Chapter 7

1. Chapter 7 The Jovian Planets: Windswept Giants

2. Introduction • Jupiter, Saturn, Uranus, and Neptune are giant planets; they are also called the jovian planets. • They are much bigger, more massive, and less dense than the inner, terrestrial planets. • Their internal structure is entirely different from that of the four inner planets. • In this chapter, we also discuss a set of moons of these giant planets, some of which range in diameter between ½ and ¼ the size of the Earth, as large as Mercury or Pluto. • Close-up space observations have shown that these moons are themselves interesting objects for study.

3. Introduction • Jupiter is the largest planet in our Solar System (see figure). • Some of its very numerous moons are close in size to the terrestrial planets and show fascinating surface structure. • Saturn has long been famous for its beautiful rings. • We now know, however, that each of the other giant planets also has rings. • When seen close-up, as on the opposite page, the astonishing detail in the rings is very beautiful.

4. Introduction • Uranus and Neptune were known for a long time to us as mere points in the sky. • Spacecraft views have transformed them into objects with more character. • The age of first exploration of the giant planets, with spacecraft that simply flew by the planets, is over. • We now are in the stage of space missions to orbit the planets, with a Jupiter orbiter having recently completed its mission and a Saturn orbiter that started collecting data in 2004. • These missions study the planets, their rings, their moons, and their magnetic fields in a much more detailed manner than before.

5. 7.1 Jupiter • Jupiter, the largest and most massive planet, dominates the Sun’s planetary system. • It alone contains two-thirds of the mass in the Solar System outside of the Sun, 318 times as much mass as the Earth (but only 0.001 times the Sun’s mass). • Jupiter has at least 52 moons of its own and so is a miniature “planetary system” (that is, several planet-like objects orbiting a central object) in itself. • It is often seen as a bright object in our night sky, and observations with even a small telescope reveal bands of clouds across its surface and show four of its moons, the Galilean satellites.

6. 7.1 Jupiter • Jupiter is more than 11 times greater in diameter than the Earth. • From its mass and volume, we calculate its density to be 1.3 g/cm3, not much greater than the 1 g/cm3 density of water. • This low density tells us that any core of heavy elements (such as iron) makes up only a small fraction of Jupiter’s mass. • Jupiter, rather, is mainly composed of the light elements hydrogen and helium. • Jupiter’s chemical composition is closer to that of the Sun and stars than it is to that of the Earth (see figure), so its origin can be traced directly back to the solar nebula with much less modification than the terrestrial planets underwent.

7. 7.1 Jupiter • Jupiter has no crust. • At deeper and deeper levels, its gas just gets denser and denser, turning mushy and eventually liquefying about 20,000 km (15 per cent of the way) down. • Jupiter’s core, inaccessible to direct study, is calculated to be made of heavy elements and to be larger and perhaps 10 times more massive than Earth. • Jupiter’s “surface” (actually, the top of the clouds that we see) rotates in about 10 hours, though different latitudes rotate at slightly different speeds. • Regions with different speeds correspond to different bands; Jupiter has a half-dozen jet streams while Earth has only one in each hemisphere.

8. 7.1 Jupiter • Jupiter’s clouds are in constant turmoil; the shapes and distribution of bands can change within days. • The bright bands are called “zones” and the dark bands are called “belts,” but the strongest winds appear on the boundaries between them. • The zones seem to be covered by a uniformly high cloud deck. • The belts have both towering convective clouds and lightning, as well as clear spaces that allow glimpses of the deeper atmosphere. • The most prominent feature of Jupiter’s surface is a large reddish oval known as the Great Red Spot. • It is two to three times larger in diameter than the Earth. • Other, smaller spots are also present. • Jupiter emits radio waves, which indicates that it has a strong magnetic field and strong “radiation belts. • Actually, these are belts of magnetic fields filled with trapped energetic particles—large-scale versions of the Van Allen belts of Earth (see the discussion in Chapter 6).

