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theses

Jupiter’s moon Europa is a differentiated planetary body comprised of a metallic core, a silicate mantle, and an outer layer of H2O. This shell can be further subdivided into an icy surface shell and a liquid subsurface ocean; details about the physical properties and structural parameters, such as the thickness, of the surface ice shell are poorly understood. Further, Europa has one of the youngest geologic surfaces in our solar system with an age of 40 - 90 Ma, implying an intense history of resurfacing. Although this young surface exhibits many extensional features such as dilational bands at which new surface material is created, evidence of corresponding contractional features is sparse. Recently, however, evidence for subduction on Europa has been observed. In this model, brittle, denser, conductive surface ice is thrust and subducted into a warmer, less dense and convecting subsurface ice. This process could be responsible for recycling aging terranes on a large scale. However, the basic physical parameters needed to initiate subduction on Europa, such as thickness of the brittle conductive layer, deformation rates, and orientation of pre-existing zones of weaknesses, are not well understood. In this study, we used a set of physical analogue paraffin wax experiments to examine the conditions that could lead to the initiation of subduction on Europa, such as the conductive layer thickness, deformation rate, and orientation of the pre-existing zone of weakness. The wax is heated from below and cooled with N2 from above so that a solid conductive lid forms above a molten convecting wax layer, similar to proposed ice models of Europa. Our results indicate that subduction could be initiated over a broad range of surface thicknesses and deformation rates above a minimum conductive layer thickness. In general, thicker conductive layers and faster deformation rates more readily initiate subduction. However, subduction in our experiments is strongly dependent on the orientation of the pre-existing zones of weakness; inclined pre-existing zones of weakness allow subduction over a wider range of thicknesses and deformation rates compared to vertical ones. Below a minimum critical conductive layer thickness, the conductive layer behaves ductilely during shortening, resulting in a previously undescribed process called ductile roll back, in which surface material is pushed, rolled, and eventually subducted into the subsurface at a pre-existing zone of weakness. Both ductile roll back and subduction in the experiments can accommodate a significant fraction of boundary displacement and, thus, could play a critical role in resurfacing Europa throughout its geologic history. These results establish a conceptual framework for the recognition and study of contractional surface features on Europa, which in turn have significant implications for Europa’s thermal history and evolution, habitability, and future space-craft missions.

ETD_8529

M.S.

Includes bibliographical references

by Michael William Klaser

doi:10.7282/T389190S

ETD graduate

The author owns the copyright to this work.