The Curious Central Peaks of Iapetus

The curious, 20-km tall central ridge of Iapetus, a.k.a. the Voyager Mountains

Saturn’s 914-mile (1471-km) -wide Iapetus (pronounced eye-AH-pe-tus) has a particularly curious feature: a chain of 20-kilometer (12-mile) high mountains encircling the moon’s equator. On the anti-Saturnian side of Iapetus, the ridge appears to break up, forming distinct, partially bright mountains. The Voyager I and Voyager II spacecraft provided the first knowledge of the peaks, and they are informally referred to as the Voyager Mountains.

This image was acquired by Cassini during a close flyby of the moon back in September of 2007.

Long considered a mystery, recent studies suggest that the ridge was formed from the slow degradation of the orbit of a ring that once surrounded Iapetus, possibly the remnants of an even smaller sub-satellite that got shattered by collisions or tidal forces. As the particles “gently” impacted Iapetus along the equator, a mountain chain was built up.

Stains of dark material on Iapetus’ icy surface, imaged by Cassini in 2007. (Color-composite by J. Major.) NASA/JPL/SSI

Iapetus is also well-known for its other odd feature — a two-toned coloration sometimes compared to a traditional “yin-yang” symbol. Nearly half of the moon is bright and icy, while the other half is coated with a dark material. It’s thought that the moon has swept up dark material blown off the distant moon Phoebe, which is constantly being weathered down due to its retrograde motion through a recently-discovered diffuse ring that exists around Saturn. As this darker material falls onto Iapetus, it affects how the moon absorbs heat radiation from the Sun and helps transport icy, brighter material across its surface through sublimation, thus creating a feedback loop that continually recolors it.

Iapetus is just another example of the many varied worlds that exist in our own Solar System!

Read more about Iapetus and the Cassini mission here, and check out an excellent post regarding the central ridge on The Planetary Society’s blog here.