Pure H2O Ice Research Articles

A comparison of ice VII formed in the H2O, NaCl–H2O, and CH3OH–H2O systems: Implications for H2O-rich planets

High-pressure H2O polymorphs, namely ice VI, ice VII, and ice X, are hypothesized to make up a considerable portion of the interiors of large icy satellites and select extra-solar planets. The incorporation of foreign ions or molecules into these high-pressure phases is possible through ocean–ice interaction, rock–ice interaction at depth, or processes that occurred during accretion. Recent research concerning the effects charged ions have on ice VII has shown that these ions notably affect the structure of ice VII (Frank et al., 2006; Klotz et al., 2009). This study was designed to determine the effects of a molecular impurity on ice VII and compare those effects to both pure H2O ice and ice with an ionic impurity. Ice samples were formed in this study via compression in a diamond anvil cell from either H2O, a 1.60mol% NaCl aqueous solution, a 1.60mol% CH3OH aqueous solution, or a 5.00mol% CH3OH aqueous solution and were compressed up to 71GPa at room temperature. Ice formed from pure H2O had no impurities whereas ices formed in the NaCl–H2O and CH3OH–H2O systems contained the impurities Na+ and Cl− and CH3OH, respectively. Pressure–volume relations were observed in situ by using synchrotron based X-ray diffraction and were used to determine the equations of state for ices formed in the H2O, NaCl–H2O and CH3OH–H2O systems. The data illustrate that ice VII formed from a NaCl-bearing aqueous solution exhibited a depressed volume when compared to that of H2O-only ice VII at any given pressure, whereas ice VII formed from CH3OH-bearing aqueous solutions showed an opposite trend, with an increase in volume relative to that of pure ice VII. The ices within planetary bodies will most likely have both ionic and molecular impurities and the trends outlined in this study can be used to improve density profiles of H2O-rich planetary bodies.

Observations of the northern seasonal polar cap on Mars III: CRISM/HiRISE observations of spring sublimation

We analyze a series of targeted CRISM and HiRISE observations of seven regions of interest at high latitudes in the Northern polar regions of Mars. These data allow us to investigate the temporal evolution of the composition of the seasonal ice cap during spring, with a special emphasis on peculiar phenomena occurring in the dune fields and in the vicinity of the scarps of the North Polar Layered Deposits (NPLDs). The strength of the spectral signature of CO2 ice continuously decreases during spring whereas the one of H2O ice first shows a strong increase until Ls=50°. This evolution is consistent with a scenario previously established from analysis of OMEGA data, in which a thin layer of pure H2O ice progressively develops at the surface of the volatile layer. During early spring (Ls<10°), widespread jet activity is observed by HiRISE while strong spectral signatures of CO2 ice are detected by CRISM. Later, around Ls=20–40°, activity concentrates at the dune fields where CRISM also detects a spectral enrichment in CO2 ice, consistent with “Kieffer’s model” (Kieffer, H.H. [2007]. J. Geophys. Res. 112, E08005. doi:10.1029/2006JE002816) for jet activity. Effects of wind are prominent across the dune fields and seem to strongly influence the sublimation of the volatile layer. Strong winds blowing down the scarps could also be responsible for the significant spatial and temporal variability of the surface ice composition observed close to the NPLD.

Dust release and tensile strength of the non-volatile layer of cometary nuclei

We developed a thermophysical model for cometary nuclei, which is based upon the assumption that comets form by the gravitational instability of an ensemble of dust and ice aggregates. Under this condition, the tensile strength of the ice-free outer layers of a cometary nucleus can be calculated, using the dust–aggregate collision and adhesion model of Weidling et al. (Weidling, R. et al. [2012]. Icarus, 218, 688-700). Based on available laboratory data on the gas permeability and thermal conductivity of ice-free dust layers, we derived the temperature and pressure at the dust–ice interface for pure water and pure carbon dioxide ice. Comparison of the vapor pressure below the dust crust with its tensile strength allows the prediction of dust release from cometary surfaces. Our model predicts dust activity for pure CO2 ice and for heliocentric distances of ≲3AU, whereas pure H2O ice cannot explain the dust emission.

