2O Ice Research Articles

An experimental study of the formation of an ice crust and migration of water vapor in a comet's upper layers

From recent close encounters with Comets Wild-2 and Tempel 1 we learned that their surfaces are very rugged and no simple uniform layers model can be applied to them. Rather, a glaciological approach should be applied for describing their surface features and behavior. Such intrinsically rugged surface is formed in our large scale experiments, where an agglomerate of ∼200 μm gas-laden amorphous ice particles is accumulated to form a 20 cm diameter and few cm high ice sample. The density, tensile strength and thermal inertia of our ice sample were found to be very close to those found by Deep Impact for Comet Tempel 1: density 250–300 kg m −3 vs DI 350–400 kg m −3; tensile strength 2–4 kPa vs DI 1–10 kPa; thermal inertia 80 W K −1 m −2 s 1/2 vs <100 W K −1 m −2 s 1/2 and <50 W K −1 m −2 s 1/2. From the close agreement between the thermal inertias measured in our ice sample, which had no dust coverage and that of Comet Tempel 1, we deduce that the low thermal inertia is an intrinsic property of the fluffy structure of the ice as a result of its low density, with an addition by the broken terrain and not due to the formation of a dust layer. Upon warming up of the ice, water vapor migrates both outward into the coma and inward. Reaching cooler layers, the water vapor condenses, forming a denser ice crust, as we show experimentally. We also demonstrate the inward and outward flow of water vapor in the outer ice layers through the exchange between layers of D 2O ice and H 2O ice, to form HDO.

Albedo control of seasonal South Polar cap recession on Mars

Over the last few decades, General Circulation Models (GCM) have been used to simulate the current martian climate. The calibration of these GCMs with the current seasonal cycle is a crucial step in understanding the climate history of Mars. One of the main climatic signals currently used to validate GCMs is the annual atmospheric pressure cycle. It is difficult to use changes in seasonal deposits on the surface of Mars to calibrate the GCMs given the spectral ambiguities between CO 2 and H 2O ice in the visible range. With the OMEGA imaging spectrometer covering the near infra-red range, it is now possible to monitor both types of ice at a spatial resolution of about 1 km. At global scale, we determine the change with time of the Seasonal South Polar Cap (SSPC) crocus line, defining the edge of CO 2 deposits. This crocus line is not symmetric around the geographic South Pole. At local scale, we introduce the snowdrop distance, describing the local structure of the SSPC edge. Crocus line and snowdrop distance changes can now be used to calibrate GCMs. The albedo of the seasonal deposits is usually assumed to be a uniform and constant parameter of the GCMs. In this study, albedo is found to be the main parameter controlling the SSPC recession at both global and local scale. Using a defrost mass balance model (referred to as D-frost) that incorporates the effect of shadowing induced by topography, we show that the global SSPC asymmetry in the crocus line is controlled by albedo variations. At local scale, we show that the snowdrop distance is correlated with the albedo variability. Further GCM improvements should take into account these two results. We propose several possibilities for the origin of the asymmetric albedo control. The next step will be to identify and model the physical processes that create the albedo differences.

Solar infrared occultation observations by SPICAM experiment on Mars-Express: Simultaneous measurements of the vertical distributions of H 2O, CO 2 and aerosol

