News and commentary about the Great Frontiers

ISS007-E-10807 (21 July 2003) --- This view of Earth's horizon as the sunsets over the Pacific Ocean was taken by an Expedition 7 crewmember onboard the International Space Station (ISS). Anvil tops of thunderclouds are also visible. Credit: Earth Science and Remote Sensing Unit, NASA Johnson Space Center

Image Credit: ISS007-E-10807 (21 July 2003) – Earth Science and Remote Sensing Unit, NASA Johnson Space Center

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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