Iapetus, << eye AP uh tuhs, >> the third largest moon of Saturn, has a surface marked by distinct light and dark halves. Its dark half appears nearly as black as tar, and its bright half resembles dirty snow. No other body in the solar system displays such a bold contrast. Iapetus’s diameter measures roughly 900 miles (1,450 kilometers). The moon orbits Saturn every 79.3 days at an average distance of about 2,213,000 miles (3,561,000 kilometers).
Scientists think that the surface of Iapetus consists primarily of water ice. On the dark half, however, dark material covers the normally bright ice. In 2009, scientists confirmed an earlier theory that the dust which darkens one side of Iapetus originates from Saturn’s nearby moon Phoebe. Large impact craters lie within the dark terrain. The appearance and age of the craters suggest that the dark material on Iapetus’s surface settled there fairly recently, after the impacts occurred.
A vast ridge runs along the equator of Iapetus within the dark terrain. This equatorial ridge measures about 12 miles (20 kilometers) wide and extends for at least 800 miles (1,300 kilometers). Some mountainous parts of the ridge rise at least 13 miles (21 kilometers) high. Scientists do not know whether the ridge folded upward, like some mountain ranges on Earth, or whether it was built up by eruptions from the moon’s interior.
The Italian-born French astronomer Giovanni Domenico Cassini discovered Iapetus using a telescope in 1671. In the early 1980’s, the United States spacecraft Voyager 1 and Voyager 2 took photographs of Iapetus. In 2004 and 2007, the U.S. Cassini spacecraft flew by Iapetus. Cassini’s images revealed the equatorial ridge and other details in the dark terrain and in the transition area between the dark and bright regions.
See also Cassini ; Satellite (Iapetus) ; Saturn (Satellites) .
