2O Ice Research Articles

Collisions between CO, CO_2, H_2O and Ar ice nanoparticles compared by molecular dynamics simulation

Molecular dynamics simulations are used to study collisions between amorphous ice nanoparticles consisting of CO, CO_2, Ar and H_2O. The collisions are always sticking for the nanoparticle size (radius of 20 nm) considered. At higher collision velocities, the merged clusters show strong plastic deformation and material mixing in the collision zone. Collision-induced heating influences the collision outcome. Partial melting of the merged cluster in the collision zone contributes to energy dissipation and deformation. Considerable differences exist—even at comparable collision conditions—between the ices studied here. The number of ejecta emitted during the collision follows the trend in triple-point temperatures and increases exponentially with the NP temperature.

DIFFERENCE IN THE SPATIAL DISTRIBUTION BETWEEN H 2 O AND CO 2 ICES IN M 82 FOUND WITH AKARI

With AKARI, we obtain the spatially-resolved near-infrared (2.5 - 5.0 um) spectra for the nearby starburst galaxy M82. These spectra clearly show the absorption features due to interstellar ices. Based on the spectra, we created the column density maps of H_2O and CO_2 ices. As a result, we find that the spatial distribution of H_2O ice is significantly different from that of CO_2 ice; H_2O ice is widely distributed, while CO_2 ice is concentrated near the galactic center. Our result for the first time reveals variations in CO_2/H_2O ice abundance ratio on a galactic scale, suggesting that the ice-forming interstellar environment changes within a galaxy. We discuss the cause of the spatial variations in the ice abundance ratio, utilizing spectral information on the hydrogen recombination Br{\alpha} and Br{\beta} lines and the polycyclic aromatic hydrocarbon 3.3 um emission appearing in the AKARI near-infrared spectra.

Sublimation’s impact on temporal change of albedo dichotomy on Iapetus

Iapetus, one of the saturnian moons, has an extreme albedo contrast between the leading and trailing hemispheres. The origin of this albedo dichotomy has led to several hypotheses, however it remains controversial. To clarify the origin of the dichotomy, the key approach is to investigate the detailed distribution of the dark material. Recent studies of impact craters and surface temperature from Cassini spacecraft data implied that sublimation of H 2O ice can occur on Iapetus’ surface. This ice sublimation can change the albedo distribution on the moon with time. In this study, we evaluate the effect of ice sublimation and simulate the temporal change of surface albedo. We assume the dark material and the bright ice on the surface to be uniformly mixed with a certain volume fraction, and the initial albedo distribution to incorporate the dark material deposits on the surface. That is, the albedo at the apex is lowest and concentrically increases in a sinusoidal pattern. This situation simulates that dark materials existed around the Iapetus’ orbit billions of years ago, and the synchronously rotating Iapetus swept the material and then deposited it on its surface. The evolution of the surface albedo during 4.0 Gyr is simulated by estimating the surface temperature from the insolation energy on Iapetus including the effect of Saturn’s eccentricity and Iapetus’ obliquity precession, and evaluating the sublimation rate of H 2O ice from the Iapetus’ surface. As a result, we found that the distribution of the surface albedo changed dramatically after 4.0 Gyr of evolution. The sublimation has three important effects on the resultant surface albedo. First, the albedo in the leading hemisphere has significantly decreased to approach the minimum value. Second, the albedo distribution has been elongated along the equator. Third, the edge of the low albedo region has become clear. Considering the effect of ice sublimation, the current albedo distribution can be reconstructed from the sinusoidal albedo distribution, suggesting the apex–antapex cratering asymmetry as a candidate for the origin of the albedo dichotomy. From the model analysis, we obtained an important aspect that the depth of the turn-over layer where the darkening process proceeded for 4 Gyr should be an order of 10 cm, which is consistent with evaluation from the Cassini radar observations.

