and its ring system (Click on the image for a larger
the greatest surprises and the most perplexing puzzles the
two Voyagers found are in the rings.
1 found much structure in the classical A-, B- and C-rings.
Some scientists suggest that the structure might be unresolved
ringlets and gaps. Photos by Voyager 1 were of lower resolution
than those of Voyager 2, and scientists at first believed
the gaps might be created by tiny satellites orbiting within
the rings and sweeping out bands of particles. One such
gap was detected at the inner edge of the Cassini Division.
2 measurements provided the data scientists need to understand
the structure. High-resolution photos of the inner edge
of the Cassini Division showed no sign of satellites larger
than five to nine kilometers (three to six miles). No systematic
searches were conducted in other ring gaps.
2's photopolarimeter provided more surprises. The instrument
measured changes in starlight from Delta Scorpii as Voyager
2 flew above the rings and the light passed through them.
The photopolarimeter could resolve structure smaller than
300 meters (1,000 feet).
star-occultation experiment showed that few clear gaps exist
in the rings. The structure in the B-ring, instead, appears
to be variations in density waves or other, stationary,
forms of waves. Density waves are formed by the gravitational
effects of Saturn's satellites. (The resonant points are
places where a particle would orbit Saturn in one-half or
one-third the time needed by a satellite, such as Mimas.)
For example, at the 2:1 resonant point with 1980S1, a series
of outward-propagating density waves has about 60 grams
of material per square centimeter of ring area, and the
velocity of particles relative to one another is about one
millimeter per second. Small-scale structure of the rings
may therefore be transitory, although larger-scale features,
such as the Cassini and Encke Divisions, appear more permanent.
edges of the rings where the few gaps exist are so sharp
that the ring must be less than about 200 meters (650 feet)
thick there, and may be only 10 meters (33 feet) thick.
In almost every case where clear gaps do appear in the rings,
eccentric ringlets are found. All show variations in brightness.
Some differences are due to clumping or kinking, and others
to nearly complete absence of material. Some scientists
believe the only plausible explanation for the clear regions
and kinky ringlets is the presence of nearby undetected
separate, discontinuous ringlets were found in the A-ring
gap, known as Encke's Gap, about 73,000 kilometers (45,000
miles) from Saturn's cloud tops. At high resolution, at
least one of the ringlets has multiple strands.
F-ring was discovered by Pioneer 11 in 1979. Photos of the
F-ring taken by Voyager 1 showed three separate strands
that appear twisted or braided. At higher resolution, Voyager
2 found five separate strands in a region that had no apparent
braiding, and surprisingly revealed only one small region
where the F-ring appeared twisted. The photopolarimeter
found the brightest of the F-ring strands was subdivided
into at least 10 strands. The twists are believed to originate
in gravitational perturbations caused by one of two shepherding
satellites, 1980S27. Clumps in the F-ring appear uniformly
distributed around the ring every 9,000 kilometers (6,999
miles), a spacing that very nearly coincides with the relative
motion of F-ring particles and the interior shepherding
satellite in one orbital period. By analogy, similar mechanisms
might be operating for the kinky ringlets that exist in
the Encke Gap.
spokes found in the B-ring appear only at radial distances
between 43,000 kilometers (27,000 miles) and 57,000 kilometers
(35,000 miles) above Saturn's clouds. Some spokes, those
thought to be most recently formed, are narrow and have
a radial alignment, and appear to corotate with Saturn's
magneticfield in 10 hours, 39.4 minutes. The broader, less
radial spokes appear to have formed earlier than the narrow
examples and seem to follow Keplerian orbits: Individual
areas corotate at speeds governed by distances from the
center of the planet. In some cases, scientists believe
they see evidence that new spokes are reprinted over older
ones. Their formation is not restricted to regions near
the planet's shadow, but seems to favor a particular Saturnian
longitude. As both spacecraft approached Saturn, the spokes
appeared dark against a bright ring background. As the Voyagers
departed, the spokes appeared brighter than the surrounding
ring areas, indicating that the material scatters reflected
sunlight more efficiently in a forward direction, a quality
that is characteristic of fine, dust-sized particles. Spokes
are also visible at high phase angles in light reflected
from Saturn on the unilluminated underside of the rings.
challenge scientists face in understanding the rings is
that even general dimensions do not seem to remain true
at all positions around Saturn: The distance of the B-ring;s
outer edge, near a 2:1 resonance with Mimas, varies by at
least 140 kilometers (90 miles) and probably by as much
as 200 kilometers (120 miles). Furthermore, the elliptical
shape of the outer edge does not follow a Keplerian orbit,
since Saturn is at the center of the ellipse, rather than
at one focus. The gravitational effects of Mimas are most
likely responsible for the elliptical shape, as well as
for the variable width of the Huygens Gap between the B-ring
and the Cassini Division.