As we did in Chapter 8, let us begin our discussion of the Jovian planets with an overview of the general features of these bodies. After which we will go into a comparative discussion of more specific aspects.

9.1.1. Jupiter

Fifth planet from the Sun, Jupiter is the largest and most massive of the planets in the Solar System. In our night sky it glows with a bright, steady yellow light, outshining the stars. The mean diameter of Jupiter is about 11 times greater than that of Earth (Table 9.1) and Jupiter is more than 1000 times larger in volume than Earth, as one can see in Figure 9.1. Jupiter's mass, however, is barely more than 300 times that of Earth, even though it exceeds the combined masses of all the other bodies orbiting the Sun. Thus its mean density is about one-fourth that of the Earth. Because its axis is tilted only 3o from the perpendicular to its orbital plane, the planet has little seasonal change.

[Table 9.1]

[Figure 9.1]

Not all portions of the visible layers of Jupiter, which appear as alternating dark and light bands parallel to the equator, rotate in unison. The equatorial region completes its rotation several minutes sooner than adjacent higher latitudes. This phenomenon is known as differential rotation and is possible in fluid media, such as gases. It is not something one expects a solid body, such as the surface of a Terrestrial planet, to do. Figure 9.1b shows the consequences of differential rotation for the outer layers. Jupiter's rapid 10-hour rotation and low density combine to flatten the planet about 6 percent in its polar diameter. Again, this is more characteristic of a fluid body that will readily deform than of a solid body that does not easily flow. The dark-band structure is composed of reddish and brown shades with irregular patches of gray, blue, and white clouds. The light zones are primarily yellow in color. The entire band structure is constantly undergoing changes in color and intensity. Clearly what we are viewing are clouds in Jupiter's atmosphere and not a solid surface such as the Terrestrial planets have. Most striking of all the atmospheric features is the Great Red Spot, which has been observed for at least 300 years. It is immense, being about four times the size of Earth.

In the early days of radio astronomy, Jupiter was found to be an intense source of radio radiation. If this radiation were just part of the planet's thermal radiation, then Jupiter would have to be extraordinarily hot. Since it is not hot, the radiation must be due to nonthermal processes, such as free electrons spiraling about magnetic lines of force.

[Box - Voyager 1 and 2, A Space Odyssey, Figure 9.2]

9.1.2. Saturn

Its rings make Saturn, sixth planet from the Sun, one of the most remarkable objects in the heavens. Brighter than all the stars except Sirius and Canopus, it shines with a steady ashen color. Saturn is second among the planets in mass and size (Table 9.1). The mean diameter of Saturn minus its ring system is almost 10 times that of the Earth, and its mass is about 100 times greater. Its density is the lowest of any planet, 0.7 times that of water. The small mean density leads to the often quoted observation that if you could find a lake large enough, Saturn would float in it, being lighter than water. Rapid rotation (a rotation period of a little over 10 hours) and an unusually low density give it more polar flattening than any other planet, about 11 percent.

Saturn is twice as far from us as Jupiter, but the markings that we can see on the noticeably flattened disk of Saturn faintly resemble the banded cloud structure of Jupiter's atmosphere (Figure 9.3). The coloration is more restrained, and the details are less distinct. On rare occasions a bright spot may appear. Thus, as in the case of Jupiter, Saturn is a fluidlike body rather than solid, like a Terrestrial planet. As is true for Jupiter, astronomers have also detected weak radio emissions in low-frequency bursts from Saturn that are synchronized with its 10.2-hour rotation period.

[Figure 9.3]

Saturn's axis of rotation is inclined by 29o to its orbital plane. Since the plane of its rings is perpendicular to its rotation axis, the rings do not lie in the orbital plane and therefore present a varying aspect to Earth as the planet goes through its roughly 30-year orbital period. Figure 9.3 shows geometrically how this occurs, along with photographs of the planet with its rings seen at four different angles to the line of sight. When seen almost edgewise, every 15 or so years, the rings almost disappear from sight, indicating that they are very thin compared to their radius. Most of Saturn's satellites orbit in the same plane as the rings, the planet's equatorial plane, and orbit outside the rings. As is evident in Table 10.1, Titan is one of the most massive satellites in the Solar System, and it is one that has been known for some time to possess an atmosphere.

9.1.3. Uranus

"In examining the small stars in the neighborhood of H Geminorum I perceived one that appeared larger than the rest; being struck with its uncommon appearance...I suspected it to be a comet." So wrote William Herschel on the night of March 13, 1781 in his observing journal. Herschel and other astronomers first believed the newly found object to be a comet and vainly tried to derive a cometary orbit for it. It was almost a year before they realized that this was a new planet.

Uranus has a radius four times larger than that of Earth and a mass almost 15 times greater. Although Uranus is somewhat larger than the more distant Neptune, it is less massive by about 15 percent. Consequently, it has a smaller mean density than Neptune but one that is larger than that of Saturn. Its average density is slightly higher than that of water. In a large telescope the slightly flattened disk is a light apple green in color.

