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Lesson 4-03 Outer (Jovian) Planets

Page history last edited by Chai Nakpiban 11 years, 6 months ago


Lesson 4.03 – Outer (Jovian) Planets

Standard:  ES1.a Students know how the differences and similarities among the sun, the terrestrial planets, and the gas planets may have been established during the formation of the solar system.




Beyond Mercury, Venus, Earth, and Mars -- the four terrestrial planets of our solar system-- there are four giant planets that are remarkably different from the first. These Jovian planets have intrigued humans for many hundreds of years, their very names rooted in Roman mythology. This lesson will explore the characteristics of these large planets and how our knowledge of them sheds light on the universe in its entirety.




Jupiter: Giant Among Planets

Jupiter is only 1/800 as massive as the sun. Still, it is the largest planet by far. Jupiter has a mass that is 2 ½ times greater than the mass of all the other planets and moons combined. In fact, had Jupiter been about 10 times larger, it would have evolved into a small star. Jupiter rotates more rapidly than any other planet, completing one rotation in slightly less than 10 Earth-hours. The effect of this fast spin is to make its equatorial region bulge and its poles flatten slightly.



When viewed through a telescope or binoculars, Jupiter appears to be covered with alternating bands of multicolored clouds that run parallel to its equator. The most striking feature is the Great Red Spot in the southern hemisphere, shown in Figure 13A. The Great Red Spot was first discovered more than three centuries ago. However, when Pioneer 11 moved within 42,000 kilometers of Jupiter’s cloud tops, images from the orbiter indicated that the Great Red Spot is a cyclonic storm.


Figure 13 A When photographed by Voyager 2, the Great Red Spot was the size of two Earth—size circles placed side by side. B The light clouds are regions where gases are sinking and cooling. The convection currents and the rapid rotation of the planet generate high-speed winds.



Structure of Jupiter

Jupiter’s hydrogen-helium atmosphere also contains small amounts of methane, ammonia, water, and sulfur compounds. The wind systems, shown in Figure 13B, generate the light- and dark-colored bands that encircle this giant. Unlike the winds on Earth, which are driven by solar energy, Jupiter itself gives off nearly twice as much heat as it receives from the sun. Thus, the interior heat from Jupiter produces huge convection currents in the atmosphere.



Atmospheric pressure at the top of the clouds is equal to sea-level pressure on Earth. Because of Jupiter’s immense gravity, the pressure increases rapidly toward its surface. At 1000 kilometers below the clouds, the pressure is great enough to compress hydrogen gas into a liquid. Consequently, Jupiter is thought to be a gigantic ocean of liquid hydrogen. Less than halfway into Jupiter’s interior, extreme pressures cause the liquid hydrogen to turn into liquid metallic hydrogen. Jupiter is also believed to have a rocky and metallic central core.



Jupiter’s Moons

Jupiter’s satellite system, consisting of 28 moons discovered so far, resembles a miniature solar system. The four largest moons were discovered by Galileo. They travel in nearly circular orbits around the planet. To the surprise of almost everyone images from Voyagers 1 and 2 in 1979 revealed that each of the four Galilean satellites is a unique geological world. The moons are shown in Figure 14. The innermost of the Galilean moons, Io, is one of three known volcanically active bodies in our solar system. The other volcanically active bodies are Earth—and Neptune’s moon Triton. The heat source for volcanic activity on Io is thought to be tidal energy generated by a relentless “tug of war” between Jupiter and the other Galilean moons. The gravitational power of Jupiter and nearby moons pulls and pushes on Io’s tidal bulge as its orbit takes it alternately closer to and farther from Jupiter. This gravitational flexing of Io is transformed into heat energy and results in Io’s volcanic eruptions.



Figure 14 Jupiter’s Moons A Io is the innermost moon and is one of only three volcanically active bodies in the solar system. B Europa—the smallest of the Galilean moons—has an icy surface that is crossed by many linear features. C Ganymede is the largest Jovian moon, and it contains cratered areas, smooth regions, and areas covered by numerous parallel grooves. D Callisto—the outermost of the Galilean moons—is densely cratered, much like Earth’s moon.



