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Orbit: 3,561,300 km from Saturn
Orbital period: 79 days
Gravity: 0.223 m/s²
Circumference: 4,622 km
Diameter: 1460 km
Mass: 1.88e21 kg
Iapetus, occasionally Japetus, is the third-largest natural satellite of Saturn, and eleventh-largest in the Solar System.
The leading and trailing hemispheres of Iapetus are radically different. The albedo of most of the  leading hemisphere is about  .04, as dark as lampblack, whereas the  trailing hemisphere’s albedo is 0.6,   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 or  some other Saturnian body.  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.

Saturn's moon Iapetus, surface

Near the edge of the dark overlaying material

On the last day of 2004, Cassini made  its first close  encounter with Iapetus.  The images  show that the  dark  material overlays the topography, indicating that it is relatively young.   And  as in the image to the left, along the edge of the dark area  there are many  craters where only one  side is covered by the dark material; the boundary  between the two regions isn’t so sharp after all.  So far the Cassini’s data do  not resolve the puzzle of the origin of the dark material but there’s more to  come!

Saturn's moon Iapetus, equator

Iapetus’s giant equatorial ridge

Cassini’s first encounter with Iapetus also revealed another striking feature  not seen before: a ridge  13 kilometers higher than the surrounding terrain that  extends at least 1300 km almost  almost exactly parallel with Iapetus’s equator  (click the image at right for more).

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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