Planetary Satellites 377
FIGURE 6 A view of the south pole of Enceladus,
obtained by theCassinicamera. The backlit
geyser-like fountains are likely to be particles of water
ice erupting from high-pressure reservoirs of liquid
water below the surface. The geysers appear to
originate from the “tiger stripes.”they are tectonic features—cover one hemisphere. How-
ever, there is no evidence for any resurfacing events early
in its history. There does seem to be a dichotomy between
crater sizes—some regions lack large craters whereas other
regions have a preponderance of such impacts. The larger
craters may be due to a population of larger debris more
prevalent during an earlier episode of collisions.
When Cassini discovered Iapetus in 1672, he noticed that
at one point in its orbit around Saturn it was very bright, but
that on the opposite side of the orbit it nearly disappeared
from view. He correctly deduced that one hemisphere is
FIGURE 7 The heavily cratered face of Dione is shown in this
Cassiniimage. Features that appeared as bright wispy streaks in
Voyagerimages (see Fig. 4) are shown to be bright ice cliffs
formed by tectonic fractures.
composed of highly reflective material and the other side
is much darker.Voyagerimages show that the bright side,
which reflects nearly 50% of the incident radiation, is fairly
typical of a heavily cratered icy satellite. The other side,
which is centered on the direction of motion, is coated with
a material with a reflectivity of about 3–4% (Fig. 8). OtherFIGURE 8 Cassiniimage of Iapetus, showing both bright and
dark terrains. The image was obtained at a distance of 173,000
km, with a resolution of about 2 km. The equatorial band is
clearly visible in the low-albedo terrain.