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7.10: Rings, Moons, and Pluto (Exercises)

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    For Further Exploration



    Carroll, M. “Titan: What We’ve Learned about a Strange New World.” Astronomy (March 2010): 30. Nice review of Cassini mission results.

    Elliot, J. “The Warming Wisps of Triton.” Sky & Telescope (February 1999): 42. About Neptune’s intriguing moon.

    Hayes, A., “Secrets from Titan’s Seas.” Astronomy (October 2015): 24. Good review of what we now know and what puzzles us about the hydrocarbon lakes of Titan.

    Jewitt, D., et al. “The Strangest Satellites in the Solar System.” Scientific American (August 2006): 40. Small irregular moons in the outer solar system.

    Lakdawalla, E. “Ice Worlds of the Ringed Planet.” Sky & Telescope (June 2009): 27. On the Cassini mission exploration of Enceladus, Iapetus, and other moons.

    Mackenzie, D. “Is There Life under the Ice?” Astronomy (August 2001): 32. On future exploration of Europa.

    Robertson, D. “Where Goes the Rain?” Sky & Telescope (March 2013): 26. About the methane weather cycle on Titan and what Cassini experiments are telling us.

    Scharf, C. “A Universe of Dark Oceans.” Sky & Telescope (December 2014): 20. Subsurface oceans on Europa, Ganymede, Enceladus, and Titan.

    Showalter, M. “How to Catch a Moon (or Two) of Pluto.” Astronomy Beat (December 2012): On the discovery of small moons around Pluto, written by the person who discovered two of them.

    Spencer, J. “Galileo’s Closest Look at Io.” Sky & Telescope (May 2001): 40.

    Talcott, R. “Cassini Flies through Enceladus’ Geysers.” Astronomy (March 2009): 32.

    Zimmerman, R. “Does Methane Flow on Titan?” Astronomy (February 2014): 22. Ideas about lakes, channels, and rain.


    Stern, A. “Pluto: Up Close and Personal.” Astronomy (July 2015): 22. Good summary of the history of understanding Pluto and our current knowledge on the eve of the New Horizons encounter.

    Stern, A. “The Pluto System Explored.” Astronomy (November 2015): 24. Fine review of what the team learned from the first few data downloads from New Horizons.

    Tombaugh, C. “How I Found Pluto” Astronomy Beat (May 2009):


    Beatty, J. “Saturn’s Amazing Rings.” Sky & Telescope (May 2013): 18. Good 7-page summary of what we know.

    Burns, J., et al. “Bejeweled Worlds.” Scientific American (February 2002): 64. On rings throughout the solar system.

    Elliot, J., et al. “Discovering the Rings of Uranus.” Sky & Telescope (June 1977): 412.

    Esposito, L. “The Changing Shape of Planetary Rings.” Astronomy (September 1987): 6.

    Sobel, D. “Secrets of the Rings.” Discover (April 1994): 86. Discusses the outer planet ring systems.

    Tiscareno, M. “Ringworld Revelations.” Sky & Telescope (February 2007): 32. Cassini results about the rings of Saturn.


    Note: Many of the sites about planets and planetary missions listed for Other Worlds: An Introduction to the Solar System and The Giant Planets also include good information about the moons of the planets.

    Cassini Mission to Saturn: and and

    Jupiter’s Moons, at JPL:

    Neptune’s Moons, at JPL:

    New Horizons Mission: Gives the latest news bulletins and images from the Pluto encounter, plus lots of background information.

    Pluto, at JPL:

    Saturn’s Moons, at JPL:

    Uranus’ Moons, at JPL:


    Two apps you can buy for iPhones or iPads can show you the positions and features of the moons of Jupiter and Saturn for any selected date:


    Amazing Moons: 2016 NASA video on intriguing moons in our solar system (4:16).

    Briny Breath of Enceladus: Brief 2009 JPL film on the geysers of Enceladus (2:36).

    Dr. Carolyn Porco’s TED Talk on Enceladus: (3:26).

    Titan: Video from Open University, with interviews, animations, and images (8:11).

    Europa Mission: 2016 talk by two JPL scientists on NASA’s plans for a mission to Jupiter’s moon, which may have an underground liquid ocean (1:26:22).

    Great Planet Debate: Neil deGrasse Tyson debates Mark Sykes about how to characterize Pluto, in 2008 (1:14:11).

    How I Killed Pluto and Why It Had It Coming: 2011 Silicon Valley Astronomy Lecture by Michael Brown on the “demotion” of Pluto to a dwarf planet (1:27:13).

    Seeking Pluto’s Frigid Heart: Dramatic 2016 New York Times production, narrated by Dennis Overbye (7:43).

    Saturn’s Restless Rings: 2013 talk by Mark Showalter in the Silicon Valley Astronomy Lecture Series (1:30:59).


    Review Questions

    1. What are the moons of the outer planets made of, and how is their composition different from that of our Moon?
    2. Compare the geology of Callisto, Ganymede, and Titan.
    3. What is the evidence for a liquid water ocean on Europa, and why is this interesting to scientists searching for extraterrestrial life?
    4. Explain the energy source that powers the volcanoes of Io.
    5. Compare the properties of Titan’s atmosphere with those of Earth’s atmosphere.
    6. How was Pluto discovered? Why did it take so long to find it?
    7. How are Triton and Pluto similar?
    8. Describe and compare the rings of Saturn and Uranus, including their possible origins.
    9. Why were the rings of Uranus not observed directly from telescopes on the ground on Earth? How were they discovered?
    10. List at least three major differences between Pluto and the terrestrial planets.
    11. The Hubble Space Telescope images of Pluto in 2002 showed a bright spot and some darker areas around it. Now that we have the close-up New Horizons images, what did the large bright region on Pluto turn out to be?
    12. Saturn’s E ring is broad and thin, and far from Saturn. It requires fresh particles to sustain itself. What is the source of new E-ring particles?

    Thought Questions

    1. Why do you think the outer planets have such extensive systems of rings and moons, while the inner planets do not?
    2. Ganymede and Callisto were the first icy objects to be studied from a geological point of view. Summarize the main differences between their geology and that of the rocky terrestrial planets.
    3. Compare the properties of the volcanoes on Io with those of terrestrial volcanoes. Give at least two similarities and two differences.
    4. Would you expect to find more impact craters on Io or Callisto? Why?
    5. Why is it unlikely that humans will be traveling to Io? 
    6. Why do you suppose the rings of Saturn are made of bright particles, whereas the particles in the rings of Uranus and Neptune are black?
    7. Suppose you miraculously removed all of Saturn’s moons. What would happen to its rings?
    8. We have a lot of good images of the large moons of Jupiter and Saturn from the Galileo and Cassini spacecraft missions (check out NASA’s Planetary Photojournal site, at, to see the variety). Now that the New Horizons mission has gone to Pluto, why don’t we have as many good images of all sides of Pluto and Charon?
    9. In the Star Wars movie Star Wars Episode VI: Return of the Jedi, a key battle takes place on the inhabited “forest moon” Endor, which supposedly orbits around a gas giant planet. From what you have learned about planets and moons of the solar system, why would this be an unusual situation?


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