Chapter 12 Rings, Moons, and Pluto

12.6 Summary, Further Exploration, Websites

Key Concepts and Summary

Rings are composed of vast numbers of individual particles orbiting so close to a planet that its gravitational forces could have broken larger pieces apart or kept small pieces from gathering together. Saturn’s rings are broad, flat, and nearly continuous, except for a handful of gaps. The particles are mostly water ice, with typical dimensions of a few centimeters. One Saturn moon, Enceladus, is today erupting geysers of water to maintain the tenuous E Ring, which is composed of very small ice crystals. The rings of Uranus are narrow ribbons separated by wide gaps and contain much less mass. Neptune’s rings are similar but contain even less material. Much of the complex structure of the rings is due to waves and resonances induced by moons within the rings or orbiting outside them. The origin and age of each of these ring systems is still a mystery.

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:

  • Jupiter Atlas:[iter-atlas/id352033947?mt=8
  • Saturn Atlas:


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: OR 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).


  1. 1 The ring letters are assigned in the order of their discovery.


an orbital condition in which one object is subject to periodic gravitational perturbations by another, most commonly arising when two objects orbiting a third have periods of revolution that are simple multiples or fractions of each other


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