Porous Water Ice Research Articles

Sublimation of porous granular ice in vacuum

In this work sublimation of porous water ice in vacuum is investigated. The experiments were performed at low temperature gradient of the order 10 K m−1. It was found, that the surface of porous granular ice can recede much slower than of compact ice. When the porosity is 0.75, as relevant for the nuclei of comets, then the rate of recession can be about 60% lower than in the case of compact ice. In addition, preliminary experiments performed at moderate and high temperature gradient are described.

A study of the compressive mechanical properties of defect-free, porous and sintered water-ice at low and high strain rates

The dynamic strength of distilled water ice, porous distilled water ice and sintered ice powder have been measured in uniaxial compression at a range of temperatures from 173 to 263 K and at strain rates in the range 103− s−1 to 10+3 s−1 using a split Hopkinson pressure bar (SHPB) and an Instron mechanical testing machine. The peak compressive strength of ice as a function of density was found to be described by a linear relationship. The average peak compressive strength for high density ice was 49.5 ± 2.0 MPa. Low density (porous) ice was found to have a lower strength of 36.5 ± 1.9 MPa. Porous low density ice specimens also had a second peak in their stress-strain curves after failure which we attributed to the effect of internal flaws. The peak compressive strength of ice as a function of density was found to be described by a linear relationship. No effect of strain rate was discernable over the strain rate range investigated. Sintered ice powder with a density greater than 735 kg m−3 had an average peak compressive strength of 27.1 ± 1.7 MPa. Sintered ice powder with a density lower than 735 kg m−3 had a strength of 20.4 ± 1.1 MPa.

Experimental study of the penetration of oxygen and deuterium atoms into porous water ice

Context. Many interstellar molecules are thought to form on dust grains. In particular, hydrogenation is one of the major mechanisms of the formation of mantle ice. To date it is not clear if H atoms can penetrate the bulk of the ice mantle or if it only has chemical activity on the accessible surface of grains. Aims. We wish to study the efficiency of atoms deposited on the outer surface of the amorphous solid water to penetrate into the ice bulk. Methods. NO molecules react with O and H atoms. They are easily detected by infrared (IR) spectroscopy. These two properties make this molecule an ideal chemical tracer for the penetration of O and H atoms through water ice. In our experiments we first deposited a NO undercoat and covered this layer (at 40 K) with a variable amount of water ice. Then, we exposed this undercoat to D (10 K) or O (40 K) atoms, and we followed the NO consumption and the products that appeared via IR signatures, and we finally analyzed the desorption of all species through a temperature-programmed desorption technique. We experimentally characterize the accessible surface of the ice and provide a model to interpret quantitatively our measurements. Results. Water ice limits the destruction of tracer NO molecules. The thicker the ice, the more NO remains unreacted. H and O atoms lead to the same amount of NO consumption, pointing out that access to reactants for these two different atoms is identical. We discuss different possible scenarios of NO localization (in and/or on the ice) and determine how this affects our observables (IR data and desorption profiles). Conclusions. In our experimental conditions, it is not possible to measure any atom penetration through the bulk of the ice. The surface diffusion followed by reaction with NO or by self-reaction (i.e., H + H → H2) is faster than bulk diffusion. We propose lower limit values for penetration barriers. Therefore the building of astrophysical ice mantles should be mostly driven by surface reactivity.

