Voyager 1 and 2 observed Saturn's rings from a very short
distance. These observations showed that the main ring bands
we see from
Earth are actually
subdivided into literally thousands of ringlets, in a fashion
similar to the grooves of a compact disk. The images also
show that ring distribution is basically uninterrupted, from
a lower boundary corresponding to the outer layers of
Saturn's atmosphere out to the region where Saturn's inner
moons are located.
The different albedo of the several rings depends on the
size and composition of the rocks of which they are made.
The B ring, the brightest and most extended of the rings,
reflects about 80% of the incoming sunlight. It is white in
color. A detailed analysis shows that it is made of ice or
ice-covered rocks, ranging in scale from centimeters to
meters. The darker appearance and the pale coloration of
other rings can be explained by ice particles containing
impurities of other material, but it is unclear what their
exact composition is.
The origin of the rings and their distribution are a direct
consequence of the gravitational action of Saturn and its
moons on the material in the primordial nebula that formed
the system. The rings are located within Saturn's Roche
limit, within which no moon could have formed.
What does this last statement mean? Extended bodies orbiting
Saturn experience tidal forces due to the difference in the
planet's gravitational attraction between the closest and
farthest portion of their bulk. Tidal gravitational forces
tend to rip apart the objects, competing with their self-
gravity, which instead tends to keep them together. Within
the distance called Roche's limit, tides dominate and large
self-gravitating moons cannot form. The objects making up
the rings are of the size of boulders, or smaller, and they
are practically not subject to tidal forces. Such small
bodies are kept together by chemical processes, rather than
gravity.
The ringlet structure is due to the very complicated action
of Saturn's 18 moons. Let us give two separate examples.
The Cassini division occurs at a distance from Saturn
where bodies have a period of revolution that is exactly a
1:2 ratio to the period of revolution of the moon
Mimas. When particles of
the ring find themselves between Saturn and Mimas, Mimas's
action perturbs their orbit a tiny bit away from a circular
shape. Given the period ratio, these episodes happen always
in the same point of the orbit, reinforcing their effects.
The net result is that boulder-size rocks are swept away from
the Cassini division. Another, simpler example, is the Encke
division, due to the presence of a small moon in its midst.
