Fine-grained Ice Research Articles

The relationship between the permeability and liquid water content of polycrystalline temperate ice

Abstract To better constrain meltwater transport and ice viscosity in temperate glaciers, particularly in ice stream shear margins, we use a custom permeameter to study the untested model relationship between the permeability of temperate ice and its liquid water content. The permeability of lab-made ice of two mean grain diameters (1.8 and 4.2 mm) is measured, and water content is controlled with the ice salinity and measured calorimetrically. Fluorescein dye is added to through-flowing, chilled water to highlight flow pathways through the ice after experiments. As predicted by a simple model, permeability increases with approximately the square of the water content and by about three orders of magnitude across water contents of 0.1–4.4%. However, permeability values are less than those of the model by average factors of 2.6 and 4.1 for the finer and coarser ice, respectively. This discrepancy is likely due to tortuous, truncated or air-clogged veins. The order-of-magnitude agreement between measured and modeled values may indicate that reduced permeability from these factors is nearly compensated by preferential flow in oversized veins that are isolated or arborescent. Both kinds of preferred flow pathways are observed but the latter only in fine-grained ice at water contents > 2%.

A single wheel test rig for ocean world rovers

Ocean Worlds such as Europa and Enceladus are known to harbor subsurface liquid water oceans under their icy crust and are high-priority targets for in situ exploration. Compared to the Moon and Mars, Ocean Worlds likely present a significantly more challenging environment for surface mobility systems due to the extremely cold temperature, high radiation dosage, and poorly constrained material properties under these conditions. Small-diameter wheels such as those used by Mars rovers are prone to slip-sinkage in loose soil and damage from sharp rock and ice formations. A 4-wheel rover with a simple drive system and large deployable compliant tires is proposed as a solution for extreme terrain mobility on Ocean World surfaces. The present work describes the design and construction of a single wheel test rig and a prototype large-diameter deployable wheel for Ocean World rovers and initial test results. The test rig allows independent control of the vertical load, slip ratio, slip angle, and camber angle, and accommodates large-diameter deployable wheels. The test rig features a modular test bed that can simulate varied surface features such as fine-grained ice, smooth hard ice, sharp ice formations, and large ice boulder fields.

Tensile strength and elastic properties of fine-grained ice aggregates: Implications for crater formation on small icy bodies

The mechanical and elastic properties of ice aggregates are important in the physics of avalanches and crater formation on icy bodies, such as icy satellites and cometary nuclei. Here we conducted uniaxial tensile tests and elastic-wave velocity measurements on artificial fine-grained ice aggregates (snow) to infer the potential for crater formation on small icy bodies. The uniaxial tensile tests on the artificial snow with filling factors (f) in the 0.30–0.59 range at −15 °C demonstrate that the tensile strength (Yt) depends on its filling factor; we obtained the empirical equation Yt=103.5f3.5 (in kPa) based on our results, which is consistent with the upper limit of natural snow’s tensile strength. The compressional- and shear-wave velocities of artificial snow with f≥ 0.4 were measured via the ultrasonic pulse velocity method. The elastic-wave velocities decrease linearly with decreasing f values. Our calculations for the Young’s moduli of the artificial snow from the elastic-wave velocity measurements are 10–40 times higher than those from the tensile tests, which indicate the rate-dependent properties of the fine-grained ice aggregates. We propose a tensile strength estimation of a cometary surface via an artificial impact based on our results and a crater-scaling law in the strength-dominated regime.

