Building the Ridge on Iapetus: Impacts Can Be Constructive!

Iapetus has a continuous ridge along the equator that extends for more than 110° in longitude. Parts of the ridge rise as much as 20 km above the surrounding terrains. Earlier Voyager observations revealed mountains on the anti-Saturn side of Iapetus with as much as 25-km in relief, extending from 180°W-220°W which may be a continuation of this ridge. Most models for the formation of this enigmatic ridge are endogenic, generally requiring the formation of a fast-spinning Iapetus with an oblate shape due to the rotation speed. Though abundant, many of these models require specific scenarios and have constraining parameters in order to generate a ridge comparable to what is seen today. An exogenic formation mechanism has also been proposed, that the ridge represents the remains of an early ring system around Iapetus that collapsed onto the surface. Thus far, none of the models have conclusively identified the origin of the ridge. We assume an exogenic origin for the ridge, derived from a collapsing disk of debris around Iapetus, without invoking any specific model for the generation of the debris disk, to determine whether it is possible to generate a ridge of the size and shape as observed. Here, the impact of the collapsing debris is simulated using the CTH hydrocode. Pi-scaling calculations suggest that extremely oblique impact angles (1-10°) are needed to add to ridge topography. These extreme impact angles severely reduce the cratering efficiency compared to a vertical impact, adding material rather than eroding it during crater formation. Furthermore, material is likely to be excavated at low angles, enhancing downrange accumulation. Multiple impacts from debris pieces will heighten this effect. Because infalling debris is predicted to impact at extremely low angles, both of these effects might have contributed to ridge formation on Iapetus. The extreme grazing angles of the impacts modeled here decouple much of the projectile energy, and impact heating of the surface is not significant. These models suggest that a collapsing disk of debris should have been able to build topography to create a ridge around Iapetus. Continuing work examining multiple impacts and projectile shape and size effects is ongoing to further constrain what a debris generated ridge may look like to provide possible observations for testing.