Impacts onto H 2O ice: Scaling laws for melting, vaporization, excavation, and final crater size

Shock-induced melting and vaporization of H 2O ice during planetary impact events are widespread phenomena. Here, we investigate the mass of shock-produced liquid water remaining within impact craters for the wide range of impact conditions and target properties encountered in the Solar System. Using the CTH shock physics code and the new 5-phase model equation of state for H 2O, we calculate the shock pressure field generated by an impact and fit scaling laws for melting and vaporization as a function of projectile mass, impact velocity, impact angle, initial temperature, and porosity. Melt production nearly scales with impact energy, and natural variations in impact parameters result in only a factor of two change in the predicted mass of melt. A fit to the π-scaling law for the transient cavity and transient-to-final crater diameter scaling are determined from recent simulations of the entire cratering process in ice. Combining melt production with π-scaling and the modified Maxwell Z-model for excavation, less than half of the melt is ejected during formation of the transient crater. For impact energies less than about 2 × 10 20 J and impact velocities less than about 5 km s −1, the remaining melt lines the final crater floor. However, for larger impact energies and higher impact velocities, the phenomenon of discontinuous excavation in H 2O ice concentrates the impact melt into a small plug in the center of the crater floor.

Winter and spring evolution of northern seasonal deposits on Mars from OMEGA on Mars Express

The OMEGA visible/near-infrared imaging spectrometer on Mars Express has observed the retreat of the northern seasonal deposits during Martian year 27-28 from the period of maximum extension, close to the northern winter solstice, to the end of the retreat at L s 95°. We present the temporal and spatial distributions of both CO 2 and H 2O ices and propose a scenario that describes the winter and spring evolution of the northern seasonal deposits. During winter, the CO 2-rich condensates are initially transparent and could be in slab form. A water ice annulus surrounds the sublimating CO 2 ice, extending over 6° of latitude at L s 320°, decreasing to 2° at L s 350°, and gradually increasing to 4.5° at L s 50°. This annulus first consists of thin frost as observed by the Viking Lander 2 and is then overlaid by H 2O grains trapped in the CO 2-rich ice layer and released during CO 2 sublimation. By L s 50, H 2O ice spectrally dominates most of the deposits. In order to hide the still several tens of centimeters thick CO 2 ice layer in central areas of the cap we propose the buildup of an optically thick top layer of H 2O ice from ice grains previously embedded in the CO 2 ice and by cold trapping of water vapor from the sublimating water ice annulus. The CO 2 ice signature locally reappears between L s 50 and 70. What emerges from our observations is a very active surface-atmosphere water cycle. These data provide additional constraints to the general circulation models simulating the Martian climate. Copyright 2011 by the American Geophysical Union.

Water ice in the dark dune spots of Richardson crater on Mars

Interfacial liquid water has been hypothesized to form during the seasonal evolution of the dark dune spots observed in the high latitudes of Mars. In this study we assess the presence, nature and properties of ices – in particular water ice – that occur within these spots using HIRISE and CRISM observations, as well as the LMD Global Climate Model. Our studies focus on Richardson crater (72°S, 179°E) and cover southern spring and summer ( L S =175–341°). Three units have been identified of these spots: dark core, gray ring and bright halo. Each unit show characteristic changes as the season progress. In winter, the whole area is covered by CO 2 ice with H 2O ice contamination. Dark spots form during late winter and early spring. During spring, the dark spots are located in a 10 cm thick depression compared to the surrounding bright ice-rich layer. They are spectrally characterized by weak CO 2 ice signatures that probably result from spatial mixing of CO 2 ice-rich and ice-free regions within pixels, and from mixing of surface signatures due to aerosols scattering. The bright halo shaped by winds shows stronger CO 2 absorptions than the average ice-covered terrain, which is consistent with a formation process involving CO 2 re-condensation. According to spectral, morphological and modeling considerations, the gray ring is composed of a thin layer of a few tens of μm of water ice. Two sources/processes could participate to the enrichment of water ice in the gray ring unit: (i) water ice condensation at the surface in early fall (prior to the condensation of a CO 2-rich winter layer) or during wintertime (due to cold trapping of the CO 2 layer) and (ii) ejection of dust grains surrounded by water ice by the geyser activity responsible for the dark spot. In any case, water ice remains longer in the gray ring unit after the complete sublimation of the CO 2. Finally, we also looked for liquid water in the near-IR CRISM spectra using linear unmixing modeling but found no conclusive evidence for it.