9. 7.1a Spacecraft to Jupiter • Our understanding of Jupiter was revolutionized in the 1970s, when first Pioneer 10 (1973) and Pioneer 11 (1974) and then Voyager 1 and Voyager 2 (both in 1979) flew past it. • The Galileo spacecraft arrived at Jupiter in 1995, when it dropped a probe into Jupiter’s atmosphere and went into orbit in the Jupiter system. • The spacecraft plunged into Jupiter’s atmosphere on September 21, 2003, ending a tremendously successful mission. (It was sent on that course in large part to avoid the possibility that it could eventually hit and contaminate the Galilean satellite Europa, on which some scientists speculate that life may exist, as we will discuss in Section 7.1g(ii).) • The Cassini spacecraft flew by Jupiter, en route to Saturn, in 2000 –2001 (see figure). • Each spacecraft carried many types of instruments to measure various properties of Jupiter, its satellites, and the space around them.

10. 7.1b The Great Red Spot • The Great Red Spot is a gaseous “island” a few times larger across than the Earth (see figure). • It is the vortex of a violent, long-lasting storm, similar to large storms on Earth, and drifts about slowly with respect to the clouds as the planet rotates. • From the sense of its rotation (counterclockwise rather than clockwise in the southern hemisphere), measured from time-lapse photographs, we can tell that it is a pressure high rather than a low. • We also see how it interacts with surrounding clouds and smaller spots. • The Great Red Spot has been visible for at least 150 years, and maybe even 300 years. • Sometimes it is relatively prominent and colorful, and at other times the color may even disappear for a few years.

11. 7.1b The Great Red Spot • Why has the Great Red Spot lasted this long? • Heat, energy flowing into the storm from below it, partly maintains its energy supply. • The storm also contains more mass than hurricanes on Earth, which makes it more stable. • Furthermore, unlike Earth, Jupiter has no continents or other structure to break up the storm. • Also, we do not know how much energy the Spot gains from the circulation of Jupiter’s upper atmosphere and eddies (rotating regions) in it. • Until we can sample lower levels of Jupiter’s atmosphere, we will not be able to decide definitively. • Studying the eddies (swirls) in Jupiter’s atmosphere helps us interpret features on Earth. • For example, one hypothesis to explain Jupiter’s spots holds that they are similar to circulating rings that break off from the Gulf Stream in the Atlantic Ocean.

12. 7.1c Jupiter’s Atmosphere • Heat emanating from Jupiter’s interior churns the atmosphere. (In the Earth’s atmosphere, on the other hand, most of the energy comes from the outside—from the Sun.) • Pockets of gas rise and fall, through the process of convection, as described for the Sun in Chapter 10. • The bright bands (“zones”) and dark bands (“belts”) on Jupiter represent different cloud layers (see figure). • Wind velocities show that each hemisphere of Jupiter has a half-dozen currents blowing eastward or westward. • The Earth, in contrast, has only one westward current at low latitudes (the trade winds) and one eastward current at middle latitudes (the jet stream).

13. 7.1c Jupiter’s Atmosphere • On December 7, 1995, a probe dropped from the Galileo spacecraft transmitted data for 57 minutes as it fell through Jupiter’s atmosphere. • It gave us accurate measurements of Jupiter’s composition; the heights of the cloud layers; and the variations of temperature, density, and pressure. • It went through about 600 km of Jupiter’s atmosphere, only about 1 per cent of Jupiter’s radius. • The probe found that Jupiter’s winds were stronger than expected and increased with depth, which shows that the energy that drives them comes from below. • Extensive lightning storms, including giant-sized lightning strikes called “superbolts,” were discovered from the Voyagers. • The Galileo spacecraft photographed giant thunderclouds on Jupiter, which indicates that some regions are relatively wet and others relatively dry. • The probe found less water vapor than expected, probably because it fell through a dry region.