The infrared AOTF spectrometer is a part of the SPICAM experiment onboard the Mars-Express ESA mission. The instrument has a capability of solar occultations and operates in the spectral range of 1–1.7 μm with a spectral resolution of ∼3.5 cm −1. We report results from 24 orbits obtained during MY28 at L s 130°–160°, and the latitude range of 40°–55° N. For these orbits the atmospheric density from 1.43 μm CO 2 band, water vapor mixing ratio based on 1.38 μm absorption, and aerosol opacities were retrieved simultaneously. The vertical resolution of measurements is better than 3.5 km. Aerosol vertical extinction profiles were obtained at 10 wavelengths in the altitude range from 10 to 60 km. The interpretation using Mie scattering theory with adopted refraction indices of dust and H 2O ice allows to retrieve particle size ( r eff ∼ 0.5 – 1 μm ) and number density (∼1 cm −3 at 15–30 km) profiles. The haze top is generally below 40 km, except the longitude range of 320°–50° E, where high-altitude clouds at 50–60 km were detected. Optical properties of these clouds are compatible with ice particles (effective radius r eff = 0.1 – 0.3 μm , number density N ∼ 10 cm − 3 ) distributed with variance ν eff = 0.1 – 0.2 μm . The vertical optical depth of the clouds is below 0.001 at 1 μm. The atmospheric density profiles are retrieved from CO 2 band in the altitude range of 10–90 km, and H 2O mixing ratio is determined at 15–50 km. Unless a supersaturation of the water vapor occurs in the martian atmosphere, the H 2O mixing ratio indicates ∼5 K warmer atmosphere at 25–45 km than predicted by models.

Existence of ferroelectric ice on planets—A neutron diffraction study

From neutron powder-diffraction experiments, we have studied the growth process of ferroelectric ice XI (eleven) with deuteron-ordered arrangements. We measured time-resolved neutron diffraction of 0.001-M KOD-doped D 2O ice. We observed the growth of ice XI at 72–74 K. The mass fraction f (the ratio of mass of ice XI to that of the doped ice) linearly increased with time for about 2 days. The de-transformed ice Ih, obtained after warming above the transition temperature of 76 K, retransformed to ice XI at 66 K. The observed increase of f with time is in good agreement with the nucleation process of the ordering and the constant growth of the ordered domain. A large fraction of ice Ih sample doped with sodium hydroxide changed to ice XI at 68 K. The results suggest that large quantities of ice on cold icy bodies in our solar system are able to transform to ice XI, which may be detectable by space telescope and planetary exploration.

Experimental and Theoretical Investigation of HC5N Adsorption on Amorphous Ice Surface: Simulation of the Interstellar Chemistry

HC 5N adsorbed on amorphous water ice at 10 K presents an interaction with the ice surface and induces the restructuring of the ice amorphous bulk. Warming up the sample induces the HC 5N desorption from the H 2O ice film, between 120 and 160 K, and the associated desorption energy is 90 kJ/mol. This value is in good agreement with that calculated E d (80 kJ/mol) and gives evidence that the amorphous ice surface is essentially dynamic. From theoretical calculations, it is shown that the HC 5N moiety presents a curvature and is no more linear and stabilized by two strong N...H bonds (2.09 and 2.29 A) and one H...O bond (1.84 A).

Proton segregation on a growing ice interface

Hydronium segregates to the surface of H 2O (D 2O) ice films grown on Pt(1 1 1) above 151 K (158 K). This is observed via the voltage that develops across the films, utilizing work function measurements. The dependence of this voltage on the film’s thickness is explained by a simple equilibrium model: as the film grows, most of the surface ions migrate so as to remain at the ice–vacuum interface, while a fixed percentage (∼0.05%) take the thermodynamically–unfavored route, to become incorporated into the growing bulk ice. This model implies a ΔG of about +0.1 eV for the movement of ions from the ice surface into the bulk ice.

Study of the luminescence of H 2O and D 2O ices induced by charged-particle bombardment

We measured the optical emission of H 2O and D 2O ices in visible region (300–500 nm) induced by energetic hydrogen ions (H +, H 2 +, and H 3 +) irradiation. Our analysis of the data of ion-stimulated luminescence (ISL) shows that all spectra of ISL emission are identical, independent of projectile. We show that all lines in the ISL emission spectrum may be assigned to decays from excited particles and/or fragments of H, H 2, OH, and H 2O. From the independence of emission spectrum on projectile we conclude that the final process causing the emissions may be attributed to the interaction between H + (and/or H) and the water molecules.