Thermoelastic properties of ice VII and its high-pressure polymorphs: Implications for dynamics of cold slab subduction in the lower mantle

Acoustic velocities in polycrystalline H 2O ice have been measured at room temperature in a pressure range 6–60 GPa by a Brillouin scattering method. Synchrotron X-ray diffraction measurements were also conducted simultaneously with the Brillouin scattering measurements in a pressure range 40–60 GPa. The obtained elastic moduli of high-pressure ice indicate that bcc-structured ice undergoes two transitions related to a change in the hydrogen bonding state at approximately 40 GPa and 58 GPa, i.e. transitions of ice VII to the pre-transitional state of ice VII at 40 GPa and to the dynamically disordered ice X at 58 GPa, respectively. This observation is consistent with previous spectroscopic studies and X-ray diffraction studies. Pressure dependencies of adiabatic elastic moduli and specific heat for ice VII and its high-pressure polymorphs were obtained from the acoustic velocity and volume data measured in this study. Isothermal compression data were obtained from previous studies. The relationships between pressure and adiabatic elastic moduli for ice VII were obtained as follows: K s = 4.0(2) + 8.51(4) × P − 0.081(2) × P 2 + 5.2(4) × 10 − 4 × P 3; μ s = 14(2) + 1.7(3) × P − 0.04(2) × P 2 + 5(3) × 10 − 4 × P 3. An empirical relationship between specific heat and pressure at room temperature was obtained for ice VII as follows: C p = 3.3(2) + 22.1(1) × e − 0.058(2) P . This result implies that the transition from ice VII to the dynamically disordered ice X is accompanied with a discontinuous change in several thermodynamic properties of ice. The elasticity difference between ice VII and the dynamically disordered ice X may affect the dynamics of cold subducting slabs in Earth's lower mantle and the interiors of icy planets. The thermoelastic properties of high-pressure polymorphs of ice obtained in this study could contribute to clarifying the dynamics and the evolution of Earth and icy planets and satellites.

Constraints on mechanisms for the growth of gully alcoves in Gasa crater, Mars, from two-dimensional stability assessments of rock slopes

The value of slope stability analyses for gaining insight into the geologic conditions that would facilitate the growth of gully alcoves on Mars is demonstrated in Gasa crater. Two-dimensional limit equilibrium methods are used in conjunction with high-resolution topography derived from stereo High Resolution Imaging Science Experiment (HiRISE) imagery. These analyses reveal three conditions that may produce observed alcove morphologies through slope failure: (1) a ca. >10 m thick surface layer that is either saturated with H 2O ground ice or contains no groundwater/ice at all, above a zone of melting H 2O ice or groundwater and under dynamic loading (i.e., seismicity), (2) a 1–10 m thick surface layer that is saturated with either melting H 2O ice or groundwater and under dynamic loading, or (3) a >100 m thick surface layer that is saturated with either melting H 2O ice or groundwater and under static loading. This finding of three plausible scenarios for slope failure demonstrates how the triggering mechanisms and characteristics of future alcove growth would be affected by prevailing environmental conditions. HiRISE images also reveal normal faults and other fractures tangential to the crowns of some gully alcoves that are interpreted to be the result of slope instability, which may facilitate future slope movement. Stability analyses show that the most failure-prone slopes in this area are found in alcoves that are adjacent to crown fractures. Accordingly, crown fractures appear to be a useful indicator of those alcoves that should be monitored for future landslide activity.

The effect of internal inhomogeneity on the activity of comet nuclei – Application to Comet 67P/Churyumov–Gerasimenko

We present thermal evolution calculations of inhomogeneous asymmetric initial configurations of a spherical model of Comet 67P/Churyumov–Gerasimenko, using a fully 3-dimensional numerical code. The initial composition is amorphous H 2O ice and dust, in a “layered-pile” configuration, where layers differing in ice/dust ratio and thermal properties extend over a fraction of the surface area and about 10 m in depth and may overlap. We analyze the effect of one such layer, as well as the combined effect of many layers, randomly distributed. We find that internal inhomogeneities affect both the surface temperature and the activity pattern of the comet. In particular, they may lead to outbursts at large heliocentric distances and also to activity on the night-side of the nucleus. The rates of ablation and depths of dust mantle and crystalline ice outer layer as functions of longitude and latitude are shown to be affected as well.

