An improved understanding of the mechanisms and factors affecting glacial flow is crucial to better predict sea level rise. Glacial ice often contains impurities such as the presence of small insoluble particles. Mixtures of ice and dust can be found in many places throughout the world, specifically in areas of high latitude and altitude (Moore, 2014). This study aims to understand the effect of entrained insoluble debris on processes of glacial motion. Glaciers move through a combination of internal ice deformation and basal sliding. Internal ice deformation, the flow of individual ice grains, has been found to be grain-size dependent in both field and laboratory studies (Goldsby and Kohlstedt, 2001). In an attempt to better understand ice grain size, this study considers the effect of debris on grain growth. Samples of pure ice and ice with debris were fabricated with a standard protocol and maintained at -5°C for controlled annealing. Microstructural characterization was preformed using a light microscope to image the samples, and calculating the average grain sizes using a linear-intercept method. The ice with debris was found to have smaller grain sizes, thought to be associated with grain-boundary pinning. Extrapolated values were used with a flow law, projecting that ice with debris will have lower viscosity, thus flow faster. To address basal sliding, the other form of glacial movement, we conducted a second phase of study. Basal sliding, the process of a glacier sliding over the bedrock, is influenced by the presence of meltwater at the base of the glacier (Hoffman et al., 2011). Frictional heating, from ice-on-rock friction, was studied as a factor affecting meltwater production. We conducted a simple 1D computer model using laboratory friction measurements of ice with entrained debris (Zoet et al., 2013). We find that debris content and frictional heating are directly proportional. Trials run at faster glacial velocities also show larger amounts of frictional heating. As frictional heating may increase meltwater, glaciers with debris may slide faster over bedrock. Overall, by better understanding the motion of debris-rich glaciers, we can focus our attention to areas around the world at risk, and better predict/prepare for sea level rise.

Basal slip along glaciers and ice streams can be significantly modified by external time-dependent forcing, although it is not clear why some systems are more sensitive to tidal stresses. We have conducted a series of laboratory experiments to explore the effect of time varying load point velocity on ice-on-rock friction. Varying the load point velocity induces shear stress forcing, making this an analogous simulation of aspects of ice stream tidal modulation. Ambient pressure, double-direct shear experiments were conducted in a cryogenic servo-controlled biaxial deformation apparatus at temperatures between -2°C and -16°C. In addition to a background, median velocity (1 and 10 μm/s), a sinusoidal velocity was applied to the central sliding sample over a range of periods and amplitudes. Normal stress was held constant over each run (0.1, 0.5 or 1 MPa) and the shear stress was measured. Over the range of parameters studied, the full spectrum of slip behavior from creeping to slow-slip to stick-slip was observed, similar to the diversity of sliding styles observed in Antarctic and Greenland ice streams. Under conditions in which the amplitude of oscillation is equal to the median velocity, significant healing occurs as velocity approaches zero, causing a high-amplitude change in friction. The amplitude of the event increases with increasing period (i.e. hold time). At high normal stress, velocity oscillations force an otherwise stable system to behave unstably, with consistently-timed events during every cycle. Rate-state friction parameters determined from velocity steps show that the ice-rock interface is velocity strengthening. A companion paper describes a method of analyzing the oscillatory data directly. Forward modeling of a sinusoidally-driven slider block, using rate-and-state dependent friction formulation and experimentally derived parameters, successfully predicts the experimental output in all but a few cases.

Frictional strength of polycrystalline ice and ice-ammonia mixtures have been measured in the laboratory in order to constrain fault behavior on the south polar region of Enceladus. These faults, known as the tiger stripes, are associated with anomalous temperature gradients and active jets, which have been linked to tidally induced stresses on the faults. The temperature dependence of fault stability is used to map a seismogenic zone within the brittle layer of Enceladus’ icy shell which is estimated as the upper 4 km. The results from our experimental study and models of frictional heating are used to infer fault strength and heat generation within the brittle layer. The friction experiments were conducted using polycrystalline ice and ice-ammonia mixtures in a custom servo-hydraulic biaxial deformation apparatus with a liquid nitrogen-cooled cryostat at icy satellite conditions. The frictional response was measured in steady-state as well as at dynamic conditions, as a function of frequency, amplitude, normal stress, and temperature. For the modelling, a simple 1-D numerical frictional heating model was constructed showing the change in temperature on a fault during sliding. The estimations were based on the solution for heat diffusion through a fault of finite thickness, where temperatures were calculated for different time/space combinations as functions of time during/after slip and inside/outside the slipping zone. Using estimated fault depth and slip distance from previous studies, as well as our experimentally determined friction coefficient, frictional heating with depth was determined with varying values for fault width and slip velocity. Frictional heating models of pure polycrystalline ice will also be compared to that of ice-ammonia partial melt in order to constrain the depth of partial melt within the icy shell, characterizing a possible oxidant transport mechanism into the subsurface ocean. Although we consider sliding on Enceladus, results in this study are additionally applicable to Europa, where instead sulfuric acid may provide the deep eutectic second phase. The identification of the source of seismicity in a tidally loaded fault system could benefit interpretation of seismic observations in future missions to icy satellites.