Low-temperature friction experiments on ice-salt mixtures: Implications for the strength of ice plate boundaries on Europa

While recent observational studies have suggested that the ice shell on Europa is undergoing active plate subduction, some theoretical studies have concluded that the high frictional strength of ice would prevent such icy plates from subducting. Constraining the frictional behavior of mixtures of ice and non-ice materials that exist on the surface is therefore key to understanding the frictional strength of plate boundaries on Europa. Here we perform friction experiments on 1 mm thick layers of gouge comprised of mixtures of ice-Ih and magnesium chloride with 0, 17, 29.5 and 47 wt% salt under low-temperature conditions (-40 to -10 degrees C). The experiments are conducted using a biaxial testing machine with constant normal stresses of either 2.5 or 5.0 MPa, and a constant shear velocity of 3 mu m/s. Our experimental results for pure ice and ice-salt mixtures yield steady-state friction coefficients ranging from 0.72 +/- 0.02 (-39.2 +/- 0.7 degrees C) to 0.47 +/- 0.01 (-16.4 +/- 0.2 degrees C), and 0.51 +/- 0.01 (-39.8 +/- 1.4 degrees C) to 0.10 +/- 0.02 (-16.9 +/- 0.5 degrees C), respectively. These differences in friction coefficient indicate that non-ice materials can have a significant effect on the friction coefficient along the plate boundaries on Europa. Spatial variations in the frictional strength of plate boundaries are expected to be localized near the base of the conductive layer, as the friction coefficient of the ice-salt mixture exhibits a strong temperature dependence. The observed stick-slip behavior at low temperatures in our experiments indicates the potential for occurrence of intermittent stick-slip along the shallower parts of the plate boundaries, whereas steady-state sliding occurs along the deeper parts.