Post contributed by Charlene Detelich, Department of Geological Sciences, University of Alaska Anchorage, USA.

Voyager flybys of the Saturnian system in the early 1980s discovered a ring of mountains on Saturn’s icy, two-toned moon, Iapetus. These mountains are up to 20 km tall, over twice the height of Mt. Everest, yet Iapetus is almost 650 times smaller than Earth. Later flybys from the Cassini spacecraft (Image 1) revealed these mountains to be part of a ridge encircling nearly all of Iapetus’s equator, resulting in the moon’s bizarre walnut shape. Numerous endogenic (internal) processes have been proposed for the origin of the equatorial ridge, including tidal spindown, convection, despinning coupled with global volume change, global contraction, or intrusion. However, a recent study by Detelich et al. (2021) found that evidence from photogeological mapping and crater statistics support an exogenic (external) origin best explained by the accretion of an ancient ring system onto Iapetus’s surface.

Image 1: Cassini image PIA08376 (https://photojournal.jpl.nasa.gov/catalog/PIA08376) taken on September 10^th, 2007 shows the enigmatic equatorial ridge of Saturnian moon, Iapetus. The ridge runs vertically down the center of the image before disappearing into the shadows of the non-lit portion of Iapetus. Image enhanced by Charlene Detelich.