Pwyll and Manannán Craters as a Laboratory for Constraining Irradiation Timescales on Europa

We examine high-spatial-resolution Galileo/Near-Infrared Mapping Spectrometer observations of the young (∼1 My–∼20 My) impact features, Pwyll and Manannán craters, on Europa’s trailing hemisphere in an effort to constrain irradiation timescales. We characterize their composition using a linear spectral modeling analysis, and find that both craters and their ejecta are depleted in hydrated sulfuric acid relative to nearby older terrain. This suggests that the radiolytic sulfur cycle has not yet had enough time to build up an equilibrium concentration of H2SO4, and places a strong lower limit of the age of the craters on the equilibrium timescale of the radiolytic sulfur cycle on Europa's trailing hemisphere. Additionally, we find that the dark and red material seen in the craters and proximal ejecta of Pwyll and Manannán show the spectroscopic signature of hydrated, presumably endogenic salts. This suggests that the irradiation-induced darkening and reddening of endogenic salts thought to occur on Europa’s trailing hemisphere has already happened at Pwyll and Manannán, thereby placing an upper limit on the timescale by which salts are irradiation reddened.