Several theories have been put forth to explain the ridge. Two theories suggest that the ridge was pushed up with internal compressive forces or from a crack in its crust.

Astronomers now theorize that  the ridge formed due to the extremely fast orbital spin (something like five to 17 hours) of the icy moon when it was first forming. Radioactive materials also helped to heat the interior of the body, which cracked and stretched its crust as it spun rapidly around.

The spin produced curvy lines into the body’s surface, which froze permanently in place. The most obvious one is the one located on its equator. Iapetus is now a slower rotating moon, making a complete rotation once every 79.3 earth-days. Astronomers studying Iapetus indicate that such a rapid rotating period early in its life, followed by a slower one today, may help them determine how planets and their moons formed and evolved in the solar system.

Iapetus, Saturn’s third largest moon, was discovered by Italian-French astronomer Giovanni Cassini in 1671. It is composed mostly of ice, with only a small amount of rocky materials. It is heavily cratered. Iapetus is about 1,472 kilometers in diameter.

The NASA Cassini spacecraft discovered the presence of this equatorial ridge in late December–early January of 2004 when it approached within 123,000 kilometers (76,400 miles) of its surface. The ridge was found to be about 20 kilometers (12 miles) wide and around 13 to 20 kilometers (8 to 12 miles) high—making it one of the highest mountains known in the solar system. The mountainous ridge runs about 1,300 kilometers (800 miles) on the equator.

Also unusual is the fact that one side of the moon is lighter in color than the other side. Scientists are wondering if this unusual coloring combination is connected to the equatorial ridge. The darker region is called Cassini Regio, which has a slightly reddish-brown look and contains much carbon, while the lighter area is named Roncevaux Terra, which is brightly whitish in appearance.

Cassini is continuing its mission to investigate Saturn and its moons. It is scheduled for a flyby of Iapetus on September 10, 2007, from about 1,100 kilometers (700 miles) away. The new information collected at this time should help scientists better analyze these unusual features of Iapetus.

Information about the NASA Cassini-Huygens mission is located at: https://www.nasa.gov/mission_pages/cassini/main/