The formation and stability of buried polar CO2 deposits on Mars

Shallow Radar soundings of the south polar layered deposits (SPLD) have revealed buried CO2 ice with as many as three distinct layers separated by thinner layers of water ice (Bierson et al., 2016). This layering is suggestive of formation by cyclical processes such as obliquity, eccentricity and advance of perihelion. If obliquity is the main driver, then there were many opportunities for CO2 deposits to have formed within the last 3.5-4 Myr. To persist, however, CO2 deposits must be followed by the deposition of an insulating layer of porous water ice that can seal in the CO2 to survive the obliquity maxima that follow. We suggest that the existing deposits were formed within the last 350 kyr. Each obliquity minimum was quickly followed by a period in which perihelion occurred in the northern summer (southern winter) solstice. Under these conditions, an enhanced deposition of low porosity, fine-grained water ice could occur on the winter pole. A similar series of depositions and burials by water ice could have occurred between 2.75 and 2.2 million years ago, but these deposits were unlikely to have survived the many high obliquity swings that occurred between 2.2 Ma and 400 ka. The formation and burial of CO2 ice could also have occurred in similar fashion earlier during the last ∼ 100 Myr in which the southern polar layered deposits were formed. Although at current times the geothermal flux may be too low to cause basal melting of CO2 deposits, in earlier times, deposits formed in deeper scarps and valleys could have led to basal CO2 melting and sequestration into the regolith. By extension, basal melting and sequestration in the regolith may provide an explanation for the fate of the thick CO2 atmosphere (pCO2 ≳ 0.5 bar) implied by climate models of early Mars.

New porous water ice metastable at atmospheric pressure obtained by emptying a hydrogen-filled ice.

The properties of some forms of water ice reserve still intriguing surprises. Besides the several stable or metastable phases of pure ice, solid mixtures of water with gases are precursors of other ices, as in some cases they may be emptied, leaving a metastable hydrogen-bound water structure. We present here the first characterization of a new form of ice, obtained from the crystalline solid compound of water and molecular hydrogen called C0-structure filled ice. By means of Raman spectroscopy, we measure the hydrogen release at different temperatures and succeed in rapidly removing all the hydrogen molecules, obtaining a new form of ice (ice XVII). Its structure is determined by means of neutron diffraction measurements. Of paramount interest is that the emptied crystal can adsorb again hydrogen and release it repeatedly, showing a temperature-dependent hysteresis.

Present and past glaciation on Pluto

Modern N2 ice glaciers flow from highlands to the east of the 750×1400km2 lowland of Sputnik Planum [SP] and merge with the ices of similar composition on SP. We explore the possibility that glaciation may be fed by N2 sublimation from SP followed by redeposition on the highlands. The uplands to the northeast, north, and west of SP have been erosionally sculpted into a variety of dissected terrains that feature linear depressions (valleys), locally in dendritic networks. We interpret these dissected terrains to have been carved by N2 glaciers formerly covering the uplands. Depositional glacial landforms (moraines, eskers, outwash) have not been identified, however. N2 glaciation would have a different erosional manifestation because the substrate (porous water ice and CH4-rich mantles) probably has lower density than N2, and also because of the lack of freeze-thaw weathering. If sufficiently thick (1–4km), N2 glaciers might have experienced basal melting. Past flow of N2 glaciers from the highlands into SP may have detached and transported the prominent mountainous water ice mountains along the western border of SP.

N2 AND CO DESORPTION ENERGIES FROM WATER ICE

ABSTRACT The relative desorption energies of CO and N2 are key to interpretations of observed interstellar CO and N2 abundance patterns, including the well-documented CO and N2H+ anti-correlations in disks, protostars, and molecular cloud cores. Based on laboratory experiments on pure CO and N2 ice desorption, the difference between CO and N2 desorption energies is small; the N2-to-CO desorption energy ratio is 0.93 ± 0.03. Interstellar ices are not pure, however, and in this study we explore the effect of water ice on the desorption energy ratio of the two molecules. We present temperature programmed desorption experiments of different coverages of 13CO and 15N2 on porous and compact amorphous water ices and, for reference, of pure ices. In all experiments, 15N2 desorption begins a few degrees before the onset of 13CO desorption. The 15N2 and 13CO energy barriers are 770 and 866 K for the pure ices, 1034–1143 K and 1155–1298 K for different submonolayer coverages on compact water ice, and 1435 and 1575 K for ∼1 ML of ice on top of porous water ice. For all equivalent experiments, the N2-to-CO desorption energy ratio is consistently 0.9. Whenever CO and N2 ice reside in similar ice environments (e.g., experience a similar degree of interaction with water ice) their desorption temperatures should thus be within a few degrees of one another. A smaller N2-to-CO desorption energy ratio may be present in interstellar and circumstellar environments if the average CO ice molecules interacts more with water ice compared to the average N2 molecules.