Water ice at low temperatures and pressures: New Raman results

Within the two types of experiments, both securing high sample’s purity, solid phases of water were studied in the region of low temperatures (above 8 K) and low pressure, conditions similar to the very upper atmosphere. Spectral features characterizing amorphous and crystalline H 2O and D 2O ices were recognized in both types of experiments. Careful analyses of the results discovered change of the slope in the temperature dependence of all deconvoluted bands at nearly 84 K for the first time. We attribute this observation to the ice XI ↔ I h phase transition that was previously reported only for doped, not for pure water.

Ultrafast IR spectroscopy of ice–water phase transition

Superheating and bulk melting of neat H 2O ice are studied using 2-color IR spectroscopy with sub-picosecond pulses. The data are compared with similar experiments in 15 M HDO in D 2O. After calibration of our spectroscopic thermometer a maximum superheating of the solid phase up to 330 ± 10 K is observed after ultrafast temperature jumps. For energy depositions beyond the limit of superheating a strong evidence for partial melting in two steps is observed. The ultrafast thermal melting leads to formation of water clusters in the excited ice lattice and is followed by secondary melting at the generated phase boundaries. The latter process is found to consume energy amounts in agreement with the latent heat of melting and is accompanied by an accelerated temperature and pressure decrease of the residual ice component. The bottleneck of the initial melting process seems to be the thermalization time of the hydrogen-bonded network of 7.0 ± 1 ps.

Annealing effects on hydrogen ordering in KOD-doped ice observed using neutron diffraction

We measured the neutron powder diffraction of 0.013 M KOD-doped D 2O ice to investigate the formation process of ice XI, a hydrogen-ordered phase of ice Ih. The doped ice Ih transformed to ice XI after annealing at 57 and subsequently at 68 K. The mass fraction of ice XI to that of the doped ice ( f) was estimated using Rietveld analysis for each sample. The f value of the doped ice, which had once experienced being ice XI ( f = 0.23), was larger than that of the doped ice, which had never experienced being ice XI ( f = 0.14). Results indicate that small hydrogen-ordered domains remained in the ice Ih, which had once transformed to ice XI, and accelerated the phase transition from ice Ih to ice XI. Results further suggest that large amounts of ice on icy bodies in our solar system can transform to ice XI, which might be detectable using infrared telescopes or planetary exploration in the near future.

Experimental study on the rheological properties of polycrystalline solid nitrogen and methane: Implications for tectonic processes on Triton

To investigate the evolution of any processes on planetary surfaces in the outer Solar System, the rheological properties of non-water ices were studied by means of a sound velocity measurement system and a uniaxial deformation apparatus. A pulse transmission method was used to obtain longitudinal ( V p) and transverse ( V s) wave velocities through solid nitrogen and methane at temperatures ranging from 5 to 64 K and from 5 to 90 K, respectively. The measured velocities confirmed that the solid methane and solid nitrogen samples were non-porous polycrystalline samples without any cracks and bubbles inside. Compression tests at constant strain-rate were performed for solid nitrogen and methane at temperatures of 5–56 K and 5–77 K, respectively, at strain-rates of 10 −4–10 −2 s −1. Both brittle and ductile behavior was observed for solid nitrogen and methane under these conditions. The maximum strength of solid nitrogen was observed to be 9 MPa in the brittle failure mode, and that of solid methane was 10 MPa. These low strengths cannot support cantaloupe structures with the topographic undulation larger than several kilometers found on Triton’s surface, suggesting that other materials such as H 2O ice could underlay solid methane and nitrogen and support these structures.

