Key Terms for Review

A ring

B ring

belt

C ring

Cassini division

differential rotation

Encke division

F ring

Galilean moon (satellite)

Great Dark Spot

Great Red Spot

hydrocarbon

liquid metallic hydrogen

occultation

polymer

prograde orbit

resonance

retrograde orbit

ringlet

Roche limit

shepherd satellite (moon)

spoke

zone (atmospheric)

Review Questions

Question 8.1

Which is the most massive planet in the solar system?

  1. Earth
  2. Neptune
  3. Saturn
  4. Jupiter
  5. Mercury

Question 8.2

Which of the following planets does not have rings? Choose only one.

  1. Mars
  2. Uranus
  3. Neptune
  4. Saturn
  5. Jupiter

Question 8.3

Which is the least massive planet in the solar system?

  1. Mercury
  2. Mars
  3. Uranus
  4. Jupiter
  5. Venus

Question 8.4

Which planet is presently known to have the most moons?

  1. Mars
  2. Saturn
  3. Uranus
  4. Jupiter
  5. Neptune

Question 8.5

Describe the appearance of Jupiter’s atmosphere. Which features are long-lived and which are relatively fleeting?

Question 8.6

To test your knowledge of Jupiter’s belt and zone structure, do Interactive Exercise 8.1 on the assigned Web site. You can print out your results, if required.

Question 8.7

What causes the belts and zones in Jupiter’s atmosphere?

Question 8.8

To test your knowledge of Jupiter’s internal structure, do Interactive Exercise 8.2 on the assigned Web site. You can print out your results, if required.

Question 8.9

What is liquid metallic hydrogen? Which planets contain this substance? What produces this form of hydrogen?

Question 8.10

Compare and contrast the surface features of the four Galilean satellites, discussing their geologic activity and their evolution.

Question 8.11

To test your knowledge of the Galilean moons, do Interactive Exercise 8.3 on the assigned Web site. You can print out your results, if required.

Question 8.12

What energy source powers Io’s volcanoes?

Question 8.13

Why are numerous impact craters found on Ganymede and Callisto but not on Io or Europa?

Question 8.14

Describe the structure of Saturn’s rings. What are they made of?

Question 8.15

To test your knowledge of Saturn’s rings, do Interactive Exercise 8.4 on the assigned Web site. You can print out your results, if required.

Question 8.16

Why do features in Saturn’s atmosphere appear to be much fainter and more “washed out” than comparable features in Jupiter’s atmosphere?

Question 8.17

Explain how shepherd satellites affect some planetary rings. Is “shepherd satellite” an appropriate term for these objects? Explain your answer.

Question 8.18

Describe Titan’s atmosphere. What effect does sunlight have on it?

Question 8.19

Describe Titan’s surface.

Question 8.20

Describe the seasons on Uranus. Why are the Uranian seasons different from those on any other planet?

Question 8.21

Briefly describe the evidence that supports the idea that Uranus was struck by a large planetlike object several billion years ago.

Question 8.22

Why are Uranus and Neptune distinctly bluer than Jupiter and Saturn?

Question 8.23

Compare the ring systems of Saturn and Uranus. Why were Uranus’s rings unnoticed until the 1970s?

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Question 8.24

How do the orientations of Uranus’s and Neptune’s magnetic axes differ from those of the other planets?

Question 8.25

To test your knowledge of planetary magnetic fields, do Interactive Exercise 8.5 on the assigned Web site. You can print out your results, if required.

Question 8.26

Explain why Triton will never collide with Neptune, even though Triton is spiraling toward that planet.

Advanced Questions

Question 8.27

Consult the Internet or a magazine such as Sky & Telescope or Astronomy to determine which satellite missions are now under way. What data and pictures have they sent back that update information presented in this chapter?

Question 8.28

Long before the Voyager flybys, Earth-based astronomers reported that Io appeared brighter than usual for a few hours after emerging from Jupiter’s shadow. Explain this brief brightening of Io.

Question 8.29

Compare and contrast Valhalla on Callisto with Caloris Basin on Mercury.

Question 8.30

As seen by Earth-based observers, the intervals between successive edge-on presentations of Saturn’s rings alternate between 13 years 9 months and 15 years 9 months. Why are these two intervals not equal?

