Ice Lattice Research Articles

Numerical simulation of microstructural damage and tensile strength of snow

This contribution uses finite‐element analysis to simulate microstructural failure processes and the tensile strength of snow. The 3‐D structure of snow was imaged by microtomography. Modeling procedures used the elastic properties of ice with bond fracture assumptions as inputs. The microstructure experiences combined tensile and compressive stresses in response to macroscopic tensile stress. The simulated nonlocalized failure of ice lattice bonds before or after reaching peak stress creates a pseudo‐plastic yield curve. This explains the occurrence of acoustic events observed in advance of global failure. The measured and simulated average tensile strengths differed by 35%, a typical range for strength measurements in snow given its low Weibull modulus. The simulation successfully explains damage, fracture nucleation, and strength according to the geometry of the microstructure of snow and the mechanical properties of ice. This novel method can be applied to more complex snow structures including the weak layers that cause avalanches.

142 Ice growth in the presence of an antifreeze protein

Antifreeze proteins (AFPs) have evolved in cold-adapted organisms to control ice crystal growth when exposed to sub-zero temperature conditions. It has been suggested that the effect of the proteins results in small ice crystal size, thus avoiding the mechanical damage to frozen tissues and cells caused by large ice grains. The polar diatom Fragilariopsis cylindrus , a dominant species within sea-ice assemblages, produces AFPs. We expressed in E. coli a recombinant form of this protein and isolated it by affinity chromatography. We studied its effect on ice grain size after shock-freezing and subsequent annealing, and under slow freezing conditions. Shock-freezing (−40 °C) produced small sized crystals, and during annealing at −4 °C AFPs successfully inhibited recrystallization already at low concentrations (1.2 μM), as observed at light microscopy and using the Otago optical recrystallometer. However, slow ice growth at −5 °C, more likely to resemble natural freezing conditions, surprisingly resulted in the formation of larger crystals in the presence of AFPs than in the negative controls. Further characteristic microstructural features, such as gradual c -axis transition within individual grains and sublimation etching patterns, were observed under crossed polarizers and at light microscopy. These features are possibly due to the incorporation of proteins into the ice lattice during growth, causing local defects. Our observations remain to be clarified, but should be taken into account when considering the biological role of AFPs as well as potential industrial applications of the proteins. Source of funding: Public funding by German AiF/IGF. Conflict of interest: None declared. maddalena.bayer@awi.de

010 The control of ice nucleation and growth in tissues and organs

Vitrification is a promising general approach to cryopreservation of clinically relevant tissues and organs that are sensitive to ice formation. However, the vitrification of larger samples requires cryoprotectant concentrations sufficiently high to avoid ice formation at relatively low cooling and warming rates. In an effort to avoid the toxicity that usually accompanies higher concentrations, certain extracellular additives have been identified that can improve vitrification tendency while minimizing toxicity, not just in solution but also in whole tissues and organs. Such additives act either through interaction with water in bulk solution (a class we call specialty cryoprotectants) or through apparently specific interactions with ice-like surfaces (a class we call ice blockers). Ice blockers act either through binding to ice nucleators (anti-nucleators) or by binding directly to ice (ice growth inhibitors, whose apparent generalized nucleation inhibition effect is a side effect of ice crystal binding). Unlike specialty cryoprotectants, ice blockers, presumably due to their specificity, tend to be active even at low concentrations (e.g., 0.1% w/v or less), and this property is strong evidence that a compound is an ice blocker rather than a specialty cryoprotectant. Antifreeze proteins are the prototypical and most well-known ice-active ice blockers, but their high cost and other potential disadvantages have stimulated interest in smaller non-protein alternatives. Certain flavonol glycosides and the polymer polyglycerol (PGL) are nucleation inhibitors that are active at low concentrations. More recently polyampholyte polymers with a 65% carboxylate mole fraction and polyglycidol block copolymers have been shown to have ice blocking activity. Polyvinyl alcohol (PVA), originally thought to be only a nucleation inhibitor, has since been shown in the literature to exhibit recrystallization inhibition, thermal hysteresis, and altered ice crystal morphology in its thermal hysteresis zone, all characteristics previously only observed for antifreeze proteins. PVA also raises the homogeneous nucleation temperature of water in emulsions, a behavior consistent with a structural match to ice at nanometer scales. Molecular dynamics simulations retrospectively showed that a minimum energy conformation of PVA can bind to the basal plane of ice, with hydrophobic methylene groups positioned to intercalate between water molecules in the basal plane. Zirconium acetate, which also alters ice crystal morphology and inhibits recrystallization, was also proposed to be active through bonding to the ice lattice. The diequatorial conformation of cis-1,3-cyclohexandiol (CHD) has also generated interest from the prospective observation that its hydroxyls are favorably positioned for binding to water molecules in the basal plane. One half molar (7.6% w/v) CHD was shown to almost double the thermal hysteresis activity of an antifreeze glycoprotein compared to no effect using 1,4-cyclohexanediol as a control, and 6% w/v CHD added to a vitrification solution also greatly slowed the growth of ice compared to 6% sucrose. However, some conventional solutes such as glycerol and propylene glycol can have similar effects, and the need for multi-percent concentrations is typical for a specialty cryoprotectant. In conclusion, ice blockers and specialty cryoprotectants are becoming increasingly indispensable and increasingly interesting additives for large-scale cryopreservation by vitrification. Source of funding: Supported by 21st Century Medicine, Inc. Conflict of interest: 21st Century Medicine is a supplier of PGL and PVA ice blockers. gfahy@21cm.com