Jupiter’s Rings

Jupiter’s ring system was one of the most unexpected discoveries made by Voyager 1. By analyzing how these rings scatter light, researchers concluded that the rings are composed of fine, dark particles, similar in size to smoke particles. The faint nature of the rings also indicates that these minute fragments are widely dispersed. The particles are thought to be fragments blasted by meteorite impacts from the surfaces of Metis and Adrastea, two small moons of Jupiter.



Saturn: The Elegant Planet

Requiring 29.46 Earth-years to make one revolution, Saturn is almost twice as far from the sun as Jupiter. However, its atmosphere, composition, and internal structure are thought to be remarkably similar to Jupiter’s. The most prominent feature of Saturn is its system of rings, shown in Figure 15. In 1610, Galileo used a primitive telescope and first saw the structures that were later found to be the rings. They appeared as two small bodies adjacent to the planet. Their ring nature was explained 50 years later by the Dutch astronomer Christian Huygens.

Figure 15 Saturn’s Rings Saturn’s rings fall into two categories based on particle density. The main rings (A and B) are densely packed. In contrast, the outer rings are composed of widely dispersed particles.



Features of Saturn

In 1980 and 1981, flyby missions of the nuclear-powered Voyagers 1 and 2 spacecraft came within 100,000 kilometers of Saturn. More information was gained in a few days than had been acquired since Galileo first viewed this elegant planet.



1.    Saturn’s atmosphere is very active, with winds roaring at up to 1500 kilometers per hour.

2.    Large cyclonic “storms” similar to Jupiter’s Great Red Spot, although smaller, occur in Saturn’s atmosphere.

3.    Eleven additional moons were discovered.

4.    The rings of Saturn were found to be more complex than expected.



More recently, observations from ground-based telescopes, the Hubble Space Telescope, and Cassini have added to our knowledge of Saturn’s ring and moon system. When the positions of Earth and Saturn allowed the rings to be viewed edge-on—thereby reducing the glare from the main rings—Saturn’s faintest rings and satellites became visible.



Saturn’s Rings

Until the discovery that Jupiter, Uranus, and Neptune also have ring systems, this phenomenon was thought to be unique to Saturn. Although the four known ring systems differ in detail, they share many attributes. They all consist of multiple concentric rings separated by gaps of various widths. In addition, each ring is composed of individual particles—“moonlets” of ice and rock—that circle the planet while regularly impacting one another.



Most rings fall into one of two categories based on particle density. Saturn’s main rings, designated A and B in Figure 15, and the bright rings of Uranus are tightly packed and contain “moonlets” that range in size from a few centimeters to several meters. These particles are thought to collide frequently as they orbit the parent planet. Despite the fact that Saturn’s dense rings stretch across several hundred kilometers, they are very thin, perhaps less than 100 meters from top to bottom.



At the other extreme, the faintest rings, such as Jupiter’s ring system and Saturn’s outermost rings, are composed of very fine particles that are widely dispersed. Saturn’s outermost rings are designated E in Figure 15. In addition to having very low particle densities, these rings tend to be thicker than Saturn’s bright rings.



Saturn’s Moons

Saturn’s satellite system consists of 31 moons, some of which are shown in Figure 16. Titan is the largest moon and is bigger than Mercury. It is the second-largest moon in the solar system. Titan and Neptune’s Triton are the only moons in the solar system known to have substantial atmospheres. Because of its dense gaseous cover, the atmospheric pressure at Titan’s surface is about 1.5 times that at Earth’s surface. Another moon, Phoebe, exhibits retrograde motion. It, like other moons with retrograde orbits, is most likely a captured asteroid or large planetesimal left over from the formation of the planets.



Figure 16 Saturn’s Moon This image of Saturn shows several of its moons.



