiter. It is quite likely that the inner two Galilean moons, Io and Europa, have a rocky composition, possibly similar to the crusts of the terrestrial planets. The two outer Galilean 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. Figure 11.12 compares the appearances and sizes of the four Galilean satellites. Figure 11.13 shows two of the moons photographed against the background of their parent planet.

Figure 11.12 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 11.13 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: Io and Europa are each 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.)

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. But let us point out that not all of the properties of the Galilean moons find analogs in the inner solar system. For example, because of Jupiter's tidal effect, all four Galilean satellites are in states of synchronous rotation, so that 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. 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

Back 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 11.14, 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 11.15, 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 the Earth's insides.

Figure 11.14 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 11.15 One of Io's volcanoes was caught in the act of erupting while the Voyager spacecraft flew past this fascinating moon. Surface features here are resolved to within a few kilometers. In (a), the volcano's umbrellalike profile shows clearly against the darkness of space. The plume measures about 100 km high and 300 km across. In (b), several jets of volcanic ejecta (dark against Io's brighter surface) can be discerned as Voyager prepares to "overfly" another volcano.

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 11.14 and 11.15 are volcanoes.) Its surface is exceptionally smooth, apparently the result of molten matter constantly filling 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.

Io's volcanism has a major effect on Jupiter's magnetosphere. All of 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 11.16; see also Figure 11.11), 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 of its atoms are ionized.) It is quite easily detectable from Earth, but before Voyager its origin was unclear. 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 11.16 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 the 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. 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 in and of themselves, 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. 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, Io is constantly energized by the ever-changing distortion of its interior. This generation of large amounts of heat within Io ultimately causes huge jets of gas and molten rock to squirt out of the surface. 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 the Earth, but the term volcano has stuck. Researchers estimate that the total amount of heat generated in 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 11.17) is a very different world from Io. Lying outside Io's orbit, 670,000 km (9.4 Jupiter radii) from Jupiter, it has relatively few craters on its surface, suggesting geologic youth. Recent activity must have erased the scars of ancient meteoritic impacts. Europa's surface does display 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 the Earth's polar oceans.

Figure 11.17 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 criss-crossing the surface is uncertain. The resolution of the Voyager mosaic in (a) is about 5 km. The two images below it (b and c) display even finer detail.

Some researchers have theorized that Europa is covered completely by an ocean of liquid water whose top is frozen at the low temperatures that prevail so far from the Sun. The cracks are attributed to the tidal influence of Jupiter and the gravitational pulls of the other Galilean satellites, although these forces are weaker than those powering Io's volcanic activity. Other planetary scientists suggest that Europa's fractured surface is instead related to some form of tectonic activity, one involving ice rather than rock. If the markings truly are fault lines of ice, then this moon is probably still quite active. If Europa does have a liquid ocean below the ice, it opens up many interesting avenues of speculation into the possible development of life there.

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). Their densities are each 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 11.18, 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 11.18 is called Galileo Regio.

Figure 11.18 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 maria on the Moon. Intense meteoritic bombardment caused liquid water--Ganymede's counterpart to our own Moon's molten lava--to upwell from the interior, flooding 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 11.19) that may have resulted from crustal tectonic motion, much as the Earth's surface undergoes mountain building and faulting at plate boundaries. Ganymede's large size indicates that its original radioactivity probably helped to heat and differentiate its partly rocky 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.

Figure 11.19 "Grooved terrain" on Ganymede may have been caused by a process similar to plate tectonics on Earth. The resolution of the detailed image (b) is about 3 km.

Callisto, shown in Figure 11.20, 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 11.20. 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 11.20 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 here is around 10 km.

Yet another remarkable finding of the 1979 Voyager missions was the discovery of a faint ring of matter encircling Jupiter in the plane of the planet's equator (see Figure 11.21). This ring lies roughly 50,000 km above the top cloud layer of the planet, inside the orbit of the innermost moon. A thin sheet of material may extend all the way down to Jupiter's cloud tops, but most of the ring is confined within a region only a few thousand kilometers across. The outer edge of the ring is quite sharply defined. In the direction perpendicular to the equatorial plane, the ring is only a few tens of kilometers thick. The small, dark particles that make up the ring may have been chipped off by meteorite impacts on two small moons--Metis and Adastrea, discovered by Voyager--that lie very close to the ring itself. Despite its different appearance and structure, Jupiter's ring can perhaps be best understood by studying the most famous ringed planet--Saturn--so we will postpone further discussion of ring properties until the next chapter.

Figure 11.21 Jupiter's faint ring, as photographed (nearly edge-on) by Voyager 2. The ring, made up of dark fragments of rock and dust, possibly chipped off the innermost moons by meteorites, was unknown before the two Voyager spacecraft arrived at the planet. It lies in Jupiter's equatorial plane, only 50,000 km above the cloud tops.