Electron irradiation of solid nitrous oxide

We report the results of chemical processing induced by 1 keV electron irradiation of a solid N 2O ice sample formed at 25 K. Ozone and several oxides of nitrogen (NO 2, N 2O 2, N 2O 3, N 2O 4 and N 2O 5) were observed to form during the irradiation period. Such reactions have important implications for the ice chemistry of outer solar system planets/satellites and interstellar ices.

A historical search for habitable ice at the Phoenix landing site

A time-resolved energy balance model in the latitude range targeted by Phoenix, and extending back in time over the past 10 Ma, has been developed and used to predict the time-varying temperature field in ground ice over scales ranging from minutes to millions of years. The temperature history is compared to the population doubling times of terrestrial psychrophiles as a function of temperature, and the lifetime of analog microbe spores against de-activation by galactic cosmic rays (GCR), in order to assess the habitability of ground ice and surrounding materials that may be sampled by Phoenix. Metrics are derived to quantify “habitability” and compare different model configurations, including total and maximum continuous time, per year, that ground ice temperatures exceed various thresholds, maximum and average dormancy periods, and maximum and average consecutive growing seasons. The key unknowns in assessing the position, and hence the temperature, of the ground ice table at high northern latitude is the fate of the perennial north polar cap at high obliquity. If enough H 2O ice can persist at polar latitudes to buffer at least the high-latitude atmosphere at all orbital configurations, ground ice is found to be relatively shallow over much of the past 10 Ma, and regularly achieves temperatures in excess of those required for the growth of terrestrial psychrophiles. The dry overburden expected at the landing site can easily be sampled by Phoenix, and includes the “sweet spot” that is characterized by the optimal habitability metrics over the past 10 Ma. If the atmosphere is buffered only by low-latitude ice deposits at obliquities greater than about 30°, the frequency and duration of habitable ice is considerably diminished, and the intervening dormancy periods, during which cosmic ray damage accumulates, are correspondingly longer. In all cases, the maximum dormancy period that must be survived by putative martian psychrophiles is at least an order of magnitude greater than the amount of time required to reduce terrestrial psychrophile spore viability by 10 −6 ( ∼ 7 × 10 4 years). Depending on the fate of high-obliquity polar ice, the maximum dormancy period can exceed 4 × 10 6 years, a factor of 60 longer than terrestrial psychrophile spore lifetimes. Habitability of martian ground ice is therefore dependent on putative martian psychrophiles developing robustness against GCR deactivation at least an order of magnitude greater than their terrestrial counterparts. Simulations of ground ice throughout the 65° N–72° N latitude range accessible to Phoenix suggest that higher-latitude ground ice has better habitability metrics, although the discrepancy is less than an order of magnitude for all metrics and across the entire latitude range.

Carbon dioxide segregation in 1:4 and 1:9 CO 2:H 2O ices

The mid-infrared spectra of mixed vapor deposited ices of CO 2 and H 2O were studied as a function of both deposition temperature and warming from 15 to 100 K. The spectra of ices deposited at 15 K show marked changes on warming beginning at 60 K. These changes are consistent with CO 2 segregating within the ice matrix into pure CO 2 domains. Ices deposited at 60 and 70 K show a greater degree of segregation, as high as 90% for 1:4 CO 2:H 2O ice mixtures deposited at 70 K. As the ice is warmed above 80 K, preferential sublimation of the segregated CO 2 is observed. The kinetics of the segregation process is also examined. The segregation of the CO 2 as the ice is warmed corresponds to temperatures at which the structure of the water ice matrix changes from the high density amorphous phase to the low density amorphous phase. We show how these microstructural changes in the ice have a profound effect on the photochemistry induced by ultraviolet irradiation. These experimental results provide a framework in which observations of CO 2 on the icy bodies of the outer Solar System can be considered.

