Geology Reference
In-Depth Information
Figure 9.2. A diagramof themajor and someminor satellites of Saturn and the primary ring systems. The distances shown are on a logarithmic
scale from the center of Saturn.
Figure 9.3. Comparative sizes of
some of Saturn
s satellites; not
shown are Saturn
'
s Titan, which
is half again as large as Earth
'
s
Moon, and the smaller moons of
Saturn (fromMoore et al.,
2004
).
Reprinted from Icarus, 171,
Moore, J. M., Schenk, P. M.,
Bruesch, L. S., Asphaug, E., and
McKinnon, W. B., Large impact
features on middle-sized icy
satellites, 421
'
443, 2004, with
permission from Elsevier.
-
All but two of the major moons are in synchronous
rotation around Saturn, which, analogously to the Galilean
satellites, leads to the terms sub-Saturn, anti-Saturn, lead-
ing,andtrailing hemispheres. Voyager and Cassini
images have been merged to generate global or near-global
maps of many of the satellites, along with place names
(Roatsch et al.,
2009
). The prime meridian (zero longitude)
for each moon is de
ned by a prominent feature, such as a
crater, near the center of the Saturn-facing hemisphere, with
longitudes being numbered to the west.
There are no obvious patterns relating the sizes or
densities of the satellites to their distance from Saturn or
to the degree of geologic activity among them. For sim-
plicity, the moons are discussed beginning with the two
satellites of high geologic interest, Titan and Enceladus;
intermediate-size moons (
Fig. 9.3
) are given in the order
of their orbital distance from Saturn, and then some of the
small moons are discussed.
9.5 Titan
Titan is a remarkable moon. At 5,150 km in diameter, it is
larger than Mercury and is in the same size class as
Ganymede and Callisto. Like those satellites of Jupiter,
its mass suggests that it is composed of about 45% water
(mostly ice, but possibly some liquid) and 55% rocky
material. What makes it remarkable is its dense atmo-
sphere and surface conditions that allow liquids to exist.
Although the satellite was the
first to be discovered in the
saturnian system (it was found by Christiaan Huygens in
1655), the presence of an atmosphere was not suspected