The non-random walk of chiral magnetic charge carriers in artificial spin ice

The flow of magnetic charge carriers (dubbed magnetic monopoles) through frustrated spin ice lattices, governed simply by Coulombic forces, represents a new direction in electromagnetism. Artificial spin ice nanoarrays realise this effect at room temperature, where the magnetic charge is carried by domain walls. Control of domain wall path is one important element of utilizing this new medium. By imaging the transit of domain walls across different connected 2D honeycomb structures we contribute an important aspect which will enable that control to be realized. Although apparently equivalent paths are presented to a domain wall as it approaches a Y-shaped vertex from a bar parallel to the field, we observe a stark non-random path distribution, which we attribute to the chirality of the magnetic charges. These observations are supported by detailed statistical modelling and micromagnetic simulations. The identification of chiral control to magnetic charge path selectivity invites analogy with spintronics.

LabVIEW-operated Novel Nanoliter Osmometer for Ice Binding Protein Investigations

Ice-binding proteins (IBPs), including antifreeze proteins, ice structuring proteins, thermal hysteresis proteins, and ice recrystallization inhibition proteins, are found in cold-adapted organisms and protect them from freeze injuries by interacting with ice crystals. IBPs are found in a variety of organism, including fish1, plants2, 3, arthropods4, 5, fungi6, and bacteria7. IBPs adsorb to the surfaces of ice crystals and prevent water molecules from joining the ice lattice at the IBP adsorption location. Ice that grows on the crystal surface between the adsorbed IBPs develops a high curvature that lowers the temperature at which the ice crystals grow, a phenomenon referred to as the Gibbs-Thomson effect. This depression creates a gap (thermal hysteresis, TH) between the melting point and the nonequilibrium freezing point, within which ice growth is arrested8-10, see Figure 1. One of the main tools used in IBP research is the nanoliter osmometer, which facilitates measurements of the TH activities of IBP solutions. Nanoliter osmometers, such as the Clifton instrument (Clifton Technical Physics, Hartford, NY,) and Otago instrument (Otago Osmometers, Dunedin, New Zealand), were designed to measure the osmolarity of a solution by measuring the melting point depression of droplets with nanoliter volumes. These devices were used to measure the osmolarities of biological samples, such as tears11, and were found to be useful in IBP research. Manual control over these nanoliter osmometers limited the experimental possibilities. Temperature rate changes could not be controlled reliably, the temperature range of the Clifton instrument was limited to 4,000 mOsmol (about -7.5 °C), and temperature recordings as a function of time were not an available option for these instruments.We designed a custom-made computer-controlled nanoliter osmometer system using a LabVIEW platform (National Instruments). The cold stage, described previously9, 10, contains a metal block through which water circulates, thereby functioning as a heat sink, see Figure 2. Attached to this block are thermoelectric coolers that may be driven using a commercial temperature controller that can be controlled via LabVIEW modules, see Figure 3. Further details are provided below. The major advantage of this system is its sensitive temperature control, see Figure 4. Automated temperature control permits the coordination of a fixed temperature ramp with a video microscopy output containing additional experimental details.To study the time dependence of the TH activity, we tested a 58 kDa hyperactive IBP from the Antarctic bacterium Marinomonas primoryensis (MpIBP)12. This protein was tagged with enhanced green fluorescence proteins (eGFP) in a construct developed by Peter Davies' group (Queens University)10. We showed that the temperature change profile affected the TH activity. Excellent control over the temperature profile in these experiments significantly improved the TH measurements. The nanoliter osmometer additionally allowed us to test the recrystallization inhibition of IBPs5, 13. In general, recrystallization is a phenomenon in which large crystals grow larger at the expense of small crystals. IBPs efficiently inhibit recrystallization, even at low concentrations14, 15. We used our LabVIEW-controlled osmometer to quantitatively follow the recrystallization of ice and to enforce a constant ice fraction using simultaneous real-time video analysis of the images and temperature feedback from the sample chamber13. The real-time calculations offer additional control options during an experimental procedure. A stage for an inverted microscope was developed to accommodate temperature-controlled microfluidic devices, which will be described elsewhere16.The Cold Stage SystemThe cold stage assembly (Figure 2) consists of a set of thermoelectric coolers that cool a copper plate. Heat is removed from the stage by flowing cold water through a closed compartment under the thermoelectric coolers. A 4 mm diameter hole in the middle of the copper plate serves as a viewing window. A 1 mm diameter in-plane hole was drilled to fit the thermistor. A custom-made copper disc (7 mm in diameter) with several holes (500 μm in diameter) was placed on the copper plate and aligned with the viewing window. Air was pumped at a flow rate of 35 ml/sec and dried using Drierite (W.A. Hammond). The dry air was used to ensure a dry environment at the cooling stage. The stage was connected via a 9 pin connection outlet to a temperature controller (Model 3040 or 3150, Newport Corporation, Irvine, California, US). The temperature controller was connected via a cable to a computer GPIB-PCI card (National instruments, Austin, Texas, USA).

LabVIEW-operated Novel Nanoliter Osmometer for Ice Binding Protein Investigations