Ion irradiation of H 2O ice on top of sulfurous solid residues and its relevance to the Galilean satellites

In this paper we present the results of new experiments of ion irradiation of water ice deposited on top of a solid sulfurous residue to study the potential formation of SO 2 at the interface ice/refractory material and discuss the possibility that this mechanism accounts for the sulfur dioxide ice detected on the surfaces of the Galilean satellites. In situ infrared spectroscopy was the used experimental technique. We have irradiated a thin film of H 2O frost on a sulfurous layer with 200 keV of He + at 80 K. The used sulfurous residue was obtained by irradiation of frozen SO 2 at 16 K and it is used as a template of sulfur bearing solid materials. We have not found evidences of the efficient formation of SO 2 after irradiation of H 2O ice on top of the sulfurous residue. An upper limit to the production yield of SO 2, Y ⩽ 6.3 × 10 −7 molecules cm −2 of interface area for each 100 eV of energy absorbed in 1 cm 3 of ice-covered residue, has been estimated. These results have relevance in the context of the surfaces of the icy Galilean satellites in which SO 2 was detected. Our results show that radiolysis of mixtures of water ice and refractory sulfurous materials is not the primary formation mechanism responsible for the SO 2 present on the surfaces of the Galilean satellites.

A close look at Saturn's rings with Cassini VIMS

Soon after the Cassini–Huygens spacecraft entered orbit about Saturn on 1 July 2004, its Visual and Infrared Mapping Spectrometer obtained two continuous spectral scans across the rings, covering the wavelength range 0.35–5.1 μm, at a spatial resolution of 15–25 km. The first scan covers the outer C and inner B rings, while the second covers the Cassini Division and the entire A ring. Comparisons of the VIMS radial reflectance profile at 1.08 μm with similar profiles at a wavelength of 0.45 μm assembled from Voyager images show very little change in ring structure over the intervening 24 years, with the exception of a few features already known to be noncircular. A model for single-scattering by a classical, many-particle-thick slab of material with normal optical depths derived from the Voyager photopolarimeter stellar occultation is found to provide an excellent fit to the observed VIMS reflectance profiles for the C ring and Cassini Division, and an acceptable fit for the inner B ring. The A ring deviates significantly from such a model, consistent with previous suggestions that this region may be closer to a monolayer. An additional complication here is the azimuthally-variable average optical depth associated with “self-gravity wakes” in this region and the fact that much of the A ring may be a mixture of almost opaque wakes and relatively transparent interwake zones. Consistently with previous studies, we find that the near-infrared spectra of all main ring regions are dominated by water ice, with a typical regolith grain radius of 5–20 μm, while the steep decrease in visual reflectance shortward of 0.6 μm is suggestive of an organic contaminant, perhaps tholin-like. Although no materials other than H 2O ice have been identified with any certainty in the VIMS spectra of the rings, significant radial variations are seen in the strength of the water–ice absorption bands. Across the boundary between the C and B rings, over a radial range of ∼7000 km, the near-IR band depths strengthen considerably. A very similar pattern is seen across the outer half of the Cassini Division and into the inner A ring, accompanied by a steepening of the red slope in the visible spectrum shortward of 0.55 μm. We attribute these trends—as well as smaller-scale variations associated with strong density waves in the A ring—to differing grain sizes in the tholin-contaminated icy regolith that covers the surfaces of the decimeter-to-meter sized ring particles. On the largest scale, the spectral variations seen by VIMS suggest that the rings may be divided into two larger ‘ring complexes,’ with similar internal variations in structure, optical depth, particle size, regolith texture and composition. The inner complex comprises the C and B rings, while the outer comprises the Cassini Division and A ring.