Question 8.31

Compare and contrast the internal structures of Jupiter and Saturn with the internal structures of Uranus and Neptune. Can you propose an explanation for why the differences between these two pairs of planets occurred?

Question 8.32

Neptune has more mass than Uranus, but Uranus is the larger of the two planets. Reconcile these two facts.

Discussion Questions

Question 8.33

Suppose that you were planning a mission to Jupiter, using an airplanelike vehicle that would spend many days, even months, flying through the Jovian clouds. What observations, measurements, and analyses should this aircraft make? What dangers might it encounter, and what design problems would you have to overcome?

Question 8.34

Discuss why astronomers believe that Europa, Ganymede, and Callisto may harbor some sort of marine life. Why do they not expect any life on the surfaces of these worlds?

Question 8.35

Suppose you were planning separate missions to each of Jupiter’s Galilean moons. What questions would you want these missions to answer, and what kinds of data would you want your spacecraft to send back? Given the different environments on the four satellites, how would the designs of the four spacecraft differ?

Question 8.36

NASA and the Jet Propulsion Laboratory have tentative plans to place spacecraft in orbit about Uranus and Neptune in this century. What kinds of data should be collected, and what questions would you like to see answered by these missions?

What If…

Question 8.37

Jupiter, at its present location, were a star? What would Earth be like? (Hint: Recall that to be a star, Jupiter would have to have 75 times more mass than it has today.)

Question 8.38

Jupiter had formed at one-third its present distance of 5.2 AU from the Sun? What would Earth be like?

Question 8.39

Io were struck by another object of similar size? (Hint: You can create a variety of different scenarios by imagining the impacting body striking from different directions, with different speeds, and at different angles.)

Question 8.40

Jupiter were orbiting in the opposite direction than it actually is? What effects might this have on the other planets? Would this change affect Earth? If so, how?

Web Questions

Question 8.41

Moving Weather Systems on Jupiter. Access and view the video “The Great Red Spot” in Chapter 8 of the Discovering the Universe Web site. a. Near the bottom of the video window you will see a white oval moving from left to right. By stepping through the video one frame at a time, estimate how long it takes this oval to move a distance equal to its horizontal dimension. (Hint: You can keep track of time by noticing how many frames it takes a feature in the Great Red Spot, at the center of the video window, to move in a complete circle around the center of the spot. The actual time for this feature to complete a circle is about 6 days.) b. The horizontal dimension of the white oval is about 4000 km. At what approximate speed (in km/h) does the white oval move? (Remember that speed = distance/time.)

Question 8.42

Search the Web, especially the Web sites at NASA’s Jet Propulsion Laboratory and at the European Space Agency, for information about the current status of the Cassini mission. When did Cassini arrive at Saturn? Where in the Saturn system is it presently? What are the current plans for its tour of Saturn’s satellites? What plans have been adopted for the Cassini extended mission, which began in 2008?

Question 8.43

In 2004, astronomers reported the discovery of two new moons of Saturn. Search the Web for information about these. How were they discovered? Have the observations been confirmed? How large are these moons? What sort of orbits do they follow?

Question 8.44

The Rotation Rate of Saturn. Access and view the video “Saturn from the Hubble Space Telescope” in Chapter 8 of the Discovering the Universe Web site. The total time that actually elapses in this video is 42.6 h. Using this information, identify and follow an atmospheric feature and determine the rotation period of Saturn.

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Got It?

Question 8.45

How would descending into Jupiter differ from descending from space onto the Earth?

Question 8.46

Why does the moon Io have active volcanoes while our Moon does not?

Question 8.47

Is Cassini’s division in Saturn’s rings empty? Justify your answer.

Question 8.48

Jupiter’s Great Red Spot is most accurately described by which of the following?

  1. an impact site
  2. a dormant volcano
  3. an active volcano
  4. a hurricane
  5. a tornado

Observing Projects

Question 8.49

Consult such magazines as Sky & Telescope and Astronomy or Starry Night™ software to determine whether Jupiter is currently visible in the night sky. If so, make arrangements to view the planet through a telescope. What magnifying power seems to give you the best view? Draw a picture of what you see. Can you see any belts and zones? How many? Can you see the Great Red Spot?

