How Iapetus Got Its Ridge

Saturn’s moon Iapetus is bizarre for many reasons, one of which is its 13-kilometre-high ridge along its equator (which makes Iapetus look rather like a walnut). A scientific paper now in press has a theory as to how it happened, which Emily Lakdawalla explains in detail: Iapetus may once have had its own moon (or submoon), which was drawn too close, torn apart, and formed into a ring. The ring particles de-orbited gently over time, building up the ridge. Image credit: NASA/JPL/Space Science Institute.