Low-speed friction and brittle compressive failure of ice: fundamental processes in ice mechanics

Frictional sliding plays a fundamental role in the mechanics of ice, on scales small and large. Whether across opposing faces of microcracks within laboratory specimens, across strike-slip like features within the sea ice cover on the Arctic Ocean, or across faults within the crust of icy satellites of the Outer Solar System, sliding of ice upon itself is central to the process of brittle compressive failure. In that regard, ice is similar to rock and to other crystalline materials. This review addresses the role of sliding in brittle compressive failure and then focuses on the nature of friction per se. Both static friction and low-velocity (<0.1 m s(-1)) kinetic friction are described at temperatures from near the equilibrium melting point to as low as -175 degrees C. The corresponding coefficients of friction are discussed in terms of the interaction and deformation of asperities that protrude from opposing surfaces. The coefficients appear to be governed largely by asperity creep at lower velocities and by creep plus localised melting at higher velocities. At still higher velocities (beyond the scope of this review), friction of warm ice is governed by hydrodynamic effects. The review closes with comments on the compressive strength of ice, on the deformation of the arctic sea ice cover and on the tectonic evolution of Saturn's Enceladus.