Photo-desorbed species produced by the UV/EUV irradiation of an H2O:CO2:NH3 ice mixture

An H2O:CO2:NH3=1:1:1 ice mixture, used as a model mixture for cometary and interstellar ices, was irradiated with ultraviolet (UV)/extreme ultraviolet (EUV) photons in the broad 4–20eV (62–310nm) energy range at 16K. The desorbed species were detected in situ by mass spectrometry during photo-irradiation, and a quartz microbalance was used as a substrate to measure the mass of material remaining on the surface. The total mass desorption for this H2O:CO2:NH3=1:1:1 ice mixture at 16K was measured to be 1.8×10−18μgphoton−1, which is comparable to the 1.5×10−18μgphoton−1 measured for pure H2O ice irradiated under the same conditions. The main desorbed species produced during the photolysis of the ices were H2, NH2•, OH•, CO, and O2, along with the starting components H2O, NH3, and CO2. We also tentatively assigned minor mass peaks to larger species such as OCN•/OCN−, HNCO, CH4, H2CO, CH3OH, and HCOOH. This result supports the scenario in which complex organic molecules can be formed in cometary and/or astrophysical ices and desorbed to the gas phase, and helps to better understand the photochemical processes occurring at the surface of Solar System icy bodies such as comets, as well as in cold astrophysical environments such as star-forming regions and protostars.

Mechanism for the abiotic synthesis of uracil via UV-induced oxidation of pyrimidine in pure H2O ices under astrophysical conditions

The UV photoirradiation of pyrimidine in pure H(2)O ices has been explored using second-order Moller-Plesset perturbation theory and density functional theory methods, and compared with experimental results. Mechanisms studied include those starting with neutral pyrimidine or cationic pyrimidine radicals, and reacting with OH radical. The ab initio calculations reveal that the formation of some key species, including the nucleobase uracil, is energetically favored over others. The presence of one or several water molecules is necessary in order to abstract a proton which leads to the final products. Formation of many of the photoproducts in UV-irradiated H(2)O:pyrimidine=20:1 ice mixtures was established in a previous experimental study. Among all the products, uracil is predicted by quantum chemical calculations to be the most favored, and has been identified in experimental samples by two independent chromatography techniques. The results of the present study strongly support the scenario in which prebiotic molecules, such as the nucleobase uracil, can be formed under abiotic processes in astrophysically relevant environments, namely in condensed phase on the surface of icy, cold grains before being delivered to the telluric planets, like Earth.

Photochemistry of carbon monoxide and methanol in water and nitric acid hydrate ices: A NEXAFS study

Soft X-ray induced chemistry of H(2)O, CO and CH(3)OH and the effects of the water and nitric acid hydrate (HNO(3).1.65H(2)O) matrix on the photochemistry of CO and CH(3)OH have been investigated using NEXAFS spectroscopy. For pure H(2)O, CO and CH(3)OH ices, we show that the destruction rates are strongly limited by back reactions, leading to strikingly high survival rates of these molecules upon the harsh irradiation conditions to which they are submitted. We also evidence the interplay between the photochemical reactions of CO and CH(3)OH and those of the matrix. The OH and O radicals released by the photolysis of H(2)O and HNO(3) react with the CO and CH(3)OH and their fragments, considerably reducing the survival rates compared to pure CO and pure CH(3)OH ices, especially in presence of nitric acid, and dramatically enhancing the formation of CO(2) at the expense of CO. Because NEXAFS spectroscopy allows identifying which reactions are important among those possible, it emerges a simple picture of the photochemical routes of CO and CH(3)OH in the H(2)O and HNO(3)/H(2)O environments.

The Dark Red Spot on KBO Haumea

AbstractKuiper belt object 136108 Haumea is one of the most fascinating bodies in our solar system. Approximately 2000 × 1600 × 1000 km in size, it is one of the largest Kuiper belt objects (KBOs) and an unusually elongated one for its size. The shape of Haumea is the result of rotational deformation due to its extremely short 3.9-hour rotation period. Unlike other 1000 km-scale KBOs which are coated in methane ice the surface of Haumea is covered in almost pure H2O-ice. The bulk density of Haumea, estimated around 2.6 g cm−3, suggests a more rocky interior composition, different from the H2O-ice surface. Recently, Haumea has become the second KBO after Pluto to show observable signs of surface features. A region darker and redder than the average surface of Haumea has been identified, the composition and origin of which remain unknown. I discuss this recent finding and what it may tell us about Haumea.

