Geology Reference
In-Depth Information
Figure 9.45. A Cassini image of Phoebe showing the heavily crater
surface (NASA PIA06064).
Figure 9.46. A mosaic of six Cassini images of Hyperion; the dark
material in the
floors of many of the craters is likely a lag deposit left
from the sublimation of ice (NASA PIA07761).
s small satellites are found inside the orbit
of Mimas. Two, Janus and Epimetheus, are called co-orbital
satellites because their orbits are within 50 kmof each other.
They are thought to be remnants of a larger moon broken
apart by impact collision. Three additional small moons,
Calypso, Telesto, and Helene, are Lagrangian satellites. It
has long been known that a small object can have the same
orbit and speed as a larger object, so long as it maintains a
60° arc ahead of or behind the larger object in positions
called Lagrangian points. Tethys has two such small objects
in its orbit; Dione has one.
Finally, Hyperion is a small, irregular-shaped object
( Fig. 9.46 ) whose elliptical orbit is just outside that of
Titan. Measuring 350 km by 235 km by 200 km, this
heavily cratered object includes both angular and rounded
parts. This moon is very dark, and, although its mass has
not been determined, it is presumed to be composed of ice
and rock. In addition to craters as large as 120 km, the
surface displays scarps that may be part of a connected,
arcuate feature nearly 300 km long that could be the
remnant of a crater. Because of its irregular shape, heavily
cratered surface, and a possible huge impact scar,
Hyperion is considered by most investigators to be the
remnant of a much larger object that was shattered by
collisional or impact processes.
Five of Saturn
'
9.8 The ring system
Saturn ' s rings occur in sets ( Figs. 9.1 and 9.2 ) and are
composed mostly of water-ice, which ranges in size from
tiny dust specks to 10m boulders. The subtle ice colors
include reddish tan and brown tones due to impurities,
such as iron oxide, or to structural damage in the ice
crystals caused by ultraviolet radiation from the Sun.
Saturn ' s rings extend from about 53,000 km to nearly
360,000 km above Saturn, and the entire system is very
thin (averaging 20m thick) so that, when viewed from the
side, it looks like a knife edge. Despite the enormous
extent of the rings, if all of the ring material were accu-
mulated, it would form a satellite less than 500 km across.
Ring elements are designated by letters that are based
on the sequence of their discovery rather than on their
position in relation to Saturn. The innermost (the D Ring)
is barely visible and consists of very narrow ringlets
composed of
fine-granulated particles that appear to
be
inward from the C Ring toward Saturn. The
C Ring consists of light and dark bands of centimeter- to
meter-sized particles but also includes some larger
objects. This ring contains little or no dust, most of it
leaking
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