Nearly 3 billion kilometers from Earth, Uranus presents an almost featureless appearance. Although a few atmospheric features have been reported, no extensive data on them exists. As with Jupiter and Saturn, we are probably seeing clouds in Uranus's atmosphere rather than a solid surface.

Uranus's rotation is peculiar in that its axis is tilted 98o to the perpendicular to its orbital plane--that is, it lies on its "side," so that we see it rotate in the reverse direction barely. For Uranus the retrograde rotation is due to the peculiar inclination of its axis, whereas for Venus it is a true reverse rotation. When its axis is along our line of sight every 42 years (half the sidereal period), we observe either its sunlit northern or southern hemisphere, while the opposite hemisphere is dark. One-quarter or three-quarters of its period later (21 years or 62 years), its axis is at right angles to our line of sight, and we observe both the northern and southern hemispheres, as shown in Figure 9.4.

[Figure 9.4]

The reason for the axis of rotation to be lying nearly in the orbital plane of the planet is probably the result of several forces conspiring to push the planet into this state. Since the planet has most likely been in this state since shortly after its formation, we can only speculate that these forces would have been gravitational interactions among its components, collisions with interplanetary debris, and the retarding drag forces exerted by gas left over from the planet's formation.

The two brightest satellites of Uranus, Titania and Oberon, were discovered by William Herschel in 1787, only 6 years after he discovered the planet itself. In all, the planet has five known satellites, which are visible in the infrared photograph in Figure 9.5. All the satellites move in nearly circular orbits that lie close to the equatorial plane of Uranus, the same plane as the ring system, but well outside the rings. In these respects, Uranus is similar to Saturn. The ring system was accidentally discovered in 1977 as a result of observations from an airborne telescope that was being used to remeasure Uranus's diameter and study its atmosphere as the planet passed over a background star. The expectations of future findings produced by such accidental events as the discovery of Uranus's rings contribute to the excitement and allure of science among not only the general public, but scientists as well.

[Figure 9.5]

9.1.4. Neptune

After Uranus had been discovered accidentally, astronomers were long perplexed that even allowing for the perturbations of Jupiter and Saturn, Uranus's orbital motion was less predictable than that of other planets. The discrepancy was finally resolved in 1845 and 1846 by two astronomers, John Adams (1819-1892) in England and Urbain Leverrier (18ll-1877) in France. By a brilliant application of Newton's law of gravitation they arrived independently at the conclusion that there must be a disturbing body beyond the orbit of Uranus.

Leverrier's results were communicated to Johann Galle (1812-1919) of the Berlin Observatory, who received the information on September 23, 1846. Within half an hour after beginning his search, Galle located the new planet among a group of eight stars whose positions had been charted on a recently prepared map. Recent historical research suggests that Galileo probably saw Neptune in December of 1612 and January of 1613, fully 233 years before Galle found it, but he did not recognize that it was a planet. We know that Neptune passed extremely close to Jupiter, which Galileo was observing during that time.

Looking at Neptune through a telescope, we see a slightly flattened, bluish-green, almost featureless disk. Observers at times have reported irregular, indistinct markings and a bright equatorial zone, although observations of the planet are very difficult to make and subject to some degree of doubt.

Neptune's diameter is about 3.5 times that of Earth. Its mass is 17 times greater, and its mean density is one-third that of Earth. Neptune is about 30 AU from the Sun or 30 times Earth's distance. Thus the angular diameter of the Sun is one-thirtieth of what it is from Earth. For us the Sun has an angular diameter of 0.5o, or 30 minutes of arc, so that from Neptune its angular diameter is 1 minute of arc. We have tried to illustrate this very pronounced difference in Figure 9.6. At a distance of 30 AU, Neptune's orbital, or sidereal, period is almost 165 years. Thus it has yet to complete one orbit of the Sun since its discovery in 1846.

[Figure 9.6]

The larger of Neptune's two satellites, Triton, was discovered less than a month after the planet. It orbits Neptune in about 6 days in a direction opposite to the planet's eastward rotation. The orbital plane in which Triton moves is inclined to the equatorial plane of Neptune. Triton appears to be somewhat larger than the Moon, but its mass is only about 80 percent that of the Moon, producing a lower mean density.

There are many aspects of a planet's atmosphere that astronomers want to know about, such as chemical composition, temperature, density, cloud composition, winds, and how these change with height, position over the surface, and time. Many of these details are not available for Earth's atmosphere, much less for the atmospheres of other planets. But from Voyager 1 and 2 we have learned a great deal about the atmosphere of the Jovian planets (Figure 9.7). Probably the most fundamental piece of information necessary for understanding a planet's atmosphere is the atmosphere's vertical temperature structure, such as shown in Figure 7.8 for Earth and Figure 9.8 for Jupiter and Saturn. On the way up through Jupiter's and Saturn's tropospheres, the measured temperature profile first declines and then increases into the stratosphere, where photons from the Sun can be directly absorbed.

