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E.P. Turtle, C.B. Phillips, H.J. Melosh, A.S. McEwen (LPL, Univ. of Arizona)
Many of the geologic features on Europa's surface suggest that it consists of only a thin layer of solid ice over a more fluid interior. Europa's few impact craters exhibit a variety of morphologies which vary with crater size. Craters with diameters between ~5 and ~24 km are complex, while the two largest impact structures are surrounded by several concentric fractures. Crater morphology is influenced by the target's material properties and near-surface structure. So, we can use transitions in crater morphology to investigate the nature of Europa's near-surface.
Previously we simulated the collapse of transient craters 13 and 26 km in diameter (comparable to the complex crater Pwyll and the multiple ring crater Callanish) in an ice lithosphere over a low viscosity Newtonian fluid. (In these simulations it is assumed that Europa consists of a thin layer of ice over a more fluid interior, a structure which is consistent with, but not necessary for, the formation of multiple ring impact craters.) We found that the lithosphere had to be between 6 and 15 km thick in order for crater collapse to generate stress fields consistent with the formation of concentric fractures around craters the size of Callanish, and not around craters the size of Pwyll.
Our early numerical models were oversimplified. We have therefore refined them substantially to be as realistic as possible: incorporating the rheological effects of a temperature gradient in the lithosphere, decreasing the size of the elements, and taking into account the curvature of the Europan surface. These more detailed simulations confirm that the lithosphere must be at least ~6 km thick in order for concentric extensional fractures to form around craters the size of Callanish, and not around craters the size of Pwyll.