Ice-binding proteins (IBPs), including antifreeze proteins, ice structuring proteins, thermal hysteresis proteins, and ice recrystallization inhibition proteins, are found in cold-adapted organisms and protect them from freeze injuries by interacting with ice crystals. IBPs are found in a variety of organism, including fish1, plants2, 3, arthropods4, 5, fungi6, and bacteria7. IBPs adsorb to the surfaces of ice crystals and prevent water molecules from joining the ice lattice at the IBP adsorption location. Ice that grows on the crystal surface between the adsorbed IBPs develops a high curvature that lowers the temperature at which the ice crystals grow, a phenomenon referred to as the Gibbs-Thomson effect. This depression creates a gap (thermal hysteresis, TH) between the melting point and the nonequilibrium freezing point, within which ice growth is arrested8-10, see Figure 1. One of the main tools used in IBP research is the nanoliter osmometer, which facilitates measurements of the TH activities of IBP solutions. Nanoliter osmometers, such as the Clifton instrument (Clifton Technical Physics, Hartford, NY,) and Otago instrument (Otago Osmometers, Dunedin, New Zealand), were designed to measure the osmolarity of a solution by measuring the melting point depression of droplets with nanoliter volumes. These devices were used to measure the osmolarities of biological samples, such as tears11, and were found to be useful in IBP research. Manual control over these nanoliter osmometers limited the experimental possibilities. Temperature rate changes could not be controlled reliably, the temperature range of the Clifton instrument was limited to 4,000 mOsmol (about -7.5 °C), and temperature recordings as a function of time were not an available option for these instruments. We designed a custom-made computer-controlled nanoliter osmometer system using a LabVIEW platform (National Instruments). The cold stage, described previously9, 10, contains a metal block through which water circulates, thereby functioning as a heat sink, see Figure 2. Attached to this block are thermoelectric coolers that may be driven using a commercial temperature controller that can be controlled via LabVIEW modules, see Figure 3. Further details are provided below. The major advantage of this system is its sensitive temperature control, see Figure 4. Automated temperature control permits the coordination of a fixed temperature ramp with a video microscopy output containing additional experimental details. To study the time dependence of the TH activity, we tested a 58 kDa hyperactive IBP from the Antarctic bacterium Marinomonas primoryensis (MpIBP)12. This protein was tagged with enhanced green fluorescence proteins (eGFP) in a construct developed by Peter Davies' group (Queens University)10. We showed that the temperature change profile affected the TH activity. Excellent control over the temperature profile in these experiments significantly improved the TH measurements. The nanoliter osmometer additionally allowed us to test the recrystallization inhibition of IBPs5, 13. In general, recrystallization is a phenomenon in which large crystals grow larger at the expense of small crystals. IBPs efficiently inhibit recrystallization, even at low concentrations14, 15. We used our LabVIEW-controlled osmometer to quantitatively follow the recrystallization of ice and to enforce a constant ice fraction using simultaneous real-time video analysis of the images and temperature feedback from the sample chamber13. The real-time calculations offer additional control options during an experimental procedure. A stage for an inverted microscope was developed to accommodate temperature-controlled