[Figure 9.7]

9.2.1. Composition of Jupiter's and Saturn's Atmospheres

The first constituents of Jupiter's atmosphere to be identified were methane and ammonia in the 1930s. Some 30 years later, the most abundant element, hydrogen, was identified and estimated to be 1000 times more prevalent than methane and ammonia. From these identifications, estimates for the hydrogen, carbon, nitrogen, and oxygen abundances indicate that Jupiter's chemical composition (and similarly for Saturn) is more like that of the Sun than like that of the Terrestrial planets. In the 1970s and 1980s, primarily through infrared observations, several additional molecules were found to be minute constituents of Jupiter's atmosphere. Many of these molecules are probably also present in Saturn's atmosphere, but Saturn is colder than Jupiter, so that some compounds are probably frozen into solid crystals; thus they are not in a gaseous state capable of being observed spectroscopically. Table 9.2 summarizes the abundances of the major constituents of Jupiter's and Saturn's atmospheres (compare with Table 8.2 for the Terrestrial planets).

[Table 9.2]

Helium, the second most abundant element in the composition of the Sun, and presumably of Jupiter and Saturn, is not directly observable by spectroscopic means. Data from Pioneer and Voyager missions provide means for indirect determinations of the number of helium atoms per unit volume; the values derived, 10 percent for Jupiter and 6 percent for Saturn, are consistent with the solar composition hypothesis.

The most conspicuous aspect of Jupiter's and Saturn's atmospheres in visible light is their clouds. Knowing something about the atmosphere's vertical temperature profile and chemical composition provides clues as to what are the basic constituents of the clouds. For Jupiter and Saturn there appear to be three distinct cloud layers, as we have tried to show in Figure 9.8. The lowest layer is composed of water ice crystals or possibly liquid drops, the next of ammonium hydrosulfide crystals, and the highest of ammonia crystals. The middle one can also be thought of as a compound of the more elementary molecules ammonia and hydrogen sulfide. All the molecules forming the basic cloud particles should produce white particles, so other molecules must be responsible for coloring the clouds, which are red, yellow, brown, blue, and white. The most likely coloring agent is sulfur, which forms a variety of colored particles depending on molecular structure. This supposition has not been confirmed.

[Figure 9.8]

Infrared images of Jupiter and Saturn show that cloud color also correlates with altitude. Seen from outside, blue clouds lie at the deepest levels in the atmosphere and are visible only through holes in the upper clouds (Figure 9.8). Brown clouds are the next highest, above which lie white clouds, and finally, red clouds are the top layer. Compared to Jupiter, the greater spread in altitude for clouds in Saturn's atmosphere results from the smaller mass of Saturn, whose gravity is not as effective in compressing the atmosphere as is the more massive Jupiter.

9.2.2. Atmospheric Dynamics for Jupiter and Saturn

The alternate light- and dark-colored cloud bands paralleling Jupiter's and Saturn's equator are constantly undergoing changes in color and intensity. Apparently this is because of the formation or dissolution of clouds of differing chemical compositions at different altitudes. There are the large-scale patterns, such as the bands themselves and the Great Red Spot on Jupiter, that last for years and sometimes centuries. This complex behavior betrays an involved atmospheric dynamics for both planets.

The dominant observable motions in the atmospheres are alternating eastward (direction of rotation) and westward winds that correlate with the colored bands. As shown in Figure 9.9, Jupiter has five or six eastward and westward moving wind streams in each hemisphere, while Saturn has fewer but stronger ones. These winds are measured relative to each planet's rotation. In the case of Earth, there is only one low-latitude westward wind stream, known as the trade winds, and one midlatitude eastward-moving jet stream. Jupiter and Saturn also have some vertical streaming.

[Figure 9.9]

Evidence suggests that these east-west winds have been constant in latitude and velocity for the last 80 or 90 years. However, cloud bands with which they correlate are changing, as for example when small eddies between wind streams are sheared apart in 1 or 2 days. As one can see in Figure 9.10, eddies are deviations in what are otherwise alternating streams flowing east or west in the atmosphere. Where steady winds have velocities up to 100 m/s, eddy velocities are a few tens of meters per second.

[Figure 9.10]

Cloud motions on a small scale are by no means orderly, as is evident in Figure 9.11. Voyager scientists were unprepared for the diversity and sometimes large turbulent motions in clouds observable in spacecraft photographs. Surprisingly, photographs failed to reveal cloud features smaller than about 100 km across. Narrow bands appear to coalesce and widen, while wide bands break apart. Material even seems to be transferring between bands.