14. 7.1d Jupiter’s Interior • Most of Jupiter’s interior is in liquid form. Jupiter’s central temperature may be between 13,000 and 35,000 K. • The central pressure is 100 million times the pressure of the Earth’s atmosphere measured at our sea level due to Jupiter’s great mass pressing in. (The Earth’s central pressure is 4 million times its atmosphere’s pressure, and Earth’s central temperatures are several thousand degrees.) • Because of this high pressure, Jupiter’s interior is probably composed of ultra-compressed hydrogen surrounding a rocky core consisting of perhaps 10 Earth masses of iron and silicates (see figure).

15. 7.1d Jupiter’s Interior • Jupiter radiates 1.6 times as much heat as it receives from the Sun. • It must have an internal energy source—perhaps the energy remaining from its collapse from a primordial gas cloud 20 million km across or from the accretion of matter long ago. • Jupiter is undoubtedly still contracting inside and this process also liberates energy. • It lacks the mass necessary by a factor of about 75, however, to have heated up enough to become a star, generating energy by nuclear processes (see Chapter 12). • It is therefore not “almost a star,” contrary to some popular accounts.

16. 7.1e Jupiter’s Magnetic Field • The space missions showed that Jupiter’s tremendous magnetic field is even more intense than many scientists had expected (see figure). • At the height of Jupiter’s clouds, the magnetic field strength is 10 times that of the Earth, which itself has a rather strong field.

17. 7.1e Jupiter’s Magnetic Field • The inner field is shaped like a doughnut, containing several shells of charged particles, like giant versions of the Earth’s Van Allen belts (see figure, right). • The outer region of Jupiter’s magnetic field interacts with the particles flowing outward from the Sun. • When this solar wind is strong, Jupiter’s outer magnetic field (shaped like a pancake) is pushed in. • When the high-energy particles interact with Jupiter’s magnetic field, radio emission results. • Jupiter’s magnetic field leads Jupiter to have giant auroras (see figure, left).

18. 7.1f Jupiter’s Ring • Though Jupiter wasn’t expected to have a ring, Voyager 1 was programmed to look for one just in case; Saturn’s rings, of course, were well known, and Uranus’s rings had been discovered only a few years earlier during ground-based observations. • The Voyager 1 photograph indeed showed a wispy ring of material around Jupiter at about 1.8 times Jupiter’s radius, inside the orbit of its innermost moon.

19. 7.1f Jupiter’s Ring • As a result, Voyager 2 was targeted to take a series of photographs of the ring. • From the far side looking back, the ring appeared unexpectedly bright, probably because small particles in the ring scattered the light toward the spacecraft. • Within the main ring, fainter material appears to extend down to Jupiter’s cloud tops (see figure). • The ring particles were knocked off Jupiter’s inner moons by micrometeorites. • Whatever their origin, the individual particles probably remain in the ring only temporarily.

20. 7.1g Jupiter’s Amazing Satellites • Four of the innermost satellites were discovered by Galileo in 1610 when he first looked at Jupiter through his small telescope. • These four moons (Io, Europa, Ganymede, and Callisto) are called the Galilean satellites (see figure). • One of these moons, Ganymede, 5276 km in diameter, is the largest satellite in the Solar System and is larger than the planet Mercury.

21. 7.1g Jupiter’s Amazing Satellites • The Galilean satellites have played a very important role in the history of astronomy. • The fact that these particular satellites were noticed to be going around another planet, like a solar system in miniature, supported Copernicus’s Sun-centered model of the Solar System. • Not everything revolved around the Earth! • It was fitting to name the Galileo spacecraft after the discoverer of Jupiter’s moons. • Jupiter also has dozens of other satellites, some known or discovered from Earth and others discovered by the Voyagers. • None of these other satellites is even 10 per cent the diameter of the smallest Galilean satellite.

