[Illustration: In only twelve hours, Saturn’s satellites grew from
names in ancient mythology into dazzling worlds with personae of
their own. As Voyager 1 sailed through the Saturn system, it
returned photographs of Mimas, Enceladus, Tethys, Dione, and
Rhea—all part of a class of intermediate-sized icy bodies heretofore
unstudied by planetary spacecraft. All but Enceladus show heavily
cratered surfaces, evidence of aeons of meteorite bombardment.
Enceladus hints at internal processes, as yet unidentified, which
may have erased from its surface the evidence of early
bombardment—but we must await Voyager 2’s arrival next August to
better understand this body.]
[Illustration: 11/9/80 4.5 million km (2.8 million mi)
The surface of giant Titan, now dethroned from its seat as the solar
system’s largest satellite (Jupiter’s Ganymede is larger), remains
an enigma, shrouded beneath thick layers of haze.]
[Illustration: 11/12/80 22,000 km (14,000 mi)
Tiny moons—three new ones and three confirmed from previous
sightings—may tell us much about ring dynamics since gravitational
forces from satellites probably influence the ring structure. Two of
these tiny moons are on the verge of collision in the same orbit,
while several others appear to bound the A- and F-Rings. Iapetus,
whose two hemispheres differ dramatically in brightness, was
photographed in its orbit, almost 3.6 million kilometers (2.2
million miles) from the planet.]
[Illustration: 11/12/80 425,000 km (264,000 mi)
Mimas, Saturn’s innermost large satellite, has an impact crater
covering more than one quarter the diameter of the entire moon.
Nowhere else in the solar system has such a disproportionately large
feature been seen. In fact, it is believed that any impact larger
than this would probably have shattered Mimas into two or more
fragments. The crater has a raised rim and central peak, typical of
large impact structures on terrestrial planets. Additional smaller
craters, 15 to 45 kilometers (10 to 30 miles) in diameter, can be
seen scattered across the surface, particularly along the
terminator. Mimas is one of the small, low density Saturnian
satellites implying that it is composed primarily of ice.]
[Illustration: 11/12/80 130,000 km (80,000 mi)
Mimas’ other side shows a uniformly and heavily cratered surface—a
record of the bombardment that occurred throughout the solar system
in its early history 4.5 billion years ago. A long, narrow trough
about 5 kilometers (3 miles) wide crosses from northeast to
southwest. Mimas’ surface is very reflective (about 60 percent),
indicating that it consists largely of ice, which has been chipped
and pulverized by aeons of meteoritic bombardment. Such a surface on
a small, low mass moon would probably resemble light, powdery snow.
Features as small as 3 kilometers (2 miles) across are visible.]
[Illustration: 11/12/80 650,000 km (400,000 mi)
Enceladus appears to be largely devoid of craters or other major
surface relief, suggesting that perhaps internal processes may have
erased such structures. This satellite will be seen better by
Voyager 2 when it flies past Saturn in August 1981.]
[Illustration: 11/12/80 1.2 million km (750,000 mi)
This heavily cratered surface of Tethys faces toward Saturn and
includes a large valley about 750 kilometers (500 miles) long and 60
kilometers (40 miles) wide. The craters are the result of impacts,
and the valley appears to be a large fracture of unknown origin.
Tethys has a diameter of 1050 kilometers (650 miles), about
one-third that of Earth’s Moon. The smallest features visible in
this picture are about 24 kilometers (15 miles) across.]
[Illustration: 11/12/80 700,000 km (435,000 mi)
Dione reveals two distinctly different hemispheres. The photograph
shows Dione’s trailing side. Bright radiating patterns are probably
rays of debris thrown out of impact craters; other bright areas may
be topographic ridges and valleys.]
[Illustration: 11/12/80 162,000 km (101,000 mi)
Dione’s other hemisphere (mosaic) also has many impact craters—the
record of cosmic collisions. The largest crater is less than 100
kilometers (60 miles) in diameter and includes a well-developed
central peak. Sinuous valleys (seen near each pole) are probably the
result of crustal fracturing in the moon’s icy crust. Dione’s
diameter is only 1100 kilometers (700 miles), much smaller than any
of Jupiter’s icy moons.]
