INSIGHTS INTO IMPACT-DERIVED PROCESSES AND THE SUBSURFACE OF EUROPA FROM PWYLL AND MANANNÁN IMPACT CRATERS

Europa has a very sparsely cratered surface, but these impact craters can reveal important insights into the subsurface structure, such as the ice shell thickness, which is crucial for understanding habitability (e.g., Chyba+Phillips 2001). Here we study the impact craters Pwyll and Manannán, which occur at the transition diameter (~20 km) between more predictable and more unusual crater morphologies, which is sometimes attributed to the presence of the subsurface ocean (e.g., Schenk+Turtle 2009, Moore+1998). We use three different, but complementary, techniques: geologic mapping (based on Galileo SSI clear, color and stereo data), IR spectral analysis (based on Galileo NIMS data) and geophysical modeling (based on inputs from Galileo data and geochemical modeling). These complementary investigations provide a comprehensive picture of the evolution of Pwyll and Manannán craters on the surface and at depth. The interiors of both craters contain broken up and down-dropped terrace material around the edges, high-standing massif material nearer the center, and crater floor material, which sometimes display lobate margins, throughout. Analogous to Occator crater on Ceres, we interpret the crater floor materials as an impact melt slurry that flowed under a solid carapace for geologically short timescales, before finally solidifying in place, based on Fagents 2003 and Scully+2019 (and references therein). Geophysical modeling predicts how the composition of impact-induced melt chambers beneath Pwyll and Manannán evolved as they froze, the products of which were then sequentially emplaced on the surface as the crater floor materials. Both craters have proximal and distal ejecta: the proximal ejecta (sourced more deeply) has distinct morphological and color properties while the distal ejecta (sourced more shallowly) is distinguished by morphology alone. IR spectral analyses of Pwyll indicate more non-ice material in the proximal than distal ejecta, which indicates a more water-ice-rich layer at shallow depths, followed by a more non-water-ice-rich layer deeper in the subsurface. The interior of Pwyll contains the highest concentrations of non-water-ice material in the Pwyll region, and displays some correlations with contacts between geologic units.