Amorphous Water Ice Research Articles

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.

SURVIVAL OF AMORPHOUS WATER ICE ON CENTAURS

Centaurs are believed to be Kuiper Belt objects in transition between Jupiter and Neptune before possibly becoming Jupiter family comets. Some indirect observational evidence is consistent with the presence of amorphous water ice in Centaurs. Some of them also display a cometary activity, probably triggered by the crystallization of the amorphous water ice, as suggested by Jewitt and this work. Indeed, we investigate the survival of amorphous water ice against crystallization, using a fully three-dimensional thermal evolution model. Simulations are performed for varying heliocentric distances and obliquities. They suggest that crystallization can be triggered as far as 16 AU, though amorphous ice can survive beyond 10 AU. The phase transition is an efficient source of outgassing up to 10-12 AU, which is broadly consistent with the observations of the active Centaurs. The most extreme case is 167P/CINEOS, which barely crystallizes in our simulations. However, amorphous ice can be preserved inside Centaurs in many heliocentric distance-obliquity combinations, below a ~5-10 m crystallized crust. We also find that outgassing due to crystallization cannot be sustained for a time longer than 104-104 years, leading to the hypothesis that active Centaurs might have recently suffered from orbital changes. This could be supported by both observations (although limited) and dynamical studies.

RADIATION SYNTHESIS OF CARBON DIOXIDE IN ICE-COATED CARBON: IMPLICATIONS FOR INTERSTELLAR GRAINS AND ICY MOONS

We report the synthesis of carbon dioxide on an amorphous carbon-13 substrate coated with amorphous water ice from irradiation with 100 keV protons at 20 K and 120 K. The quantitative studies show that the CO{sub 2} is dispersed in the ice; its column density increases with ion fluence to a maximum value (in 10{sup 15} molecules cm{sup -2}) of {approx}1 at 20 K and {approx}3 at 120 K. The initial yield is 0.05 (0.1) CO{sub 2} per incident H{sup +} at 20 (120) K. The CO{sub 2} destruction process, which limits the maximum column density, occurs with an effective cross section of {approx}2.5 (4.1) Multiplication-Sign 10{sup -17} cm{sup 2} at 20 (120) K. We discuss radiation-induced oxidation by reactions of radicals in water with the carbon surface and demonstrate that these reactions can be a significant source of condensed carbon dioxide in interstellar grains and in icy satellites in the outer solar system.

A cometary nucleus model taking into account all phase changes of water ice: amorphous, crystalline, and clathrate

Context. Current theories, models of cometary nuclei and of ice formation in the protoplanetary disk, and laboratory studies suggest that cometary materials could be formed of pure crystalline water ice, amorphous water ice, clathrate hydrate, or a mixture of these structures of water ice. However, current models of cometary nuclei consider only two forms of ice during the thermodynamic evolution of comets: amorphous and crystalline water ices. Aims. In this work, we have developed a model of cometary nucleus that takes into account all water ice structures and phase changes in order to predict the outgassing profile of volatile molecules that could be measured by the Rosetta mission and can be used to constrain the structural type of ice existing in the interior of the Comet 67P/Churyumov-Gerasimenko, the target comet of the Rosetta mission, and, hopefully, its initial composition.Methods. We added the physic of formation/dissociation of clathrate hydrates in addition to others physical processes that are taken into account in models without clathrate hydrates. All thermal changes, as well as the release and trapping of gas with water phase changes are taken into account.Results. This model describes heat transmission, latent heat exchanges, all water ices structures and transitions (amorphous-to-pure crystalline, amorphous-to-clathrate hydrates and pure crystalline-to-clathrate hydrates and vise versa), sublimation/recondensation of volatile molecules in the nucleus, gas diffusion, gas released and trapped by crystallization and clathrate formation/dissociation processes, as well as gas and dust release and mantle formation at the surface. Applying this model to the comet 67P/Churyumov-Gerasimenko, results show different outgassing profiles of volatiles molecules from the nucleus depending on the water ice structure, the distribution of volatile molecules between the “trapped” and “condensed” states in the nucleus and the thermal inertia of its porous matrix. Conclusions. Given these results, we pretend that this model is able to constrain the water ice structure and chemical composition in comets from outgassing profiles of volatile molecules, and especially those of the target comet of the Rosetta mission.

