Saturn’s shepherd moon

Saturn's moon Daphnis in the Keeler Gap in Saturn's rings

One of Saturn's two shepherd moons, Daphnis (upper left), inhabits the Keeler Gap in the planet's famous ring system. The moon's gravity causes a wave-like effect on the inner and outer edge of the Gap.

Saturn has more than 60 moons of many different sizes and shapes. Most of them orbit well outside the realm of the planet’s rings, but some live within the rings.

One such is Daphnis, a tiny 7-kilometre-diameter body that circles Saturn within a gap in the rings known as the Keeler Gap, which itself is only 42 kilometres wide.

The moon is named after a figure from Greek mythology. Daphnis was a shepherd, the some of Hermes and brother of Pan. (Saturn’s other shepherd moon is named Pan.)

Daphnis orbit is not perfectly circular; it is ever so slightly elliptical. Plus, it doesn’t orbit cleanly in the same plane as the rings, but has an inclined orbit that makes it range up to about 8 kilometres above and about 8 kilometres below the ring plane.

As it zips along through the Keeler Gap, Daphnis’ gravity disturbs the material in the rings on each edge of the Gap, resulting in the edges forming a “wavy” appearance. The wave on the inside edge of the Gap moves ahead of Daphnis, while the wave on the outer edge of the Gap lags behind, due to the different speeds at which Daphnis and material in the inner and outer edges circle Saturn.

Scientists had suspected that an undiscovered moon was causing the wavy edges of the Keeler Gap, but it wasn’t until May 2005 that it was spotted. The Cassini Imaging Science Team made the discovery on May 6, 2005 from images obtained five days earlier. It was subsequently spotted in other images taken on May 2, and earlier images taken in April 2005.

The image shown here was taken on July 5, 2010 by NASA’s Cassini spacecraft, and transmitted to Earth the following day.

Story by Jonathan Nally, Editor,

Image courtesy NASA / JPL / Space Science Institute.

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