ESTIMATING REGOLITH THICKNESS ON ENCELADUS USING THE ANGLE OF REPOSE OF INDIVIDUAL PITS WITHIN PIT CHAINS

Pit chains on Mars have been documented to be closely associated with extension along high-angle, buried normal faults. Drainage of overlying loose regolith into the dilational space along the fault plane results in aligned pits in the regolith. Pit chains have also been observed on Earth, Venus, and small bodies Phobos, Eros, Gaspra, Ida, and Vesta. Enceladus is the only body in the outer solar system where pit chains have been positively identified, and therefore the only icy body to exhibit a demonstrable layer of loose regolith. Pit chains on Enceladus form predominantly within the cratered terrains on the Saturn and anti-Saturn hemispheres, and form multiple sets of parallel pit chains that share the same relative age, orientation, and morphology. Pits provide a useful proxy to measure the thickness of surficial regolith and thus to probe planetary surfaces that have been modified by regolith accumulation. A previously published proxy experimentally determined a one-to-one correlation between average pit spacing along a single pit chain and regolith depth, although why such a correlation would occur is unclear. We compare the pit spacing proxy to a new proxy that infers regolith depths from the maximum pit diameter within an individual chain, which is physically controlled by the regolith thickness and angle of repose. We initially estimated a range of angles of repose (20°–40°), but this resulted in a wide spread of possible regolith depths at each pit. We refined our approach by accurately measuring the angle of repose at each pit using an established methodology. Assuming circular pits, the sun angle, shadow length, and pit diameter were used to calculate the angle of repose. Our results reveal a global spatial variability in angles of repose, with lower angles (<50°) in the cratered terrains, and higher angles (50°–80°) in tectonized terrains. Thus, surface material is more competent in the tectonized terrains, perhaps explaining the dearth of pits. Regolith thickness in the cratered terrains is variable (~50–1000 m). Higher values occur in the tectonized terrains (1000–3500 m), but only near the boundary with the south polar terrain (SPT). Enceladus’s regolith is derived primarily from fall-back from the SPT plumes, and models suggest that regolith will preferentially accumulate thicker deposits nearer to the SPT.