Full crystallographic orientation (<i>c</i> and <i>a</i> axes) of warm, coarse-grained ice in a shear-dominated setting: a case study, Storglaciären, Sweden

Abstract. Microstructures provide key insights into understanding the mechanical behavior of ice. Crystallographic preferred orientation (CPO) develops during plastic deformation as ice deforms dominantly by dislocation glide on the basal plane, modified and often intensified by dynamic recrystallization. CPO patterns in fine-grained ice have been relatively well characterized and understood in experiments and nature, whereas CPO patterns in “warm” (T>-10∘C), coarse-grained, natural ice remain enigmatic. Previous microstructural studies of coarse-grained ice have been limited to c-axis orientations using light optical measurements. We present the first study of a axes as well as c axes in such ice by application of cryo-electron backscatter diffraction (EBSD) and do so in a shear-dominated setting. We have done this by developing a new sample preparation technique of constructing composite sections, to allow us to use EBSD to obtain a representative, bulk CPO on coarse-grained ice. We draw attention to the well-known issue of interlocking grains of complex shape and suggest that a grain sampling bias of large, branching crystals that appear multiple times as island grains in thin sections may result in the typical multimaxima CPOs previously identified in warm, coarse-grained ice that has been subjected to prolonged shear. CPOs combined from multiple samples of highly sheared ice from Storglaciären provide a more comprehensive picture of the microstructure and yield a pronounced cluster of c axes sub-normal to the shear plane and elongate or split in a plane normal to the shear direction as well as a concomitant girdle of a axes parallel to the shear plane with a maximum perpendicular to the shear direction. This pattern compares well with patterns produced by subsampling datasets from ice sheared in laboratory experiments at high homologous temperatures up to strains of ∼1.5. Shear strains in the margin of Storglaciären are much higher than those in experimental work. At much lower natural strain rates, dynamic recrystallization, particularly grain boundary migration, may have been more effective so that the CPO represents a small, final fraction of the shear history. A key result of this study is that multimaxima CPOs in coarse-grained ice reported in previous work may be due to limited sample sizes and a sampling bias related to the presence of island grains of a single host that appear several times in a thin section.

Using a composite flow law to model deformation in the NEEM deep ice core, Greenland – Part 2: The role of grain size and premelting on ice deformation at high homologous temperature

Abstract. The ice microstructure in the lower part of the North Greenland Eemian Ice Drilling (NEEM) ice core consists of relatively fine-grained ice with a single maximum crystallographic preferred orientation (CPO) alternated by much coarser-grained ice with a partial (great circle) girdle or multi-maxima CPO. In this study, the grain-size-sensitive (GSS) composite flow law of Goldsby and Kohlstedt (2001) was used to study the effects of grain size and premelting (liquid-like layer along the grain boundaries) on strain rate in the lower part of the NEEM ice core. The results show that the strain rates predicted in the fine-grained layers are about an order of magnitude higher than in the much coarser-grained layers. The dominant deformation mechanisms, based on the flow relation of Goldsby and Kohlstedt (2001), between the layers is also different, with basal slip rate limited by grain boundary sliding (GBS-limited creep) being the dominant deformation mechanism in the finer-grained layers, while GBS-limited creep and dislocation creep (basal slip rate limited by non-basal slip) contribute both roughly equally to bulk strain in the coarse-grained layers. Due to the large difference in microstructure between finer-grained ice and the coarse-grained ice at premelting temperatures (T>262 K), it is expected that the fine-grained layers deform at high strain rates, while the coarse-grained layers are relatively stagnant. The difference in microstructure, and consequently in viscosity, between impurity-rich and low-impurity ice can have important consequences for ice dynamics close to the bedrock.

Effects of microparticles on deformation and microstructural evolution of fine-grained ice

ABSTRACTWe investigated the effects of microparticles and grain size on the microstructural evolutions and mechanical properties of polycrystalline ice. Uniaxial compression tests were conducted using fine-grained pure ice and silica-dispersed ice under various conditions. Deformation behavior of fine-grained ice was found to be characterized by stress exponent n ≈ 2 and activation energy Q ≈ 60 kJ mol−1. The derived strain rates of fine-grained ice were ≈ 1 order of magnitude larger than those of coarse-grained ice obtained in previous studies, and they were found to be independent of particle dispersion and dependent on the mean grain size of ice, with grain size exponent p ≈ 1.4. Work hardening was observed in dislocation creep, while the strain rate continued to decrease. These results indicate that the deformation mechanism of fine-grained ice is different from typical dislocation creep, often associated with n = 3. Although microparticles restricted grain growth, there was little direct effect on the deformation of fine-grained ice. Microstructural observations of the ice samples indicated that the grain boundaries were straight and that the subgrain boundary densities increased after deformation. Our experiments suggest that grain size and boundaries play important roles in the deformation processes of polycrystalline ice.

