Small in Number but Mighty in Significance: Impact Craters as Windows Into Europa's Subsurface

Impact craters are rare on the icy ocean world Europa: only 24 ≧ 10 km in diameter are currently observed. Craters ≳20 km in diameter, such as Pwyll and Manannán, display unusual morphologies, perhaps because they excavate down close to the subsurface ocean. Previous work suggests that Manannán may have impacted into liquid, slushy ice or thinner ice than at Pwyll. Here we show that impact craters, via their intrinsic morphologic and compositional properties, as well as impact-excavation of otherwise unobservable materials, provide windows into subsurface structure and composition, making them natural laboratories for investigating Europa's habitability. We find that both Pwyll and Manannán excavated dark material from ∼3.1 km deep, which could be intriguing non-ice materials such as H₂SO₄ or hydrated minerals. We calculated that Pwyll impacted into a solid ice shell of ≧10.7 km thickness. We hypothesize that Manannán may have impacted into less viscous material, and detail how this can be tested with future Europa Clipper data. We also hypothesize that a reservoir of sub-surface impact-induced material sourced the lobate material within Pwyll. This hypothesis can also be tested with Europa Clipper data and, if confirmed, would provide targets to observe salts derived from potentially some of the most processed brines on Europa.