Iapetus is the seventeenth of Saturn¹'s known satellites and the third largest:

　　orbit:    3,561,300 km from Saturn

　　diameter: 1460 km

　　mass:     1.88e21 kg

　　In Greek mythology² Iapetus was a Titan, the son of Uranus³, the father of Prometheus and Atlas⁴ and an ancestor of the human race.

　　Discovered by Cassini in 1671.

　　With a density⁵ of only 1.1, Iapetus must be composed almost entirely⁶ of water ice.

　　Trailing side of Iapetus from Voyager 2 The leading and trailing hemispheres of Iapetus are radically⁷ different. The albedo of most of the leading hemisphere is about .04, as dark as lampblack, whereas the trailing hemisphere's albedo is .6, as bright as Europa. This difference is so striking that Cassini noted⁸ 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 inexplicably⁹ sharp.

　　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 relatively¹⁰ young. And as in the image to the left, along the edge of the dark area there are many craters¹¹ 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!

　　Iapetus's giant equatorial ridge¹² Cassini's first encounter with Iapetus also revealed another striking feature not seen before: a ridge 13 kilometers higher than the surrounding terrain¹³ that extends at least 1300 km almost almost exactly parallel with Iapetus's equator.

　　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.

　　Iapetus 是土星已知卫星中距土星第十七近、第三大的一颗：

　　公转轨道：距土星 3,561,300 千米

　　卫星直径：1460 千米

　　质量：1.88e21 千克

　　希腊神话中，Iapetus是一个巨人，乌拉若斯的儿子，普罗米修斯和Atlas的父亲，人类的祖先。

　　它由卡西尼于1671年发现。

　　密度仅为1.1，土卫八的大部分肯定是由冰水组成的。

　　正对公转和反对公转的半球完全不同。正转半球的反照率在0.03到0.05之间，与煤烟一样暗，反面为1.5，几乎同木卫四一样亮。这些不同点太明显，使得卡西尼注意到了，说出了“我只能见到土卫八的一面，另一面则不能。”的话。

　　对此的一个解释是正转半球被土卫九上撒下的煤质尘埃覆盖。然而，土卫八的颜色与土卫九的并不相配。另一个可能是因为土卫八内部的活跃作用引起。这个难题加上这两个半球分界处的线是无法形容的形状，增添了人们的疑惑。

　　所有的土星的卫星，除了土卫八与土卫九，处于土星赤道平面。土卫八的倾斜角近15度。