Massive ice avalanches on Iapetus mobilized by friction reduction during flash heating

Long-runout landslides are debris flows or avalanches that travel much farther than expected. They apparently exhibit friction coefficients much lower than either the static or sliding values that are generally accepted for geologic materials. Many friction-reduction mechanisms have been proposed for such landslides observed on Earth and Mars. Here we analyse images from the Cassini mission and report numerous long-runout landslides on Iapetus, an icy satellite of exceptional topographic relief. Its extremely cold, airless surface provides an excellent laboratory for studying long-runout landslides, as influence by trapped atmosphere or groundwater-two proposed friction-reduction mechanisms-is negligible. We use the ratio of drop height to runout length as an approximation for the friction coefficient of landslide material. We find that on Iapetus this ratio falls between 0.1 and 0.3, but does not decrease with increasing length as seen on Earth and Mars. We show that this lack of dependence is consistent with localized frictional heating in ice rubble such that sliding surfaces are slippery. Friction along tectonic faults on icy bodies may be similarly reduced.