Pressure dependent trace gas trapping in amorphous water ice at 77 K: Implications for determining conditions of comet formation

The nature of cometary volatile materials is subject to debate. Theoretical models of cometary nuclei and laboratory studies suggest that these objects could be made of amorphous water ice in addition to other volatile molecules and refractory grains. This water ice structure has the ability to encapsulate the gases of surrounding environment, reflecting the physical and chemical conditions during their deposition. Therefore, the knowledge of the chemical composition of volatile molecules trapped in amorphous water ice provides a tool for probing the formation environment of cometary ice grains. Experimental studies of gas trapping efficiency in amorphous water ice have been previously conducted mostly under kinetic conditions, where dynamic pumping and temperature gradients prevented rigorous calibrations. In this work, we investigated the trapping efficiencies of Ar, CO, CH4, Kr and N2 by depositing water vapor as ice in the presence of trace gases in a volume submerged in liquid nitrogen at 77K. The gas trapping efficiencies were determined simply by monitoring the pressure difference of the trace gases before and after the deposition of a known amount of water molecules as amorphous ice.Our results show that the trapped gas to water molecule ratio in amorphous ice is controlled primarily by the partial pressure of the gas during water ice deposition, and is independent of the ice deposition rate as well as the gas to water ratio in the vapor phase. The trapping efficiencies of gases decrease in the order of Kr>CH4>CO>Ar>N2 in accordance with previous studies. Assuming that the water ice structure of comets is at least partially amorphous water ice at the time of their formation, these results suggest that the total pressure and composition of the surrounding environment of amorphous ice formation are significant controlling factors of trace gas concentrations in cometary ice. This further indicates that the evolution of the solar nebula and timing of cometary ice condensation can also be important parameters in linking the volatile contents of comets and their formation process.

SURVIVAL DEPTH OF ORGANICS IN ICES UNDER LOW-ENERGY ELECTRON RADIATION (⩽2 keV)

Icy surfaces in our solar system are continually modified and sputtered with electrons, ions, and photons from solar wind, cosmic rays, and local magnetospheres in the cases of Jovian and Saturnian satellites. In addition to their prevalence, electrons specifically are expected to be a principal radiolytic agent on these satellites. Among energetic particles (electrons and ions), electrons penetrate by far the deepest into the ice and could cause damage to organic material of possible prebiotic and even biological importance. To determine if organic matter could survive and be detected through remote sensing or in situ explorations on these surfaces, such as water ice-rich Europa, it is important to obtain accurate data quantifying electron-induced chemistry and damage depths of organics at varying incident electron energies. Experiments reported here address the quantification issue at lower electron energies (100 eV-2 keV) through rigorous laboratory data analysis obtained using a novel methodology. A polycyclic aromatic hydrocarbon molecule, pyrene, embedded in amorphous water ice films of controlled thicknesses served as an organic probe. UV-VIS spectroscopic measurements enabled quantitative assessment of organic matter survival depths in water ice. Eight ices of various thicknesses were studied to determine damage depths more accurately. The electron damage depths weremore » found to be linear, approximately 110 nm keV{sup -1}, in the tested range which is noticeably higher than predictions by Monte Carlo simulations by up to 100%. We conclude that computational simulations underestimate electron damage depths in the energy region {<=}2 keV. If this trend holds at higher electron energies as well, present models utilizing radiation-induced organic chemistry in icy solar system bodies need to be revisited. For interstellar ices of a few micron thicknesses, we conclude that low-energy electrons generated through photoionization processes in the interstellar medium could penetrate through ice grains significantly and trigger organic reactions several hundred nanometers deep-bulk chemistry thus competing with surface chemistry of astrophysical ice grains.« less

Nanopatterning on Nonplanar and Fragile Substrates with Ice Resists

Electron beam (e-beam) lithography using polymer resists is an important technology that provides the spatial resolution needed for nanodevice fabrication. But it is often desirable to pattern nonplanar structures on which polymeric resists cannot be reliably applied. Furthermore, fragile substrates, such as free-standing nanotubes or thin films, cannot tolerate the vigorous mechanical scrubbing procedures required to remove all residual traces of the polymer resist. Here we demonstrate several examples where e-beam lithography using an amorphous ice resist eliminates both of these difficulties and enables the fabrication of unique nanoscale device structures in a process we call ice lithography. (1, 2) We demonstrate the fabrication of micro- and nanostructures on the tip of atomic force microscope probes, microcantilevers, transmission electron microscopy grids, and suspended single-walled carbon nanotubes. Our results show that by using amorphous water ice as an e-beam resist, a new generation of nanodevice structures can be fabricated on nonplanar or fragile substrates.

