The Prometheus Effect

This mosaic of 15 Cassini images of Saturn's F ring shows how the moon Prometheus creates a gore in the ring once every 14.7 hours, as it approaches and recedes from the F ring on its eccentric orbit.

The individual images have been processed to make the ring appear as if it has been straightened, making it easier to see the ring's structure. The mosaic shows a region 147,000 kilometers (91,000 miles) along the ring (horizontal direction in the image); this represents about 60 degrees of longitude around the ring. The region seen here is about 1,500 kilometers (900 miles) across (vertical direction). The first and last images in the mosaic were taken approximately 2.5 hours apart.

Each dark channel, or "gore," is clearly visible across more than 1,000 kilometers (600 miles) of the ring and is due to the gravitational effect of Prometheus (102 kilometers, or 63 miles across), even though the moon does not enter the F ring. The channels have different tilts because the ring particles closer to Prometheus (overexposed, stretched, and just visible at the bottom right of the image) move slower with respect to the moon than those farther away. This causes the channels to shear with time, their slopes becoming greater, and gives the overall visual impression of drapes of ring material. The channels at the right are the youngest and have near-vertical slopes, while those at the left are the oldest and have near-horizontal slopes.

This phenomenon has not previously been detected in any other planetary ring system, but computer simulations of the system prove that the disturbance is caused by a simple gravitational interaction. The eccentric orbit of Prometheus is gradually moving so that the moon will eventually come even closer in its closest approach to the eccentric F ring. Scientists calculate that its perturbations of the F ring will reach a maximum in December 2009.

The images in this mosaic were taken using the Cassini spacecraft narrow-angle camera on April 13, 2005, at a distance of approximately 1.1 million kilometers (700,000 miles) from Saturn. The resolution in the original images, before reprojection, was 6 kilometers (4 miles) per pixel.

The Cassini-Huygens 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. The Cassini imaging team homepage is at http://ciclops.org.