The ultraviolet reflectance of Enceladus: Implications for surface composition

The reflectance of Saturn’s moon Enceladus has been measured at far ultraviolet (FUV) wavelengths (115–190 nm) by Cassini’s Ultraviolet Imaging Spectrograph (UVIS). At visible and near infrared (VNIR) wavelengths Enceladus’ reflectance spectrum is very bright, consistent with a surface composed primarily of H 2O ice. At FUV wavelengths, however, Enceladus is surprisingly dark – darker than would be expected for pure water ice. Previous analyses have focused on the VNIR spectrum, comparing it to pure water ice (Cruikshank, D.P., Owen, T.C., Dalle Ore, C., Geballe, T.R., Roush, T.L., de Bergh, C., Sandford, S.A., Poulet, F., Benedix, G.K., Emery, J.P. [2005] Icarus, 175, 268–283) or pure water ice plus a small amount of NH 3 (Emery, J.P., Burr, D.M., Cruikshank, D.P., Brown, R.H., Dalton, J.B. [2005] Astron. Astrophys., 435, 353–362) or NH 3 hydrate (Verbiscer, A.J., Peterson, D.E., Skrutskie, M.F., Cushing, M., Helfenstein, P., Nelson, M.J., Smith, J.D., Wilson, J.C. [2006] Icarus, 182, 211–223). We compare Enceladus’ FUV spectrum to existing laboratory measurements of the reflectance spectra of candidate species, and to spectral models. We find that the low FUV reflectance of Enceladus can be explained by the presence of a small amount of NH 3 and a small amount of a tholin in addition to H 2O ice on the surface. The presence of these three species (H 2O, NH 3, and a tholin) appears to satisfy not only the low FUV reflectance and spectral shape, but also the middle-ultraviolet to visible wavelength brightness and spectral shape. We expect that ammonia in the Enceladus plume is transported across the surface to provide a global coating.

Shock and post-shock temperatures in an ice–quartz mixture: implications for melting during planetary impact events

Melting of H 2O ice during planetary impact events is a widespread phenomenon. On planetary surfaces, ice is often mixed with other materials; yet, at present, the partitioning of energy between the components of a shocked mixture is still an open question in the shock physics community. Knowledge of how much energy is partitioned into the ice component is necessary to predict and interpret a wide range of processes, including shock-induced melting and chemistry. In this work, we construct a conceptual framework for the thermodynamic pathways of the components in a shocked hydrodynamic mixture by defining three broad regimes based on the characteristic length scale of the mixture compared to the thickness of the shock front: (1) small length scale mixtures where pressure and temperature equilibrate immediately behind the shock front; (2) intermediate length scales where pressure but not thermal equilibration is achieved behind the shock front; and (3) long length scales where pressure equilibration requires multiple shock wave reflections. We conduct shock wave experiments, reaching pressures from 8 to 23 GPa, in an H 2O ice–SiO 2 quartz mixture in the intermediate length scale regime. In each experiment, all the parameters required to address the question of energy partitioning were determined: the shock velocity in the mixture, the shock front thickness, and the shock and post-shock temperatures of the H 2O component. The measured pressure is in agreement with the bulk compressibility of the mixture. The shock and post-shock temperatures of the H 2O component indicate that the ice was shocked close to the principal Hugoniot. Therefore, in the intermediate length scale regime, the partitioning of shock energy is defined initially by the Hugoniots of the components at the equilibrated pressure. We discuss energy partitioning in mixtures over the wide range of length and time scales encountered during planetary impact events and identify the current challenges in calculating the volume of melted ice. In some cases, the criteria for shock-induced melting of ice in a mixture are the same as for pure ice.

Enhanced CO 2 trapping in water ice via atmospheric deposition with relevance to Mars

It has been suggested that inclusions of CO 2 or CO 2 clathrate hydrates may comprise a portion of the polar deposits on Mars. Here we present results from an experimental study in which CO 2 molecules were trapped in water ice deposited from CO 2/H 2O atmospheres at temperatures relevant for the polar regions of Mars. Fourier-Transform Infrared spectroscopy was used to monitor the phase of the condensed ice, and temperature programmed desorption was used to quantify the ratio of species in the generated ice films. Our results show that when H 2O ice is deposited at 140–165 K, CO 2 is trapped in large quantities, greater than expected based on lower temperature studies in amorphous ice. The trapping occurs at pressures well below the condensation point for pure CO 2 ice, and therefore this mechanism may allow for CO 2 deposition at the poles during warmer periods. The amount of trapped CO 2 varied from 3% to 16% by mass at 160 K, depending on the substrate studied. Substrates studied were a tetrahydrofuran (C 4H 8O) base clathrate and Fe–montmorillonite clay, an analog for Mars soil. Experimental evidence indicates that the ice structures are likely CO 2 clathrate hydrates. These results have implications for the CO 2 content, overall composition, and density of the polar deposits on Mars.