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.

Formation of water on silicate surfaces under interstellar conditions

We explore experimentally the formation of water molecules from O2 and D atoms on bare grains composed of amorphous silicates analogous to those in diffuse interstellar clouds. We provide the fractions of D2 O and D2 O2 molecules formed on the silicate surface held at 10 K from the O2 + D pathway using RAIRS and TPD techniques. For comparison, we also study the formation of water molecules on surfaces covered with amorphous water ice representing the dense clouds.

Changes in the morphology of interstellar ice analogues after hydrogen atom exposure

Interstellar water ice is mainly amorphous, but the nature of its morphology still remains poorly known. The experimental study described in this work focuses on how relevant changes of the ice morphology result from atomic hydrogen exposure and subsequent recombination. We show that there is an exponential decrease in the porosity of the amorphous water ice sample following hydrogen-atom irradiation. These and other laboratory results lead us to suggest that water ice in space is almost certainly amorphous and non-porous (compact).

THERMAL SHADOWS AND COMPOSITIONAL STRUCTURE IN COMET NUCLEI

We use a fully 3-dimensional thermal evolution model to examine the effects of a non-uniform surface albedo on the subsurface thermal structure of comets. Surface albedo markings cast "thermal shadows", with strong lateral thermal gradients. Corresponding compositional gradients can be strong, especially if the crystallization of amorphous water ice is triggered in the hottest regions. We show that the spatial extent of the structure depends mainly on the obliquity, ther- mal conductivity and heliocentric distance. In some circumstances, subsurface structure caused by the thermal shadows of surface features can be maintained for more than 10 Myr, the median transport time from the Kuiper Belt to the inner solar system. Non-uniform compositional structure can be an evolutionary product and does not necessarily imply that comets consist of building blocks accumulated in different regions of the protoplanetary disk.

Formation of H2+ by Ultra-Low-Energy Collisions of Protons with Water Ice Surfaces

The molecular ion of dihydrogen (H2+) is produced by 1 eV collisions of protons (H+) on amorphous water ice surfaces. The reaction is also observed on crystalline ice surfaces, but with lower efficiency. Collisions of D+ on amorphous H2O and D2O ices yield D2+ on the former, subsequent to isotope exchange on the H2O surface. Ultra-low-energy collision-induced dihydrogen ion production is also observed from alkanol surfaces, with decreasing efficiency as the alkyl chain length increases. There is no corresponding reaction on solid hexane. This endothermic reaction, with implications for interstellar chemistry and plasma etching processes, is proposed to occur as a result of stabilization of the other reaction product, a hydroxyl radical (OH•), on water surfaces through hydrogen-bonding interactions with the surface. These results point to an interesting chemistry involving ultra-low-energy ions on molecular solids.

Water Ice Formation and the o/p Ratio

AbstractWater is observed in many astrophysical environments in both gas and solid phase. Water ice, for its specific properties, is probably the most important template that structures the gas-solid interaction. In cold environments, its synthesis is supposed to occur directly in the solid phase and then water acts as a catalytic matrix for subsequent synthesis of various molecules. When the medium begins to warm again, water sublimates and nourishes the gas phase, as occurs for example in comets or in star forming regions. Over the last four years, water formation on cold surfaces has been studied experimentally. Different precursors (O, O2, O3. . .) have been used to understand the complex mechanisms that take place. Although numerous questions remain unanswered, at present, it is clear that water is easily formed by different pathways, and that the ice formed has an amorphous structure. The recent observations of the ortho/para ratio of water with Herschel satellite have similarities with the previous o/p ratio observations of water in comets. Some experimental work have been recently reported in this domain, mostly rare gas matrix studies where nuclear spin conversion is measured even at 4.2 K. H2 molecules adsorbed on amorphous solid water ice also exhibit a nuclear spin conversion in presence of a tiny fraction of O2. Finally, I will discuss if microphysics properties of water desorption can explain the o/p ratio values observed.