A NEW DETERMINATION OF THE BINDING ENERGY OF ATOMIC OXYGEN ON DUST GRAIN SURFACES: EXPERIMENTAL RESULTS AND SIMULATIONS

The energy to desorb atomic oxygen from an interstellar dust grain surface, $E_{\rm des}$, is an important controlling parameter in gas-grain models; its value impacts the temperature range over which oxygen resides on a dust grain. However, no prior measurement has been done of the desorption energy. We report the first direct measurement of $E_{\rm des}$ for atomic oxygen from dust grain analogs. The values of $E_{\rm des}$ are $1660\pm 60$~K and $1850\pm 90$~K for porous amorphous water ice and for a bare amorphous silicate film, respectively, or about twice the value previously adopted in simulations of the chemical evolution of a cloud. We use the new values to study oxygen chemistry as a function of depth in a molecular cloud. For $n=10^4$ cm$^{-3}$ and $G_0$=10$^2$ ($G_0$=1 is the average local interstellar radiation field), the main result of the adoption of the higher oxygen binding energy is that H$_2$O can form on grains at lower visual extinction $A_{\rm V}$, closer to the cloud surface. A higher binding energy of O results in more formation of OH and H$_2$O on grains, which are subsequently desorbed by FUV radiation, with consequences for gas-phase chemistry. For higher values of $n$ and $G_0$, the higher binding energy can lead to a large increase in the column of H$_2$O but a decrease in the column of O$_2$.

Physisorption and desorption of H2, HD and D2 on amorphous solid water ice. Effect on mixing isotopologue on statistical population of adsorption sites.

We study the adsorption and desorption of three isotopologues of molecular hydrogen mixed on 10 ML of porous amorphous water ice (ASW) deposited at 10 K. Thermally programmed desorption (TPD) of H2, D2 and HD adsorbed at 10 K have been performed with different mixings. Various coverages of H2, HD and D2 have been explored and a model taking into account all species adsorbed on the surface is presented in detail. The model we propose allows to extract the parameters required to fully reproduce the desorption of H2, HD and D2 for various coverages and mixtures in the sub-monolayer regime. The model is based on a statistical description of the process in a grand-canonical ensemble where adsorbed molecules are described following a Fermi-Dirac distribution.

Compaction of porous ices rich in water by swift heavy ions

Porous water ice and water ice mixtures H2O:X (X=CO, CO2 and CH4) produced at 15K, with film thicknesses in the 0.5–1μm range, were irradiated by swift ions and monitored by mid-infrared spectroscopy (FTIR). The analysis of the evolution of the pure water ice infrared absorption on ion beam dose reveals a strong correlation among three quantities: (i) the absorbance of the most intense band (3250cm−1), (ii) the wavelength of the maximum absorbance of this band and (iii) the absorbance of the OH-dangling bonds. This correlation is interpreted as indications of the water ice compaction by irradiation: as the beam fluence increases, the ice porosity decreases, the dangling bond peaks collapse and the area and position of the 3250cm−1 band vary exponentially, all of them evolving with the same compaction cross section (σc). The linear dependence σc∝Se (Se being the electronic stopping power) is observed for both pure and mixed water ices, confirming previous results. We suggests that the infrared absorption A-value varies with dose as (1-ζe-D/D0) during the compaction process (D0=0.2eV/molec being the effective energy density to eliminate the OH-db, and ζ is a parameter characterizing the porosity). These findings may be used as a diagnostic tool to probe the morphology of water ices occurring in the outer Solar System and in the ISM.