Ultraviolet observations of Phoebe from the Cassini UVIS

Observations of Saturn's distant moon Phoebe were made at far-ultraviolet (FUV) (1100–1900 Å) and extreme-ultraviolet (EUV) (600–1100 Å) wavelengths by the Cassini Ultraviolet Imaging Spectrograph (UVIS) during the Cassini spacecraft flyby on June 11, 2004. These are the first UV spectra of Phoebe and the first detection of water ice on a Solar System surface using FUV wavelengths. The characteristics of water ice in the FUV are presented, and Hapke models are used to interpret the spectra in terms of composition and grain size; the use of both areal and intimate mixing models is explored. Non-ice species used in these models include carbon, ice tholin, Triton tholin, poly-HCN and kerogen. Satisfactory disk-integrated fits are obtained for intimate mixtures of ∼ 10 % H 2O plus a non-ice species. Spatially resolved regions of higher ( ∼ 20 % ) and lower ( ∼ 5 % ) H 2O ice concentrations are also detected. Phoebe does not display any evidence of volatile activity. Upper limits on atomic oxygen and carbon are 5 × 10 11 and 2 × 10 12 atoms / cm 2 , respectively, for solar photon scattering. The UVIS detection of water ice on Phoebe, and the ice amounts detected, are consistent with IR measurements and contribute to the evidence for a Phoebe origin in the outer Solar System rather than in the main asteroid belt.

Time-dependent oxygen isotopic effects of CO self shielding across the solar protoplanetary disk

Optically thin surfaces of the solar circumstellar disk were likely sites for generating 16O isotope variability in the early Solar System. Astrochemical reaction network calculations predict that a robust feature of these photoactive horizons of the disk was conversion of CO gas to 16O-poor (high Δ 17O) H 2O ice on a timescale of 10 5 yr. Within several AU of the central star ultraviolet fluxes were too great for the oxygen isotopic effects of CO photodissociation to be sequestered in H 2O, meaning that the CO self shielding oxygen isotopic effect was an outer disk phenomenon. Calculations depicting transport in the circumstellar disk suggest that CO photodissociation at disk surfaces triggered a wave of high-Δ 17O H 2O that passed from surface regions through the outer disk and into the rocky planet-forming region on a timescale of 10 5 to 10 6 yr.

Spatial variability, composition and thickness of the seasonal north polar cap of Mars in mid-spring

