Origins of Dark Material in Crater Ejecta on Europa

The origins of low-albedo hydrate material on the surface of Europa have been a source of great curiosity since Voyager first returned close-up images of the icy moon. Sulfur originating from Io is known to contribute an exogenic flux of dark material, primarily to Europa's trailing hemisphere. At regional and local scales, hydrated salt compounds are concentrated within chaos, ridges, and pits, suggesting an endogenic source. Many of Europa's impact craters also exhibit dark ejecta, the origins of which are unknown. We used imaging data from NASA's Galileo mission and models to investigate impact features and their possible contributions to the exogenic deposition of dark material. Our study examines the ejecta of large impact craters to determine whether intrinsically dark impactors could be responsible for this low-albedo component in the crater's ejecta. Using Galileo images, we identified a number of craters with this dark ejecta characteristic. Using crater scaling laws, we then estimated the impactor size for each crater and, using a radiative transfer model, determined the contribution of the dark ejecta due to the impactor. Our preliminary results show that, by comparing the ratio of dark vs light material in the ejecta to the ratio of impactor mass vs ejecta mass, the concentrations of dark material found in the ejecta of these craters cannot be solely attributed to the impactor itself. This result indicates that dark crater ejecta on Europa must be at least partly endogenic; subsurface material excavation and/or chemical or physical alteration due to impact forces are likely sources. Further study is planned to consider these alternate mechanisms as they relate to the relatively limited number of impact sites found on the surface today.