Water formation through O2 + D pathway on cold silicate and amorphous water ice surfaces of interstellar interest

The formation of the first monolayer of water molecules on bare dust grains is of primary importance to understand the growth of the icy mantles that cover dust in the interstellar medium. In this work, we explore experimentally the formation of water molecules from O(2) + D reaction on bare silicate surfaces that simulates the grains present in the diffuse interstellar clouds at visual extinctions (A(V) < 3 mag). For comparison, we also study the formation of water molecules on surfaces covered with amorphous water ice representing the dense clouds (A(V) ≥ 3 mag). Our studies focus on the formation of water molecules in the sub-monolayer and monolayer regimes using reflection absorption infrared spectroscopy and temperature-programmed desorption techniques. We provide the fractions of the products, such as D(2)O and D(2)O(2) molecules formed on three astrophysically relevant surfaces held at 10 K (amorphous olivine-type silicate, porous amorphous water ice, and nonporous amorphous water ice). Our results showed that the formation of D(2)O molecules occurs with an efficiency of about 55%-60% on nonporous amorphous water ice and about 18% on bare silicate grains surfaces. We explain the low efficiency of D(2)O water formation on the silicate surfaces by the desorption upon formation of certain products once the reaction occurs between O(2) and D atoms on the surface. A kinetic model taking into account the chemical desorption of newly formed water supports our conclusions.

EXPERIMENTAL INVESTIGATION OF THE ORTHO/PARA RATIO OF NEWLY FORMED MOLECULAR HYDROGEN ON AMORPHOUS SOLID WATER

Several astronomical observations have shown that the ortho/para ratio (OPR) of H2 can differ from the expected statistical value of 3 or the local thermodynamic equilibrium (LTE) value at the gas or dust temperature. It is thus important to know the OPR of H2 newly formed on dust grain surfaces, in order to clarify the dependence of the observed OPR in space on the formation process. Using an experimental setup designed to mimic interstellar medium environments, we measured the OPR of H2 and D2 formed on the surface of porous amorphous water ice held at 10 K. We report for the first time the OPR value for newly formed D2, consistent with the expected LTE value at the high-temperature limit found by previous theoretical and experimental works on the determination of the OPR upon H2 formation on surfaces at low temperature.

Morphology and crystallization kinetics of compact (HGW) and porous (ASW) amorphous water ice

An investigation of porosity and isothermal crystallization kinetics of amorphous ice produced either by background water vapour deposition (ASW) or by hyperquenching of liquid droplets (HGW) is presented. These two types of ice are relevant for astronomical ice research (Gálvez et al., Astrophys. J., 2010, 724, 539) and are studied here for the first time under comparable experimental conditions. From CH(4) isothermal adsorption experiments at 40 K, surface areas of 280 ± 30 m(2) g(-1) for the ASW deposits and of 40 ± 12 m(2) g(-1) for comparable HGW samples were obtained. The crystallization kinetics was studied at 150 K by following the evolution of the band shape of the OD stretching vibration in HDO doped ASW and HGW samples generated at 14 K, 40 K and 90 K. Comparable rate constants of ∼7 × 10(-4) s(-1) were obtained in all cases. However a significant difference was found between the n Avrami parameter of the samples generated at 14 K (n∼ 1) and that of the rest (n > 2). This result hints at the possible existence of a structurally different form of amorphous ice for very low generation temperatures, already suggested in previous literature works.

The origin of the Martian moons revisited

The origin of the Martian moons, Phobos and Deimos, is still an open issue: either they are asteroids captured by Mars or they formed in situ from a circum-Mars debris disk. The capture scenario mainly relies on the remote-sensing observations of their surfaces, which suggest that the moon material is similar to outer-belt asteroid material. This scenario, however, requires high tidal dissipation rates inside the moons to account for their current orbits around Mars. Although the in situ formation scenarios have not been studied in great details, no observational constraints argue against them. Little attention has been paid to the internal structure of the moons, yet it is pertinent for explaining their origin. The low density of the moons indicates that their interior contains significant amounts of porous material and/or water ice. The porous content is estimated to be in the range of 30–60% of the volume for both moons. This high porosity enhances the tidal dissipation rate but not sufficiently to meet the requirement of the capture scenario. On the other hand, a large porosity is a natural consequence of re-accretion of debris at Mars’ orbit, thus providing support to the in situ formation scenarios. The low density also allows for abundant water ice inside the moons, which might significantly increase the tidal dissipation rate in their interiors, possibly to a sufficient level for the capture scenario. Precise measurements of the rotation and gravity field of the moons are needed to tightly constrain their internal structure in order to help answering the question of the origin.

