The Solar System and Its Place in the Galaxy 17
close to the planet. In fact, Phobos, the larger and closer
satellite, orbits Mars faster than the planet rotates. Both of
the martian satellites have surface compositions that appear
to be similar to carbonaceous chondrites in composition.
This has resulted in speculation that the satellites are cap-
tured asteroids. A problem with this hypothesis is that Mars
is located close to the inner edge of the asteroid belt, where
silicate asteroids dominate the population, and where car-
bonaceous asteroids are relatively rare. Also, both satellites
are located very close to the planet and in near-circular or-
bits, which is unusual for captured objects.
In contrast to the satellites of the terrestrial planets,
the satellites of the giant planets are numerous and are
arranged in complex systems. Jupiter has four major satel-
lites, easily visible in small telescopes from Earth, and 58
known lesser satellites. The discovery of the four major
satellites by Galileo in 1610, now known as the Galilean
satellites, was one of the early confirmations of the Coper-
nican theory of a heliocentric solar system. The innermost
Galilean satellite, Io, is about the same size as the Earth’s
Moon and has active vulcanism on its surface as a result
of Jupiter’s tidal perturbation and the gravitational inter-
action with Europa and Ganymede (see Section 2.1). The
next satellite outward is Europa, somewhat smaller than Io,
which appears to have a thin ice crust overlying a possi-
ble liquid water ocean, also the result of tidal heating by
Jupiter and the satellite–satellite interactions. Estimates of
the age of the surface of Europa, based on counting im-
pact craters, are very young, suggesting that the thin ice
crust may repeatedly break up and reform. The next satel-
lite outward from Jupiter is Ganymede, the largest satellite
in the solar system, even larger than the planet Mercury.
Ganymede is another icy satellite and shows evidence of
tectonic activity and of being partially resurfaced at some
time(s) in its past. The final Galilean satellite is Callisto, an-
other icy satellite that appears to preserve an impact record
of comets and asteroids dating back to the origin of the
solar system. As previously noted, the orbits of the inner
three Galilean satellites are locked into a 4:2:1 mean-motion
resonance.
The lesser satellites of Jupiter include 4 within the orbit
of Io, and 54 at very large distance from the planet. The
latter are mostly in retrograde orbits, which suggest that
they are likely captured comets and asteroids. The orbital
parameters of many of these satellites fall into several tightly
associated groups. This suggests that each group consists of
fragments of a larger object that was disrupted, most likely
by a collision with another asteroid or comet. Possibly, the
collision occurred within the gravitational sphere of Jupiter,
which then could have led to the dynamical capture of the
fragments.
All of the close-orbiting jovian satellites (out to the or-
bit of Callisto) appear to be in synchronous rotation with
Jupiter. However, rotation periods have been determined
for two of the outer satellites, Himalia and Elara, and these
appear to be around 10 to 12 hours, much shorter than their
∼250 day periods of revolution about the planet.
Saturn’s satellite system is very different from Jupiter’s
in that it contains only one large satellite, Titan, compara-
ble in size to the Galilean satellites, 8 intermediate-sized
satellites, and 47 smaller satellites. Titan is the only satellite
in the solar system with a substantial atmosphere. Clouds
of organic residue in its atmosphere prevent easy viewing
of the surface of that moon, though theCassinispacecraft
has had success in viewing the surface at infrared and radar
wavelengths. The atmosphere is primarily nitrogen and also
contains methane and possibly argon. The surface temper-
ature on Titan has been measured at 94 K, and the surface
pressure is 1.5 bar.Cassinihas revealed on Titan a complex
surface morphology that includes rivers, lakes, and possible
cryo-vulcanism.
The intermediate and smaller satellites of Saturn all ap-
pear to have icy compositions and have undergone sub-
stantial processing, possibly as a result of tidal heating and
also due to collisions. Orbital resonances exist between sev-
eral pairs of satellites, and most are in synchronous rotation
with Saturn. An interesting exception is Hyperion, which
is a highly nonspherical body and which appears to be in
chaotic rotation. Another moon, Enceladus, has a ring of
material in its orbit that likely has come from the satellite,
either as a result of a recent massive impact or as a result
of active vulcanism on the icy satellite;Cassinihas found
ice geysers near Enceladus’ south pole. Two other satel-
lites, Dione and Tethys, have companion satellites in the
same orbit, which oscillate about the Trojan-libration points
for the Saturn–Dione (1 companion) and Saturn–Tethys
(2 companions) systems, respectively. Yet another particu-
larly interesting satellite of Saturn is Iapetus, which is dark
on one hemisphere and bright on the other and has a narrow
ridge circling the satellite at its equator. The reason(s) for
the unusual dichotomy in surface albedos or the equatorial
ridge are not known.
Saturn has one very distant, intermediate-sized satellite,
Phoebe, which is in a retrograde orbit and which is sus-
pected of being a captured early solar system planetesimal,
albeit a very large one. Phoebe is not in synchronous ro-
tation, but rather has a period of about 10 hours. The 47
known small satellites of Saturn include 10 embedded in
or immediately adjacent to the planet’s ring system, the
three Trojan-type librators, and 34 in distant orbits. As with
Jupiter, the majority of these distant objects are in retro-
grade orbits and some are in groups, which suggests that
they are collisional fragments.
The Uranian system consists of five intermediate-sized
satellites and 22 smaller ones. Again, these are all icy bod-
ies. These satellites also exhibit evidence of past heating and
possible tectonic activity. The satellite Miranda is particu-
larly unusual in that it exhibits a wide variety of complex