Pan and Daphnis

Figure 1: An image of Saturn and its rings taken by the Cassini spacecraft on 15 April 2008. Credit: NASA/JPL/Space Science Institute.

 Figure 2: A mosaic of a portion of Saturn’s rings, showing the A Ring, and the positions of the Encke Gap and Keeler Gap. Credit: NASA/JPL/Space Science Institute.

Pan and Daphnis are two inner moons of Saturn that orbit within gaps in Saturn’s A Ring. Pan orbits within the 325 km wide Encke Gap and Daphnis orbits within the 42 km wide Keeler Gap. Pan is a small walnut-shaped moon measuring 34.4 by 31.4 by 20.8 km in size. Daphnis is smaller than Pan and it measures 8.6 by 8.2 by 6.4 km in size. Pan was discovered in 1990 from images taken by the Voyager 2 spacecraft and Daphnis was discovered in 2005 from images taken by the Cassini spacecraft.

Pan’s walnut-shape is due to the presence of an equatorial ridge that was formed when the moon swept up ring material from the Encke Gap. Daphnis probably has an equatorial ridge as well. Pan takes 13.8 hours to circle Saturn while Daphnis takes 14.3 hours. The orbits of both moons have slight inclinations which cause then to move above and below Saturn’s ring plane. Pan speeds around Saturn at 19.9 km/s and from its surface, Saturn would span a whopping 53.6°.

Figure 3: Saturn’s moon Pan casting a slender shadow onto the A Ring. This image was taken by the Cassini spacecraft as Saturn was approaching its August 2009 equinox. Credit: NASA/JPL/Space Science Institute.

 Figure 4: The gravitational pull from Saturn’s moon Daphnis perturbs the orbits of the ring particles along the Keeler Gap and sculpts them into vertical structures. These structures cast long shadows across the A Ring in this image taken by the Cassini spacecraft as Saturn was approaching its August 2009 equinox. Credit: NASA/JPL/Space Science Institute.

References:

- Thomas, P. C. (July 2010), “Sizes, shapes, and derived properties of the saturnian satellites after the Cassini nominal mission”, Icarus 208 (1): 395-401

- Porco, C.C.; Thomas, P.C.; Weiss, J.W.; Richardson, D.C. (2007), “Saturn’s Small Inner Satellites: Clues to Their Origins”, Science 318: 1602-1607