Abstract The planetary fourier spectrometer (PFS) for the Mars express mission (MEX) is an infrared spectrometer operating in the wavelength range from 1.2 to 45 μm by means of two spectral channels, called SWC (short wavelength channel) and LWC (long wavelength channel), covering, respectively, 1.2–5.5 and 5.5–45 μm. The middle-spring Martian north polar cap (Ls∼40°) has been observed by PFS/MEX in illuminated conditions during orbit 452. The SWC spectra are here used to study the cap composition in terms of CO2 ice, H2O ice and dust content. Significant spectral variation is noted in the cap interior, and regions of varying CO2 ice grain sizes, water frost abundance, CO2 ice cover and dust contamination can be distinguished. In addition, we correlate the infrared spectra with an image acquired during the same orbit by the OMEGA imaging spectrometer and with the altimetry from MOLA data. Many of the spectra variations correlate with heterogeneities noted in the image, although significant spectral variations are not discernible in the visible. The data have been divided into five regions with different latitude ranges and strong similarities in the spectra, and then averaged. Bi-directional reflectance models have been run with the appropriate lighting geometry and used to fit the observed data, allowing for CO2 ice and H2O ice grain sizes, dust and H2O ice contaminations in the form of intimate granular mixtures and spatial mixtures. A wide annulus of dusty water ice surrounds the recessing CO2 seasonal cap. The inner cap exhibits a layered structure with a thin CO2 layer with varying concentrations of dark dust, on top of an H2O ice underneath ground. In the best-fits, the ices beneath the top layer have been considered as spatial mixtures. The results are still very good everywhere in the spectral range, except where the CO2 ice absorption coefficients are such that even a thin layer is enough to totally absorb the incoming radiation (i.e. the band is saturated). This only happens around 3800 cm−1, inside the strong 2.7-μm CO2 ice absorption band. The effect of finite snow depth has been investigated through a layered albedo model. The thickness of the CO2 ice deposits increases with latitude, ranging from 0.5–1 g cm−2 within region II to 60–80 g cm−2 within the highest-latitude (up to 84°N) region V. Region I is at the cap edge and extends from 65°N to 72°N latitude. No CO2 ice is present in this region, which consists of relatively large grains of water ice (20 μm), highly contaminated by dust (0.15 wt%). The adjacent region II is a narrow region [76–79°N] right at the edge of the north residual polar cap. This region is very distinct in the OMEGA image, where it appears to surround the whole residual cap. The CO2 ice features are barely visible in these spectra, except for the strong saturated 2.7 μm band. It basically consists of a thin layer of 5-mm CO2 ice on top of an H2O ice layer with the same composition as region I. A third interesting region III is found all along the shoulder of the residual cap [79–81°N]. It extends over 1.5 km in altitude and over only 2° of latitude and consists of CO2 ice with a large dust content. It is an admixture of CO2 ice (3–4 mm), with several tens of ppm by mass of water ice and more than 2 ppt by mass of dust. The surface temperatures have been retrieved from the LWC spectra for each observation. We found an increase in the surface temperature in this region, indicating a spatial mixture of cold CO2 ice and warmer dust/H2O ice. Region IV is close to the top of the residual cap [81–84°N]; it is much brighter than region III, with a dust content 10 times lower than the latter. The CO2 grain size is 3 mm and strong CO2 ice features are present in the data, indicating a thicker CO2 ice layer than in region II (1–2 g cm−2). The final region V is right at the top of the residual cap (⩾84°N). It is “pure” CO2 ice (no dust) of 5 mm grain sizes, with 30 ppm by weight of water ice. The CO2 ice features are very pronounced and the 2.7 μm band is saturated. The optical thickness is close to the semi-infinite limit (30–40 g cm−2). Assuming a snowpack density of 0.5 g cm−3, we get a minimum thickness of 1–2 cm for the top-layer of regions II and III, 4–10 cm for region IV, and ⩾60–80 cm thickness for region V. These values are in close agreement with several recent results for the south seasonal polar cap. These results should provide new, useful constraints in models of the Martian climate system and volatile cycles.

Comet Hale–Bopp in outburst: Imaging the dynamics of icy particles with HST/NICMOS

Comet Hale–Bopp was imaged at wavelengths from 1.87 to 2.22 μm by HST/NICMOS in post-perihelion observations starting on UT 1997 August 27.95. Diffraction-limited (∼0 . ″ 2) images were obtained at high signal-to-noise (∼1500) to probe the composition and dynamics of the inner coma and also the size and activity of the nucleus. The velocities of several unusual morphological features over a 1.7 h period, indicate that a significant outburst occurred 7.4 h prior to these images while the comet was at a heliocentric distance of 2.49 AU. Similar features are also apparent after re-analysis of pre-perihelion ground-based images. The inner coma (radius ≲ 2500 km ) is dominated by an “arc” feature, which expanded and became more diffuse with time. This feature can be modeled as the bright central portion of a “jet of outburst” from a near-equatorial region of the nucleus. Less prominent, time-variable linear and circular morphologies are also apparent. The expansion rates of both the arc feature and the circular morphologies imply a common origin and also suggest a grain size distribution with two broad maxima. In addition, several static linear features extend to the edge of the field of view (21,100 km). Radial brightness profiles are highly asymmetric and only approach a ρ −1 decline at distances ⩾15,000 km. Images in a narrow-band filter at 2.04 μm exhibit a ∼4% absorption feature relative to nearly simultaneous images at wavelengths of 2.22, 1.90, and 1.87 μm. This absorption is attributed to H 2O ice in the coma grains. The spatial distribution and expansion velocity of the absorption at 2.04 μm indicate that these grains are associated with the outburst. The constancy of the absorption feature indicates no appreciable sublimation over 1.7 h. The unresolved nucleus has a flux density consistent with a 40 ± 10 km diameter assuming a 4% geometric albedo.