IR Spectroscopic Testing of Surfaces in Water Ice and in Icy Mixtures with Prussic Acid or Ammonia

An experimental technique is developed for spectral studies of ice films deposited at 77 K onto the ZnSe inner windows of a cell, so that the spectra could be registered in the presence of gaseous adsorbate. Pure H2O or D2O ice as well as HCN/D2O and ND3/D2O mixed icy films with different dopant/water ratios (1:10, 1:5, and 1:1) were investigated. The surface area of ice deposited at 77 K H2O estimated from adsorption measurements was about 160 m2/g. Bands of dangling hydroxyl groups disappear on raising the temperature up to 130-160 K when the changes of bulk absorption provide evidence for a phase transition from amorphous to polycrystalline ice. Surface properties of icy films were characterized by low-temperature adsorption of CO and CHF3. Insertion of small doses of HCN or ammonia does not change the acid/base strength of dangling hydroxyl groups or coordinately unsaturated surface oxygen atoms, but it changes the proportion between the concentrations of these sites as compared with pure water ice. For high dopant concentrations, the dangling hydroxyls were not observed, and the dominant adsorption sites for CO are likely to be the unsaturated oxygen atoms, while serious structural changes occur in the bulk of ices.

Effects of CO2 on H2O band profiles and band strengths in mixed H2O:CO2 ices

H2O is the most abundant component of astrophysical ices. In most lines of sight it is not possible to fit both the H2O 3 um stretching, the 6 um bending and the 13 um libration band intensities with a single pure H2O spectrum. Recent Spitzer observations have revealed CO2 ice in high abundances and it has been suggested that CO2 mixed into H2O ice can affect relative strengths of the 3 um and 6 um bands. We used laboratory infrared transmission spectroscopy of H2O:CO2 ice mixtures to investigate the effects of CO2 on H2O ice spectral features at 15-135 K. We find that the H2O peak profiles and band strengths are significantly different in H2O:CO2 ice mixtures compared to pure H2O ice. In all H2O:CO2 mixtures, a strong free-OH stretching band appears around 2.73 um, which can be used to put an upper limit on the CO2 concentration in the H2O ice. The H2O bending mode profile also changes drastically with CO2 concentration; the broad pure H2O band gives way to two narrow bands as the CO2 concentration is increased. This makes it crucial to constrain the environment of H2O ice to enable correct assignments of other species contributing to the interstellar 6 um absorption band. The amount of CO2 present in the H2O ice of B5:IRS1 is estimated by simultaneously comparing the H2O stretching and bending regions and the CO2 bending mode to laboratory spectra of H2O, CO2, H2O:CO2 and HCOOH.

Diffusion of HDO in Pure and Acid-Doped Ice Films

In these experiments, a few bilayers of D(2)O were vapor-deposited on a pure crystalline H(2)O ice film or an ice film doped with a small amount of HCl. Upon deposition, H/D isotopic exchange quickly converted the D(2)O layer into an HDO-rich mixture layer. Infrared absorption spectroscopy followed the changes of the HDO from the initial HDO mixture layer to HDO isolated in the H(2)O ice film. This was possible because isolated HDO in H(2)O ice has a unique, sharp peak in the O-D stretch region that can be distinguished from the broad peak due to the initial HDO mixture layer. The absorbance of isolated HDO displayed first-order kinetics and was attributed to diffusion of HDO from the HDO-rich mixture layer into the underlying H(2)O ice film. While negligible diffusion was observed for pure ice films and for ice films with HCl concentrations up to 1 x 10(-4) mole fraction, diffusion of HDO occurred for higher concentrations of (2-20) x 10(-4) mole fraction HCl with a concentration-independent rate constant. The diffusion under these conditions followed Arrhenius behavior for T = 135-145 K yielding E(a) = 25 +/- 5 kJ/mol. The mechanism for the HDO diffusion involves either (i) molecular self-diffusion or (ii) long-range H/D diffusion by a series of multiple proton hop and orientational turn steps. While these spectroscopic results compare favorably with recent studies of molecular self-diffusion in low-temperature ice films, the diffusion results from all the ice film studies at low temperatures (ca. T < 170 K) differ from earlier bulk ice studies at higher temperatures (ca. T > 220 K). A comparison and discussion of the various diffusion studies are included in this report.

