A buckling model for the flattening and equatorial ridge of Iapetus

The excess flattening and equatorial ridge of Iapetus has been explained by a model in which the moon formed at a high spin rate, achieved isostatic equilibrium by very rapid interior heating caused by short-lived radioactive isotopes (SLRI), and subsequently cooled, locking in an equatorial ridge and excess flattening with respect to an equilibrium shape at its present spin rate. Here we propose an alternate model that does not require an unusually high initial spin rate or the SLRI. The initial formation of Iapetus results in a slightly oblate spheroid with porosity > 10%. Radioactive heating by longlived isotopes warms the interior to about 200 K, at which point it becomes ductile and the interior compacts by 10% while the 100 km-thick exterior shell remains strong. The shell must deform to match the reduced volume of the interior, and we propose that this deformation occurs as buckling along the equator. For shell thickness > 100 km, the dominant buckling mode is a second degree harmonic. The final shape of the collapsed sphere matches the observed shape, described as an oblate ellipse, except along the equator where strain concentration forms a broad ridge.