Cassini images have revealed, in some of the gaps in Saturn's rings, the presence of previously unseen faint rings ...possible indicators of small yet-unseen moons.
Image A is a contrast-stretched view of the 270 kilometer (170 mile) -wide Maxwell gap in Saturn's C ring. The right arrow points to the optically thick Maxwell ringlet; the left arrow points to the new diffuse ring seen interior to it.
Image B is a view of the approximately 350 kilometer (220 mile) -wide Huygens gap, between the outer edge of the B ring (on the left) and the dark bands (on the right) in the Cassini division. The right arrow points to the optically thick Huygens ring; the left arrow points to the new diffuse ring interior to it.
Image C is a view of the ringlets inside the Encke gap. Some of these had been seen by Voyager, but this contrast-enhanced Cassini lit-side image shows the presence of three major ringlets and a rather tenuous one. The center ringlet, which in this image has the highest optical depth among the ringlets, is coincident with Pan's orbit, implying that here, the ring particles are being maintained in horseshoe orbits. It, along with other ringlets in the Encke gap, exhibit longitudinal variations in brightness, suggestive of the particle accumulation that occurs in corotation sites associated with corotation resonances.
In Image D, which is a composite of several wide angle images taken of the lit-side of the rings after orbit insertion, there is clear indication of material extending about 400 kilometers (250 miles) beyond the edge of the overexposed A ring (on the right), as well as two diffuse rings: a 300 kilometer (190 mile) -wide ring of material, R/2004 S1, in the orbit of Atlas (left-most arrow) and another ring, R/2004 S2 (4), comparable to the Atlas ring and immediately interior to Prometheus's periapse distance (right-most arrow). These rings had been reported earlier and are comparable to the jovian ring. Prometheus's apoapse distance corresponds to the inner sharp boundary of the F ring's envelope. These observations indicate that Prometheus has swept material from the region occupied by its orbit.
It is not clear at present whether the origin of all these low-optical depth ringlets is the same. The association of the Atlas ring with Atlas and the main Encke ringlet with Pan would suggest that these rings derive from their associated moon. In other cases, a ring may exist because the material (or small parent bodies within it) are shepherded by a larger moon also present in the gap. The particles in many or all of these diffuse ringlets may have substantial fractions of micrometer-sized dust, implying that non-gravitational forces also may affect the ringlets' dynamics. In any case, the presence of narrow, diffuse ringlets in gaps like Maxwell and Huygens, along with the major Maxwell and Huygens ringlets, and the additional narrow ringlets in the Encke gap, suggests that there may be yet unseen moonlets in these gaps.
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 Cassini-Huygens mission for NASA's Science Mission Directorate, Washington, D.C. The imaging team consists of scientists from the US, England, France, and Germany. The imaging operations center and team lead (Dr. C. Porco) are based at the Space Science Institute in Boulder, Colo.