18 Encyclopedia of the Solar System
terrains. It has been suggested that Miranda, and possi-
bly many other icy satellites, were collisionally disrupted at
some time in their history, and the debris then reaccreted in
orbit to form the currently observed satellites, but preserved
some of the older morphology. Such disruption/reaccretion
phases may have even reoccurred on several occasions for
a particular satellite over the history of the solar system. Of
the smaller Uranian satellites, 13 are embedded in the ring
system and 9 are in distant, mostly retrograde orbits. Again,
these are likely captured objects.
Neptune’s satellite system consists of one large icy satel-
lite, Triton, and 12 smaller ones. Triton is somewhat larger
than Pluto and is unusual in that it is in a retrograde orbit.
As a result, the tidal interaction with Neptune is causing the
satellite’s orbit to decay, and eventually Triton will be torn
apart by the planet’s gravity when it passes within theRoche
limit. The retrograde orbit is often cited as evidence that
Triton must have been captured from interplanetary space
and did not actually form in orbit around the planet. Despite
its tremendous distance from the Sun, Triton’s icy surface
displays a number of unusual terrain types that strongly sug-
gest thermal processing and possibly even current activity.
TheVoyager 2spacecraft photographed what appeared to
be plumes from “ice volcanoes” on Triton.
The lesser satellites of Neptune include 6 that are either
in or adjacent to the ring system and 6 in distant orbits,
evenly split between direct and retrograde.
Among the dwarf planets, Ceres has no known satellites.
Pluto has one very large satellite, Charon, which is slightly
more than half the size of Pluto, and two smaller satellites,
Nix and Hydra, each estimated to be∼40–60 km in diam-
eter. The Pluto–Charon system is fully tidally evolved. This
means that Pluto and Charon each rotate with the same
period, 6.38723 days, which is also the revolution period of
the satellite in its orbit. As a result, Pluto and Charon al-
ways show the same faces to each other. It is suspected that
the Pluto–Charon system was formed by a giant impact be-
tween two large Kuiper belt objects. The third dwarf planet,
Eris, also has an intermediate-sized satellite, Dysnomia,
about 300–400 km in diameter.
In addition to their satellite systems, all of the jovian plan-
ets have ring systems (Fig. 8). As with the satellite systems,
each ring system is distinctly different from its neighbors.
Jupiter has a single ring at 1.72–1.81 planetary radii, dis-
covered by the Voyager 1 spacecraft. The ring has several
components, related to the four small satellites in or close
to the ring. The micron-sized ring particles appear to be
sputtered material off the embedded satellites.
Saturn has an immense, broad ring system extending
between 1.11 and 2.27 planetary radii, easily seen in a small
telescope from Earth. The ring system consists of three
major rings, known as A, B, and C ordered from the outside
in toward the planet, a diffuse ring labeled D inside the C
ring and extending down almost to the top of the Saturnian
atmosphere, and several other narrow, individual rings.
Closer examination by theVoyagerspacecraft revealed
that the A, B, and C rings were each composed of thousands
of individual ringlets. This complex structure is the re-
sult of mean-motion resonances with the many Saturnian
satellites, as well as with small satellites embedded within
the rings themselves. Some of the small satellites act as
gravitational “shepherds,” focusing the ring particles into
narrow ringlets. Additional narrow and diffuse rings are lo-
cated outside the main ring system.
The Uranian ring system was discovered accidentally in
1977 during observation of a stellar occultation by Uranus.
A symmetric pattern of five narrow dips in the stellar signal
was seen on both sides of the planet. Later observations
of other stellar occultations found an additional five narrow
rings.Voyager 2detected several more, fainter, diffuse rings
and provided detailed imaging of the entire ring system.
The success with finding Uranus’ rings led to similar
searches for a ring system around Neptune using stel-
lar occultations. Rings were detected but were not always
symmetric about the planet, suggesting gaps in the rings.
SubsequentVoyager 2imaging revealed large azimuthal
concentrations of material in one of the six detected rings.
All of the ring systems are within the Roche limits of their
respective planets, at distances where tidal forces from the
planet will disrupt any solid body, unless it is small enough
and strong enough to be held together by its own material
strength. This has led to the general belief that the rings are
disrupted satellites, or possibly material that could never
successfully form into satellites. Ring particles have typical
sizes ranging from micron-sized dust to meter-sized objects
and appear to be made primarily of icy materials, though
in some cases contaminated with carbonaceous materials.
Jupiter’s ring is an exception because it appears to be com-
posed of carbonaceous and silicate materials, with no ice.
Another component of the solar system is the zodiacal
dust cloud, a huge, continuous cloud of fine dust extending
throughout the planetary region and generally concentrated
toward the ecliptic plane. The cloud consists of dust grains
liberated from comets as the nucleus ices sublimate and
from collisions between asteroids. Comets are estimated to
account for about two thirds of the total material in the
zodiacal cloud, with asteroid collisions providing the rest.
Dynamical processes tend to spread the dust uniformly
around the Sun, though some structure is visible as a re-
sult of the most recent asteroid collisions. These structures,
or bands as they are also known, are each associated with
specific asteroid collisional families.
Dust particles will typically burn up due to friction with
the atmosphere when they encounter the Earth, appearing
as visible meteors. However, particles less than about 50
μm in radius have sufficiently large area-to-mass ratios that
they can be decelerated high in the atmosphere at an alti-
tude of about 100 km and can radiate away the energy gen-
erated by friction without vaporizing the particles. These
particles then settle slowly through the atmosphere and