Phase equilibrium in the system CO2‐H2O: Application to Mars

Thermodynamic analysis of the system CO2‐H2O shows that although CO2clathrate and liquid CO2are stable over most of the pressure and temperature range expected for the Martian regolith, hydraulic equilibrium of the regolith pore gas with the overlying atmosphere, a condition most likely in equatorial regions, will destabilize both phases. Condensation on Mars comes in four predictable assemblages: pure H2O ice, CO2clathrate (hydrate), a eutectic mixture of solid CO2and either clathrate or H2O ice (if the clathrate fails to nucleate), and under limited conditions, pure solid CO2. Seasonal frosts are likely to consist of thin layers of water ice and/or clathrate, overlain by the eutectic assemblage. Growing ice caps are also likely to be layered, with sets of ice (lower) and eutectic (upper) layers interspersed with pure solid CO2layers. The saturated water content of a 1‐bar CO2atmosphere with surface temperatures &gt;273 K would have reached 20 precipitable millimeters. Basal melting, triggered by trapped heat flow from the interior, limits the thickness and composition of the polar ice caps and is to be expected during periods of low obliquity, when there is extensive cooling and condensation at the poles. Introduction of liquid CO2into the Martian regolith by basal melting at the poles may be a major mechanism for loss of the ancient greenhouse atmosphere and its long‐term sequestration. Explosive release of CO2fluid coexisting with groundwater may trigger outflow channel floods and reset atmospheric isotope ratios.

Sp i t zer Mid-Infrared Spectroscopy of Ices toward Extincted Background Stars

A powerful way to directly observe the solid-state inventory of dense molecular clouds is by infrared spectroscopy of background stars. We present Spitzer IRS 5-20 μm spectra of ices toward stars behind the Serpens and Taurus molecular clouds, probing visual extinctions of 10-34 mag. These data provide the first complete inventory of solid-state material in dense clouds before star formation begins. The spectra show prominent 6.0 and 6.85 μm bands. In contrast to some young stellar objects (YSOs), most (~75%) of the 6.0 μm band is explained by the bending mode of pure H2O ice. In realistic mixtures this number increases to 85%, because the peak strength of the H2O bending mode is very sensitive to the molecular environment. The strength of the 6.85 μm band is comparable to what is observed toward YSOs. Thus, the production of the carrier of this band does not depend on the energetic input of a nearby source. The spectra show large abundances of CO and CO2 (20%-40% with respect to H2O ice). Compared with YSOs, the band profile of the 15 μm CO2 bending mode lacks the signatures of crystallization, confirming the cold, pristine nature of these lines of sight. After the dominant species are removed, there are residuals that suggest the presence of minor species such as HCOOH and possibly NH3. Clearly, models of star formation should begin with dust models already coated with a fairly complex mixture of ices.

Pure ices IV and XII from high-density amorphous ice

High-density amorphous ice (HDA), made by compression of hexagonal ice at 77 K, was heated at a constant pressure of 0.81 GPa up to 183 K and its phase transition followed by displacement-temperature curves. The crystalline phases, recovered at 77 K and 1 bar, were characterized by X-ray diffraction. Pure D2O ice IV and nearly pure H2O ice IV were formed on slow heating at a rate of 0.4 K min–1, whereas pure H2O ice XII and D2O ice XII were formed on fast heating at [Formula: see text] 15 K min–1. On heating HDA at rates in between these two values a mixture of ice IV and ice XII was obtained, where their relative yields depended in a systematic manner on the heating rate. Conversion of HDA into either ice IV or ice XII is an example of a "parallel reaction" where the relative yields of ice IV and ice XII can be controlled by temperature, i.e., by the rate of heating, in our approach. It is conceivable that a similar behaviour occurs on crystallization of the related pressure-amorphized silica. PACS Nos.: 61.10-i, 64.60-i, 64.70-p

