378 Encyclopedia of the Solar System
aspects of Iapetus are unusual. It is the only large Saturnian
satellite in a highly inclined orbit, and it is less dense than
objects of similar albedo. This latter fact implies a higher
fraction of ice or possibly methane or ammonia in its inte-
rior.
Two models exist for the origin of the dark material: It
derived from an exogenic (external) source, or it was en-
dogenically (internally) created. One scenario for the exo-
genic deposit of material entails dark particles being ejected
from Phoebe and drifting inward to coat Iapetus. The ma-
jor problem with this model is that the dark material on
Iapetus is redder than Phoebe, although the material could
have undergone chemical changes after its expulsion from
Phoebe to make it redder. Recent observations also show
that many of the outer retrograde satellites are similar in
color to the dark hemisphere of Iapetus, so the dust may
have come from those satellites as well. One observation
lending credence to an internal origin is the concentration
of material on crater floors, which implies an infilling mech-
anism. TheCassiniimages obtained on December 31, 2004,
reveal a gradual transition between the bright and dark re-
gions, which is more consistent with an exogenic model. The
Cassinivisual and infrared spectrometer also detected car-
bon dioxide and organics in the dark material. TheCassini
cameras also captured a unique geologic feature on Iapetus:
an equatorial ridge 20 km wide and 13 km high extending
over at least one hemisphere of the satellite.
4.3.2 HYPERION AND PHOEBE
Telescopic observations showed that the surface of Hype-
rion, which lies between the orbits of Iapetus and Titan,
is covered with ice. Because Hyperion has a visual geo-
metric albedo of 0.30, this ice must be mixed with a sig-
nificant amount of darker, rocky material. Its composition
may be similar to D-type asteroids. Although Hyperion is
only slightly smaller than Mimas, it has a highly irregular
shape (see Table 1). This suggests, along with the satel-
lite’s battered appearance, that it has been subjected to in-
tense bombardment and fragmentation.Cassiniimages of
the satellite (Fig. 9) show craters that appear to have been
deeply eroded, perhaps by sublimation of ice by darker,
hotter deposits in the crater floors. Hyperion is the only
satellite known to be in chaotic rotation—perhaps a colli-
sion within the last few million years knocked it out of a
tidally locked orbit. [SeeChaoticMotion in theSolar
System.]
Saturn’s outer satellite Phoebe, a dark object (see
Table 1) with a surface composition similar to that of C-
type asteroids (but apparently with more organic material),
moves in a highly inclined, retrograde orbit, suggesting that
it is a captured object. The spectral signature of water ice
was detected by ground-based telescopes. Although it is
smaller than Hyperion, Phoebe has a more nearly spherical
shape. Figure 10 shows an image of the satellite obtained
byCassini–Huygensin July 2004. The heavily battered sur-
FIGURE 9 Cassiniimage of Hyperion, showing deep craters
and a mottled, porous surface. The image was obtained at a
distance of about 62,000 km, with a resolution of about 700 m.
The largest crater on its surface is approximately 120 km in
diameter and 10 km deep.face has a number of unusual conical craters, and the largest
crater reveals higher albedo ice cliffs on its rims. Carbon
dioxide and organic material was detected byCassini, sug-
gesting that the satellite formed in the outer solar system,
perhaps as far out as the Kuiper Belt.FIGURE 10 Cassiniphotomosaic of Phoebe, obtained at
distances ranging from 16,000 to 12,000 km, with a
corresponding resolution of about 150 m. The heavily cratered
surface shows no hint of geological resurfacing. Icy cliffs are
evident in the large crater at the top of the image, and the crater
to the right of center shows evidence for layering near its rim.