Delayed formation of the equatorial ridge on Iapetus from a subsatellite created in a giant impact

The great equatorial ridge on Saturn’s moon Iapetus is arguably the most perplexing landform in the solar system. The ridge is a mountain range up to 20 km tall and sitting on the equator of Iapetus, and explaining its creation is an unresolved challenge. Models of its formation must satisfy three critical observations: why the ridge (1) sits exactly on the equator, (2) is found only on the equator, and (3) is thus far found only on Iapetus. We argue that all previously proposed models fail to satisfy these observations, and we expand upon our previous proposal that the ridge ultimately formed from an ancient giant impact that produced a subsatellite around Iapetus. The orbit of this subsatellite would then decay, once Iapetus itself had despun due to tides raised by Saturn, until tidal forces from Iapetus tore the subsatellite apart. The resultant debris formed a transient ring around Iapetus, the material of which rained down on the surface to build the ridge. By sequestering the material in a subsatellite with a tidally evolving orbit, formation of the ridge is delayed, which increases the likelihood of preservation against the high-impact flux early in the solar system’s history and allows the ridge to form on thick, stiff lithosphere (heat flow likely <1 mW m 2) required to support this massive load without apparent flexure. This mechanism thus explains the three critical observations.