JOVIAN SYSTEM DATA ANALYSIS PROGRAM PROPOSAL SUMMARY

Europa's icy surface has been deformed by a variety of tectonic processes: ridge formation, plate rotation and pull-apart, apparent diapiric intrusion, and wholesale crustal overturn (chaos formation). Each of these reflects the response of the ice shell to tidal forces and heat flowing from the rocky interior. Understanding the nature of these responses should provide powerful constraints on the thermal and mechanical structure of the ice shell, e.g., its thickness, whether or not a subsurface ocean exists (though this point is less and and less debated), whether the ocean is accessible to radar sounding, etc. This proposal seeks to analyze the overall pattern of lineaments and plate rotation, as revealed by imaging during the primary mission, and use the derived stress and strain pattern and, to the extent possible, its time history to constrain the stress sources responsible (e.g., tidal flexing and nonsynchronous rotation). The latter will be numerically modeled so as to better understand the complex interplay of multiple stress sources, stress relief during shell failure, and shell thickness variations. Understanding diapirism and chaos formation requires a better understanding of the response of Europa's (presumed) floating ice shell to radiogenic and tidal heating. This will be accomplished by state-of-the-art numerical convection modeling in a thin shell, where both temperature-dependent viscosity and temperature-dependent tidal heating can be incorporated. Specifically, we propose to 1) perform a stratigraphic analysis of the lineament/ridge pattern as revealed by Galileo imagery to determine the pattern and (partial) time history of deformation; 2) relate these analyses to elastoviscoplastic finite element models of tidally flexed and/or synchronously rotating ice shells; 3) perform a stratigraphic analysis of spreading and rotation centers (wedge-shaped band regions) to establish regional strain histories; 4) relate these to the shell deformation models above and to possible convective patterns in the lower levels of the ice shell as described below; 5) simulate possible convective modes in Europa's ice shell using 2-D and 3-D cartesian finite element calculations, and thereby establish whether quasi-steady convective patterns are established or whether diapiric instability is the dominant heat transfer mode and whether thermal runaways are possible; 6) relate the convection results to the SSI/NIMS evidence for diapirism and chaos disruption regions. The significance of the proposed work for NASA OSS interests is that it should allow for a greater understanding of the history of the Europan shell and its interaction with the presumed ocean underneath, along with direct bearing on future exploration of Europa, whether by remote sensing (radar sounding) or in situ investigations. Relevant citations: Leith, A.C., and W.B. McKinnon: Is there evidence for polar wander on Europa?, Icarus 120, 387-398, 1996; McKinnon, W.B.: On the thickness and stability of the Europan ice shell, EOS Trans. AGU 78 (Fall Meeting Suppl.), F416, 1997; McKinnon, W.B.: Geodynamics of icy satellites, in Solar System Ices (Kluwer Academic Pub.), 525-550, 1998; Dombard, A.J., and W.B. McKinnon: Formation of grooved terrain on Ganymede: Extensional instability mediated by cold, superplastic creep, Icarus, in press; Gurnis, M.: A reassessment of the heat transport by variable viscosity convection with plates and lids, Geophys. Res. Lett. 16, 179-182,1989; Zhong, S., and M. Gurnis: Interaction of weak faults and non-Newtonian rheology produces plate tectonics in a 3D model of mantle flow, Nature 383, 245-247, 1996.