MODELING AN EXOGENIC ORIGIN FOR THE EQUATORIAL RIDGE ON IAPETUS

Saturn’s moon Iapetus displays unusual geologic features, including a long equatorial ridge that encircles most of the body. This ridge can reach heights up to 20 km, making it the tallest topographic feature in the solar system as a fraction of the size of the host body. There are two main theories for the formation of this ridge: that it is formed endogenically (e.g., related to the despinning of Iapetus), or that it is formed exogenically by the collapse of an early ring to the surface. Most previous work has examined a tectonic origin for the ridge, but studies have found it challenging to generate stresses necessary to form the ridge topography. Here, instead, we examine the exogenic hypothesis to investigate the plausibility of the ridge being formed from infalling ring and satellite material.

This study examines the cratering efficiency of the expected distribution of ring particles that would have been created from ejecta of early impacts on Iapetus to determine whether they could form a ridge on the surface of Iapetus. Assuming water ice for both projectile and target material, pi-scaling calculations suggest that extremely oblique impact angles are needed to add to ridge topography. These extreme impact angles reduce the cratering efficiency, adding material rather than eroding it during crater formation; further, material is likely to be excavated at low angles, leading to accumulation downrange as well. Because infalling debris is predicted to impact at extremely low angles, both of these effects might have contributed to ridge formation on Iapetus.

Infalling material will also deliver heat to the surface of Iapetus, which can lead to a weakening of the moon’s surface and near subsurface material. Using the CTH code from Sandia National Labs, hydrocode models of extremely oblique impacts are used to examine the heat generated by the infalling material to assess the plausibility of a thermally-weakened surface supporting a large surface ridge.