Molecular dynamics simulations of D2O ice photodesorption

Molecular dynamics (MD) calculations have been performed to study the ultraviolet (UV) photodissociation of D(2)O in an amorphous D(2)O ice surface at 10, 20, 60, and 90 K, in order to investigate the influence of isotope effects on the photodesorption processes. As for H(2)O, the main processes after UV photodissociation are trapping and desorption of either fragments or D(2)O molecules. Trapping mainly takes place in the deeper monolayers of the ice, whereas desorption occurs in the uppermost layers. There are three desorption processes: D atom, OD radical, and D(2)O molecule photodesorption. D(2)O desorption takes places either by direct desorption of a recombined D(2)O molecule, or when an energetic D atom produced by photodissociation kicks a surrounding D(2)O molecule out of the surface by transferring part of its momentum. Desorption probabilities are calculated for photoexcitation of D(2)O in the top four monolayers and are compared quantitatively with those for H(2)O obtained from previous MD simulations of UV photodissociation of amorphous water ice at different ice temperatures [Arasa et al., J. Chem. Phys. 132, 184510 (2010)]. The main conclusions are the same, but the average D atom photodesorption probability is smaller than that of the H atom (by about a factor of 0.9) because D has lower kinetic energy than H, whereas the average OD radical photodesorption probability is larger than that of OH (by about a factor of 2.5-2.9 depending on ice temperature) because OD has higher translational energy than OH for every ice temperature studied. The average D(2)O photodesorption probability is larger than that of H(2)O (by about a factor of 1.4-2.3 depending on ice temperature), and this is entirely due to a larger contribution of the D(2)O kick-out mechanism. This is an isotope effect: the kick-out mechanism is more efficient for D(2)O ice, because the D atom formed after D(2)O photodissociation has a larger momentum than photogenerated H atoms from H(2)O, and D transfers momentum more easily to D(2)O than H to H(2)O. The total (OD + D(2)O) yield has been compared with experiments and the total (OH + H(2)O) yield from previous simulations. We find better agreement when we compare experimental yields with calculated yields for D(2)O ice than when we compare with calculated yields for H(2)O ice.

HCl Adsorption and Ionization on Amorphous and Crystalline H2O Films below 50 K

Molecular beams were used to grow amorphous and crystalline H(2)O films and to dose HCl upon their surface. The adsorption state of HCl on the ice films was probed with infrared spectroscopy. A Zundel continuum is clearly observed for exposures up to the saturation HCl coverage on ice upon which features centered near 2530, 2120, 1760, and 1220 cm(-1) are superimposed. The band centered near 2530 cm(-1) is observed only when the HCl adlayer is in direct contact with amorphous solid water or crystalline ice films at temperatures as low as 20 K. The spectral signature of solid HCl (amorphous or crystalline) was identified only after saturation of the adsorption sites in the first layer or when HCl was deposited onto a rare gas spacer layer between the HCl and ice film. These observations strongly support conclusions from recent electron spectroscopy work that reported ionic dissociation of the first layer HCl adsorbed onto the ice surface is spontaneous.

Highly efficient electron-stimulated desorption of benzene from amorphous solid water ice

The desorption of benzene ( C 6 H 6 ) from the surface of compact amorphous solid water (ASW) during irradiation with electrons in the range 100–350 eV has been investigated. Two desorption components, with particularly large cross-sections, were present in the observed desorption signal. The fast component, with a cross-section of > 10 - 15 cm 2 , is attributed to desorption of isolated C 6 H 6 molecules that are π -hydrogen bonded to small clusters of water ( H 2 O ) molecules on the compact ASW surface. The slower component, with a cross-section of ca. 10 - 16 cm 2 , is attributed to a more complex desorption process involving larger C 6 H 6 islands on the compact ASW surface. Possible desorption mechanisms are discussed.

A THERMAL EVOLUTION MODEL OF CENTAUR 10199 CHARIKLO

Centaur 10199 Chariklo appears to have a varying spectral behavior. While three different spectral studies detect the presence of water ice at the surface, two more recent studies do not detect any absorption bands. In this article, we consider the possibility that Chariklo undergoes cometary activity that could be responsible for the observed spectral variations. We simulate its thermal evolution, finding that crystalline water ice should be present in the object core, and amorphous water ice should be found at the surface. Upon entering the inner solar system, Chariklo might experience some cometary activity due to ice crystallization if the obliquity is high, due to the adjustment of the internal structure to a new thermal equilibrium. No other activity is expected from this source, unless an external source like an impact provides the heat needed. In the case of such an event, we find that dust emitted in a coma is unlikely to be responsible for the observed spectral variations. In contrast, water ice grains in the coma would reproduce this pattern, meaning that the water ice detected after Chariklo's discovery was present in these grains and not on the object surface. Nonetheless, any activity would require an external additional heat source to be triggered, through an outburst, which might favor the spatial variations hypothesis.