[Figure 9.11]

Conspicuous in Jupiter's southern hemisphere is the oval-shaped Great Red Spot, measuring some 14,000 x 40,000 km. Although it has always been present since its telescopic discovery three centuries ago, it does vary both in size and intensity. Its sense of circulation, and that in other ovals in the southern hemisphere, is counterclockwise, whereas those ovals in the northern hemisphere rotate clockwise. This suggests that such ovals are high-pressure cells analogous to those in the Earth's atmosphere. Small white clouds can been seen circulating around the Great Red Spot over periods of a week or so, whereas by comparison, the interior is relatively calm.

Saturn also has oval-shaped circulation cells in its atmosphere. Shown in Figure 9.12 is a brown oval photographed during the August 1981 flyby of Saturn by Voyager 2. It is not known whether the eddies and ovals on both Jupiter and Saturn extend as deep into the planet as do the wind streams. However, the long-term persistence of the winds and the short life for eddies and ovals are probably related to the mass of material involved in the phenomena. Thus the winds probably extend deep into the planet, while the shorter-lived eddies are relatively shallow structures. However, this is still quite speculative.

[Figure 9.12]

In the 1930s, spectroscopic studies revealed methane in the atmospheres of Uranus and Neptune, as in those of Jupiter and Saturn. Since then, hydrogen has been identified, helium has been inferred indirectly, and some other hydrogen-containing molecules have also been discovered. Greatly distant from the Sun, Uranus and Neptune are very cold, and thus a number of molecular combinations are probably frozen into a crystal or liquid-drop form.

Important for Uranus is the fact that its axis of rotation lies almost in its orbital plane, causing regions near the poles to remain alternately in sunlight or darkness for periods approaching 42 years. What effect such a phenomenon has on the overall structure of the atmosphere and how much of a difference it produces between Uranus and Neptune is only now being considered.

9.3.1. Voyager 2 at Uranus

Although alike in many respects, ground-based and Voyager 2 evidence suggests that the atmospheres of Uranus and Neptune are not highly similar to each other or to those of Jupiter and Saturn. Voyager 2 made its closest approach to Uranus on January 24, 1986, coming within 80,000 km of the planet. The striking aspect of the planet as seen from the advantageous position of Voyager 2 or from the distant Earth is how bland and featureless the blue-green planet appears. Prior to Voyager 2, ground-based data had been interpreted to say that there were no clouds in the Uranian atmosphere unlike Jupiter's and Saturn's atmospheres. Although Voyager 2 did final confirm the existence of clouds in the planet's atmosphere, they are considerable smaller than the planet's diameter and only about five percent brighter than the background atmosphere (Figure 9.13). Icy materials, formed from hydrogen, carbon, nitrogen, and oxygen, appear to be the principle constituents of Uranus, and at the very low temperatures in the Uranian atmosphere these compounds condense to form clouds of ice crystals. Methane freezes at the lowest temperatures, so that the top cloud layers are probably composed of methane ice crystals. These methane clouds are probably extensive enough to obscure the underlying ammonia and water clouds. This would explain why in infrared spectra of the planet there are no signatures of these two molecules. Finally, the blue-green cast to Uranus is due to selective absorption of the reddish sunlight by methane molecules in its atmosphere.

[Figure 9.13]

On Earth the heating of equatorial zones and the consequent temperature decline toward the poles produces the strong eastward moving jet streams at mid-latitudes. But on Uranus sunlight comes at times almost down the rotation axis into the polar regions. Thus it was questionable whether or not Voyager 2 would find wind patterns similar to those in the atmospheres of Jupiter and Saturn. Voyager evidence, however, shows that indeed there are east-west type of winds in the Uranian atmosphere. The amount of solar radiant energy arriving at Uranus is so weak in comparison to that at the Earth's distance that the winds may not be caused by unequal heating as in the case of Earth.

9.3.2. Neptune, A Visit by Voyager 2 in 1989

The general comments made about models of planetary interiors in Section 8.6 are applicable to the Jovian planets as well as to the Terrestrials. Larger masses and the fact that the Jovian planets contain far more easily vaporized materials than do the Terrestrial planets mean that the internal structures of the Jovian planets are not like those of the Terrestrials. Jupiter and Saturn are the only planets composed primarily of hydrogen and helium (as is the Sun): Only hydrogen and helium could give Jupiter and Saturn their mean densities of 1.31 and 0.69 g/cm3, respectively, for the temperatures and pressures that characterize each planet. However, the masses of Uranus and Neptune are 5 and 6 percent, respectively, that of Jupiter, while their mean densities are about equal to or larger than Jupiter's. This indicates that whereas Jupiter and Saturn are composed primarily of hydrogen and helium, the percentages of carbon, nitrogen, oxygen, and possibly silicon, and iron in Uranus and Neptune must be greater than in Jupiter. That is, Uranus and Neptune do not have solar compositions, which is almost exclusively the gaseous materials mentioned in Chapter 6, but rather they have larger fractions of icy and rocky materials in their composition.