microfluidic devices, which will be described elsewhere16. The Cold Stage System The cold stage assembly (Figure 2) consists of a set of thermoelectric coolers that cool a copper plate. Heat is removed from the stage by flowing cold water through a closed compartment under the thermoelectric coolers. A 4 mm diameter hole in the middle of the copper plate serves as a viewing window. A 1 mm diameter in-plane hole was drilled to fit the thermistor. A custom-made copper disc (7 mm in diameter) with several holes (500 μm in diameter) was placed on the copper plate and aligned with the viewing window. Air was pumped at a flow rate of 35 ml/sec and dried using Drierite (W.A. Hammond). The dry air was used to ensure a dry environment at the cooling stage. The stage was connected via a 9 pin connection outlet to a temperature controller (Model 3040 or 3150, Newport Corporation, Irvine, California, US). The temperature controller was connected via a cable to a computer GPIB-PCI card (National instruments, Austin, Texas, USA).

Electrical response of ammonium-rich water ice

AbstractThe electrical properties of water ice impact the study of diverse frozen environments, in particular the radar sounding of ice masses. The high-frequency response of meteoric polar ice depends partly on the bulk concentration of ammonium (NH4+), but the nature of this response has been unclear. Here we use broadband dielectric spectroscopy to investigate the electrical response of laboratory-frozen solutions. By analyzing the relaxation frequency of these samples and its temperature dependence, we show that the mobility of Bjerrum D-defects formed in the ice lattice by ammonium is 1.4 ±0.8 x 10–9m2 V–1 s–1 at -20°C, or about an order of magnitude smaller than that of Bjerrum L-defects formed by chloride. However, co-substitution of both ions increases the ice-lattice solubility of chloride by a factor of ∼7, causing an enhanced conductivity response due to greater concentrations of Bjerrum L-defects. Thus, despite its low mobility, ammonium can also affect the high-frequency electrical response of polar ice, but its covariance with chloride must be considered.

Emergent magnetic monopoles in frustrated magnetic systems

Emergent magnetic monopoles in frustrated magnetic systems

Ice growth from supercooled aqueous solutions of reactive oxygen species

Classical molecular dynamics simulations and potential of mean force calculations were performed to study (1) the adsorption of several reactive oxygen species (ROSs, namely · OH, · HO2 and H2O2) on atmospheric air/ice interfaces and (2) the growth of ice from supercooled aqueous solutions of these ROSs. Free energy minima were observed for these ROSs at the air/ice interfaces. The presence of ROSs in supercooled water during the freezing process leads to the formation of quasi-liquid layers (QLLs) that were thicker than those formed from freezing of pure supercooled water. The ROSs in the supercooled water were always displaced to the air/ice interface during the freezing process at 270 K, but if the freezing process is carried out at 260 K, a significant fraction of hydroperoxy and hydrogen peroxide become trapped by the growing ice lattice. We also studied freezing of supercooled aqueous solutions containing ROSs and benzene, with 1-octanal at the interfaces with air. The structure of ice is not significantly altered by hydroperoxy, hydrogen peroxide, or benzene being trapped in the ice lattice. The presence of 1-octanal at the interfaces does not alter the trends described above; however, thinner QLLs are formed after the freezing process, and 1-octanal tends to slow down the dynamics of ROSs and aromatics at the air/ice interface.

