Using images from the Cassini space probe, scientists from Brown University and the Lunar and Planetary Institute have gathered data which suggests that an equatorial ridge present on one of Saturn’s moons, Iapetus, may have formed through external processes. The results can be found in a preprint version of the paper which has been submitted to the journal Icarus.
Saturn possesses a diverse array of icy moons, or satellites, which differ considerably in size and origin. Of the 62 discovered so far 53 have been officially named. The third largest moon, Iapetus, was discovered in 1671 by Giovanni Domenico Cassini and is nicknamed the yin and yang moon due to its distinctive two-tone coloring. Iapetus also possesses an impressive ridge system along its equator; some areas are reported to reach a height of 20 kilometers and 70 kilometers in width.
What has intrigued scientists over the years is how this ridge system came to be. One possibility is that it was perhaps formed by tectonic forces or volcanism, which could explain its equatorial location. Another is that rather than being a result of processes occurring on the moon itself, external (exogenic) processes such as debris infall may be responsible. One such theory is that it could be derived from an early ring system which fell onto the moon.
In order to shed light on the origins of the ridge, scientists used images from the Cassini space probe to generate topographic profiles of the ridge, which they then used to measure peak morphology and slope angle. They found that the most common peak shape was triangular and also found evidence of slope angles close to the angle of repose, which is the maximum angle at which material on the slope will remain in place without sliding down.
Although this data cannot prove what formed this ridge, the scientists believe that it provides evidence that it may be exogenic rather than endogenic in origin. This is because geologic activity would produce less uniform, shallower peaks. What seems more likely at this stage is that a ring may have once been orbiting the moon which was the result of a collision event. This ring may have then fallen onto the equator of the planet, producing the peaks.