Sublimation of cometary ices in the presence of organic volatiles II

Evolution of the surfaces of cometary nuclei is determined by the sublimation of ice. The rate of sublimation is commonly calculated using the simple Hertz–Knudsen formula, which should be corrected by a temperature dependent sublimation coefficient αs (Kossacki et al., 1999; Gundlach et al., 2011; Kossacki and Leliwa-Kopystynski, 2014). In Kossacki et al. (2017) the temperature dependence of αs for water ice with acetone and methanol present in comets was analyzed. In this work the influence of propanal at the temperature ~195–~230 K and formamide at the temperature ~200–~235 K were investigated. Experiments were performed with slab ice and porous fine-grained ice. It is found, that the presence of propanal reduces the sublimation coefficient at T < 207 K and enhances at T > 208 K. The presence of formamide reduced the sublimation coefficient in the whole investigated temperature range. Porous fine-grained ice sublimed up to four times slower than non-porous one.

Model scale ice — Part B: Numerical model

The model-scale ice of the Aalto University ice tank is fine-grained ice, which is used in experimental research on Arctic marine structures. However, the behavior of model-scale ice is not much explored and a deeper understanding of the model-scale ice mechanics is required to appraise the validity of full-scale and model-scale ice property tests. Therefore, this paper presents a numerical model that accounts for the model-scale ice micro-structure and a variety of the physical effects under load. The numerical model accounts for the non-homogeneity of the model-scale ice and is based on damage mechanics. The basic mechanical properties, i.e. grain size, elastic strain-modulus, Poisson's ratio, compressive strength and tensile strength, are determined experimentally, and the free parameters of the damage model are determined based on experimental evidence and by reverse engineering until the compliance of the numerical and experimental response is achieved. Furthermore, the response sensitivity on variations of the Poisson's ratio and the non-homogeneities is analyzed. Ultimately, the failure stresses of the numerical model are compared with the experimentally-determined strength values. As a result, an experimentally-calibrated numerical model of the model-scale ice is obtained. In the future, this numerical model can be used to evaluate marine designs prior to testing, thus allowing for inexpensive simulation-based design of arctic marine structures.

Phase Equilibrium for Clathrate Hydrates Formed with Methane, Ethane, Propane, or Carbon Dioxide at Temperatures below the Freezing Point of Water

This paper reports the three-phase (ice + hydrate + guest-rich vapor) equilibrium pressure−temperature conditions at temperatures (243 to 273) K in the systems of water and each of the following guest gases: methane, ethane, propane, and carbon dioxide. The measurements were also performed for the water-rich liquid + hydrate + guest-rich vapor three-phase equilibrium conditions at temperatures above 273 K. The pressure ranges of the present measurements in the four systems are (0.971 to 2.471) MPa in the methane system, (0.122 to 0.637) MPa in the ethane system, (41.0 to 280.0) kPa in the propane system, and (0.364 to 0.963) MPa in the carbon dioxide system. On the basis of the obtained three-phase equilibrium data, the quadruple points for the ice + water-rich liquid + hydrate + guest-rich vapor were also determined in the respective systems. The measurements were carried out using the batch, isochoric procedure. Fine-grained ice powders with diameters of (1 to 2) mm were used to form the hydrate. The me...