Ice cloud processing of ultra-viscous/glassy aerosol particles leads to enhanced ice nucleation ability

Abstract. The ice nucleation potential of airborne glassy aqueous aerosol particles has been investigated by controlled expansion cooling cycles in the AIDA aerosol and cloud chamber of the Karlsruhe Institute of Technology at temperatures between 247 and 216 K. Four different solutes were used as proxies for oxygenated organic matter found in the atmosphere: raffinose, 4-hydroxy-3-methoxy-DL-mandelic acid (HMMA), levoglucosan, and a multi-component mixture of raffinose with five dicarboxylic acids and ammonium sulphate. Similar to previous experiments with citric acid aerosols, all particles were found to nucleate ice heterogeneously before reaching the homogeneous freezing threshold provided that the freezing cycles were started well below the respective glass transition temperatures of the compounds; this is discussed in detail in a separate article. In this contribution, we identify a further mechanism by which glassy aerosols can promote ice nucleation below the homogeneous freezing limit. If the glassy aerosol particles are probed in freezing cycles started only a few degrees below their respective glass transition temperatures, they enter the liquid regime of the state diagram upon increasing relative humidity (moisture-induced glass-to-liquid transition) before being able to act as heterogeneous ice nuclei. Ice formation then only occurs by homogeneous freezing at elevated supersaturation levels. When ice forms the remaining solution freeze concentrates and re-vitrifies. If these ice cloud processed glassy aerosol particles are then probed in a second freezing cycle at the same temperature, they catalyse ice formation at a supersaturation threshold between 5 and 30% with respect to ice. By analogy with the enhanced ice nucleation ability of insoluble ice nuclei like mineral dusts after they nucleate ice once, we refer to this phenomenon as pre-activation. We propose a number of possible explanations for why glassy aerosol particles that have re-vitrified in contact with the ice crystals during the preceding homogeneous freezing cycle exhibit pre-activation: they may retain small ice embryos in pores, have footprints on their surface which match the ice lattice, or simply have a much greater surface area or different surface microstructure compared to the unprocessed glassy aerosol particles. Pre-activation must be considered for the correct interpretation of experimental results on the heterogeneous ice nucleation ability of glassy aerosol particles and may provide a mechanism of producing a population of extremely efficient ice nuclei in the upper troposphere.

Emission of Ag2 dimers from a substrate during vacuum deposition of the mixture of silver and water vapor

The work presents calculation of Ag2 dimers emission from the substrate into the vapor medium in case of joint deposition of silver and water vapors on ideal substrate formally modeling the crystal of water ice in terms of energy properties. It is assumed that the dimers are formed on the condensation surface as a result of random collisions of atoms at their surface migration, and the dimers emission is conditioned by thermal fluctuations of crystal lattice of water ice. The calculations based on the modified Langmuir adsorption model allowed concluding that emission of silver and water dimers takes place in the entire range of the studied water vapor pressure, binding energy of silver-water, and crystal temperature. Dynamics of emission from the beginning of deposition and dependence of dimers emission on micro-roughness of the condensate surface have been investigated. Statistical processing of results has shown that the probability of dimers emission from the condensate surface is determined not only by the value of the binding energy between the dimer and condensate but by configuration of the nearest dimer environment on the condensation surface. It has been found that there is a certain value of micro-roughness of condensation surface providing the maximal intensity of dimer emission. Dimers emission from the surfaces bordering on the flows of vapor mixtures contaminates the flows with unsuspected admixtures. The latter one makes investigation of this phenomenon important for aeromechanics of vapor and gas mixtures.