[Illustration: 11/13/80 80,000 km (50,000 mi)
Craters stand shoulder-to-shoulder on the surface of Saturn’s
satellite Rhea, seen in this mosaic of the highest-resolution
pictures of the north polar region. Rhea is 1500 kilometers (950
miles) in diameter and is the most heavily cratered Saturn moon. The
largest crater, made by the impact of cosmic debris, is about 300
kilometers (190 miles) in diameter.]
[Illustration: 11/12/80 128,000 km (79,500 mi)
Impact craters on the ancient surface of Rhea closely resemble those
on Mercury and Earth’s Moon. Many of the craters have central peaks
formed by rebound of the floor during the explosive formation of the
crater. Some craters are old and degraded by later impacts. Many
have sharp rims and appear relatively fresh, while others are very
shallow and have subdued rims, indicative of their antiquity. White
areas on the edges of several of the craters are probably fresh ice
exposed on steep slopes or possibly deposited by volatiles leaking
from fractured regions. Surface features as small as 2.5 kilometers
(1.5 miles) in diameter are visible.]
[Illustration: 11/9/80 4.5 million km (2.8 million mi)
Titan is a large, bizarre satellite. It is larger (almost 5120
kilometers or 3180 miles in diameter) than the planet Mercury and
possesses a dense atmosphere of unique composition. Voyager 1’s
cameras show Titan’s surface to be totally obscured by a thick layer
of atmospheric haze. In the full-disk photograph, only two features
are visible: a faint boundary between the southern and darker
northern hemispheres and a dark “hood” overlying Titan’s north polar
region.]
[Illustration: 11/12/80 435,000 km (270,000 mi)
This hood and greater detail in the haze layers are shown in the
higher resolution photograph.]
[Illustration: 11/10/80 4.6 million km (2.8 million mi)
Little detail can be seen in this distant view of Hyperion, the
satellite which orbits just beyond Titan. Voyager 2 will observe
Hyperion at a closer range.]
[Illustration: 11/12/80 3.2 million km (1.9 million mi)
Saturn’s satellite Iapetus displays a large, circular feature about
200 kilometers (120 miles) across with a dark spot in its center.
The circular feature is probably a large impact structure outlined
by dark material, possibly thrown out by the impact. The satellite’s
leading hemisphere is to the left, and the trailing hemisphere,
which is four to five times brighter, is to the right. Iapetus’
diameter is 1450 kilometers (900 miles).]
[Illustration: 11/12/80 177,000 km (110,000 mi)
Two satellites (Saturn’s tenth and eleventh) revolve in nearly
identical orbits 151,000 kilometers (94,000 miles) from Saturn’s
center. The satellites are each 100 to 200 kilometers in diameter,
larger than the distance separating their orbits, and they are
currently approaching one another at a rate which promises collision
in about two years. Such a collision, however, will probably be
averted by orbital changes induced by the satellites’ mutual
gravitational interactions as they near one another. The trailing
co-orbital satellite, seen in this photograph, has a very irregular
outline (the Sun is shining from the left). This color composite was
produced from three exposures taken over a period of more than six
minutes. During this period, a thin shadow, cast by a previously
unknown ring, moved across the satellite causing the “rainbow”
pattern shown here.]
[Illustration: 10/25/80 25 million km (16 million mi)
Two smaller satellites—Saturn’s thirteenth and fourteenth moons—were
discovered on October 25, 1980, in images taken to study the dark
“spokes” within Saturn’s B-Ring. The smaller, inner satellite has a
diameter of about 500 kilometers (300 miles) and is visible just
outside the A-Ring, near the bottom of the picture. It travels in an
orbit between the A-Ring and the F-Ring (not visible in this
photograph). The second satellite, seen to the left, travels just
outside the F-Ring and is about 600 kilometers (400 miles) in
diameter. Scientists believe the dimensions of the narrow F-Ring may
be determined by these two satellites, which orbit on either edge of
the ring.]