The cratered surface of Saturn's moon Mimas
Enceladus, Tethys, Dione, and Rhea are approximately spherical
in shape and appear to be composed mostly of water ice.
Enceladus reflects almost 100 percent of the sunlight that
strikes it. All five satellites represent a size range that
had not been explored before.
Tethys, Dione, and Rhea are all cratered; Enceladus appears
to have by far the most active surface of any satellite
in the system (with the possible exception of Titan, whose
surface was not photographed). At least five types of terrain
have been identified on Enceladus. Although craters can
be seen across portions of its surface, the lack of craters
in other areas implies an age less than a few hundred million
years for the youngest regions. It seems likely that parts
of the surface are still undergoing change, since some areas
are covered by ridged plains with no evidence of cratering
down to the limit of resolution of Voyager 2's cameras (2
kilometers or 1.2 miles). A pattern of linear faults crisscrosses
other areas. It is not likely that a satellite as small
as Enceladus could have enough radioactive material to produce
the modification. A more likely source of heating appears
to be tidal interaction with Saturn, caused by perturbations
in Enceladus' orbit by Dione (like Jupiter's satellite Io).
Theories of tidal heating do not predict generation of enough
energy to explain all the heating that must have occurred.
Because it reflects so much sunlight, Enceladus' current
surface temperature is only 72 Kelvins (-330 degrees Fahrenheit).
of Mimas show a huge impact crater. The crater, named Herschel,
is 130 kilometers (80 miles) wide, one-third the diameter
of Mimas. Herschel is 10 kilometers (6 miles) deep, with
a central mountain almost as high as Mount Everest on Earth.
of Tethys taken by Voyager 2 show an even larger impact
crater, named Odysseus, nearly one-third the diameter of
Tethys and larger than Mimas. In contrast to Mimas' Herschel,
the floor of Odysseus returned to about the original shape
of the surface, most likely a result of Tethys' larger gravity
and the relative fluidity of water ice. A gigantic fracture
covers three-fourths of Tethys' circumference. The fissure
is about the size scientists would predict if Tethys were
once fluid and its crust hardened before the interior, although
the expansion of theinterior due to freezing would not be
expected to cause only one large crack. The canyon has been
named Ithaca Chasma. Tethys' surface temperature is 86 Kelvins
(-305 degrees Fahrenheit).
shows no evidence of internal activity. Its irregular shape
causes an unusual phenomenon: Each time Hyperion passes
Titan, the larger satellite's gravity gives Hyperion a tug
and it tumbles erratically, changing orientation. The irregular
shape of Hyperion and evidence of bombardment by meteors
make it appear to be the oldest surface in the Saturn system.
has long been known to have large differences in surface
brightness. Brightness of the surface material on the trailing
side has been measured at 50 percent, while material on
the leading side reflects only 5 percent of the sunlight.
Most dark material is distributed in a pattern directly
centered on the leading surface, causing conjecture that
dark material in orbit around Saturn was swept up by Iapetus.
The trailing face of Iapetus, however, has craters with
dark floors. That implies that the dark material originated
in the satellite's interior. It is possible that the dark
material on the leading hemisphere was exposed by ablation
(erosion) of a thin, overlying, bright surface covering.
2 photographed Phoebe after passing Saturn. Phoebe orbits
Saturn in a retrograde direction (opposite to the direction
of the other satellites' orbits) in a plane much closer
to the ecliptic than to Saturn's equatorial plane. Voyager
2 found that Phoebe has a roughly circular shape, and reflects
about 6 percent of the sunlight. It also is quite red. Phoebe
rotates on its axis about once in nine hours. Thus, unlike
the other Saturnian satellites (except Hyperion), it does
not always show the same face to the planet. If, as scientists
believe, Phoebe is a captured asteroid with its composition
unmodified since its formation in the outer solar system,
it is the first such object that has been photographed at
close enough range to show shape and surface brightness.
Dione and Rhea have bright, wispy streaks that stand out
against an already-bright surface. The streaks are probably
the results of ice that evolved from the interior along
fractures in the crust.