Uranus: The Sideways Planet

A unique feature of Uranus is that it rotates “on its side.” Instead of being generally perpendicular to the plane of its orbit like the other planets, Uranus’s axis of rotation lies nearly parallel with the plane of its orbit. Its rotational motion, therefore, has the appearance of rolling, rather than the top-like spinning of the other planets. Uranus’s spin may have been altered by a giant impact.



A surprise discovery in 1977 revealed that Uranus has a ring system. This find occurred as Uranus passed in front of a distant star and blocked its view. Observers saw the star “wink” briefly both before and after Uranus passed by. Later studies indicate that Uranus has at least nine distinct ring belts.



Spectacular views from Voyager 2, such as seen in Figure 17, show the varied terrains of the five largest moons of Uranus. Some have long, deep canyons and linear scars, whereas others possess large, smooth areas on otherwise crater-riddled surfaces. Miranda, the innermost of the five largest moons, has a greater variety of landforms than anybody yet examined in the solar system.



Figure 17 This image of Titania, one of Uranus’s moons, was taken by Voyager 2 from a distance of 1 million kilometers.



Neptune: The Windy Planet

As shown in Figure 18, Neptune has a dynamic atmosphere, much like those of Jupiter and Saturn. Winds exceeding 1000 kilometers per hour encircle Neptune, making it one of the windiest places in the solar system. It also has an Earth-size blemish called the Great Dark Spot that is reminiscent of Jupiter’s Great Red Spot. The Great Dark Spot is assumed to be a large rotating storm. About five years after the Great Dark Spot was discovered, it vanished, only to be replaced by another dark spot in the planet’s northern hemisphere.




Figure 18 The Great Dark Spot of Neptune is visible in the center of the left of the image. Bright cirrus-like clouds that travel at high speeds around the planet are also visible. Identifying What is the Great Dark Spot?



Perhaps most surprising are the white, cirrus-like clouds that occupy a layer about 50 kilometers above the main cloud deck. The clouds are most likely frozen methane. Neptune has 13 known moons. Voyager images revealed that the bluish planet also has a ring system.



Triton, Neptune’s largest moon, is nearly the size of Earth’s moon. Triton is the only large moon in the solar system that exhibits retrograde motion. This motion indicates that Triton formed independently of Neptune and was gravitationally captured.



Triton also has the lowest surface temperature yet measured on any body in the solar system at −200°C. Its atmosphere is mostly nitrogen with a little methane. Despite low surface temperatures, Triton displays volcanic-like activity.



Pluto: Planet X

Pluto lies on the fringe of the solar system, almost 40 times farther from the sun than Earth. It is 10,000 times too dim to be visible to the unaided eye. Because of its great distance and slow orbital speed, it takes Pluto 248 Earth-years to orbit the sun. Since its discovery in 1930, it has completed about one-fourth of a revolution. Pluto’s orbit is highly eccentric, causing it to occasionally travel inside the orbit of Neptune, where it resided from 1979 through February 1999.



In 1978 the moon Charon was discovered orbiting Pluto. Because of its close proximity to the planet, the best ground-based images of Charon show it only as an elongated bulge. In 1990 the Hubble Space Telescope produced a clearer image of the two icy worlds, shown in Figure 19. Charon orbits Pluto once every 6.4 Earth-days at a distance 20 times closer to Pluto than our moon is to Earth.



Figure 19 This Hubble image shows Pluto and its moon Charon.



Current data indicate that Pluto has a diameter of approximately 2300 kilometers, making it the smallest planet in the solar system. Charon is about 1300 kilometers across, exceptionally large in proportion to its parent.



The average temperature of Pluto is estimated at −210°C, which is cold enough to solidify most gases that might be present. Thus, Pluto might best be described as a dirty iceball of frozen gases with lesser amounts of rocky substances.




1.    Take notes on the above information.

2.    Turn in your notes.




Take the 4.03 Quiz

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