Ice Growth from Supercooled Aqueous Solutions of Benzene, Naphthalene, and Phenanthrene

Classical molecular dynamics (MD) were performed to investigate the growth of ice from supercooled aqueous solutions of benzene, naphthalene, or phenanthrene. The main objective of this study is to explore the fate of those aromatic molecules after freezing of the supercooled aqueous solutions, i.e., if these molecules become trapped inside the ice lattice or if they are displaced to the QLL or to the interface with air. Ice growth from supercooled aqueous solutions of benzene, naphthalene, or phenanthrene result in the formation of quasi-liquid layers (QLLs) at the air/ice interface that are thicker than those observed when pure supercooled water freezes. Naphthalene and phenanthrene molecules in the supercooled aqueous solutions are displaced to the air/ice interface during the freezing process at both 270 and 260 K; no incorporation of these aromatics into the ice lattice is observed throughout the freezing process. Similar trends were observed during freezing of supercooled aqueous solutions of benzene at 270 K. In contrast, a fraction of the benzene molecules become trapped inside the ice lattice during the freezing process at 260 K, with the rest of the benzene molecules being displaced to the air/ice interface. These results suggest that the size of the aromatic molecule in the supercooled aqueous solution is an important parameter in determining whether these molecules become trapped inside the ice crystals. Finally, we also report potential of mean force (PMF) calculations aimed at studying the adsorption of gas-phase benzene and phenanthrene on atmospheric air/ice interfaces. Our PMF calculations indicate the presence of deep free energy minima for both benzene and phenanthrene at the air/ice interface, with these molecules adopting a flat orientation at the air/ice interface.

Structure of water layers on hydrogen-covered Pt electrodes

The structure of water layers above hydrogen-covered Pt(111) surfaces at room temperature has been studied by ab initio molecular dynamics simulations based on periodic density functional theory calculations. Fully hydrogen-covered Pt(111) with additionally either a hydrogen vacancy or another hydrogen adatom have been considered. The resulting structures have been analyzed in detail as a function of the hydrogen coverage. In particular, the thermal disorder in the water layer is examined in terms of deviations from the ice lattice, orientational inhomogeneity within a water bilayer, as well as the onset of proton transfer. On hydrogen-covered Pt(111), the water layer is located at a much larger distance from the Pt atoms than on the pure metal surfaces. Surprisingly, the more weakly bound water layer on the hydrogen-covered Pt(111) electrode exhibits a greater order than the water layer on clean Pt(111) which is attributed to the stronger water–water interaction above hydrogen-covered Pt(111). The relevance of our findings for the understanding of electrochemical electrode/water interfaces is discussed.

Magnetic interactions and reversal of artificial square spin ices

Artificial spin ices are nanoscale geometrically engineered systems that mimic the behavior of bulk spin ices at room temperature. We describe the nanoscale magnetic interactions in a square spin ice lattice by an experimentally verified model that accounts for the correct shape of the magnetic islands. Magnetic force microscopy measurements on lithographically fabricated lattices are compared to Monte Carlo simulations of the reversal process of two lattices with different lattice spacings. Lattice node statistics and correlations show significant differences in the reversal mechanism for lattices with different spacings. The effect of structural variations is also compared for the two lattice reversals.