9.4.1. Jupiter and Saturn: The Sunlike Composition Planets

As was noted in Chapter 5, the rapid rotation of Jupiter and Saturn, coupled with their composition of low-density materials, argues that their internal structures are more fluid than solid. Another significant factor is that Jupiter and Saturn give off more heat than they receive from the Sun. In the case of Jupiter, the heat given off is about 1.5 to 2 times the amount received from the Sun, and for Saturn, it is between 2 and 3 times the amount. Hence Jupiter and Saturn have internal sources of heat. It is extremely unlikely that the heat source is anything as exotic as that in the Sun and the stars; Jupiter and Saturn are not small stars. But it is fair to say that they are more like the Sun than like the Earth, and they are clearly an intermediate type of body. The internal heat source probably results from the conversion of gravitational potential energy into thermal energy as the two planets contracted during their formation and after. In fact, it is likely that they are still contracting--but very slowly.

Models for Jupiter's and Saturn's internal structure are shown in Figure 9.14, along with ones for Uranus and Neptune. Both Jupiter and Saturn have dense cores of rocky and icy materials--rather than compressed hydrogen and helium. The core is about 4 percent of the mass of Jupiter and 25 percent of the mass of Saturn, with temperatures in the range of 20,000 to 30,000 K and densities ranging from 10 to 20 g/cm3. Surrounding the core is a layer existing under a pressure in excess of 3 millions times Earth's atmospheric pressure. In it hydrogen and helium behave more like liquid metals than solids. The upper boundary of the metallic liquid zone is rather abrupt, giving way to a molecular liquid mantle of hydrogen and helium. Through both the metallic and molecular liquid zones, which are 96 and 75 percent, respectively, of the masses of Jupiter and Saturn, the temperature and density decrease. The molecular liquid mantles gradually change to molecular gases, which are then the atmospheres of the two planets, as shown for Jupiter in Figure 9.15.

[Figure 9.14]

[Figure 9.15]

9.4.2. Uranus and Neptune: Similar Yet Different

Like Jupiter and Saturn, Uranus and Neptune have a three-layered structure, but unlike the Solar System giants, each layer is of quite different chemical composition. The core of each planet is probably a rocky (iron and silicates primarily) and icy (methane, ammonia, and water principally) material. For Uranus, the pressure of overlying layers may not be sufficient to make the core solid, but it remains a thick, viscous liquid with convective motions in it. However, Neptune's greater mean density suggests that its core is solid.

Surrounding the core of each planet is a liquid mantle of water, methane, and ammonia, in which there may be some convective motions for Neptune but not for Uranus. Finally, each planet has a thick crust of hydrogen and helium that is compressed by gravity into a very dense gas. The crusts gradually give way to low-density atmospheres. Thus, like Jupiter and Saturn, these planets have no solid surface surrounded by a thin atmosphere as the Terrestrial planets have.

9.5.1. Jupiter's Magnetosphere

Jupiter is the strongest radio emitter in the Solar System after the Sun, emitting both thermal and nonthermal radiation. At times its radio emission exceeds even the Sun's in intensity. The nonthermal radiation is a type of synchrotron radiation, and it results from Jupiter's having a magnetic field and energetic free electrons in radiation belts that spiral around the magnetic field lines. These radiation belts are analogous to Earth's Van Allen belts (Section 7.4). There are occasional bursts having energies up to 1017 erg/s. The bursts are more intense when the nearest Galilean satellite, Io, appears on one side of Jupiter as viewed from Earth. Why should the position of Io make a difference? We suspect that it is due to the motion of Io through Jupiter's magnetic field, disturbing the field and the electrons trapped in it.

Pioneer space probes ran into the bow-shock wave formed by the solar wind's interaction with Jupiter's magnetic field as far out as 108 Jupiter radii. Data from the two Pioneer craft and the two Voyagers indicate that the boundary of the magnetosphere in the direction of the Sun varies between about 50 and 100 Jupiter radii. The planet's inner radiation belt is like Earth's Van Allen belts but from 5000 to 10,000 times more intense.

Farther out, the magnetic field flattens into a disk that extends several million kilometers from the planet, and its long tail, flowing out opposite to the direction of the Sun, extends an unknown distance beyond the orbit of Saturn. The shape is influenced by the large centrifugal force that results from the planet's rapid rotation.

9.5.2. Saturn's Magnetosphere

Saturn's magnetic field also defines a zone about it, or a magnetosphere, in which it can control the motions of subatomic particles. The Saturnian magnetosphere is intermediate in size, and its intensity lies somewhere between that of Jupiter and Earth. All three are based on a common framework of physical principles, but each possesses its own distinctive character.

Prior to the late summer of 1979, astronomers could only speculate on the magnetic field and radiation belts around Saturn. During that summer, Pioneer 11 detected the boundary of the magnetosphere lying some 24 Saturnian radii from the planet (its rings extend about 6 radii from the planet). Saturn's magnetosphere is apparently more disklike than that of the Earth, which is more spherical but less so than Jupiter's larger magnetosphere.

