Estimates of Europa’s Ice Shell Thickness from Elastically-supported Topography

The thickness of Europa’s solid ice shell is uncertain, and has important implications for Europa’s habitability and thermal history. Here we obtain an estimate for the ice shell thickness from observations of a plateau SW of Cilix impact crater (3◦N, 182◦W ). A shell thickness of ≈ 25 km is compatible with our observations; a conservative lower bound is 6 km. Fig 1 shows a Galileo image of the area, with Cilix in the northern portion. Fig 1 also shows a digital elevation model (DEM) of the same area, derived by combining the 120 m/pixel image shown with six 70 m/pixel Galileo images to form stereo pairs [Giese et al., 1999]. Of particular interest is a 30-40 km wide plateau-shaped feature in the southern portion of the image, which is bounded on both the NW and SE sides by shallow topographic lows. Fig 2 shows a series of topographic profiles across this plateau. It demonstrates relatively steep scarps (slope ≈ 2◦) to the north and south, with a moat outboard of the scarps. Figure 3a shows the mean topography of profiles p1-p6, obtained by stacking them using the northern plateau scarp as a fixed point. The dashed lines show the standard deviation about the mean. The bold line is a best-fit model flexural profile, calculated by varying the assumed trapezoidal load geometry and the elastic thickness of the underlying plate until the misfit is minimized. The best-fit elastic thickness te is 6.0 km, and the fit to the topography is generally within one standard deviation, except at the far southern end of the profile. A study by Figueredo et al. [2002] of Murias Chaos, Europa, produced a slightly smaller value of te ≈ 4± 2 km, but these authors did not relate this value to the total solid shell thickness. Figure 3b shows the local minimum value of the RMS misfit H as a function of te (solid line). The existence of a minimum misfit,Hmin is apparent for the stacked profile; it is also clear thatH increases rapidly for values of te less than the best-fit value. For a misfit range of 1.2Hmin, the acceptable range of te is 4.6-9.4 km. The dotted lines in Figure 3b are misfit plots of the fit to the individual profiles p1,p3 and p5 of Figure 2. They all show that small values of te produce unacceptably high misfits. Best fit te values for these profiles range from 4.9 km to 11.9 km. The depth to which ice can maintain long-term elastic strength depends on both temperature and strain rate. At low strain rates or high temperatures, the ice will deform in a ductile fashion and the elastic stresses will be relaxed. There is thus a characteristic temperature TR which defines the base of the elastic layer [Nimmo et al., 2002]. For a conducting shell, the increase in temperature with depth depends on the total shell thickness; thus the elastic thickness may be used to constrain the crustal thickness. For the conducting part of the ice shell, in which conductivity varies as 1/T , we have