Measurements of regolith thicknesses on Enceladus: Uncovering the record of plume activity

Abstract

Tectonic pit chains are geologic landforms that form in loose unconsolidated regolith and are observed on many solar system bodies. In the outer solar system pit chains are most clearly observed on Enceladus. In this paper, we test three techniques for using pit chain morphometry to probe the regolith thickness across Enceladus accumulated as fall-back from its plume eruptions. We favor the precision of the Slope Angle Method (SAM), allowing for spatially related heterogeneities in regolith characteristics, and we measured regolith thicknesses up to 700 m (mean = 250 m, n = 116). We combine models of plume deposition rates with observed regolith thicknesses and assume lunar-like crater-generated regolith production. We constrain the time needed to deposit the observed sequences of regolith to extrapolate the age of the plume source and its liquid reservoir. The porosity and density of the plume-generated regolith are unconstrained parameters that affect the rate at which the regolith is deposited. Assuming a constant deposition mass flux, we explored the parameter space of regolith density (0.02–0.92 g/cm3) and porosity (0–90%) that would deposit a thick enough regolith to match the pit chain observations. Only by using the lowest density (0.2 g/cm3) and highest porosity (90%) are we able to reproduce the maximum regolith thicknesses within the age of the solar system. We interpret the long deposition times needed to match regolith thicknesses to suggest that Enceladus's mass flux from the plume needs to have been significantly higher, that the regolith is possibly a low-density/high-porosity material, and/or additional regolith-forming processes (like alternative vent localities) exist, or have existed.