Question 8.50

Make arrangements to view Jupiter’s Great Red Spot through a telescope. Consult the Sky & Telescope Web site, which lists the times when the center of the Great Red Spot passes across Jupiter’s central region, as seen from Earth. The Great Red Spot is well placed for viewing for 50 minutes before and after this time. You will need a refractor with an objective lens of at least 15 cm (6 in.) diameter or a reflector with an objective of at least 20 cm (8 in.) diameter. The use of a pale blue or green filter can increase the color contrast and make the spot more visible. Sketch Jupiter based on your observations.

Question 8.51

If Jupiter is visible at night, observe it through a pair of binoculars or a telescope. Can you see all four Galilean moons? Make a drawing of what you observe. Use Starry Night™, set to the time of your observations, to identify the moons. Click the Home button in the toolbar and click the Stop button. Then use the Find pane to locate Jupiter. Close the Find pane and use the Zoom controls in the toolbar to set a field of view about 25′ wide. Select Labels > Planets-Moons from the menu. You should be able to see all the moons labeled in their present positions. Compare the locations of the moons during your observations with the equivalent view on Starry NightTM to identify these moons.

Question 8.52

Use Starry Night™ to examine Jupiter. Open Favourites > Explorations > Jupiter. Select Options > Solar System > Planets-Moons… from the menu. In the Planets-Moons Options dialog window, click the check box for Surface guides and click OK to remove the pole sticks and equator from the image. Click the Stop button and then use the Zoom controls and location scroller to examine Jupiter’s atmosphere. (a) Describe the appearance of the atmosphere of the planet. (b) Compare the number of white ovals and brown ovals in Jupiter’s southern hemisphere (the hemisphere in which the Great Red Spot is located) with the number of white ovals and brown ovals visible in the northern hemisphere. What general rule can you infer from these observations about the abundance of these storms in the two hemispheres?

Question 8.53

Use Starry Night™ to observe the appearance of Jupiter. Open Favourites > Explorations > Jupiter and use the Time Flow controls to determine the rotation period of the planet. (Hint: You may want to track the motion of an easily recognizable feature, such as the Great Red Spot.) (a) What is the approximate rotation period of Jupiter? (b) Measure Jupiter’s equatorial radius and its polar radius. You can use the angular separation tool to make these measurements. (Hint: Click on the cursor selection tool at the left side of the toolbar and select the angular measurement tool from the dropdown menu.) Measure from the center of the planet to its east or west limb (edge) along the equator of the planet to determine its equatorial radius and then measure from the center of the planet to its north or south pole to measure the polar radius. Are these two radii the same? If not, which is larger? Offer an explanation for this observation.

Question 8.54

Consult such magazines as Sky & Telescope or Astronomy or Starry NightTM software to determine whether Saturn is currently visible in the night sky. If so, view Saturn through a small telescope. Make a sketch of what you see. Estimate the angle at which the rings are tilted to your line of sight. Can you see the Cassini division? Can you see any belts or zones in Saturn’s clouds? Can you spot a faint, starlike object near Saturn? Could this be Saturn’s moon Titan? What observations could you perform to test whether this starlike object is a Saturnian satellite?

Question 8.55

Use Starry Night™ to examine the rings of Saturn. Open Favourites > Explorations > Saturn and use the location scroller to view Saturn so that you are looking straight down on the plane of the rings. (a) Draw a copy of what you see and label the different rings and divisions. (b) Using the location scroller, adjust the view so that Saturn’s rings appear edge-on and then rotate the image until the Sun comes into view. Which of the following is correct?

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  • (i) The Sun is in the same plane as the rings of Saturn.
  • (ii) The rings are in Saturn’s equatorial plane.
  • (iii) Neither (i) nor (ii) is correct.
  • (iv) Both (i) and (ii) are correct.

Question 8.56

Use the Starry Night™ program to examine the satellites of Uranus. Open Favourites > Explorations > Uranus and select View > Stars > Stars from the menu to remove the background stars from the view. Use the Elevation buttons in the Viewing Location section of the toolbar to change the distance from the planet to about 0.01 AU. You should now be able to see at least five satellites of the planet Uranus. Open the Find pane and expand the layer for Uranus. Click the checkboxes to the left of each of the listed moons to display their names and click the checkboxes to the right of the listed moons to display their orbits in the view. Use the location scroller to adjust the view so that the orbits of the moons appear edge-on. (a) Do all of the satellites appear to lie in this same plane? (Hint: Zoom in on Uranus if necessary to check your conclusion.) (b) How do you imagine that this plane relates to the plane of Uranus’s equator?

