Saturn's Moons


Hyperion (Saturn VII)
Hyperion ("hi PEER ee en") is the sixteenth of Saturn's known satellites.

orbit:1,481,100 km from Saturn
diameter:286 km (410 x 260 x 220)
mass:1.77e19 kg
Discovered by:Bond and Lassell in 1848.

Hyperion is the largest highly irregular (non-spherical) body in the solar system. Proteus is quite a bit larger but is almost spherical. It seems likely that Hyperion is a fragment of a larger body that was broken by a large impact in the distant past. Like most of Saturn's moons, Hyperion's low density indicates that it is composed of water ice with only a small amount of rock.

But unlike most of Saturn's moons, Hyperion has a low albedo (.2 - .3) indicating that it is covered by at least a thin layer of dark material. This may be material from Phoebe (which is much darker) that got past Iapetus.

The Voyager images and subsequent ground based photometry indicate that Hyperion's rotation is chaotic, i.e. its axis of rotation wobbles so much that its orientation in space is completely 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 (Titan). These factors combine to restrict the set of conditions under which stable rotation is possible. 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.
Iapetus (Saturn VIII)
Iapetus ("eye AP i tus" ) is the seventeenth of Saturn's known satellites and the third largest.

orbit:3,561,300 km from Saturn
diameter:1460 km
mass:1.88e21 kg
Discovered by:Cassini in 1671.

With a density of only 1.1, Iapetus must be 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, almost as bright as Europa. 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. However, the color of the leading half of Iapetus and that of Phoebe don't quite 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.
Phoebe (Saturn IX)
Phoebe ("FEE bee") is the outermost of Saturn's known satellites. Phoebe is almost 4 times more distant from Saturn than its nearest neighbor (Iapetus).

Orbit:12,952,000 km from Saturn
Diameter:220 km
Mass:4.0e18 kg
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.

Why is Phoebe in such an odd orbit?
Is it really the source of the dark material on Hyperion and the leading hemisphere of Iapetus? Phoebe's density is only 0.7 which is anomalously low. What is its composition?

  • Difficulties:-
    1-Will Hyperion's rotation remain chaotic forever? Under what circumstances could it become stable?

    2-Why is Iapetus' orbit not in the plane of the other moons?

    3-Why is Phoebe in such an odd orbit?

    4-Is it really the source of the dark material on Hyperion and the leading hemisphere of Iapetus?

    5-Phoebe's density is only 0.7 which is anomalously low. What is its composition?

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    Last Modified : 5 Sep. 2001
    Created By#C0115361 Team