The circumstellar dust shell of the post-AGB star HD 161796

We have modeled the complete optical to millimeter spectrum of the Post-Asymptotic Giant Branch (Post-AGB) star HD 161796 and its circumstellar dust shell. A full 2{200 m spectrum taken with the Infrared Space Observatory was used to constrain the dust properties. A good t is achieved using only 4 dust components: amorphous silicates, the crystalline silicates forsterite and enstatite, and crystalline water ice, contributing re- spectively about 63, 4, 6 and 27% to the total dust mass. The dierent dust species were assumed to be co-spatial but distinct, resulting in dierent temperatures for the dierent grain populations. We nd a temperature for the crystalline H2O ice of 70 K, which is higher than thermal equilibrium calculations of pure H2 Oi ce would give. This implies that the ice must be formed as a mantle on top of an (amorphous) silicate core. In order to form H2O ice mantles the mass loss rate must exceed some 5 10 5 M yr 1 . With a water-ice fraction of 27% a lower limit for the gas to dust mass ratio of 270 is found. At a distance of 1.2 kpc (Skinner et al. 1994) and adopting an outflow velocity of 15 km s 1 (Likkel et al. 1991) an AGB mass loss rate of (5:1 10 4 M yr 1 ) is found, which lasted 900 years and ended 430 years ago. During this phase a total of 0.46 M was expelled. The mass loss rate was high enough to account for the presence of the H2 Oi ce.

Pure Ice IV from High-Density Amorphous Ice

High-density amorphous ice (HDA), made by compression of hexagonal ice at 77 K, was heated at a constant pressure of 0.81 GPa up to 195 K and its phase transition followed by displacement-temperature curves. The crystalline phases recovered at 77 K and 1 bar were characterized by X-ray diffraction. Pure D2O ice IV and nearly pure H2O ice IV were formed on slow heating at a rate of ≈0.4 K min-1, whereas pure H2O and D2O ice XII were formed on fast heating at ≥15 K min-1. On heating HDA at rates inbetween, a mixture of ice IV and ice XII is obtained, where their relative yields depend in a systematic manner on the heating rate. Conversion of HDA into either ice IV or ice XII is an example of a “parallel reaction” where the relative yields of ice IV and ice XII can be controlled by temperature, that is in our approach by the rate of heating. It is conceivable that a similar behavior occurs on crystallization of the related pressure-amorphized silica.

Hydrocarbons, Ices, and “XCN” in the Line of Sight toward the Galactic Center

We discuss 2.8–3.9 lm spectra from the United Kingdom Infrared Telescope of seven sight lines toward IR sources near Sagittarius A* in the Galactic center (GC). In all lines of sight, the 3.0 l mH 2O ice feature is present with optical depths in the range 0.33–1.52. By constructing a simple ice model, we show that the ice profile is not fully accounted for by pure H2O ice mantles. Residual absorption is present at 2.95 and 3.2–3.6 lm. Aliphatic hydrocarbon absorption at 3.4 lm is shown to vary by a factor of 1.7, indicating significant changes in the foreground extinction across the small field. By determining the true ice profile for the GC line of sight, we reveal an additional broad absorption component around � 3.3 lm, which partially underlies the 3.4 lm aliphatic hydrocarbon feature. Its carrier resides in the diffuse interstellar medium. The width of this absorption is deduced to be at least � 100 cm � 1 , much broader than individual polycyclic aromatic hydrocarbon molecules produced in the laboratory or unidentified infrared emission features observed in the interstellar medium. The 4.62 lm ‘‘ XCN ’’ feature is detected in the molecular clouds along the line of sight toward IRS 19. In the solar neighborhood, this feature is seen only toward some deeply embedded protostars. Toward the GC, it may indicate the serendipitous presence of such an object in the line of sight to IRS 19, or it might conceivably arise from the processing of ices in the circumnuclear ring of the GC itself. Subject headings: dust, extinction — Galaxy: center — infrared: ISM: lines and bands — infrared: stars — ISM: molecules

Heterogeneous Chemistry of HOBr on Different Types of Ice and on Ice Doped with HCl, HBr, and HNO3 at 175 K &lt; T &lt; 215 K

