Geology Reference
In-Depth Information
evidently having been swept away to form the D Ring.
The B Ring is the brightest of Saturn
'
is rings and is about
2 km thick, making it thicker than any other ring. It is
composed mostly of particles ~10 cm in diameter,
although some may be as large as 10 m.
Transient, spokelike patterns appear superposed on the
B Ring. Each
“
spoke
”
is 10,000 km long and 100
-
1,000 km
wide. They radiate outward through the B Ring and appear
and disappear with time. These are thought to be clouds of
dust particles, perhaps levitated by electrostatic forces gen-
erated as the ring particles collide with each other.
The A Ring is the outermost of the large, bright rings
and consists of several narrow ringlets, with the outermost
ringlet having a sharp, well-de
ned edge. Atlas is a small
satellite discovered during the Voyager mission and is in
orbit just outside the A Ring. Referred to as a
spun outward to form the nearly pure water-ice ring par-
ticles seen today. Canup (
2010
) suggests that, as this
system evolved, the mass decreased and
“
spawned
”
icy
moons at the outer edge of the system, including Tethys.
9.9 Summary
Saturn, like Jupiter, is a giant planet composed mostly of
gasses, likely surrounding a central rocky core. It, too, has
complex cloud patterns driven by internal heating and
leading to features such as the ephemeral Great White
Spot. Of all the planets of the Solar System, Saturn has
the greatest diversity of moons and ring systems. Its largest
satellite, Titan, has long been held in fascination from a
geologic perspective. Given its dense nitrogen atmosphere,
the presence of methane, and a temperature regime in which
all three states of methane (solid, gas, and liquid) could
exist, following the Voyager results there was speculation
that Titan
'
s surface could be modi
ed by extensive pro-
cesses of gradation. This speculation was con
rmed when
the Cassini
shepherd-
ing
”
satellite, it appears to constrain material in the A Ring
by gravitational forces. Separating the A Ring from the B
Ring is the Cassini
“
division,
”
named after the Italian
astronomer Gian Domenico Cassini. First thought to be
relatively free of particles, spacecraft images show that
narrow rings are present within the division.
Pioneer data revealed the F Ring, which is ~700 kmwide
and orbits beyond the A Ring. It is bound by two
“
shep-
herding
”
satellites, Pandora and Prometheus, that appear to
keep the ring particles constrained to a very narrow orbit.
The arc-like GRing is composed of ice grains just inside the
orbit of Mimas while the wide, diffuse E Ring begins near
Mimas and extends to Enceladus. The E ring is composed
of
fine ice grains, rocky material, and carbon dioxide orig-
inating from the Enceladus eruptions. The Phoebe Ring,
discovered in 2009, resides just within the orbit of Phoebe,
making it the outermost of Saturn
'
is rings; it is thought to
originate from impacts on Phoebe or to be otherwise
derived from Phoebe.
Some planetary scientists suggest that Saturn
“
Huygens results showed extensive, integrated
fluvial networks, many of which empty into pools of liquid
methane. Vast dune-
elds attest to the action of winds in the
dense atmosphere, showing that aeolian processes are
important, especially in the equatorial region of Titan.
Although there is not yet direct evidence that gradation
processes are currently active, the shrinking of lakes sug-
gests that methane is cycling between the surface and the
atmosphere as a function of Titan season.
Cassini images revealed the presence of active geysers
on Enceladus, adding this moon to the small set of Solar
System objects that demonstrate active internally driven
processes. This discovery came as a surprise because
Enceladus seemed too small (~504 km in diameter) to
generate internal heat. The active geysers emanate from
the so-called
-
s rings
represent one or more satellites that were broken apart
by tidal forces or through collisions. Alternatively, the
rings could be composed of material that never accreted
to form one or more satellites in the early stages of the
Solar System
'
terrain of the south pole region
and can be traced more than 100 km above the surface.
The geyser plumes consist mostly of water vapor and
include other gasses and dust particles, some of which
might
“
feed
”
into Saturn
'
s E Ring. The presence of water
and its internal energy make Enceladus a potential target
for astrobiological exploration.
Saturn
'
s other moons span a wide range of sizes, orbital
characteristics, and geologic histories. While Mimas is in
the same size class as Enceladus, its heavily cratered surface
indicates little internal activity since its formation. The
presence of canyon systems is attributed to adjustments of
the ice-shell in response to the formation of the impact
“
tiger-stripe
”
s formation. In 2010, planetary scientist
Robin Canup proposed a new theory of Saturn
'
s ring
formation based on Cassini data and elegant computa-
tional modeling. She suggested that early in the history
of the Saturn system, there were large, differentiated
moons composed of ices and rocky material, comparable
to Titan. Tidal forces stripped the outer layers of ice-rich
material from the outer shell of one such object, while the
rocky part spiraled inward to Saturn. The icy material
'