Adsorbed water and thin liquid films on Mars

AbstractAt present, bulk liquid water on the surface and near-subsurface of Mars does not exist due to the scarcity of condensed- and gas-phase water, pressure and temperature constraints. Given that the nuclei of soil and ice, that is, the soil solid and ice lattice, respectively, are coated with adsorbed and/or thin liquid films of water well below 273 K and the availability of water limits biological activity, we quantify lower and upper limits for the thickness of such adsorbed/water films on the surface of the Martian regolith and for subsurface ice. These limits were calculated based on experimental and theoretical data for pure water ice and water ice containing impurities, where water ice containing impurities exhibit thin liquid film enhancements, ranging from 3 to 90. Close to the cold limit of water stability (i.e. 273 K), thin liquid film thicknesses at the surface of the Martian regolith is 0.06 nm (pure water ice) and ranges from 0.2 to 5 nm (water ice with impurities). An adsorbed water layer of 0.06 nm implies a dessicated surface as the thickness of one monolayer of water is 0.3 nm but represents 0.001–0.02% of the Martian atmospheric water vapour inventory. Taking into account the specific surface area (SSA) of surface-soil (i.e. top 1 mm of regolith and 0.06 nm adsorbed water layer), shows Martian surface-soil may contain interfacial water that represents 6–66% of the upper- and lower-limit atmospheric water vapour inventory and almost four times and 33%, the lower- and upper-limit Martian atmospheric water vapour inventory. Similarly, taking the SSA of Martian soil, the top 1 mm or regolith at 5 nm thin liquid water thickness, yields 1.10×1013and 6.50×1013litres of waters, respectively, 55–325 times larger than Mars’ atmospheric water vapour inventory. Film thicknesses of 0.2 and 5 nm represent 2.3×104–1.5×106litres of water, which is 6.0×10−7–4.0×10−4%, respectively, of a 10 prμm water vapour column, and 3.0×10−6–4.0×10−4% and 6.0×10−6–8.0×10−4%, respectively, of the Martian atmospheric water vapour inventory. Thin liquid film thicknesses on/in subsurface ice were investigated via two scenarios: (i) under the idealistic case where it is assumed that the diurnal thermal wave is equal to the temperature of ice tens of centimetres below the surface, allowing for such ice to experience temperatures close to 273 K and (ii) under the, likely, realistic scenario where the diurnal thermal wave allows for the maximum subsurface ice temperature of 235 K at 1 m depth between 30°N and 30°S. Scenario 1 yields thin liquid film thicknesses ranging from 11 to 90 nm; these amounts represent 4×106–3.0×107litres of water. For pure water ice, Scenario 2 reveals that the thickness of thin liquid films contained on/within Martian subsurface is less than 1.2 nm, several molecular layers thick. Conversely, via the effect of impurities at 235 K allows for a thin liquid film thickness on/within subsurface ice of 0.5 nm, corresponding to 6.0×104litres of water. The existence of thin films on Mars is supported by data from the Mars Exploration Rovers (MERs) Spirit and Opportunity's Alpha Proton X-ray Spectrometer instrumentation, which have detected increased levels of bromine beneath the immediate surface, suggestive of the mobilization of soluble salts by thin films of liquid water towards local cold traps. These findings show that biological activity on the Martian surface and subsurface is not limited by nanometre dimensions of available water.

Non-hexagonal ice at hexagonal surfaces: the role of lattice mismatch

It has long been known that ice nucleation usually proceeds heterogeneously on the surface of a foreign body. However, little is known at the microscopic level about which properties of a material determine its effectiveness at nucleating ice. This work focuses on the long standing, conceptually simple, view on the role of a good crystallographic match between bulk ice and the underlying substrate. We use grand canonical Monte Carlo to generate the first overlayer of water at the surface and find that the traditional view of heterogeneous nucleation does not adequately account for the array of structures that water may form at the surface. We find that, in order to describe the structures formed, a good match between the substrate and the nearest neighbour oxygen-oxygen distance is a better descriptor than a good match to the bulk ice lattice constant.

Controls on the movement and composition of firn air at the West Antarctic Ice Sheet Divide

Abstract. We sampled interstitial air from the perennial snowpack (firn) at a site near the West Antarctic Ice Sheet Divide (WAIS-D) and analyzed the air samples for a wide variety of gas species and their isotopes. We find limited convective influence (1.4–5.2 m, depending on detection method) in the shallow firn, gravitational enrichment of heavy species throughout the diffusive column in general agreement with theoretical expectations, a ~10 m thick lock-in zone beginning at ~67 m, and a total firn thickness consistent with predictions of Kaspers et al. (2004). Our modeling work shows that the air has an age spread (spectral width) of 4.8 yr for CO2 at the firn-ice transition. We also find that advection of firn air due to the 22 cm yr−1 ice-equivalent accumulation rate has a minor impact on firn air composition, causing changes that are comparable to other modeling uncertainties and intrinsic sample variability. Furthermore, estimates of Δage (the gas age/ice age difference) at WAIS-D appear to be largely unaffected by bubble closure above the lock-in zone. Within the lock-in zone, small gas species and their isotopes show evidence of size-dependent fractionation due to permeation through the ice lattice with a size threshold of 0.36 nm, as at other sites. We also see an unequivocal and unprecedented signal of oxygen isotope fractionation within the lock-in zone, which we interpret as the mass-dependent expression of a size-dependent fractionation process.