22. 7.1g Jupiter’s Amazing Satellites • Through first Voyager-spacecraft and then Galileo-spacecraft close-ups (see figure), the satellites of Jupiter have become known to us as worlds with personalities of their own. • The four Galilean satellites, in particular, were formerly known only as dots of light. • Not only the Galilean satellites, which range between 0.9 and 1.5 times the size of our own Moon, but also the smaller ones that have been imaged in detail turn out to have interesting surfaces and histories.

23. 7.1g(i) Pizza-like Io • Io, the innermost Galilean satellite, provided the biggest surprises. • Scientists knew that Io gave off particles as it went around Jupiter, and other scientists had predicted that Io’s interior would be heated by its flexing (to be discussed below). • Voyager 1 discovered that these particles resulted from active volcanoes on the satellite, a nice confirmation of the earlier ideas. • Eight volcanoes were seen actually erupting, many more than erupt on the Earth at any one time. • When Voyager 2 went by a few months later, most of the same volcanoes were still erupting.

24. 7.1g(i) Pizza-like Io • Though the Galileo spacecraft could not go close to Io for most of its mission, for fear of getting ruined because of Jupiter’s strong radiation field in that region, it could obtain high-quality images of Io and its volcanoes (see figure (a)). • Finally, it went within a few hundred kilometers of Io’s surface, and found that 100 volcanoes were erupting simultaneously. • Io’s surface (see figure (b) and (c)) has been transformed by the volcanoes, and is by far the youngest surface we have observed in the Solar System.

25. 7.1g(i) Pizza-like Io • Why does Io have so many active volcanoes? • Gravitational forces from Ganymede and Europa distort Io’s orbit slightly, which changes the tidal force on it from Jupiter in a varying fashion. • This changing tidal force flexes Io, creating heat from friction that heats the interior and leads to the volcanism. • The surface of Io is covered with sulfur and sulfur compounds, including frozen sulfur dioxide, and the thin atmosphere is full of sulfur dioxide. • It certainly wouldn’t be a pleasant place to visit! • Io’s surface, orange in color and covered with strange formations because of the sulfur, led Brad Smith, the head of the Voyager imaging team, to remark that “It’s better looking than a lot of pizzas I’ve seen.”

26. 7.1g(i) Pizza-like Io • Galileo imaging shows many mountains too tall to be supported by sulfur, so stronger types of rock must be involved, with a crust at least 30 km thick above the molten regions. • Also, Galileo’s infrared observations show that some of the volcanoes are too hot to be sulfur volcanism. • The surface changed substantially even as the Galileo spacecraft watched it. • By the time Galileo made its sixth, last, and closest pass of Io in 2001, it had raised the total of identified volcanoes to 120.

27. 7.1g(ii) Europa, a Possible Abode for Life • Europa, Jupiter’s Galilean satellite with the highest albedo (reflectivity), has a very smooth surface and is covered with narrow, dark stripes. • The lack of surface relief, mapped by the Galileo spacecraft to be no more than a couple of hundred meters high, suggests that the surface we see is ice. • The markings may be fracture systems in the ice, like fractures in the large fields of sea ice near the Earth’s north pole, as apparently verified in Galileo close-ups (see figure).

28. 7.1g(ii) Europa, a Possible Abode for Life • Some longer ridges can be traced far across Europa’s surface. • Few craters are visible, suggesting that the ice was soft enough below the crust to close in the craters. • Either internal radioactivity or, more likely, a gravitational tidal heating like that inside Io provides the heat to soften the ice. • Because Europa possibly has a liquid-water ocean and extra heating, many scientists consider it a worthy location to check for signs of life. • We can only hope that the ice crust, which may be about 10–50 km thick, is thin enough in some locations for us to be able to penetrate it to reach the ocean that may lie below—and see if life has ever existed there.