Synthesis and rheology of fine-grained ice II at high pressures and low temperatures

Synthesis and rheology of fine-grained ice II at high pressures and low temperatures

Performance and applications of an automated c-axis ice-fabric analyzer

AbstractA completely automated instrument for ice-fabric analysis has been built, tested and applied. The analysis of a thin section is performed in two steps. First, the section is loaded into the machine and is automatically imaged in about 16 min. Then, at a later time and convenient location, the grains in the section are identified from a stored image (either automatically or by the user), and the c-axis orientation of each grain is calculated at a rate of 1200 grains h−1. Comparison of automated results with manual Rigsby stage results shows excellent agreement. Diagnostics indicate that the accuracy of the new technique is 0.5° in the c-axis orientation. Two specific applications are demonstrated: (1) the ability to measure internal grain orientations that may reveal internal stress, and (2) the ability to measure very thin thin sections. The latter, combined with the ability to zoom in on small regions of a section, will be useful for examining the fabric of fine-grained (&lt; 1 mm size grains) ice.

Localization of deformation in polycrystalline ice

Since ice deforms mainly by dislocation glide on the basal planes, very strong incompatibilities exist between the grains of a polycrystal. Specially designed two-dimensional compression creep tests on different types of ice multicrystals embedded in a matrix of fine-grained isotropie ice allow to observe the strain heterogeneities which arise during the deformation of polycrystalline ice, and which are the consequence of inter-granular strain incompatibilities. The main features observed were bending and kink bands initiated at triple junctions or at geometric defects of the grain boundaries. These bands developed perpendicularly to the observed basal slip lines and provided additional degrees of freedom for the accommodation of inter-granular deformation. Mechanisms related to dislocations motion and dynamic recrystallization are proposed for the initiation and for possible development of the observed localization features.

Ice crystallographic evolution at a temperate glacier: Glacier de Tsanfleuron, Switzerland

Abstract Ice crystallographic measurements have been made on eight cores retrieved from temperate Glacier de Tsanfleuron, Switzerland. Cores are aligned approximately along a flow-parallel transect, allowing a stratigraphic model of crystallographic evolution at the glacier to be constructed. Results indicate the presence of four crystallographic units at the glacier. Unit 1 composed of homogeneous, fine-grained ice with a uniform fabric, is located within c. 20 m of the ice surface in the accumulation area of the glacier. Crystal growth within this unit occurs in the absence of significant stresses, and its rate is closely described by an Arrheniustype relationship. Unit 2 ice, characterized by the local development of coarser crystals, forms after some decades of Arrhenius growth, marking the initial influence of processes of dynamic recrystallisation. Unit 3 ice, characterized by an abrupt increase in minimum crystal size, occurs at a depth of c. 33 m throughout the glacier. In the accumulation area, this increase coincides with the first evidence of systematic fabric enhancement, interpreted in terms of dynamic recrystallisation. Unit 4 ice, characterized by large, interlocking grains with a multi-modal girdle fabric, develops within c. 10 m of the glacier bed. Here, the measured minimum crystal size is consistent with a steady-state balance between Arrhenius processes of grain growth and strain-related processes of grain-size reduction. These changes are interpreted in terms of the effects of intense, continuous deformation in this basal zone.

Grain boundary sliding in fine-grained Ice I

Grain boundary sliding in fine-grained Ice I

Borehole video observation of englacial and basal ice conditions in a temperate valley glacier

As part of a study of ice dynamics, 25 boreholes were drilled with high-pressure hot water to the base of Haut Glacier d’Arolla, Switzerland. The boreholes were distributed across a half-section of the glacier, with closest spacing towards the glacier margin. The interior structure of the glacier was investigated by lowering a miniature color video camera down 11 boreholes, and the glacier bed was observed in 3 boreholes.The video showed distinct changes in ice conditions with depth in the glacier, including ice foliations and changes in ice-bubble content and color. A basal ice layer of varying thickness was recorded in two of the three boreholes in which the glacier bed was observed. This basal ice layer was characterized by relatively sediment-rich, coarse-clear ice, and was thickest in a zone of high water-pressure fluctuations. The available evidence suggests that localized freezing at the glacier bed caused by variations in water pressure is the primary source of this basal ice layer. The near-surface transition between finegrained and coarse-bubbly ice, and between coarse-bubbly and coarse-clear ice, appeared to correlate with the depths of influence of diurnal and annual cold waves, respectively. Two types of ice foliation were identified, consisting of distinct planar coarse-clear ice, or more pervasive coarse-bubbly or fine-grained ice. An origin from crevasse closure or the transposition of sedimentary stratification provides the most likely explanation for these.

