sini Division, then the almost opaque B 
ring, and finally the semitransparent C 
ring closest to Saturn, The Cassini Di- 
vision is named after the seventeenth- 
century astronomer who discovered it. 
This gap is thought to represent the 
perturbing effects of Mimas, Saturn’s 
innermost large satellite, which com- 
pletes one orbit as the particles in the 
gap complete two orbits. One or two 
other gaps had been seen from the 
earth, but little else about Saturn’s ring 
structure had been seen or could be an- 
ticipated. 
Hence the structure viewed by Voy- 
ager came as a welcome surprise, pro- 
viding information on which to test 
theories of ring origin and evolution. 
Instead of two gaps there were hun- 
dreds. Rings (or ringlets) appeared 
within the rings, almost without limit 
as the resolution of the Voyager images 
improved. The Cassini Division, itself a 
huge structure as wide as the North 
American continent, was found to have 
gaps and rings down to ten miles wide. 
Only a narrow zone where the orbital 
period of a hypothetical particle is ex- 
actly one-half that of Mimas was found 
to be truly empty. 
A simple fractional relationship be- 
tween periods, such as exists between 
the Cassini Division and Mimas, is 
called a resonance. According to the- 
ory, whenever ring particles are reso- 
nating with a nearby satellite they begin 
to swing back and forth, nearer and far- 
ther from Saturn. The effect is like 
pushing a swing. The push can come 
every time the swing returns to its high- 
est point or every other time or with 
more complicated regularity. But if the 
push is large enough, the swinging will 
increase. Mimas exerts a gravitational 
pull, not a push, on the particles in Sat- 
urn’s ring structure, but the net result is 
the same. The swinging of ring particles 
in their orbits causes the particles to 
collide. The places where the swinging 
amplitude is largest, where there are 
resonances, are cleaned out by colli- 
sions. The trouble is that the known 
satellites have only a few strong reso- 
nances, not enough to explain the hun- 
dreds of observed gaps. 
The current view is that there are 
hundreds of unseen “pushers” hidden 
in the rings. These might be objects one 
to ten miles in diameter, orbiting along- 
side the one- to ten-foot-sized objects 
known to be present. In Voyager images 
such tiny satellites would be barely de- 
tectable against the blackness of space. 
They can apparently escape detection 
by hiding among the smaller bodies. 
Nevertheless, their existence is reason- 
ably well established, forcing us to con- 
sider a much wider size range for the 
ring particles. 
The existence of ring particles in the 
one- to ten-foot range can be inferred 
from the way the rings scatter radio 
waves at these wavelengths. The pres- 
ence of much smaller particles, of the 
order one ten-thousandth of an inch, 
was revealed by Voyager observations 
of the way the rings scatter sunlight. 
Clearly, different processes control the 
abundance of objects in these very dif- 
ferent size ranges. One theory that now 
looks attractive is that Saturn’s rings 
are the remains of a satellite or group of 
satellites that broke up, rather than a 
satellite that never formed. Gentle col- 
lisions within a cloud of particles tend 
to produce a more uniform-sized distri- 
bution, but violent collisions of large 
objects tend to produce both large and 
small fragments. 
An important concept for the study 
of rings is the so-called Roche limit, 
which defines where an orbiting object 
can hold itself together under its own 
gravity. For example, a shovelful of 
sand in empty space will evolve into a 
Voyager images revealed that two of 
the three strands in Saturn ’ s thin, 
outermost F ring are twisted and 
braided. The cause of this 
phenomenon has yet to be explained. 
46 