Of all the aspects of the Jovian planets, their ring systems are among the most captivating. Galileo first observed what we know as Saturn's rings in July of 1610, but it was not until 1655 that Christian Huygens proposed that they are a flattened disk of matter detached from the planet. In 1857, James Clerk Maxwell showed mathematically that they must consist of numerous tiny bodies in orbit around Saturn. This was experimentally demonstrated in 1895 from Doppler shifts that showed that each of the ring particles pursues its independent orbit around Saturn in accordance with Kepler's third law. The farther out from the planet, the lower are the particles' speeds, whereas a solid ring would rotate fastest at the farthest point from the planet.

It can be shown with reasonable mathematical precision that particles swarming around a planet eventually form a thin system of rings in the equatorial plane. A ring system is produced by the gravitational attraction of the planet and many gravitational interactions of ring particles with each other. Satellites of the planet play an important role in sculpting the appearance of the rings and in keeping them from spreading out in the equatorial plane. In addition, the ring system forms within several planetary radii of the planet's surface and is not able to form at greater distances. Although the same basic principles underlie the three known ring systems, Saturn's rings are much more elaborate and complex than those of Uranus and Jupiter.

9.6.1. Rings of Saturn

Three concentric rings have been known for some time and are labeled A, B, and C in order of decreasing distance from Saturn. The distance from the planet's surface is given in Table 9.3. The brightest ring, B, is separated from the somewhat fainter ring A by a dark space of about 5000 km, called Cassini's division. The faintest of the three major rings, the so-called crepe ring C, lies inside the inner edge of ring B. An exceptionally faint ring D, which lies inside the inner edge of ring C, has been found by Voyager investigations. Outside ring A other faint rings, known as E, F, and G, have been identified. The vertical extent of all the rings is less than a couple of kilometers, possibly even as thin as 100 m. Given their immense diameters, they are proportionally thousands of times thinner than a razor blade.

[Table 9.3]

The three major rings, A, B, and C, lie within the critical distance called the Roche limit, which is equal to about 2.4 Saturnian radii. This limit is named after the nineteenth-century French mathematician Edouard Roche (1820-1883), who found that inside this limit the gravitational attraction exerted by a planet on two adjacent orbiting particles is larger than the attraction of the two particles for each other. Whether the rings were formed inside the Roche limit by the breakup of a satellite, comet, or other body or whether Saturn's gravitational force prevented primordial particles from coalescing to form a satellite is unknown.

High-resolution photographs made by the Voyager spacecraft, as in Figure 9.16, surprised astronomers when they revealed that the three major rings, A, B, and C, are made up of hundreds, if not thousands, of very narrow ringlets. It had been thought that Cassini's division appears dark because it is devoid of any particles. However, photographs taken from the backside of the rings show that even Cassini's division is crammed with something like 100 ringlets (Figure 9.17). The Voyagers even provided evidence that some ringlets are not circular. In addition, ring F, shown in Figure 9.18, has knots, braids, and twists in it--which had not been predicted from the gravitational theory. We are not sure what causes this strange behavior.

[Figure 9.16]

[Figure 9.17]

[Figure 9.18]

The nature of the ring particles is suggested by the way sunlight bounces off of them, a process called scattering. The relative amounts of light scattered in various directions by the ring particles depends on both their sizes and the wavelength of light. From data collected by the Voyager spacecrafts, scientists estimate that the ring particles vary in size from a few microns up to a few tens of meters. But the most abundant particles are snowball-sized particles around 10 cm. The range of particle sizes in each of the rings, however, does not appear to be the same. For example, ring C and Cassini's division appear to contain relative few of the smaller particles. Apparently, the larger-sized particles in Cassini's division do not scatter photons in the backward direction as well as smaller particles do, so they appear dark from the sunlit side but bright from the backside.

From studies of infrared data obtained by the Voyager spacecrafts, it appears that the ring particles are water ice or at least covered with water ice. In addition, there are subtle differences in color between rings and even ringlets, which suggests that some other substance or substances are mixed with the water ice. These trace substances have not yet been identified. The existence of the color variations between rings and its persistence suggests that ringlet particles do not migrate between rings.

Probably the most unexpected aspect found by the Voyagers was wedge-shaped spokes orientated radially out from the planet in ring B. Figure 9.19 shows the spokes from the sunlit side, where they appear dark. However, looking back toward the Sun, they appear bright. They are perplexing in that if they are produced somehow by the ring particles, Keplerian motion should dissolve the spokes in a short time. However, they are seen to last close to 10 hours. Studies of light scattered from the spokes suggest that they are fine dustlike particles that are situated a few tens of meters above the rings. Nevertheless, there are still some puzzling aspects to the spokes for which we have no completely satisfactory solution.

[Figure 9.19]

9.6.2. Discovery of a Ring System For Jupiter

The notion that Jupiter possesses a ring system like that of Saturn was proposed some 20 years ago. Pioneer 11 data were interpreted as consistent with the existence of a system of tiny satellites forming a ring about Jupiter. This was at best speculation, and it was only Voyager 1's photograph of the Beehive star cluster that finally revealed the ring system.