Comet 17P/Holmes: Possibility of a CO driven explosion

This work is a continuation of our previous paper about brightening of Comet 17P/Holmes (Kossacki, K.J., Szutowicz, S. [2010]. Icarus 207, 320–340). In that paper we presented results of simulations indicating that the nonuniform crystallization of amorphous water ice itself is probably not sufficient for an explosion. In the present work we investigate the possibility that the explosion is caused by a rapid sublimation of the CO ice leading to the rise of gas pressure above the tensile strength of the nucleus. We simulated evolution of a model nucleus in the orbit of Comet 17P/Holmes. The nucleus is composed of water ice, carbon monoxide ice and dust and has the shape of an elongated ellipsoid. The simulations include crystallization of amorphous ice in the nucleus, changes of the dust mantle thickness, and sublimation of the CO ice. In our model CO is mantling grains composed of dust and amorphous water ice. Orientation of the nuclear spin axis in space is the same as derived in Moreno et al. (Moreno, F., Ortiz, J.L., Santos-Sanz, P., Morales, N., Vidal-Nunez, M.J., Lara, L.M., Gutierrez, P.J. [2008]. Astrophys. J. 677, L63–L66) for Comet Holmes during recent brightening event. Hence, the angle between the orbital and the equatorial planes of the comet is I = 95°, and the cometocentric solar longitude at perihelion is Φ = 210°. The calculations are performed for the south pole being the sub-solar point close to time of the outburst. Our computations indicate, that the CO pressure within the comet nucleus can rise to high values. When the layer between the dust mantle and the crystallization front of the amorphous water ice is very fine grained, few microns in radius, the CO pressure within the nucleus can exceed 10 kPa. This value is the lowest estimate for the tensile strength of the nucleus of Comet Holmes (Reach, W.T., Vaubaillon, J., Lisse, C.M., Holloway, M., Rho, J. [2010]. Icarus 208, 276–292). Hence, when the gas pressure reaches this value the nucleus may explode.

Photochemistry of coronene with water at 10 K: first tentative identification by infrared spectroscopy of oxygen containing coronene products

UV photochemistry of a polycyclic aromatic hydrocarbon model, coronene (C(24)H(12)), has been investigated when it is in interaction with water in argon cryogenic matrices, adsorbed on amorphous water ice films, and embedded in solid water. Photoprocessing, carried out at 10 K and λ > 235 nm by means of a high-pressure Hg arc lamp, results in the oxidation and reduction of coronene. These species have been tentatively identified as being the 1,10-dihydroxycoronene and the 1,10-coroquinone by FTIR spectroscopy with the support of isotopic experiments and DFT calculations. These photochemical products most likely form, after hydrogen bonding between C(24)H(12) and H(2)O, through ionization of the PAH and subsequent reactivity with water upon irradiation. Cations, thus generated, react subsequently with water yielding the production of oxygen containing coronene compounds. Such species are of particular interest as they may form in interstellar and early Solar System ices, and are also of astrobiological significance as they could play an important role in processes taking place in most of the living organisms.

Changes in the morphology of interstellar ice analogues after hydrogen atom exposure

The morphology of water ice in the interstellar medium is still an open question. Although accretion of gaseous water could not be the only possible origin of the observed icy mantles covering dust grains in cold molecular clouds, it is well known that water accreted from the gas phase on surfaces kept at 10 K forms ice films that exhibit a very high porosity. It is also known that in the dark clouds H(2) formation occurs on the icy surface of dust grains and that part of the energy (4.48 eV) released when adsorbed atoms react to form H(2) is deposited in the ice. The experimental study described in the present work focuses on how relevant changes of the ice morphology result from atomic hydrogen exposure and subsequent recombination. Using the temperature-programmed desorption (TPD) technique and a method of inversion analysis of TPD spectra, we show that there is an exponential decrease in the porosity of the amorphous water ice sample following D-atom irradiation. This decrease is inversely proportional to the thickness of the ice and has a value of ϕ(0) = 2 × 10(16) D-atoms cm(-2) per layer of H(2)O. We also use a model which confirms that the binding sites on the porous ice are destroyed regardless of their energy depth, and that the reduction of the porosity corresponds in fact to a reduction of the effective area. This reduction appears to be compatible with the fraction of D(2) formation energy transferred to the porous ice network. Under interstellar conditions, this effect is likely to be efficient and, together with other compaction processes, provides a good argument to believe that interstellar ice is amorphous and non-porous.