Moon Convention

People with similar jobs or interests hold conventions and meetings, so why shouldn't moons? Pandora, Prometheus, and Pan -- seen here, from right to left -- also appear to be holding some sort of convention in this image.

Some moons control the structure of nearby rings via gravitational “tugs.” The cumulative effect of the moon's tugs on the ring particles can keep the rings' edges from spreading out as they are naturally inclined to do, much like shepherds control their flock. Pan is a prototypical shepherding moon, shaping and controlling the locations of the inner and outer edges of the Encke gap through a mechanism suggested in 1978 to explain the narrow Uranian rings. However, though Prometheus and Pandora have historically been called “the F ring shepherd moons” due to their close proximity to the ring, it has long been known that the standard shepherding mechanism that works so well for Pan does not apply to these two moons.

The mechanism for keeping the F ring narrow, and the roles played -- if at all -- by Prometheus and Pandora in the F ring’s configuration are not well understood. This is an ongoing topic for study by Cassini scientists.

This view looks toward the sunlit side of the rings from about 29 degrees above the ringplane. The image was taken in visible light with the Cassini spacecraft narrow-angle camera on Jan. 2, 2015.

The view was obtained at a distance of approximately 1.6 million miles (2.6 million kilometers) from the rings and at a Sun-ring-spacecraft, or phase, angle of 86 degrees. Image scale is 10 miles (15 kilometers) per pixel.

The Cassini mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. The Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, manages the mission for NASA's Science Mission Directorate, Washington, D.C. The Cassini orbiter and its two onboard cameras were designed, developed and assembled at JPL. The imaging operations center is based at the Space Science Institute in Boulder, Colo.

For more information about the Cassini-Huygens mission visit http://saturn.jpl.nasa.gov or http://www.nasa.gov/cassini . The Cassini imaging team homepage is at http://ciclops.org .

Credit: NASA/JPL-Caltech/Space Science Institute

EDITOR’S NOTE: The mechanism for keeping the F ring narrow, and the roles played -- if at all -- by Prometheus and Pandora in the F ring’s configuration are not well understood. This is an ongoing topic for study by Cassini scientists.

When this mechanism was originally proposed to explain the existence of narrow rings, it was believed that Prometheus and Pandora were archetypal examples of this mechanism in action.

However, the situation is far more intricate and complicated than originally envisioned and that Prometheus largely controls the structure of the F ring while Pandora looks on (Cuzzi, 2014). Pan similarly sculpts its portion of the A ring. Consistent with the traditional shepherding mechanism, Pan shapes and controls the locations of the inner and outer edges of the Encke gap by turning back wayward ring particles that stray into that gap. But, instead of constraining a narrow ring, Pan also maintains a narrow gap.

Cassini images such as the one above have provided exciting evidence that the interactions between Saturn's moons and rings are far more intriguing and complicated than rings scientists envisioned prior to Cassini's arrival at Saturn. The rich dynamics between moons and rings will continue to be an ongoing topic for study by Cassini scientists through the end of the mission and beyond.