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Saturn's moons make waves in rings

NASA / JPL-Caltech / SSI

The Saturnian moon Daphnis and Pan stir ripples in the giant planet's rings due to their gravitational effect. Five-mile-wide Daphnis (lower left) is perturbing particles in Saturn's A ring, while 17-mile-wide Pan (upper right) has kicked up dark wakes in the ring propagating toward the middle of the image. This picture was taken in visible light by the Cassini spacecraft's narrow-angle camera on June 3, 2010, at a distance of about 329,000 miles from Saturn.

This image from NASA's Cassini orbiter shows why Daphnis and Pan are known as "shepherd moons": The gravitational influence of those tiny satellites help keep Saturn's giant rings in line, creating subtle ripples and waves in the process.

Five-mile-wide Daphnis, at lower left, makes its circuit around Saturn in the Keeler Gap, an open space in the planet's A ring. As it passes through, it perturbs the particles along both sides of the gap, sculpting the edges. To learn more about Daphnis' influence and watch a movie showing the shepherd at work, check out this Web page from the Cassini mission's imaging team.

Meanwhile, 17-mile-wide Pan performs a similar function in the A ring's Encke Gap at upper right. You can see the dark waves left in the moon's wake by its gravitational influence on the icy particles in the disk. The images on this Web page provide additional perspectives on Pan. Such effects, documented in detail during Cassini's eight years in the Saturnian system, explain why Daphnis was named after a shepherd in Greek mythology, while Pan was named after the god of shepherds.

More about Saturn's moons and rings:

Alan Boyle is msnbc.com's science editor. Connect with the Cosmic Log community by "liking" the log's Facebook page, following @b0yle on Twitter and adding the Cosmic Log page to your Google+ presence. You can also check out "The Case for Pluto," my book about the controversial dwarf planet and the search for new worlds.