Abstract. In order to better understand ice deformation mechanisms, we document the microstructural evolution of ice with increasing strain. We include data from experiments at relatively low temperatures (−20 and −30 ∘C), where the microstructural evolution with axial strain has never before been documented. Polycrystalline pure water ice was deformed under a constant displacement rate (strain rate ∼1.0×10-5 s−1) to progressively higher strains (∼ 3 %, 5 %, 8 %, 12 % and 20 %) at temperatures of −10, −20 and −30 ∘C. Microstructural data were generated from cryogenic electron backscattered diffraction (cryo-EBSD) analyses. All deformed samples contain subgrain (low-angle misorientations) structures with misorientation axes that lie dominantly in the basal plane, suggesting the activity of dislocation creep (glide primarily on the basal plane), recovery and subgrain rotation. Grain boundaries are lobate in all experiments, suggesting the operation of strain-induced grain boundary migration (GBM). Deformed ice samples are characterized by interlocking big and small grains and are, on average, finer grained than undeformed samples. Misorientation analyses between nearby grains in 2-D EBSD maps are consistent with some 2-D grains being different limbs of the same irregular grain in the 3-D volume. The proportion of repeated (i.e. interconnected) grains is greater in the higher-temperature experiments suggesting that grains have more irregular shapes, probably because GBM is more widespread at higher temperatures. The number of grains per unit area (accounting for multiple occurrences of the same 3-D grain) is higher in deformed samples than undeformed samples, and it increases with strain, suggesting that nucleation is involved in recrystallization. “Core-and-mantle” structures (rings of small grains surrounding big grains) occur in −20 and −30 ∘C experiments, suggesting that subgrain rotation recrystallization is active. At temperatures warmer than −20 ∘C, c axes develop a crystallographic preferred orientation (CPO) characterized by a cone (i.e. small circle) around the compression axis. We suggest the c-axis cone forms via the selective growth of grains in easy slip orientations (i.e. ∼ 45∘ to shortening direction) by GBM. The opening angle of the c-axis cone decreases with strain, suggesting strain-induced GBM is balanced by grain rotation. Furthermore, the opening angle of the c-axis cone decreases with temperature. At −30 ∘C, the c-axis CPO changes from a narrow cone to a cluster, parallel to compression, with increasing strain. This closure of the c-axis cone is interpreted as the result of a more active grain rotation together with a less effective GBM. We suggest that lattice rotation, facilitated by intracrystalline dislocation glide on the basal plane, is the dominant mechanism controlling grain rotation. Low-angle neighbour-pair misorientations, relating to subgrain boundaries, are more extensive and extend to higher misorientation angles at lower temperatures and higher strains supporting a relative increase in the importance of dislocation activity. As the temperature decreases, the overall CPO intensity decreases, primarily because the CPO of small grains is weaker. High-angle grain boundaries between small grains have misorientation axes that have distributed crystallographic orientations. This implies that, in contrast to subgrain boundaries, grain boundary misorientation is not controlled by crystallography. Nucleation during recrystallization cannot be explained by subgrain rotation recrystallization alone. Grain boundary sliding of finer grains or a different nucleation mechanism that generates grains with random orientations could explain the weaker CPO of the fine-grained fraction and the lack of crystallographic control on high-angle grain boundaries.

Abstract. Understanding ice deformation mechanisms is crucial for understanding the dynamic evolution of terrestrial and planetary ice flow. To understand better the deformation mechanisms, we document the microstructural evolution of ice with increasing strain. We include data from deformation at relatively low temperature (−20 and −30 °C) where the microstructural evolution has never before been documented. Polycrystalline pure water ice was deformed under a constant displacement rate (equal to the strain rate of ~1.0×10−5 s−1) at temperatures of −10, −20 and −30 °C to progressively higher true axial strains (~ 3, 5, 8, 12 and 20 %). Mechanical data show peak and steady-state stresses are larger at colder temperatures as expected from the temperature dependency of creep. Cryo-electron backscattered diffraction (EBSD) analyses show distinct sub-grain boundaries in all deformed samples, suggesting activation of recovery and subgrain rotation. Deformed ice samples are characterised by big grains interlocking with small grains. For each temperature series, we separated big grains from small grains using a threshold grain size, which equals to the square mean root diameter at ~ 12 % strain. Big grains are more lobate at −10 °C than at colder temperatures, suggesting grain boundary migration (GBM) is more prominent at warmer temperatures. The small grains are smaller than subgrains at −10 °C and they become similar in size at −20 and −30 °C, suggesting bulge nucleation facilitates the recrystallization process at warmer temperature and subgrain rotation recrystallization is the nucleation mechanism at colder temperatures. At temperatures warmer than −15 °C, c-axes develop a crystallographic preferred orientation (CPO) characterized by a cone (i.e., small circle) around the compression axis. We suggest the c-axis cone forms as a result of selective growth of grains at easy slip orientations (i.e., ~ 45° to shortening direction) by strain-energy driven GBM. This particular finding is consistent with previous works. The opening-angle of the c-axis cone decreases with strain, suggesting strain-induced GBM is balanced by grain rotation. Furthermore, the opening-angle of the c-axis cone decreases with temperature. At −30 °C, the c-axis CPO transits from a narrow cone to a cluster, parallel to compression, with increasing strain. This closure of the c-axis cone is interpreted as the result of a more active grain rotation together with a less effective GBM. As the temperature decreases, the overall CPO intensity decreases, facilitated by the CPO weakening in small grains. We suggest the grain size sensitivity of grain boundary sliding (GBS) favours a faster strain rate in small grains and leads to the CPO weakening at cold temperatures. CPO development cannot provide a uniform explanation for the mechanical weakening (enhancement) after peak stress. Grain size reduction, which can be observed in all deformed samples, is most likely to cause weakening (enhancement) and should be considered to have a significant control on the rheology of natural ice flow.

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Vast quantities of solid CO 2 reside in topographic basins of the south polar layered deposits (SPLD) on Mars and exhibit morphological features indicative of glacial flow. Previous experimental studies showed that CO 2 ice is 1-2 orders of magnitude weaker than water ice under Martian polar conditions. Here we present data from deformation experiments on pure, fine-grained CO 2 ice, over a broader range of temperatures than previously explored (158-213 K). The experiments confirm previous observations of highly nonlinear power law creep at larger stresses, but also show a transition to a previously unseen linear-viscous creep regime at lower stresses. We examine the viscosity of CO 2 within the SPLD and predict that the CO 2-rich deposits are modestly stronger than previously thought. Nevertheless, CO 2 ice flows much more readily than H 2 O ice, particularly on the steep flanks of SPLD topographic basins, allowing the CO 2 to pond as observed. Plain Language Summary Massive deposits of solid CO 2 (dry ice) were recently discovered at Mars's south pole and appear to be flowing like glaciers on Earth. To predict the glacial flow rate of dry ice at Martian polar conditions, we produced and then deformed samples of CO 2 ice in the laboratory. Under relatively large stresses and low temperatures, the rate of CO 2 ice flow is very sensitive to changes in stress, meaning that dry-ice glaciers will flow much faster over steep topographic slopes at Mars's south pole than over shallow slopes. This extreme sensitivity to the slope of underlying topography explains why CO 2 ice deposits are ponded within topographic basins at Mars's south pole, preserving a half-million-year long climate record.