Dione’s spectral and geological properties

We present a detailed analysis of the variations in spectral properties across the surface of Saturn’s satellite Dione using Cassini/VIMS data and their relationships to geological and/or morphological characteristics as seen in the Cassini/ISS images. This analysis focuses on a local region on Dione’s anti-saturnian hemisphere that was observed by VIMS with high spatial resolution during orbit 16 in October 2005. The results are incorporated into a global context provided by VIMS data acquired within Cassini’s first 50 orbits. Our results show that Dione’s surface is dominated by at least one global process. Bombardment by magnetospheric particles is consistent with the concentration of dark material and enhanced CO 2 absorption on the trailing hemisphere of Dione independent of the geology. Local regions within this terrain indicate a special kind of resurfacing that probably is related to large-scale impact process. In contrast, the enhanced ice signature on the leading side is associated with the extended ejecta of the fresh impact crater Creusa (∼49°N/76°W). Although no geologically active regions could be identified, Dione’s tectonized regions observed with high spatial resolution partly show some clean H 2O ice implying that tectonic processes could have continued into more recent times.

An experimental and theoretical description of the (NH 3) n−1{NH 3–H–H 2O} + cluster ions produced by fast ion bombardment

The (NH 3) n −1{NH 3–H–H 2O} + cluster series produced by 252Cf fission fragments (FF) impact onto a NH 3–H 2O ice target have been analyzed theoretically and experimentally. The ion desorption yields show a smooth dependence of the cluster population on its mass, with a relative higher abundance for n = 3. Results of DFT calculations show that two main series of cluster ions may be formed corresponding to the {H 3O} + and {NH 4H 2O} + cores. The theoretical analysis shows that the members of the (NH 3) n −1{NH 4–H 2O} + series are more stable than those of the (NH 3) n {H 3O} + series, the (NH 3) 2{NH 3–H–H 2O} + structure being the most stable one, in agreement with the experiments.

Fully 3-dimensional calculations of dust mantle formation for a model of Comet 67P/Churyumov–Gerasimenko

A fully 3-dimensional implicit numerical model for comet nucleus evolution is presented, emphasizing dust mantle formation. A spherical configuration is considered with an initial composition of amorphous H 2O ice and dust, taking into account a discrete dust-grain size distribution. The model is applied to Comet 67P/Churyumov–Gerasimenko, adopting its orbital elements, rotation period and rotation axis inclination. We find that the dust mantle thickness varies over the surface from 1 cm to about 10 cm (thus lower and higher than the diurnal skin-depth, respectively). The size distribution of ejected grains varies along the orbit and is steeper than the initial one adopted for the nucleus. The crystallization front advances inward in spurts, and its depth varies between 1 and several meters. We test the effect of the thermal conductivity on the surface temperature distribution and depths of the dust mantle and crystallization front.

Recent rheologic processes on dark polar dunes of Mars: Driven by interfacial water?

In springtime on HiRISE images of the Southern polar terrain of Mars flow-like or rheologic features were observed. Their dark color is interpreted as partly defrosted surface where the temperature is too high for CO 2 but low enough for H 2O ice to be present there. These branching streaks grow in size and can move by an average velocity of up to about 1 m/day and could terminate in pond-like accumulation features. The phenomenon may be the result of interfacial water driven rheologic processes. Liquid interfacial water can in the presence of water ice exist well below the melting point of bulk water, by melting in course of interfacial attractive pressure by intermolecular forces (van der Waals forces e.g.), curvature of water film surfaces, and e.g. by macroscopic weight, acting upon ice. This melting phenomenon can be described in terms of “premelting of ice”. It is a challenging consequence, that liquid interfacial water unavoidably must in form of nanometric layers be present in water ice containing soil in the subsurface of Mars. It is the aim of this paper to study possible rheologic consequences in relation to observations, which seem to happen at sites of dark polar dunes on Mars at present. The model in this work assumes that interfacial water accumulates at the bottom of a translucent water-ice layer above a dark and insolated ground. This is warmed up towards the melting point of water. The evolving layer of liquid interfacial water between the covering ice sheet and the heated ground is assumed to drive downward directed flow-like features on slopes, and it can, at least partially, infiltrate (seep) into a porous ground. There, in at least temporarily cooler subsurface layers, the infiltrated liquid water refreezes and forms ice. The related stress built-up is shown to be sufficient to cause destructive erosive processes. The above-mentioned processes may cause change in the structure and thickness of the covering ice and/or may cause the movement of dune grains. All these processes may explain the observed springtime growing and downward extension of the slope streaks analyzed here.