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HYPERION:
Hyperion is the sixteenth moon. It has an
orbit of 1,481,100 km from Saturn and a diameter of 286 km. The mass is
1.77e19 kg. It was discovered by Bond and Lassell in 1848. Hyperion is
the largest highly irregular or non-spherical body in the solar system.
Hyperion seems to be a fragment of a larger body that was broken by a
large impact in the past. Hyperion's low density indicates that it is
composed of water ice with only a small amount of rock. Unlike most of
Saturn's moons, Hyperion has a low albedo (.2 - .3). This means that it
is covered by a thin layer of dark material. Hyperion's rotation is
chaotic, meaning that its its axis of rotation wobbles so much that its
orientation in space is unpredictable. Hyperion is the only known body
in the solar system that rotates chaotically but simulations seem to
indicate that other irregular satellites may have done so in the past.
Hyperion is unique in that it is very irregularly shaped, has a highly
eccentric orbit, and is near another large moon. The 3:4 orbital
resonance between Titan and Hyperion may also make chaotic rotation more
likely. Hyperion's odd rotation probably accounts for the fact that
Hyperion's surface is more or less uniform, in contrast to many of
Saturn's other moons which have distinctly different leading and
trailing hemispheres.
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IAPETUS:
Iapetus is the seventeenth of
Saturn's known satellites and the third largest. It has an orbit of
3,561,300 km from Saturn and a diameter of 1460 km. The mass is 1.88e21
kg. It was discovered by Cassini in 1671. Iapetus only has a density of
1.1, meaning that it is composed almost entirely of water ice. The
leading and trailing hemispheres of Iapetus are radically different. The
albedo of the leading hemisphere is between .03 and .05, as dark as
lampblack, whereas the trailing hemisphere's albedo is .5. This
difference is so striking that Cassini noted that he could see Iapetus
only on one side of Saturn and not on the other. One explanation of this
is that the leading hemisphere is dusted with a coating of material
knocked off of Phoebe. The color of the leading half of Iapetus and that
of Phoebe do not match. Another possibility is that some active process
within Iapetus is responsible. The puzzle is compounded by the fact that
the dividing line between the two sides is inexplicably sharp. All of
Saturn's moons except for Iapetus and Phoebe are very nearly in the
plane of Saturn's equator. Iapetus is inclined almost 15 degrees.
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PHOEBE:
Phoebe is the outermost satellite. Phoebe
is almost 4 times more distant from Saturn than its nearest neighbor,
Iapetus. It has an orbit of 12,952,000 km from Saturn and a diameter of
220 km. The mass is 4.0e18 kg. Phoebe was discovered by Pickering in
1898. Most of Saturn's moons are bright but Phoebe's albedo is very low
(.05), as dark as lampblack. All of Saturn's moons except for Phoebe and
Iapetus orbit very nearly in the plane of Saturn's equator. Phoebe's
orbit is inclined almost 175° (its north pole is in the opposite
direction to Saturn's). Phoebe's eccentric, retrograde orbit and unusual
albedo indicates that it may be a captured asteroid or Kuiper Belt
object. Phoebe is also unusual in that it does not rotate synchronously
as do all the other moons of Saturn except Hyperion. Material knocked
off of Phoebe's surface by microscopic meteor impacts may be responsible
for the dark surfaces of Hyperion and the leading hemisphere of Iapetus.
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