Grain sizes, surface areas, and porosities of vapor-deposited H 2O ices used to simulate planetary icy surfaces

Mean grain sizes and specific surface areas (SSAs) of ice substrates formed by vapor deposition at low temperatures are of importance in simulating external surfaces of icy satellites in the solar system. Environmental scanning electron microscopy (ESEM) was used to obtain granule sizes and to observe the phase of ice granules prepared on borosilicate, silicon, and metallic plates. Ices prepared at a temperature lower than 140 K appear to be amorphous, and their granule sizes are typically submicrometer. At slightly warmer temperatures, near 180–200 K, ice films are composed of either hexagonal or cubic granules with sizes up to a few micrometers. When briefly annealed to even warmer temperatures, ice granule sizes approach ∼ 10 μm . SSAs of ice substrates were determined from BET (Brunauer, Emmett, and Teller) analysis of gas adsorption isotherms in the temperature range from 83.5 to 261 K. SSAs decrease drastically from 102 m 2/g at 83.5 K to 0.87 m 2/g at 150 K and further decrease slowly to 0.22 m 2/g at 261 K, suggesting that the transition from amorphous to crystalline forms occurs at ∼ 150 K . The overall decrease in SSAs is primarily due to metamorphism and sintering. These results are comparable to recent field and laboratory measurements. Possible implications for theoretical models of icy satellites of outer planets using remote sensing techniques are also discussed.

Electron irradiation of crystalline and amorphous D 2O ice

We studied the electron irradiation of crystalline and amorphous deuterated water ices at 12 K. The experiments show that molecular deuterium (D 2), molecular oxygen (O 2), and Hydrogen peroxide (D 2O 2) were produced inside the irradiated ice samples. A quantitative comparison of crystalline and amorphous ice samples showed that the production rates of D 2, O 2 and D 2O 2 in amorphous ices are systematically higher than those in crystalline samples. Reaction mechanisms and astrophysical implications are discussed.

Sublimation pressure and sublimation enthalpy of H 2O ice Ih between 0 and 273.16 K

The sublimation of water in the region of extremely low pressures and temperatures meets increasing interest for the exploration of icy cosmic bodies. At temperatures below 130 K, the shape of the sublimation curve of H 2O is not known; neither experimental data nor theoretical treatments exist for this region. Based upon theoretical upper and lower bounds for the heat capacity of water vapor in this range, a narrow region in the pressure–temperature diagram is identified which must necessarily enclose the sublimation curve down to virtually any lower pressures and temperatures. Within this region, an approximate sublimation curve is computed from the 2006 Gibbs potential of ice Ih, using available heat capacity data points of water vapor between 10 and 130 K. The theoretical zero-point limiting law of the sublimation pressure is derived. Valid between 20 and 273.16 K, correlation equations for the sublimation enthalpy and the sublimation pressure are fitted to the computed data. All quantities are expressed in the 1990 temperature scale ITS-90. Under cosmic conditions, our results suggest that the sublimation of ice is unlikely below 50 K and impossible below 23 K.

Raman spectroscopic features of hydrogen-ordering in ice XII

The Raman spectra of recovered hydrogen ordered H 2O and D 2O ice XIV were recorded and compared with the spectra of the corresponding hydrogen disordered phase (ice XII). On heating ice XIV, the transition to ice XII is observed. Subsequent cooling leads to the weak reappearance of ice XIV lattice vibrational peaks which demonstrates the reversibility of the hydrogen order ↔ disorder phase transition. However, cooling at ambient pressure produces a less ordered ice XIV than cooling under pressure which implies that pressure favours hydrogen ordering of ice XII.