The uptake kinetics of HOBr on ice with and without HX has been measured in a Teflon-coated low-pressure flow reactor (Knudsen cell) at temperatures of 175−205 K. The values of the initial uptake coefficient γ0 of HOBr on different types of pure H2O ice such as single-crystal ice, vapor-deposited ice, and samples frozen from liquid H2O range from 0.4 to 0.03 and reveal a pronounced negative temperature dependence with an activation energy of Ea = −9.7 ± 1.0 kcal/mol. The rate of HOBr uptake is independent of the type of ice. HOBr is able to sustain an equilibrium vapor pressure above an ice surface in the range from 185 to 210 K when sufficient HOBr has been adsorbed and is associated with an enthalpy change ΔH0r = −9.4 ± 1.0 kcal/mol. Adsorbed HNO3 has no influence on the HOBr uptake coefficient on ice, even when the amount of HNO3 contaminating the surface of ice is as high as 10 formal monolayers. The interaction of HOBr with HX-doped ice in the temperature range from 180 to 215 K leads to rapid format...

Equilibrium Temperatures of H2O, Co2 and NH3 Ice Grains in the Solar System

AbstractThe equilibrium temperatures of spherical grains of pure H2O, CO2 and NH3 ice in the solar system are computed at heliocentric distances ranging from 1 AU out to (&gt; 1500 AU) where solar illumination becomes negligible compared to galactic and extra-galactic light; the grain size range is from submicron to tens of meter in diameter. These results are based on recently published optical constants and on the use of the “complex angular momentum” (CAM) theory for computing the interaction of the grains with light.

Photodetachment, photodissociation, and photochemistry of surface molecules of icy solids containing NH3 and pure H2O ices

Photodetachment and photodissociation of surface molecules of pure and mixed ices containing NH3 and H2O at 90∼130 K have been investigated by using a mass spectrometer situated in a ultrahigh vacuum apparatus. NH3 molecules were excited to the lowest singlet state by an ArF laser at 193 nm (hν=6.4 eV). The detachment of NH3 molecules from pure NH3 solids was observed at a low laser fluence of 0.1 mJ/cm2. Two photons of 248 nm laser (2hν=10 eV) also detached H2O molecules from pure H2O ices. Mixed solids of NH3 and H2O released both molecules from the surface upon irradiation with 193 nm laser. The translational energies of the detached molecules were of the order of the intermolecular interactions of the constituent molecules (0.03∼0.2 eV) and the energy distributions were not changed by the increase of laser fluence. A detachment mechanism explaining all of the observations is proposed. The force acting on the detachment is ascribed to ‘‘electronic exchange repulsion between an excited molecule and a ground state molecule.’’ At a higher laser fluence (≳500 mJ/cm2), the detachment of ions was observed at m/e=17 and 18 with comparable intensities in a mixed solid of NH3 and H2O. Translational energy distributions of the ions peaked at ∼1 eV for both species. Dissociation of atomic hydrogen from the surface molecules was observed in the solid containing NH3. A fraction of the atomic hydrogen posseses translation energy (Et=17 kcal/mol=0.74 eV) which is similar to that observed in the photolysis of free NH3 molecles in a molecular beam. A very low energy component (Et=0.6 kcal/mol=0.026 eV) appeared and exhibited an intensity change depending upon the deposition and annealing conditions. Molecular hydrogen was also released with a translational temperature very close to the surface temperature. The photoproducts N2H4, N2H2, and NH2OH were detected in the mixed solid system of NH3 and H2O. Photochemical bimolecular condensation is proposed as a mechanism of the reaction forming a hydrogen molecule and a condensed molecule. The present study affords an experimental foundation of the photoejection model of physically adsorbed molecules on interstellar grains presented by Watson and Salpeter (Ref. 2).

Interstellar matrices

The mandes of cold (10 K) incerscellar dusc grains have speccroscopical and chemical properties similar to those of low temperature matrices prepared in the laboratory. This allows us to study experimentally the contribution of grains to the formation of interstellar molecules, and to interpret observed interstellar infrared spectra.In low temperature mixtures of CO, H2O and NH3 many interstellar molecules are formed by photolysis. Temperature dependent chemiluminescence is observed during warm up of these phocolyzed samples.The infrared spectrum, in particular the shape and position of the 3250 cm-1 absorption band (OH stretch), of pure H2O ice and molecular complexes of H2O, NH3 and CO ac 10 K is studied in detail and is compared with interstellar spectra.