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== Orbital characteristics ==

== Orbital characteristics ==

Iapetus is in an eccentric orbit around Saturn, at an average distance of 3,561,300 km. This distance is nearly three times the average distance of [[Titan (moon)|Titan]] and 2.5 times the average distance of [[Hyperion (moon)|Hyperion]]. Its sidereal month is about 79.33 days.

Iapetus is in an eccentric orbit around Saturn, at an average distance of 3,561,300 km. This distance is nearly three times the average distance of [[Titan (moon)|Titan]] and 2.5 times the average distance of [[Hyperion (moon)|Hyperion]]. Its sidereal month is about 79.33 days.

The orbit of Iapetus is inclined to Saturn's equator by more than 14 degrees, more than any of the other large moons of Saturn. Only the much smaller irregular moons of Saturn have orbits more inclined than that of Iapetus.

The orbit of Iapetus is inclined to Saturn's equator by more than 14 degrees, more than any of the other large moons of Saturn. Only the much smaller irregular moons of Saturn have orbits more inclined than that of Iapetus.

Iapetus is best known for the surprisingly different colors of its leading and trailing hemispheres. Its leading hemisphere has an albedo of about 0.05, while the trailing hemisphere has an albedo as high as 0.6. The dark region is named Regio Cassini, in honor of the discoverer, and the bright region is named Roncevaux Terra. The boundary between these two regions is sharp, but not perfectly so. In the border between the two regions, the dark material often covers only one side of a crater or fills the floor of another.<ref name=Arnett>Arnett, Bill. "[http://www.nineplanets.org/iapetus.html Entry for Iapetus]." ''The <s>Nine</s> 8 Planets'', May 13, 2005. Accessed June 16, 2008.</ref>

Iapetus is best known for the surprisingly different colors of its leading and trailing hemispheres. Its leading hemisphere has an albedo of about 0.05, while the trailing hemisphere has an albedo as high as 0.6. The dark region is named Regio Cassini, in honor of the discoverer, and the bright region is named Roncevaux Terra. The boundary between these two regions is sharp, but not perfectly so. In the border between the two regions, the dark material often covers only one side of a crater or fills the floor of another.<ref name=Arnett>Arnett, Bill. "[http://www.nineplanets.org/iapetus.html Entry for Iapetus]." ''The <s>Nine</s> 8 Planets'', May 13, 2005. Accessed June 16, 2008.</ref>

The origin of the dark material remains unsettled. Some astronomers speculate that the material came from [[Phoebe]], a member of the [[Norse group]] of moons. Yet the colors of Phoebe and the dark region of Iapetus are not alike.<ref name=Arnett/><ref name=Hamilton/>

The origin of the dark material remains unsettled. Some astronomers speculate that the material came from [[Phoebe]], a member of the [[Norse group]] of moons. Yet the colors of Phoebe and the dark region of Iapetus are not alike.<ref name=Hamilton/><ref name=Arnett/>

The dark material is either several meters thick or else is constantly renewed. Cassini Regio has no bright-rimmed craters consistent with erosion of the dark material.<ref name=Hamilton/>

The dark material is either several meters thick or else is constantly renewed. Cassini Regio has no bright-rimmed craters consistent with erosion of the dark material.<ref name=Hamilton/>

=== Shape and surface features ===

=== Shape and surface features ===

Iapetus is an oblate spheroid.<ref name=Thomas/> The observed oblateness is far more pronounced than astronomers would expect of a body having a sidereal day of more than 79 Earth days. Moreover, Iapetus has a ridge of mountains, 20 km high, along its equator. To assume such a shape, Iapetus ought to have had a sidereal day of 10 hours, slowing later to 16 hours. Its crust would then have had to freeze and thicken over a period of 100 million to 900 million years, according to [[uniformitarianism|uniformitarian]] theories. But during this time, Iapetus would have required a source of internal heat sufficient to melt a vast quantity of water ice. Astronomers who have considered the problem believe that this internal heat source was radioactive decay, specifically of ²⁶[[Aluminum|Al]] and ⁶⁰[[iron|Fe]].<ref name=Cowen>Cowen, Ron. "[http://www.sciencenews.org/view/feature/id/8823/title/Idiosyncratic_Iapetus Idiosyncratic Iapetus]." ''Science News'' 172:104-106, August 14, 2007. Accessed June 16, 2008.</ref>

Iapetus is an oblate spheroid.<ref name=Thomas/> The observed oblateness is far more pronounced than astronomers would expect of a body having a sidereal day of more than 79 Earth days. Moreover, Iapetus has a ridge of mountains, 20&nbsp;km high, along its equator. To assume such a shape, Iapetus ought to have had a sidereal day of 10 hours, slowing later to 16 hours. Its crust would then have had to freeze and thicken over a period of 100 million to 900 million years, according to [[uniformitarianism|uniformitarian]] theories. But during this time, Iapetus would have required a source of internal heat sufficient to melt a vast quantity of water ice. Astronomers who have considered the problem believe that this internal heat source was radioactive decay, specifically of ²⁶[[Aluminum|Al]] and ⁶⁰[[iron|Fe]].<ref name=Cowen>Cowen, Ron. "[http://www.sciencenews.org/view/feature/id/8823/title/Idiosyncratic_Iapetus Idiosyncratic Iapetus]." ''Science News'' 172:104-106, August 14, 2007. Accessed June 16, 2008.</ref>

The ridge system itself is an enigma. Astronomers have suggested at least two models for its formation:

The ridge system itself is an enigma. Astronomers have suggested at least two models for its formation: