Shepherd Moons at Work

On February 20, 2005, the Cassini spacecraft took a picture of Pandora and Prometheus hard a work shepherding water ice particles and dust into a distinct F-ring unit within Saturn’s larger ring structure.

Historically, three main rings were discovered around Saturn, labeled C, B and A from the inside out. Later, fainter rings and structures within the larger rings were detected and labeled with additional letters from the alphabet. The F-ring is located outside of the A-ring. It is now known that many of these rings are in fact made up of even thinner ringlets. A large gap called the Cassini Division between B- and A-ring is not completely devoid of particles and was a concern when Cassini first arrived at the Saturn system in 2004. Cassini passed through the gap with no damage and has been hard at work studying the complex system of rings and gaps around Saturn ever since.

Many of the moons in the system interact with the rings to create their complex structure. Ringlets may vary slightly from each other in terms of composition and average particle size. Individual grains of primarily water ice in the rings can be as small as microscopic in size or as large as a boulder. The source of material for the rings is not well understood, but evidence suggests the rings are young and made up of material from some of the moons, perhaps through ice volcanoes.

While the rings of Saturn are easily visible to Earth-based telescopes, the amount of material in them is nearly negligible. The rings’ water ice composition and wide range reflects sunlight and give them their majesty. Jupiter, Uranus, and Neptune are also known to have rings, albeit much less dramatic. Scientists will use Cassini to study the rings of Saturn in great detail over the next several years.

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Richard Leis

Richard Leis is a writer and poet. His first published poem, "Roadside Freak Show," arrives on August 21, 2017 in Impossible Archetype.  His essays about fairy tales and technology have been published on Tiny Donkey. Richard is also the Downlink Lead for the High Resolution Imaging Science Experiment (HiRISE) team at the University of Arizona. He monitors images of the Martian surface taken by the HiRISE camera located on board the Mars Reconnaissance Orbiter in orbit around Mars and helps ensure they process successfully and are validated for quick release to the science community and public. Once upon a time, Richard wrote and edited the science and technology news and commentary website Frontier Channel, hosted the RADIO Frontier Channel podcast, and organized transhumanist clubs. Follow Richard on his website (, on Goodreads (richardleis), Twitter (@richardleisjr), and Facebook (richardleisjr).