Question 8.57

Determine whether Uranus or Neptune is currently visible in the night sky. If so, make arrangements to view them through a telescope. To help you to locate these planets, use your Starry Night™ software or the star chart published each January in Sky & Telescope or Astronomy magazine, showing the paths of Uranus and Neptune against the background stars. For a more detailed view of each planet than you will see through a telescope, locate them with your Starry NightTM software and zoom to high resolution.

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WHAT IF…: We Lived on a Metal-Poor Earth?

Earth provides a wealth of building blocks necessary for the development and evolution of complex life-forms. More than 80 elements on or near Earth’s surface combine in countless ways essential for its diversity of flora and fauna. Especially important for life on Earth are metals, such as iron. The human body typically contains more than 3 grams of iron, mostly in the form of hemoglobin that helps transport oxygen through the bloodstream. A slight iron imbalance leads to anemia or toxicity, and without abundant metals, most of our technologies would never have developed.

But what if the solar system formed from an interstellar gas cloud containing fewer metals and high-mass elements—that is, a solar system with less iron, nickel, copper, and other metals crucial for life as we know it?

Metal-Rich versus Metal-Poor

Earth formed with almost 6 × 1021 metric tons of star matter, almost one-third of it iron. If the solar system formed from interstellar gas containing relatively few heavy elements, Earth would contain a much lower fraction of such elements as uranium, lead, iron, and nickel. Earth would be composed of a correspondingly higher fraction of lower-mass elements, such as carbon, nitrogen, oxygen, silicon, and aluminum. That version of Earth—let’s call it Lithia—would be profoundly different from our planet.

The Environment of Lithia

Without heavy elements like iron, Lithia’s density, gravity, and magnetic fields are much lower than Earth’s. Let’s assume that the density of Lithia is similar to that of the Moon. (We’ll assume that the Moon doesn’t change in its characteristics or its distance from Lithia.)

If Lithia has the same radius as Earth, the force of gravity on Lithia’s surface is only 60% that on Earth. That is, you would weigh 40% less on Lithia. Mobile life-forms that evolve on the metal-poor planet require much less muscle strength to get about and less bone density to resist the planet’s gravity.

With fewer radioactive elements to provide heat, the core of Lithia cooled off quickly compared with the core of Earth. As a result, Lithia has much less heat flowing upward through its mantle and crust, so most likely there is no plate tectonic activity. Lithia also has less volcanic activity. But without plate motion to spread the lava flows, volcanoes will grow to higher elevations than on Earth. The lack of crustal motions also means that pockets of high-density material will not rise from below, forming the metal ores mined on Earth.

The Moon’s orbit and the tides that it induces are different, too. With Lithia having 60% of Earth’s mass, the Moon at its present distance orbits once every 36 days instead of once every 29.5 days. Thus, the cycle of lunar phases is longer, but the cycle of lunar-induced tides is slightly shorter. Surprisingly, the tides have roughly the same height, because the tidal effects depend only on the Moon’s gravity and not Earth’s, and the distance between Earth and Lithia and their satellite is the same.

Life on Lithia

The low abundance of iron and the resulting lack of a planetary magnetic field might preclude the development of complex life-forms on Lithia. And yet, the incredible diversity of life we find on Earth suggests that evolution on Lithia might well occur. If advanced life-forms evolve on Lithia before the Sun uses up its nuclear fuel and becomes a red giant, the chemical differences between Lithia and Earth dictate that they will differ profoundly from the complex life that we find on Earth.

Without the heat of radioactive elements to keep Lithia’s core molten for billions of years and with fewer metals, Lithia will have a much weaker magnetic field—if any at all—than Earth. Inhabitants can expect auroras to grace the night sky continuously. On the other hand, high-energy particles and the radiation they create in the atmosphere will bombard any life-forms on the surface continuously and will also adversely affect the ozone layer. A larger amount of the Sun’s harmful ultraviolet radiation will reach the surface. Without more protection, life as we know it could not survive.

A Barren Landscape With a thin atmosphere and an abundance of lighter elements, such as silicon (silicon dioxide is sand), Lithia might resemble a barren desert here on Earth.