Images taken of Saturn's rings by Cassini immediately after it entered
orbit around Saturn have turned up circumstantial evidence that an unseen
moon may be orbiting dead center in the narrow Keeler gap in Saturn's
outer A ring.
The Keeler gap, a narrow gap calculated from Cassini images to be 42
kilometers (26 miles) wide, lies approximately 250 kilometers (155 miles)
inside the outer edge of the A ring. Several faint discontinuities, or
spikes, in the outer gap edge have been discovered in two narrow angle
camera images of the illuminated side of the rings taken immediately
after Saturn orbit insertion. One of these images has been mapped into a
longitude-radius system, contrast enhanced and stretched by a factor of
five, and is shown here. The longitudinal extent of the map is 4.6
degrees; the radial extent, top to bottom, is about 60 kilometers (37
miles). The most easily seen spikes are labeled A through J in this
image.
These features are similar to the spikes protruding inward from the core
of the F ring during Prometheus's passages (See PIA06143). Wisps of faint
material, vaguely reminiscent of the drapes and striations seen interior
to the F ring, are seen in association with, and between, some of these
spikes in the Keeler gap edge. These features all move in unison at the
orbital speed appropriate for particles at this location. Arguing on the
basis of their similarity to the features caused by Prometheus in the F
ring, it is likely that the features are caused by the passages of a
yet-unseen moonlet on an eccentric orbit within the Keeler gap. The
spikes are about 5 kilometers (3 miles) long. The putative moon would
have a diameter of a few kilometers.
The most statistically significant characteristic spacing of the Keeler
gap spikes has been found to be 199 kilometers (124 miles). A set of
features with this particular wavelength, could be produced if a moon on
an eccentric orbit were orbiting 21 kilometers (13 miles) away from the
gap edge; i.e., dead center in the gap.
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.