29. 7.1g(iii) Giant Ganymede • The largest satellite in the Solar System, Ganymede, shows many craters (see figure, top) alongside weird, grooved terrain (see figure, bottom). • Ganymede is bigger than Mercury but less dense; it contains large amounts of water ice surrounding a rocky core. • But an icy surface is as hard as steel in the cold conditions that far from the Sun, so it retains the craters from perhaps 4 billion years ago. • The grooved terrain is younger.

30. 7.1g(iii) Giant Ganymede • Ganymede shows many lateral displacements, where grooves have slid sideways, like those that occur in some places on Earth (for example, the San Andreas fault in California). • It is the only place besides the Earth where such faults have been found. • Thus, further studies of Ganymede may help our understanding of terrestrial earthquakes. • The Galileo spacecraft found a stronger magnetic field for Ganymede than expected, so perhaps Ganymede is more active inside than previously supposed.

31. 7.1g(iv) Pockmarked Callisto • Callisto, the outermost of Jupiter’s Galilean satellites, has so many craters (see figure) that its surface must also be the oldest. • Callisto, like Europa and Ganymede, is covered with ice. • A huge bull’s-eye formation, Valhalla, contains about 10 concentric rings, no doubt resulting from an enormous impact. • Perhaps ripples spreading from the impact froze into the ice to make Valhalla.

32. 7.1g(iv) Pockmarked Callisto • Callisto had been thought to be old and uninteresting, but observations from the Galileo spacecraft have revised the latter idea, by showing changes: • There are fewer small craters than expected, so the small craters that must have once been there were probably covered by dust that meteorite impacts eroded from larger craters, or disintegrated by themselves through electrostatic charges. • The Galileo spacecraft’s measurements of Callisto’s gravity from place to place show that its mass is concentrated more toward its center than had been thought. • This concentration indicates that heavier materials inside have sunk, and perhaps even indicates that there is an ocean below Callisto’s surface. • Callisto’s interactions with Jupiter’s magnetic field have been interpreted to back up the idea of an internal ocean.

33. 7.1g(v) Other Satellites • Galileo’s last pass near a Jupiter satellite occurred in 2002 at Amalthea, a small, inner, potato-shaped satellite. • The cameras weren’t used; Jupiter’s magnetosphere especially close to the planet was primarily studied. • From tracking the spacecraft, Amalthea’s mass was measured from its gravitational attraction. • The mass coupled with the observed volume gives the density, which turned out to be unexpectedly low, close to that of water ice. • Jet Propulsion Laboratory scientists deduced that Amalthea seems to be a loosely packed pile of rubble. • Amalthea is thus probably mostly rock with perhaps a little ice, rather than a mix of rock and iron, which would be denser. • Amalthea, and presumably other irregular satellites, seem to have been broken apart, with the pieces subsequently drawn roughly together.

34. 7.1g(v) Other Satellites • Many much smaller satellites of Jupiter are being discovered from the ground, given the existence of mosaics of sensitive CCDs (electronic detectors; see Chapter 3) that cover larger regions of the sky than previously possible and of computer processing methods to analyze the data. • In recent years, dozens of small moons, some only 2 km across, have been discovered around Jupiter, bringing Jupiter’s total of moons up to at least 52. • These “irregular” satellites are, no doubt, captured objects that were once in orbit around the Sun. • This origin is different from that of the large, regular satellites (like the Galilean moons), and from that of the set of smaller satellites in close orbits around Jupiter that apparently are remnants of collisions. • Both these latter types of moons are thought to have formed from a disk of gas and dust about the planet. • The other jovian planets also have these three types of satellites.

35. 7.1g(v) Other Satellites • Studies of Jupiter’s moons tell us about the formation of the Jupiter system, and help us better understand the early stages of the entire Solar System. • NASA’s next New Horizons mission, in a series of small spacecraft, is to visit Jupiter and its moons in the middle of the next decade. • NASA has approved a preliminary phase of the Juno mission, to be launched in 2010 to study Jupiter’s interior and atmosphere from polar orbit. • The spacecraft is to find out, from gravity studies, if the planet has an ice-rock core and to study how much water and ammonia Jupiter’s atmosphere holds.

