Planetary Satellites 375
FIGURE 3 Four small satellites of
Jupiter: Metis, Adrastea, Amalthea,
and Thebe, shown to their correct
relative sizes. All four satellites
orbit between Jupiter’s ring and
the Galilean satellites.This result implies Amalthea is probably a “rubble pile”
composed of an agglomeration of debris reaccreted from
a collision long ago. Thebe is a low albedo satellite with a
reflectivity of 4–5%. It is also reddish in color, probably due
to contamination by sulfur from Io.
Exterior to the Galilean satellites, there is a class of
four satellites moving in inclined, prograde orbits (Lysithea,
Elara, Himalia, and Leda). They are dark objects, reflecting
only 2 or 3% of incident radiation, and may be similar to
C- and D-type asteroids. Themisto is a prograde satellite
moving in an inclined orbit between the Galilean satellites
and this group of four. Beyond this family lies the prograde
S2000 J11 and the retrograde Euporie. Another family of
objects is represented by the outermost satellites, which
have highly inclined retrograde orbits. They include Sinope,
Pasiphae, Carme, and Ananke, which were all discovered
in the first half of the last century, and 21 additional objects
discovered in 1999–2001. These satellites orbit at distances
of 21 million to 24 million km from Jupiter, and they may
be captured asteroids.
The 23 additional small satellites discovered in 2003 are
all small bodies orbiting at distances between 17 million
and over 28 million km: They comprise an “extended fam-
ily” of the outer retrograde satellites of Jupiter (one satellite,
Karpo, is in an inclined prograde orbit). This large group
of retrograde satellites appears to fall into smaller families
that occupy three dynamical groups separated by their dis-
tances from Jupiter. The small outer satellites of Jupiter are
probably captured asteroids, and they most likely represent
a large reservoir of additional undiscovered satellites. [See
AppendixTableSummary.]
4.3 The Saturnian System
4.3.1 THE MEDIUM-SIZED ICY SATELLITES OF SATURN: RHEA,
DIONE, TETHYS, MIMAS, ENCELADUS, AND IAPETUS
The Saturnian system contains 47 known satellites. Exclud-
ing the giant Titan, the six largest satellites of Saturn are
smaller than the Galilean satellites but still sizable, with
radii greater than 200 km—as such they represent a unique
class of icy satellite. Earth-based telescopic measurements
showed the spectral signature of ice for all six satellites. The
satellites’ low densities and high albedos (see Table 1) im-
ply that their bulk composition is largely water ice, possibly
combined with ammonia or other volatiles. They have
smaller amounts of rocky silicates than the Galilean satel-
lites. Resurfacing has occurred on several of the satellites.
Most of what is presently known of the Saturnian system
was obtained from theVoyagerflybys in 1980 and 1981 and
theCassini–Huygensexploration of Saturn’s satellites. The
six medium-sized icy satellites are shown to relative size in
Fig. 4.
The innermost medium-sized satellite Mimas is covered
with craters, including one (named Herschel) that is as large
as a third of the satellite’s diameter (see upper left of Fig. 4).
The impacting body was probably nearly large enough to
break Mimas apart; such disruptions may have occurred
on other objects. There is a suggestion of surficial groovesFIGURE 4 The six medium-sized icy Saturnian satellites. They
are, left to right from the upper left, Mimas, Enceladus, Tethys,
Dione, Rhea, and Iapetus.