In 2005, NASA’s Cassini spacecraft observed a water-rich plume erupting from the south polar terrain of Enceladus, suggesting that this moon of Saturn possessed a subsurface ocean beneath its icy shell. Some of the erupted water in this plume escapes Enceladus to form Saturn’s diffuse E-ring, but most falls back frozen to the moon’s surface, forming a layer of loose, unconsolidated material called regolith. If the deposition rate from this plume has remained constant, then the thickness of the deposited regolith should indicate how long the plume — and its liquid reservoir — has remained active.