36. 7.2 Saturn • Saturn, like Jupiter, Uranus, and Neptune, is a giant planet. • Its diameter, without its rings, is 9 times that of Earth; its mass is 95 Earth masses. • It is a truly beautiful object in a telescope of any size. • The glory of its system of rings makes it stand out even in small telescopes. • The view from the Cassini spacecraft, now in orbit around it, is breathtaking (see figure).

37. 7.2 Saturn • The giant planets have low densities. • Saturn’s is only 0.7 g/cm3, 70 per cent the density of water (see figure). • The bulk of Saturn is hydrogen molecules and helium, reflecting Saturn’s formation directly from the solar nebula. • Saturn is thought to have a core of heavy elements, including rocky material, making up about the inner 20 per cent of its diameter. • Voyagers 1 and 2 flew by Saturn in 1980 and 1981, respectively. Cassini, a joint NASA/European Space Agency mission, arrived at Saturn in 2004. • It is orbiting the Saturn system, going up close in turn to various of Saturn’s dozens of moons. • We will be discussing Cassini observations throughout the following sections.

38. 7.2a Saturn’s Rings • The rings extend far out in Saturn’s equatorial plane, and are inclined to the planet’s orbit. • Over a 30-year period, we sometimes see them from above their northern side, sometimes from below their southern side, and at intermediate angles in between (see figures). • When seen edge-on, they are almost invisible.

39. 7.2a Saturn’s Rings • The rings of Saturn consist of material that was torn apart by Saturn’s gravity or material that failed to collect into a moon at the time when the planet and its moons were forming. • However, the rings may have formed fairly recently, within the past few hundred million years. • Every massive object has a sphere, called its Roche limit, inside of which blobs of matter do not hold together by their mutual gravity. • The forces that tend to tear the blobs apart from each other are tidal forces. • They arise, like the Earth’s tides, because some parts of an object are closer to the planet than others and are thus subject to higher gravity. • The difference between the gravity force farther in and the gravity force farther out is the tidal force.

40. 7.2a Saturn’s Rings • The radius of the Roche limit is usually 2½ to 3 times the radius of the larger body, closer to the latter for the relative densities of Saturn and its moons. • The Sun also has a Roche limit, but all the planets lie outside it. • The natural moons of the various planets lie outside their respective Roche limits. • Saturn’s rings lie inside Saturn’s Roche limit, so it is not surprising that the material in the rings is spread out rather than collected into a single orbiting satellite. • Artificial satellites that we send up to orbit the Earth are constructed of sufficiently rigid materials that they do not break up even though they are within the Earth’s Roche limit; they are held together by forces much stronger than gravity.

41. 7.2a Saturn’s Rings • Saturn has several concentric major rings visible from Earth. • The brightest ring is separated from a fainter broad outer ring by an apparent gap called Cassini’s division. (The 17th-century astronomer Jean-Dominique Cassini (1625 –1712), who moved from Italy to France in 1671, discovered several of Saturn’s moons as well as the division in the rings, the latter in 1675. It was very appropriate to name not only the ring division but also NASA’s spacecraft to Saturn after him.) • Another ring is inside the brightest ring. • We know that the rings are not solid objects, because the rotation speed of the outer rings is slower than that of rings closer to Saturn. • Radar waves bounced off the rings show that the particles in the rings are at least a few centimeters, and possibly a meter, across. • Infrared studies show that at least their outer parts consist of ice.

