Cassini images have revealed the presence of previously unseen faint rings
in some of the gaps in Saturn's rings--possible indicators of small
yet-unseen moons.
Image A is a contrast-stretched view of the 270-kilometer-wide (170 mile)
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
inside it.
Image B is a view of the approximately 350-kilometer-wide (220 miles)
Huygens gap, between the outer edge of Saturn's 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 inside it.
Image C is a view of the ringlets inside the Encke gap. Some of these had
been seen by NASA's Voyager spacecraft, 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. This finding, along
with observed variations in brightness along the ringlet, implies that
accumulations of particles in the ringlet are maintained in special
orbits that prevent them from colliding with Pan.
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
Saturn's overexposed A ring (on the right), as well as two diffuse rings:
a 300-kilometer-wide (190 mile) ring of material, R/2004 S1, in the orbit
of Atlas (left-most arrow) and another ring, R/2004 S2, comparable to the
Atlas ring and immediately interior to Prometheus's orbit (right-most
arrow). These rings had been reported earlier and are comparable to the
jovian ring. Prometheus's orbit is elliptical, and brings the moon as
close to Saturn as the outer edge of R/2004 S2 and as far away from the
planet as the inner sharp boundary of Saturn's F ring. These observations
indicate that Prometheus has swept material from the region occupied by
its orbit.
It is not clear yet 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 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 team is based at
the Space Science Institute, Boulder, Colo.
For more information about the Cassini-Huygens mission, visit
http://saturn.jpl.nasa.gov and the Cassini imaging team home page,
http://ciclops.org.
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