Borehole video observation of englacial and basal ice conditions in a temperate valley glacier

As part of a study of ice dynamics, 25 boreholes were drilled with high-pressure hot water to the base of Haut Glacier d’Arolla, Switzerland. The boreholes were distributed across a half-section of the glacier, with closest spacing towards the glacier margin. The interior structure of the glacier was investigated by lowering a miniature color video camera down 11 boreholes, and the glacier bed was observed in 3 boreholes. The video showed distinct changes in ice conditions with depth in the glacier, including ice foliations and changes in ice-bubble content and color. A basal ice layer of varying thickness was recorded in two of the three boreholes in which the glacier bed was observed. This basal ice layer was characterized by relatively sediment-rich, coarse-clear ice, and was thickest in a zone of high water-pressure fluctuations. The available evidence suggests that localized freezing at the glacier bed caused by variations in water pressure is the primary source of this basal ice layer. The near-surface transition between finegrained and coarse-bubbly ice, and between coarse-bubbly and coarse-clear ice, appeared to correlate with the depths of influence of diurnal and annual cold waves, respectively. Two types of ice foliation were identified, consisting of distinct planar coarse-clear ice, or more pervasive coarse-bubbly or fine-grained ice. An origin from crevasse closure or the transposition of sedimentary stratification provides the most likely explanation for these.

Thrusting and debris entrainment in a surging glacier: Bakaninbreen, Svalbard

Bakaninbreen, a 17 km long glacier terminating in fjord waters in central Spitsbergen (77°45′ N, 17°20′E), began to surge between the springs of 1985 and 1986. By summer 1994 the surge front had reached a position 3 km from the terminus and had almost ceased propagation. Structural investigations were undertaken to characterise the tectonic evolution of this thermally complex surge-type glacier, and the role played by thrusting and its effect on debris entrainment. Much of the glacier surface, particularly within and below the surge front, displayed transverse high-angle thrusts, defined by discrete fractures bounded by coarse clear ice. Some fractures were associated with a film of mud, whereas in others a discrete laver of diamicton, with interstitial ice several decimetres thick, was evident. Within the surge front, and genetically related to the thrusts, was a number of shear zones several metres wide. These were defined by fine-grained ice that was the product of the grinding up of crystals during shear (mylonitization). Three main sedimentary facies are associated with the thrusts: mud, gravelly mud and clast-rich muddy diamicton. The diamicton has the character of basal glacial debris: grain-size distribution ranging from clay to cobble size, clasts with a predominance in the sub-angular and sub-rounded classes, and striated and faceted clasts. Hot-water drilling through the glacier revealed several englacial layers above the surge front, and debris brought up on the drill stem suggests a basal origin. At least some of these englacial layers are probably the sub-surface continuations of the thrusts. The observed facies indicate that the glacier is moving over a soft, deformable bed and that thrusting is an important process in transferring debris to the surface, especially when the surge front is propagating down-glacier.