42. 7.2a Saturn’s Rings • The images from the Voyagers revolutionized our view of Saturn, its rings, and its moons. • The Cassini mission is providing even more detailed views, as it orbits for years instead of merely flying by (see figure on next side). • Only from spacecraft can we see the rings from a vantage point different from the one we have on Earth. • Backlighted views showed that Cassini’s division, visible as a dark (and thus apparently empty) band from Earth, appeared bright, so it must contain some particles. • The rings are thin, for when they pass in front of stars, the starlight easily shines through. • Studies of the changes in the radio signals from the Voyagers when they went behind the rings showed that the rings are only about 20 m thick. • Relative to the diameter of the rings, this is equivalent to a CD (compact disc) that is 30 km across though still its normal thickness!

43. 7.2a Saturn’s Rings

44. 7.2a Saturn’s Rings • The closer the spacecraft got to the rings, the more individual rings became apparent. • Each of the known rings was actually divided into many thinner rings. • The number of these rings (sometimes called “ringlets”) is in the hundreds of thousands. • The images from the Cassini spacecraft (see figure) surpassed even Voyager’s views of ringlets.

45. 7.2a Saturn’s Rings • The outer major ring turns out to be kept in place by a tiny satellite orbiting just outside it. • At least some of the rings are kept narrow by “shepherding” satellites that gravitationally affect the ring material, a concept that we can apply to rings of other planets. • Density waves (see figure) were seen in the ring.

46. 7.2a Saturn’s Rings • A post-Voyager theory said that many of the narrowest gaps may be swept clean by a variety of small moons. • These objects would be embedded in the rings in addition to the icy snowballs that make up most of the ring material. • The tiny moon Pan has been observed clearing out the Encke Gap in just this way. • Theorists suppose that smaller moons probably clear out several other gaps in Saturn’s rings, and Cassini is finding some of these moons.

47. 7.2b Saturn’s Atmosphere • Like Jupiter, Saturn rotates quickly on its axis; a complete period is only 10 hours, in spite of Saturn’s diameter being over 9 times greater than Earth’s. • The rapid rotation causes Saturn to be larger across the equator than from pole to pole. • This equatorial bulging makes Saturn look slightly “flattened.” • Jupiter also looks flattened, or oblate, for this reason. • The structure in Saturn’s clouds is of much lower contrast than that in Jupiter’s clouds. • It is not a surprise that the chemical reactions can be different; after all, Saturn is colder than Jupiter.

48. 7.2b Saturn’s Atmosphere • Saturn has extremely high winds, up to 1800 km /hr, 4 times faster than the winds on Jupiter. • Cassini is tracking the winds with higher precision than was previously possible. • On Saturn, the variations in wind speed do not seem to correlate with the positions of bright and dark bands, unlike the case with Jupiter (see figure). • As on Jupiter, but unlike the case for Earth, the winds seem to be driven by rotating eddies, which in turn get most of their energy from the planet’s interior. • Such differences provide a better understanding of storm systems in Earth’s atmosphere.

49. 7.2c Saturn’s Interior and Magnetic Field • Saturn radiates about twice as much energy as it absorbs from the Sun, a greater factor than for Jupiter. • One interpretation is that only ⅔ of Saturn’s internal energy remains from its formation and from its continuing contraction under gravity. • The rest would be generated by the gravitational energy released by helium sinking through the liquid hydrogen in Saturn’s interior. • The helium that sinks has condensed because Saturn, unlike Jupiter, is cold enough.

50. 7.2c Saturn’s Interior and Magnetic Field • Saturn gives off radio signals, as does Jupiter, a pre-Voyager indication to earthbound astronomers that Saturn also has a magnetic field. • The Voyagers found that the magnetic field at Saturn’s equator is only ⅔ of the field present at the Earth’s equator. • Remember, though, that Saturn is much larger than the Earth and so its equator is much farther from its center. • Saturn’s magnetic field contains belts of charged particles (analogous to Van Allen belts), which are larger than Earth’s but smaller than Jupiter’s. (Saturn’s surface magnetic field is 20 times weaker than Jupiter’s.) • These particles interact with the atmosphere near the poles and produce auroras (see figure).