Questions

Review Questions

1. If you observed the Galilean satellites through a telescope for a single night, could you notice their motions around Jupiter?

2. Why can’t the Galilean satellites be seen with the naked eye?

3. During the time it takes Ganymede to complete one orbit, how many orbits do Io and Europa complete?

4. In what ways does the system of Galilean satellites resemble our solar system? In what ways is it different?

5. No spacecraft from Earth has ever landed on any of the Galilean satellites. How, then, can we know if they are made of rock, water, or a combination of the two?

6. In what ways did the formation of the Galilean satellites mimic the formation of the planets? In what ways were the two formation processes different?

7. In the classic science-fiction film 2010: The Year We Make Contact, an alien intelligence causes Jupiter to contract so much that nuclear reactions begin at its center. As a result, Jupiter becomes a star like the Sun. Is this possible in principle? Explain your answer.

8. All of the Galilean satellites orbit Jupiter in the same direction. Furthermore, the planes of their orbits all lie within 0.5° of Jupiter’s equatorial plane. Explain why these observations are consistent with the idea that the Galilean satellites formed from a “Jovian nebula.”

9. Most of Jupiter’s smaller moons orbit in a direction opposite to Jupiter’s rotation. What does this tell us about those moons?

10. What is the source of energy that powers Io’s volcanoes? How is it related to the orbits of Io and the other Galilean satellites?

11. Io has no impact craters on its surface, while our Moon is covered with craters. What is the explanation for this difference?

12. Despite all the gases released from its interior by volcanic activity, Io does not possess a thick atmosphere. Explain why not.

13. Long before the Voyager flybys, Earth-based astronomers reported that Io appeared brighter than usual for the few hours after it emerged from Jupiter’s shadow. From what we know about the material ejected from Io’s volcanoes, suggest an explanation for this brief brightening of Io.

14. How do lavas on Io differ from typical lavas found on Earth? What does this difference tell us about Io’s interior?

15. What accounts for the different colors found on Io’s surface?

16. What is the Io torus? What is its source?

17. What is the origin of the electric current that flows through Io?

18. How was the Galileo spacecraft used to determine the internal structure of Io and the other Galilean satellites?

19. What surface features on Europa provide evidence for geologic activity?

20. What is the evidence for an ocean of liquid water beneath Europa’s icy surface? What is the evidence that substances other than water are dissolved in this ocean?

21. What aspects of Europa lead scientists to speculate that life may exist there? What precautions have been taken to prevent contaminating Europa with Earth organisms?

22. Why is ice an important constituent of Ganymede and Callisto, but not of Earth’s Moon?

23. How do scientists know that the dark terrain on Ganymede is older than the bright terrain?

24. In what ways is Ganymede like our own Moon? In what ways is it different? What are the reasons for the differences?

25. Why were scientists surprised to learn that Ganymede has a magnetic field? What does this field tell us about Ganymede’s history?

26. What leads scientists to suspect that there is liquid water beneath Ganymede’s surface?

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

28. Describe the surprising aspects of Callisto’s surface and interior that were revealed by the Galileo spacecraft. Why did these come as a surprise?

29. Compare and contrast the surface features of the four Galilean satellites. Discuss their relative geological activity and the evolution of these four satellites.

30. The larger the orbit of a Galilean satellite, the less geologic activity that satellite has. Explain why.

31. Explain how orbital resonance from the 1:2:4 ratio of the orbital periods of Io, Europa, and Ganymede is related to the geologic activity on these satellites.

32. Describe Titan’s atmosphere. Explain how the Sun’s ultraviolet radiation might have altered Titan’s original atmosphere?

33. In what ways do hydrocarbons like methane on Titan behave like water on Earth?

34. Why does the presence of methane in Titan’s atmosphere imply that Titan has had recent volcanic activity, or has some other source?

35. What is a cryovolcano? How can water be anything but frozen solid in Titan’s low temperatures?

36. What is the evidence that there were liquid hydrocarbons on Titan in the past? That there are liquid hydrocarbons now?

37. Is the bulk of Titan’s surface made of water-ice or methane-ice?

38. How would you account for the existence of the satellites of Jupiter other than the Galilean ones?

39. Which of Saturn’s moderate-sized satellites show evidence of geologic activity? What might be the energy source for this activity?

40. Explain the present-day difference in the reflectivity on parts of Iapetus. Explain what might cause this if Saturn’s largest ring—the newly discovered ring around Saturn in Figure 13-30—produces an initial difference on Iapetus’s surface.

41. Saturn’s equator is tilted by 27° from the ecliptic, while Jupiter’s equator is tilted by only 3°. Use these data to explain why we see fewer transits, eclipses, and occultations of Saturn’s satellites than of the Galilean satellites.