Thrusting and debris entrainment in a surging glacier: Bakaninbreen, Svalbard

Bakaninbreen, a 17 km long glacier terminating in fjord waters in central Spitsbergen (77°45′ N, 17°20′E), began to surge between the springs of 1985 and 1986. By summer 1994 the surge front had reached a position 3 km from the terminus and had almost ceased propagation. Structural investigations were undertaken to characterise the tectonic evolution of this thermally complex surge-type glacier, and the role played by thrusting and its effect on debris entrainment. Much of the glacier surface, particularly within and below the surge front, displayed transverse high-angle thrusts, defined by discrete fractures bounded by coarse clear ice. Some fractures were associated with a film of mud, whereas in others a discrete laver of diamicton, with interstitial ice several decimetres thick, was evident. Within the surge front, and genetically related to the thrusts, was a number of shear zones several metres wide. These were defined by fine-grained ice that was the product of the grinding up of crystals during shear (mylonitization). Three main sedimentary facies are associated with the thrusts: mud, gravelly mud and clast-rich muddy diamicton. The diamicton has the character of basal glacial debris: grain-size distribution ranging from clay to cobble size, clasts with a predominance in the sub-angular and sub-rounded classes, and striated and faceted clasts. Hot-water drilling through the glacier revealed several englacial layers above the surge front, and debris brought up on the drill stem suggests a basal origin. At least some of these englacial layers are probably the sub-surface continuations of the thrusts. The observed facies indicate that the glacier is moving over a soft, deformable bed and that thrusting is an important process in transferring debris to the surface, especially when the surge front is propagating down-glacier.

Ice fabrics from a borehole at the top of the south dome, Barnes Ice Cap, Baffin Island

Crystallographic fabrics from a borehole at the top of the south dome of Barnes Ice Cap, Baffin Island, Canada, were studied. Fabric intensity in coarse-grained ice increases with increasing depth, and the pattern changes from an initial weakly oriented one to one involving two c-axis maxima within a small circle centered on the vertical. The two maxima are aligned in a plane in the direction of maximum horizontal extension; they become distinct by about 180 m and then dominate the fabric until about 215 m; below this depth, the small-circle pattern dominates. Fine-grained ice displays similar but more variable fabric patterns, complicated by the inheritance of a vertical single maximum from the initial state. Measured surface strain rates suggest a coaxial deformation history intermediate between pure shear and uniaxial compression. Such stress configurations are known to lead to two-maximum and small-circle fabric patterns, respectively, on the basis of experiments and models involving plastic basal glide alone. With increasing strain, the angle between the two maxima, or the opening angle of the small circle, decreases in a similar manner both in the fabric diagrams for natural ice and in the fabric patterns predicted by models of plastic glide. Both the two-maximum and the small-circle patterns are considered to be equivalent to the single maximum pattern found in natural ice subjected to simple shear. In all cases, the fabric becomes well-defined at natural octahedral unit shears of about 1.0, and in all cases the fabric development is inferred to be largely controlled by basal glide.

Ice Fabrics in a Vertical Flow Plane, Barnes Ice Cap, Canada

AbstractAt a depth of about 75 m in the lower part of the accumulation area of the Barnes Ice Cap there is a change from fine-grained ice with a weakly-orientedc-axis fabric to coarser ice with a broad single-maximum fabric. At a depth of about 150 m the single maximum becomes elongate perpendicular to the direction of bubble elongation, and then splits into two distinct maxima making an angle of about 40–45° with respect to one another. At greater depths a third and finally a fourth maximum appear, forming the well known diamond pattern. Mean crystal size does not seem to increase in the transitions from one to two and thence to three maxima, but it may become more uniform. Crystal size does increase in the transition from three to four maxima, however. At the base of the glacier there is a 10–20 m thick layer of unusually-bubbly, fine-grained white ice with a strong single-maximum fabric.The depths to the transitions increase up-glacier and in place of the single-maximum fabric a small-circle pattern is found. Down-glacier the depths to the transitions decrease, systematically eliminating the higher zones. Thus in the lower part of the ablation area, ice with a four-maximum fabric appears at the surface.The independent variables governing these fabric transitions appear to be temperatureT, stressτ, and cumulative strain³oc. In a tentative stability diagram showing the fields in which given fabrics are stable inT–τ–³ocspace, multiple-maximum fabrics occur at high temperatures (&amp;gt; —10°C) and at moderate to high stresses, weakly-oriented fabrics at low stresses or low cumulative strains, broad single-maximum fabrics at moderate stresses or moderate cumulative strains, and strong single-maximum fabrics at high stresses or large cumulative strains.