Data from the Voyagers reveal a ring system composed of three parts. The primary ring, and brightest part of the system, starts abruptly at 1.81 Jupiter radii and ends, while fading gradually, 6,000 km closer to the planet. At most this primary ring is about 30 kilometers thick. There is also a tenuous sheet of material that is several times fainter than the bright ring extending from the primary ring smoothly toward to the planet's outer atmosphere. Surrounding the ring and sheet is a faint, lens-shaped halo some 20,000 km thick close to the planet. Saturn's rings are not embedded in such a halo.

The particles composing the ring appear to be smaller on the average, typically a few microns in size, than Saturn's ring particles. Also unlike Saturn's ring particles, those of Jupiter's and Uranus's ring systems are quite dark. Thus they are not water ice or coated with water ice. Evidence suggests that they are probably silicate particles whose origin is unknown.

9.6.3. Uranus's Ring System: Narrow and Dark

Occasionally, a planet will pass between the Earth and a star. Such an event is called an occultation (from the Latin word meaning "hiding"). In recent years, astronomers have carefully monitored these occultations because the time and place on Earth at which the occultation will be visible can be calculated. It requires a precise knowledge of the planet's orbit to make such a calculation, and the precision with which the prediction is confirmed by the observation in turn tells us how well we really know the planet's orbit.

As the planet begins to occult the star, its atmosphere, which is partially transparent, covers the star first, so that there is a gradual dimming of the star. If there were no atmosphere, the star's brightness would remain constant until the opaque body of the planet cut off all light; the change would be sudden, not gradual. In this manner, astronomers aboard the Kuiper Airborne Observatory, an airplane fitted with an infrared telescope, flying high over the Indian Ocean discovered a ring system around Uranus on March 10, 1977. About a half hour before the occultation was to take place, the star's light dimmed unexpectedly for a few seconds, followed by four other dips in brightness minutes later. The sequence was repeated in reverse as the star passed beyond the disk of Uranus on the other side. Since the original discovery of five rings, four less prominent rings have been identified, making a total of nine rings.

Voyager 2 confirmed that the Uranian rings are quite narrow, not more than a few up to one hundred km in width and are separated from each other by hundreds of km of virtually empty space. The radii for all the rings lie between 1.6 and 1.95 planetary radii. The rings are not all circular, but they are dark with sharp edges. Not all the rings lie in the planet's equatorial plane (Figure 9.20), although they are close. Thus the rings are orientated almost perpendicular to the planet's orbital plane. Hence the origin of the rings is closely related to that of Uranus, since the planet's equatorial plane is almost perpendicular to its orbital plane. In addition to observing the nine known rings, Voyager 2 also discovered one new ring that is quite narrow and faint and about 100 almost transparent bands that are virtually invisible from the Earth.

The particles composing the known rings are larger than had been anticipated, being from about 10 cm size up to several meters. In contrast the particles composing the newly discovered dust bands are quite small, being nearly microscopic (roughly 0.02 mm) in size. In addition to the new ring and dust bands, ten small satellites were found all of which lie closer to Uranus than the closest of the previously known satellites Miranda. Two of the new satellites are quite close to the rings. But the most interesting discovery is that the main rings and the new satellites are charcoal black. Photographing them against a black background was a monumental accomplishment. Since the ring particles are poor reflectors, they can not be coated with water (or ammonia or methane) ice. More likely, they are a silicate- or carbon-bearing material.

[Figure 9.20]

9.6.4. Does Neptune Have a Ring System?

Although Pluto is not one of the Jovian planets, it does orbit in the outer part of the Solar System. For that reason, we will include the discussion of the Pluto at this point, but we want to again emphasize that it is neither a Jovian or for that matter a Terrestrial planet. In fact, there is some question among astronomers whether or not Pluto should even be listed as a planet. Let us precede to see why there should be any debated over this point.

9.7.1. Discovery of Pluto

Spurred by the success of the discovery of Neptune in 1846 as discussed at the beginning of this chapter, astronomers searched for evidence for even more distant planets. Percival Lowell was convinced by his calculations begun in 1905 that minute discrepancies still complicated the orbit of Uranus. (Neptune had not been observed long enough to provide useful data.) He concluded that the irregularities might be caused by a planet beyond Neptune.

Several years of intermittent and unproductive search passed. Then in January of 1929, the Lowell Observatory acquired a 13-inch photographic refractor and put a young assistant, Clyde Tombaugh, to work on a new search. After a year of photographing star fields along the ecliptic and later all over the sky, Tombaugh made the historic find on photographs taken in January of 1930.

