Saturn’s New F-ring Shepherd?

Categories: Saturn system

Saturn’s New F-ring Shepherd?

Before the Cassini mission to Saturn, 31 moons had been found in the satellite system. Cassini has already identified two more, bringing the total to the second most populated planet in our solar system (Jupiter has more than 60). Recent analysis of Cassini images shows a potential third moon waiting to be confirmed as a true satellite orbiting near the outer F-ring.

Three frames showing in the squared region the movement around the F-ring of a new previously untracked object.
Credit: NASA/JPL

A small new found object, temporarily designated S/2004 S 3, has been seen orbiting Saturn’s outer F ring. The tiny object, seen in this movie centered in a green box, orbits the planet at a distance of approximately 141,000 kilometers (86,420 miles) from the center of Saturn. Its nature, moon or clump, is not presently known.

This movie is a sequence of 3 images taken with the Cassini spacecraft narrow angle camera on June 21, 2004. Images were taken every eight minutes over the course of two and one-quarter hours. Each image has been enhanced to show the presence of the newly detected object.

The size of the object has been estimated to be four to five kilometers (two to three miles) across. Because it is so small, it is not resolved and appears as a faint point of light just barely visible above the background noise. These images, which are part of a sequence specifically designed to search for small new moons in the inner Saturnian system, have not been cleaned of artifacts but have been greatly enhanced in contrast to increase visibility. Consequently, the main rings are overexposed, and many cosmic ray hits and noise patterns are clearly apparent. The image scale is approximately 40 kilometers (25 miles) per pixel.

Saturn F rings. Image Credit: NASA/JPL

The F-ring is already known to be shaped by gravitational action of the moons orbiting on both inner and outer sides of the debris ring. The third and fourth innermost moons of Saturn were unexpectedly discovered to be gravitational "shepherds." The inner moon Prometheus and the outer moon Pandora use their gravitational attraction to define Saturn ‘s outermost ring. Were any of the smaller chunks of ice and rock that compose Saturn’s F Ring to stray, the ring particle would be gravitationally pulled back into place by one of these passing moons. This complex interaction creates a ring structure with two narrow braids and several unusual knots. Whether this new object is also a shepherd moon remains to be seen. Since Prometheus orbits inside the ring and Pandora orbits outside the ring herding it into shape, it is likely that the new object also would contribute some effects to Pandora’s ability to define the fine structure of the F-ring.

A new found ring of material, S/2004 1 R, in the orbit of Saturn’s moon Atlas has been seen in this view of the region between the edge of Saturn’s A ring and the F ring.
Credit: NASA/JPL

As a milestone for marrying theoretical and observational astronomy, scientists predicted that an undiscovered ring might be formed here, and only in 1980 was their "F-ring" hypothesis confirmed by Voyager I during its Saturn flyby. The lettering scheme, "A through F", refers to the historical order of the ring divisions’ discoveries, and does not relate easily to their distance from Saturn. In the case of the thin F and G rings, the last discovered gaps, these debris fields do lie outside the more prominent "A through D" system.


Related Web Pages

Saturn Edition, Astrobiology Magaz.
Saturn’s Rings in UV
Cassini Closes In on Saturn

Saturn– JPL Cassini Main Page
Lord of the Rings
Space Science Institute, Imaging Team Boulder, Colorado
Saturn: The Closest Pass
Voyager Image Query Form
David Seal’s JPL site -Solar System Simulator