H ATOM IRRADIATION OF CARBON GRAINS UNDER SIMULATED DENSE INTERSTELLAR MEDIUM CONDITIONS: THE EVOLUTION OF ORGANICS FROM DIFFUSE INTERSTELLAR CLOUDS TO THE SOLAR SYSTEM

We present the results of experiments aimed at studying the interaction of hydrogen atoms at 80 K with carbon grains covered with a water ice layer at 12 K. The effects of H processing have been analyzed, using IR spectroscopy, as a function of the water ice layer. The results confirm that exposure of the samples to H atoms induces the activation of the band at 3.47 μm with no evidence for the formation of aromatic and aliphatic C-H bonds in the CH2 and CH3 functional groups. The formation cross section of the 3.47 μm band has been estimated from the increase of its integrated optical depth as a function of the H atom fluence. The cross section decreases with increasing thickness of the water ice layer, indicating an increase of adsorption of H atoms in the water ice layer. A penetration depth of 100 nm has been estimated for H atoms in the porous water ice covering carbon grains. Sample warm-up at room temperature causes the activation of the IR features due to the vibrations of the CH2 and CH3 aliphatic functional groups. The evolution of the 3.47 μm band carrier has been evaluated for dense and diffuse interstellar clouds, using the estimated formation cross section and assuming that the destruction cross section by energetic processing is the same as that derived for the 3.4 μm band. In both environments, the presence of the 3.47 μm band carrier is compatible with the evolutionary timescale limit imposed by fast cycling of materials between dense and diffuse regions of the interstellar medium. In diffuse regions the formation of the CH2 and CH3 aliphatic bands, inhibited in dense regions, takes place, masking the 3.47 μm band. The activation of the CH2 and CH3 aliphatic vibrational modes at the end of H processing after sample warm-up represents the first experimental evidence supporting an evolutionary connection between the interstellar carbon grain population, which is responsible for the 3.4 μm band (diffuse regions) and contributes to the absorption at 3.47 μm (dense regions), and the organics observed in interplanetary dust particles and cometary Stardust grains.

Experimental evidence for water formation on interstellar dust grains by hydrogen and oxygen atoms

Context. The synthesis of water is one necessary step in the origin and development of life. It is believed that pristine water is formed and grows on the surface of icy dust grains in dark interstellar clouds. Until now, there has been no experimental evidence whether this scenario is feasible or not on an astrophysically relevant template and by hydrogen and oxygen atom reactions. Aims. We present here the first experimental evidence of water synthesis by such a process on a realistic grain surface analogue in dense clouds, i.e., amorphous water ice. Methods. Atomic beams of oxygen and deuterium are aimed at a porous water ice substrate (H2O) held at 10 K. Products are analyzed by the temperature-programmed desorption technique. Results. We observe production of HDO and D2O, indicating that water is formed under conditions of the dense interstellar medium from hydrogen and oxygen atoms. This experiment opens up the field of a little explored complex chemistry that could occur on dust grains, believed to be the site where key processes lead to the molecular diversity and complexity observed in the Universe.

H bonds in astrophysical ices

We have studied, by infrared absorption spectroscopy, the profile (shape and peak position) of the OH dangling bond feature in pure porous amorphous solid water (ASW) and in mixtures of water with other species (CO, CO 2, O 2, N 2, H 2O 2, CH 4, SO 2, and CH 3OH) at 12–16 K. Furthermore, we have investigated the effects of ion and UV irradiation on the morphology/porosity of amorphous water ice. Thin films (about 0.25–1 μm) of porous amorphous water ice were irradiated with 200 keV H +, 30 keV He +, and 10.2 eV Lyman alpha photons at 12–16 K. We have found that the profile of the OH dangling bond feature depends on the mixture considered and that the intensity of the OH dangling bond feature decreases after irradiation. This latter result indicates that the porosity of amorphous water ice decreases after both ion and UV irradiation.