Advanced Questions

Questions preceded by an asterisk (*) involve topics discussed in Box 1-1 or Box 7-2.

42. Using the orbital data in Table 13-1, demonstrate that the Galilean satellites obey Kepler’s third law.

43. *What is the size of the smallest feature you should be able to see on a Galilean satellite through a large telescope under conditions of excellent seeing when Jupiter is near opposition? How does this compare with the best Galileo images, which have resolutions around 25 meters?

44. Using the diameter of Io (3642 km) as a scale, estimate the height to which the plume of Pele rises above the surface of Io in Figure 13-5a. (You will need to make measurements on this figure using a ruler.) Compare your answer to the value given in the figure caption.

45. Explain why the volcanic plumes in Figure 13-5b have a bluish color. (Hint: See Box 5-4.)

46. Jupiter, its magnetic field, and the charged particles that are trapped in the magnetosphere all rotate together once every 10 hours. Io takes 1.77 days to complete one orbit. Using a diagram, explain why particles from Jupiter’s magnetosphere hit Io primarily from behind (that is, on the side of Io that trails as it orbits the planet).

47. Assuming material is ejected from Io into Jupiter’s magnetosphere at the rate of 1 ton per second (1000 kg/s), how long will it be before Io loses 10 percent of its mass? How does your answer compare with the age of the solar system?

48. Volcanic eruptions from Io add about 1 ton of material to Jupiter’s magnetosphere every second. Based on the information given in Section 13-4, what fraction of the material ejected from Io’s volcanoes goes into Jupiter’s magnetosphere?

49. How long does it take for Ganymede to entirely enter or entirely leave Jupiter’s shadow? Assume that the shadow has a sharp edge.

50. Figure 13-16 shows that Ganymede’s dark terrain has both more craters and larger craters than the bright terrain. Explain what this tells you about the sizes of meteoroids present in the solar system in the distant past and in the more recent past.

51. Put a few cubes of ice in a glass, then fill the glass with warm water to make the ice cubes crack. The fracture lines in each cube are clearly visible because they reflect light. Use this observation to suggest why Ganymede’s icy bright terrain, which may have been badly fractured by the stresses that produced the grooves (Figure 13-16), is 25 percent more reflective than the dark terrain.

52. *Find the escape speed on Titan. What is the limiting molecular weight of gases that could be retained by Titan’s gravity? (Hint: Use the ideas presented in Box 7-2 and assume an average atmospheric temperature of 95 K.)

53. Many of the gases in the atmosphere of Titan, such as methane, ethane, and acetylene, are highly flammable. Why, then, doesn’t Titan’s atmosphere catch fire? (Hint: What gas in our atmosphere is needed to make wood, coal, or gasoline burn?)

54. Suppose Earth’s Moon were removed and replaced in its orbit by Titan. What changes would you expect to occur in Titan’s atmosphere? Would solar eclipses be more or less common as seen from Earth? Explain your answers.

55. At infrared wavelengths, the Hubble Space Telescope can see details on Titan’s surface as small as 580 km (360 mi) across. Determine the angular resolution of the Hubble Space Telescope using infrared light. If visible light is used, is the angular resolution better, worse, or the same? Explain your answer.

56. (a) To an observer on Enceladus, what is the time interval between successive oppositions of Dione? Explain your answer. (b) As seen from Enceladus, what is the angular diameter of Dione at opposition? How does this compare to the angular diameter of the Moon as seen from Earth (about ½°)?

Discussion Questions

57. If you could replace our Moon with Io, and if Io could maintain its present amount of volcanic activity, what changes would this cause in our nighttime sky? Do you think that Io could in fact remain volcanically active in this case? Why or why not?

58. Speculate on the possibility that Europa, Ganymede, or Callisto might harbor some sort of life. Explain your reasoning.

59. Comment on the suggestion that Titan may harbor life-forms.

60. Jupiter’s satellite Io and Saturn’s satellite Enceladus are both geologically active, and both are in 2-to-1 resonances with other satellites. However, the amount of geologic activity of Enceladus is far less than on Io. Discuss some possible reasons for this difference.

61. Imagine that you are in charge of planning a successor to the Cassini spacecraft to further explore the Saturnian system. In your opinion, which satellites in the system should be examined more closely? What data should be collected? What kinds of questions should the new mission attempt to answer?

Web/eBook Questions

62. Various spacecraft missions have been proposed to explore Europa in greater detail. Search the World Wide Web for information about these. How would these missions test for the presence of an ocean beneath Europa’s surface?

63. Two new satellites of Jupiter were discovered in 2011. Search the World Wide Web for information about these satellites. What do the orbital characteristics of these moons tell you about their origins?