Friction in Cold Ice Within Outer Solar System Satellites With Reference to Thermal Weakening at High Sliding Velocities

The icy shells of Enceladus and Europa consist from top down of cold (similar to 100 K) ice at low (<0.1 MPa) pressure, cold ice at high pressure up to similar to 10 MPa, and warm ice near 273 K the base. The pressure similar to 10 MPa and temperature near 273 K of basal ice within Enceladus and Europa are similar to that within terrestrial glaciers that are known to have seismicity (icequakes). Warm ice easily melts during sliding so its icequakes are qualitatively explained and expected on these satellites if the macroscopic strain rates are comparable to those within terrestrial glaciers subjected to oceanic tides. However, cold ice at high pressures does not readily macroscopically melt during sliding. A dynamic weakening mechanism for crustal faults in rock may be applicable to Enceladus and Europa. Micron-scale real contacts support similar to 0.4-GPa shear tractions and normal tractions on rapidly sliding ice faults. At sliding velocities above similar to 0.1 m/s, the asperity tips of the contacts become hot and weak in ice. The macroscopic friction depends on the average strength of the asperity tips during the lifetimes of contact. The strength of the asperity tips self-organizes so that frictional heating balances the heat lost from the asperity tip by conduction. The macroscopic coefficient of friction at coseismic sliding velocities decreases to a modest fraction of the low-velocity coefficient of friction, but does not approach zero. This velocity-weakening mechanism likely allows major icequakes within the cold interiors of Europa and Enceladus.