The top half of this composite 
photograph shows Saturn 's rings as 
seen from the northern, or sunlit, 
side. The bottom image shows the 
same rings viewed from the southern, 
or unlit, side. The thick, opaque 
portions of the rings appear bright 
when viewed from the north and dark 
when viewed from the south. 
Conversely, the thin, semitransparent 
parts of the rings appear dark when 
seen from the north and bright 
when seen from the south. These 
images were made by Voyager 1 
in November 1980, when the 
spacecraft was approximately 
1,898,000 miles from Saturn. 
charges, in which charge built up by the 
motion of ring particles through Sat- 
urn's magnetic field suddenly flows ra- 
dially. The sparks created by the 
electrical discharges would be made 
visible by the motion of small particles 
out of the ring plane. Right now, it is 
not known whether the spokes are just 
another curious feature of Saturn’s 
rings or whether the associated electri- 
cal forces play an important role in ring 
structure and history. 
Electrical forces have also been in- 
voked, partly in desperation, to explain 
another puzzling phenomenon, namely 
the intricate structure in Saturn’s thin 
F ring, located just outside the A ring. 
The F ring contains three intermittent 
strands, two of which appear to be 
twisted. The instantaneous shape of the 
strands does not follow a simple orbital 
trajectory, so perhaps the whole pat- 
tern of kinks and braids orbits Saturn 
together. But then why doesn’t the 
structure spread into a more conven- 
tional flat ring? Once again, hidden sat- 
ellites embedded in the ring structure 
may provide the answer. Electrical 
forces may also be important, since 
much of the F ring structure is revealed 
by scattering from small particles. 
Moving outward from the rings and 
small satellites, we encounter the larger 
satellites Mimas, Enceladus, Tethys, 
Dione, Rhea, Titan, Hyperion, and Ia- 
petus. Except for Titan, whose diame- 
ter is 3,000 miles, all have diameters 
from 180 to 900 miles; all these satel- 
lites have densities close to that of water 
and are at least partly covered by water 
ice. Considering their small size and 
similar compositions, the surfaces of 
these satellites are surprisingly differ- 
ent. Mimas has a single large crater, 
with a diameter one-third that of the 
satellite. The impact of the collision 
that created the crater must have come 
close to breaking the satellite apart and 
was probably capable of melting a por- 
tion of the crust, causing substantial re- 
surfacing. The absence of intermedi- 
ate-sized craters probably indicates 
that at least one such resurfacing oc- 
curred after the final heavy bombard- 
ment during satellite accretion. En- 
celadus was not visited by Voyager 1, 
but viewed from across the Saturn sys- 
tem it appears to be almost featureless. 
It is also the brightest of the icy satel- 
lites. Tethys has a density almost equal 
to that of water. A long deep valley on 
the surface of the satellite implies geo- 
logic activity at some time in the past. 
Dione and Rhea both have bright, wis- 
py markings on their dark, trailing 
hemispheres, the “protected” sides in 
their orbital motion around Saturn. 
These markings suggest that a low-den- 
sity material, probably ice, was forced 
up through fissures in the surface. 
Bombardment of the leading hemi- 
spheres has since removed the mark- 
ings from the front sides. Dione has a 
low overall crater population, with a 
flat, plainslike surface that must have 
come after the heavy bombardment 
phase so as to cover any preexisting cra- 
tered terrain. 
Basic questions about these five icy 
satellites are, Why do they have such 
low densities (implying that they are 
mostly water ice)? and Why are they so 
small in relation to Jupiter’s four Gali- 
lean satellites? The answers may lie in 
the difference in mass between Jupiter 
and Saturn. Saturn, being less massive, 
did not contract during its formation as 
soon as Jupiter, and so may have incor- 
porated more of the late-arriving satel- 
lite material. Jupiter, contracting 
sooner, may have left more satellite 
material in orbit. Also, Saturn’s early 
temperatures were never as high as Ju- 
piter’s, so that water was not evaporat- 
ed away as it was close to Jupiter. These 
ideas are supported by computer mod- 
els of a collapsing cloud of planetary 
material. 
Saturn’s giant satellite Titan is a 
unique object. Larger than our moon, it 
is the only satellite in the solar system 
known to possess an atmosphere. Its 
bulk density is consistent with a rock- 
ice mixture. The surface of the satellite 
is entirely obscured by a dark haze of 
extremely small particles. Before Voy- 
ager got there, the only gas known to be 
in Titan’s atmosphere was methane 
(CH„). The satellite’s surface pressure 
and possible other gaseous constituents 
were unknown. The “slowing down” of 
Voyager’s radio signal as the spacecraft 
passed behind Titan made possible a 
determination of surface pressure. In- 
frared and ultraviolet spectra taken by 
Voyager established the abundance of 
gases. Titan’s atmosphere turned out to 
be mostly nitrogen (N 2 ), like the 
earth’s. The mass of an air column at 
the surface is more than one-tenth that 
on Venus and over ten times that on the 
earth. The speed and direction of the 
winds are unknown, but there is a faint 
pattern of colored clouds arranged in 
bands. 
It is remarkable that all planetary at- 
mospheres, including Titan’s, are made 
primarily of the same four elements: 
carbon (C), hydrogen (H), oxygen (O), 
48 
