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.

Emily S. Martin of the Smithsonian Institution and a team of researchers sought to infer regolith thicknesses on Enceladus from tectonic pit chains. These features form when fractures open, creating a space into which regolith can drain and forming a series (chain) of aligned circular depressions within the regolith. The depth of a pit provides a lower bound on regolith thickness at that location. By investigating 116 pits, the research team measured an average regolith thickness of 250 meters, with some locations exhibiting up to 700 meters of regolith.

After accounting for other regolith sources, Martin and her team assumed a constant mass deposition rate (inferred from the current plume eruption rate). They explored a range of regolith densities and porosities, which are unconstrained but also affect the deposition rate. They found that only the most low-density and high-porosity regolith scenarios could reproduce the maximum regolith thickness observed within the age of the solar system. This suggests that Enceladus’s cryovolcanic activity has varied with time and that its plume erupted at significantly higher rates and/or at additional vent locations in the past, both of which would increase the mass flux. READ MORE