PUSH, PULL, SHEAR, OR OVERPRINT? QUANTITATIVE STRAIN ANALYSIS OF RIDGES ON EUROPA (Invited Presentation)

The detection of an induced magnetic field reveals the presence of a global saltwater ocean beneath the icy shell of Jupiter’s moon Europa, implying the possibility of habitable environments and the emergence of life. Recent geologic resurfacing, expressed in part as globally distributed linear surface features including ridges, the focus of this study, may have brought evidence of that ocean closer to the surface. We study here the kinematics of ridges to better understand their origin and relation with the underlying ocean. Ridges have been classified as double ridges or ridge complexes. Proposed formation mechanisms for double ridges include incremental ice wedging, the surface expression of a crystallizing water intrusion, or the result of heating caused by repeated strike-slip motion on a vertical fault. Previous studies, which include reconstructions of ridges, have suggested that double ridges may accommodate contraction, which might relate to the drainage of melt from shear heating. Conversely, it has also been proposed that double ridges and ridge complexes evolved from simple cracks or troughs into progressively more complex structures with increasing extension. Locations on the surface of Europa where double ridges transition into ridge complexes argue for a common origin for all types of ridges. This study presents an approach to determine the shear and normal strains associated with the formation of surface features on Europa, including ridges. This geometrical method uses the orientation and offset of cracks crossed by the studied ridge. It makes no assumptions about the ridge formation mechanism and allows for the possibility that part of the ridge is resurfaced without strain. We find small amounts of shear and normal strains associated with the formations of both double ridges and ridge complexes, with slightly more normal strain than shear strain. However, extension does not explain the entire width of the feature observed, so some portion of ridge formation must be controlled by overprinting, or deformation in place. This observation of an initial width involved in ridge formation is consistent with proposed formation models involving water transport within Europa’s ice shell, such as a cryovolcanic intrusion, like a dike or warm diapir.