Pluto's great distance from the Sun (a semimajor axis of about 40 AU) and its very long sidereal period of almost 248 years have made it a difficult planet to study. Pluto's large eccentricity carries it as close to the Sun as about 30 AU and as far away as almost 50 AU, a variation of almost 20 AU, or about 3 billion kilometers. Thus during a portion of its orbit it is closer to the Sun than Neptune is. In fact, Pluto has been closer to the Sun than Neptune since the winter of 1978 and 1979 and will remain so until the spring of 1999. If you refer to Figure 3.5, you can imagine Pluto now moving along that portion of its orbit that is north of the plane of the ecliptic (on the right-hand side). Pluto reaches aphelion while it is south of the ecliptic plane (on the left-hand side). At that point it will be about 14 AU below the plane of the ecliptic, which is a greater distance than Saturn is from the Sun. Pluto's crossing of Neptune's orbital plane is done well above or below it so that there is no chance for the two planets to collide.

9.7.2. What Kind of Body Is Pluto?

At the time of its discovery it was estimated that Pluto was approximately Earthlike in size and mean density. Thus there was the possibility that a Terrestrial planet had been found in the outer Solar System. Since then, with a longer period for study, estimates of Pluto's mass and radius have decreased substantially (Table 9.4). It is by means of Pluto's gravitational attraction for Neptune and--to a lesser extent--Uranus that astronomers have tried to estimate its mass. This means that they must know its orbit, which is a difficult undertaking. Our best estimates at this time suggest that Pluto's mass is too small to have ever produced the effects on Uranus' orbit that presumably lead to its discovery. Thus in some sense the discovery of Pluto is owed to the systematic search made by Tombaugh and not to the predicative powers of Newtonian gravitational theory.

[Table 9.4]

The planet's small disk, measured with difficulty even by a large telescope, is less than 4/10 Earth's diameter (Table 9.4). Pluto's mass is at most a few percent that of Earth's. For comparison Pluto's mass is just 1/500 of Earth's mass, while Mar's mass is 1/9 of Earth's, Mercury's 1/18 of Earth's, and the Moon's 1/80 of Earth's. Quite clearly this small mass does not qualify Pluto for planet status; it is only the historical precedent of having classified Pluto as being a planet for so long that prevents the changing its status. It is likely that Pluto is an icy body because of its low mean density, which the best estimates place around 1 g/cm3. This prompts the belief by astronomers that Pluto is composed primarily of water in solid form, or possibly a mixture of water and other ices. A suggested structure for Pluto is a small rocky core covered by an extensive water ice mantle with a crust of methane ice.

Pluto's brightness varies slightly, presumably because sunlight is reflected unevenly from its surface--perhaps because of light and dark areas. Photoelectric observations of these variations reveal that the period of Pluto's rotation is 6.4 days. The nature of the brightness variation also suggests that Pluto's axis of rotation is highly inclined relative to its orbital plane. There is a good chance that Pluto's satellite orbits in the equatorial plane of the planet. If so, then, as shown in Figure 9.21, Pluto's axis of rotation is almost in its orbital plane, like that of Uranus.

[Figure 9.21]

Recent infrared observations suggested a surface composition dominated by ices, primarily solid methane. Pluto's low surface gravity and extremely low temperature mean that its atmosphere is a tenuous one of methane and possibly neon; indeed, recent infrared observations have spectroscopically detected methane in Pluto's atmosphere. However, its observed reflectivity is not consistent with a prediction of methane ice, or frost, alone. That is, the dark patches are possibly a rocky silicate material.

9.7.3. Charon, Satellite of Pluto

Pluto has one satellite, Charon, shown in the photograph in Figure 9.22, which was taken in June of 1978. (The discovery was made during the examination of photographs taken as part of the routine task of refining data on the planet's orbital motion.) In Figure 9.22, the image of Pluto is elongated, with a bulge at the top that is the unresolved image of Charon. Although such photographs do not resolve the satellite, Charon's existence has been confirmed by observing a sequence of its eclipses of Pluto, which became visible on Earth in 1985 and should continue to be visible until 1991, and then will not be seen again for 124 years. From these eclipse observations and speckle photographs, Charon's estimated diameter is around 1200 km.

[Figure 9.22]

If Pluto and its satellite have equal mean densities, the satellite is about 10 percent the mass of Pluto, which would make it by far the largest satellite in the Solar System in comparison with its parent planet (the Moon is about 1.2 percent of the Earth's mass). The satellite orbits Pluto in an approximately circular orbit inclined to that of Pluto, as Figure 9.14b shows, at an estimated distance of 19,400 km and with an orbital period of 6.4 days.

Is it possible that even more remote bodies lie beyond Pluto? As we try to show in Chapter 11, there is no physical reason that other objects similar to Pluto cannot exist. In fact, for 13 years after the discovery of Pluto, Tombaugh continued to search for more trans-Neptunian planets. His and subsequent efforts have shown that if one or several are out there, they are extremely faint, and it is unlikely that they could be anything like the size of Jupiter. If they are there, it is likely that they are more like the minor bodies of the Solar System to be discussed in the next chapter.