Laboratory evidence for the non-detection of excited nascent H2 in dark clouds

Abstract There has always been a great deal of interest in the formation of H2 as well as in the binding energy released upon its formation on the surface of dust grains. The present work aims at collecting experimental evidence for how the bond energy budget of H2 is distributed between the reaction site and the internal energy of the molecule. So far, the non-detection of excited nascent H2 in dense quiescent clouds could be a sign that either predictions of emission line intensities are not correct or the de-excitation of the newly formed molecules proceeds rapidly on the grain surface itself. In this Letter, we present experimental evidence that interstellar molecular hydrogen is formed and then rapidly de-excited on the surface of porous water ice mantles. In addition, although we detect ro-vibrationally excited nascent molecules desorbing from a bare non-porous (compact) water ice film, we demonstrate that the amount of excited nascent hydrogen molecules is significantly reduced no matter the morphology of the water ice substrate at 10 K (both on non-porous and on porous water ice) in a regime of high molecular coverage as is the case in dark molecular clouds.

Melting and Sublimation of Planetary Ices Under Low Pressure Conditions: Laboratory Experiments with a Melting Probe Prototype

One possibility to explore the subsurface layers of icy bodies is to use a probe with a “hot tip", which is able to penetrate ice layers by melting. Such probes have been built and used in the past for the exploration of terrestrial polar ice sheets and may also become useful tools to explore other icy layers in the Solar System. Examples for such layers are the polar areas of Mars or the icy crust of Jupiter’s moon Europa. However, while on Earth a heated probe launched into an ice sheet always causes melting with subsequent refreezing, the behaviour of such a probe in a low pressure environment is quite different. We report on the results of some experiments with a simple “melting probe" prototype with two different kinds of hot tips in a vacuum environment. For one of the tips the probe moved into two types of ice samples: (i) compact water ice and (ii) porous water ice with a snow (firn) like texture. It was also found that the penetration behaviour was basically different for the two sample types even when the same kind of tip was used. While in the porous sample the ice was only subliming, the phase changes occurring during the interaction of the tip with the compact ice are much more complex. Here alternating phases of melting and sublimation occur. The absence of the liquid phase has severe consequences on the performance of a “melting probe" under vacuum conditions: In this environment we find a high thermal resistance between the probe surface and the underlying ice. Therefore, only a low percentage of the heat that is generated in the tip is used to melt or sublime the ice, the bulk of the power is transferred towards the rear end of the probe. This is particularly a problem in the initial phases of an ice penetration experiment, when the probe has not yet penetrated the ice over its whole length. In the compact ice sample, phases could be observed, where a high enough gas pressure had built up locally underneath the probe, so that melting becomes possible. Only during these melting periods the thermal contact between the probe and the ice is good and in consequence the melting probe works effectively.

Mobility of D atoms on porous amorphous water ice surfaces under interstellar conditions

Aims. The mobility of H atoms on the surface of interstellar dust grains at low temperature is still a matter of debate. In dense clouds, the hydrogenation of adsorbed species (i.e., CO), as well as the subsequent deuteration of the accreted molecules depend on the mobility of H atoms on water ice. Astrochemical models widely assume that H atoms are mobile on the surface of dust grains even if controversy still exists. We present here direct experimental evidence of the mobility of H atoms on porous water ice surfaces at 10 K. Methods. In a UHV chamber, O2 is deposited on a porous amorphous water ice substrate. Then D atoms are deposited onto the surface held at 10 K. Temperature-Programmed Desorption (TPD) is used and desorptions of O2 and D2 are simultaneously monitored. Results. We find that the amount of O2 that desorb during the TPD diminishes if we increase the deposition time of D atoms. O2 is thus destroyed by D atoms even though these molecules have previously diffused inside the pores of thick water ice. Our results can be easily interpreted if D is mobile at 10 K on the water ice surface. A simple rate equation model fits our experimental data and best fit curves were obtained for a D atoms diffusion barrier of 22(+-)2 meV. Therefore hydrogenation can take place efficiently on interstellar dust grains. These experimental results are in line with most calculations and validate the hypothesis used in several models.