H2O and D2 mixtures under pressure: Spectroscopy and proton exchange kinetics

We have investigated the pressure-induced spectral changes and the proton exchange reactions of D(2)-H(2)O mixtures to 64 GPa using micro-Raman spectroscopy. The results show the profound difference in the rotational and vibrational Raman spectra of hydrogen isotopes from those of the pure samples, showing the vibrational modes at higher frequencies and continuing to increase with pressure without apparent turnover. This indicates the repulsive nature of D(2)-H(2)O interaction without hydrogen bonds between the two and, thus, interstitial fillings of D(2) molecules into the bcc-like ice lattice. The spectral analysis using the Morse potential yields a hydrogen bond distance of 0.734 Å at 6 GPa--slightly shorter than that in pure--attributed to the repulsive interaction. The pressure-dependent spectral changes suggest that the proton-ordering transition in the ice lattice occurs over a large pressure range between 28 and 50 GPa, which is substantially lower than that of pure ice (40-80 GPa). This again indicates the presence of high internal pressure arising from the repulsive interaction. The Raman spectra show evidences that the proton exchange occurs in various phases including in solid D(2) and H(2)O mixtures. Based on the time-dependent spectral changes, we obtained the proton exchange rates of k ~ 0.085 h(-1) at 0.2 GPa in fluid D(2) and water mixtures, k ~ 0.03 h(-1) and 0.003 h(-1) at 2 GPa and 4 GPa, respectively, in fluid D(2)-ice mixtures, and k ~ 10(-3) h(-1) at 8 GPa in solid D(2) and ice mixtures.

Reply to the comment on “Pressure-induced phase transition of ice in aqueous KOH solution”

The conclusions that have been drawn in the previous papers [1–4] regarding the pressure-induced phase transition of ice in aqueous electrolyte salt solutions were drawn on the basis of the compressions of quick pre-cooled samples. I suppose that the main difference in the results between Yoshimura and Handle et al. lies in the (initial) cooling rate of the sample, i.e. path (1) in Figure 1 in the original paper [3]. Supporting evidence for this is that if I cooled the sample solution at a slower cooling rate (e.g. 5 K/min), I found that the samples show a “normal” transition behavior (i.e. Ih-HDA-VII) at the reported pressures, as stated in the previous papers [2–4]. This is probably because the cooling rate should influence the morphology, the dimension and the distribution of ice crystals. In actual fact, methods reported for the rapid quenching of aqueous solutions were developed for the purpose of achieving small ice crystals [5]. The transition of the ice crystal size from macroscopic to nanometer dimensions occurs discontinuously at a critical cooling rate (and also over a very limited temperature range) [6]. I propose that most of the salt segregates at grain boundaries, and such segregation would lead to very small particles of ice in the sample and reduce the size of the ice crystals. It is possible that, this effect could cause the unexpected transition. In the measurements, I used a specially designed cooling apparatus (flow-type liquid N2 cryostat) as shown in Figure 1. First, the whole of a small-size diamond anvil cell (DAC) (SRDAC-KYO-3-1, Kyowa Co. Ltd; screw-type diamond anvil cell, o.d. = 25 mm, height = 55 mm) with the sample solution was directly immersed into liquid nitrogen. In doing so, the overall cooling rate of ∼150 K/min was achieved. The samples were milky in appearance either because of the inclusions of salts or because of the concentrated salt solution. In this procedure, small transient clusters (seeds) may exist in the sample before the transition upon compression. On the other hand, generally if we cool the sample slowly, the solutes are largely rejected into the liquid and do not make the solid solution upon freezing of aqueous solutions. It should be, however, noted that Klotz et al. [7] recently reported that it is possible for salts to enter the ice lattice structure and form the salty ice under high pressure at lower temperatures. Then, the sample was compressed isothermally keeping liquid nitrogen temperature during the in situ Raman measurements

Exclusion of Nitrate to the Air–Ice Interface During Freezing

During freezing, the majority of solutes are rejected from the growing ice lattice and are concentrated at grain boundaries or nodes within the ice crystal or at the ice crystal surface itself. The degree of solute enrichment as well as the location of the rejected solutes has important consequences for reactions occurring in or on frozen media. We have used glancing-angle Raman spectroscopy to probe the exclusion of nitrate to the air–ice interface during freezing. This work represents the first use of this technique to measure solutes at the ice surface. Our results show that nitrate is excluded to the ice surface but not to the extent predicted by equilibrium thermodynamics. These findings have important implications for understanding the mechanism of snowpack nitrate photolysis.