Ice Mixtures Research Articles

Detecting and characterizing the abundance and form of water-ice in permanently-shadowed regions of the moon using a three-band lidar system

We investigated the ability of a three-band lidar instrument (with wavelengths of 532, 1064, and 1560 nm) to detect and quantify different types of regolith ± water ice mixtures for both mare and highlands regions. A variety of samples were spectrally characterized, focusing on variables such as type of lunar regolith (simulant, lunar meteorite, highland vs mare), grain size (<45 μm, 45–1000 μm, <1 mm), water ice surface coverage and porosity of regolith (discontinuous, continuous, packed), and water ice percentages in intimate mixtures vs areal mixtures. Spectral metrics included absolute reflectance (532, 1064, 1560 nm) and reflectance ratios (1560/532, 1064/532, 1560/1064 nm). The ability to detect water ice in the presence of different types of regolith is critically dependent on the optical properties of end members, the selected wavelength, and the physical nature of the samples, such as grain size and areal vs intimate mixtures. Given the differences in the spectroscopic properties of the end members, both absolute reflectance and reflectance ratios have different predictive powers and both are required for robust analysis. The 1560 nm region is the most sensitive of the three wavelength regions to water ice content, as it falls well within a major water ice absorption band. Spectral contrast affects the ability to detect water ice. It is most sensitive for characterizing water ice + highland materials because the latter are brighter than water ice in this wavelength region. Powdered lunar materials are red-sloped across the 532 to 1064 to 1560 nm regions. By contrast, water ice shows a large downturn in reflectance between 1064 and 1560 nm. Detection limits for water ice in many situations can be as low as 2% and are lower for areal vs intimate mixtures. We found that porosity and angle of incidence with the surface have only minor effects on spectral reflectance properties of regolith. Further confirmation of water ice could be realized by comparing lidar data acquired by transects across permanently-shadowed regions and seeing how absolute reflectance and reflectance ratios vary between sunlit and adjacent shadowed regions. If it is known or assumed that regolith properties do not change as one enters a PSR, with the exception of the presence of surficial water ice, changes in reflectivity and reflectance ratios could provide compelling evidence for the presence of water ice.

COMPRESSING AIR IN A TANK PARTIALLY FILLED FROZEN WATER AND ITS USE TO DRAIN WATER FROM SOLAR COLLECTORS

The use of the effect of increasing the volume of ice when freezing water to compress the air in a tank partially filled with water is considered. The aim of the study is to create conditions for the movement of the ice surface and determine the final air pressure after freezing water. A method of air compression has been developed and theoretical and experimental studies of the final air pressure have been performed. Experimental tests of a patented method of air compression with the provision of a movable ice surface by applying thermal insulation to the air chamber at the top of the tank were carried out. For this purpose, two air compressors of different sizes are made using cold air and a mixture of ice and salt. Obtained excess air pressures from 0.3 to 0.5 bar. The thermodynamic analysis of the process is carried out. The equation for determining the pressure at complete freezing of water, with a constant cross-section of the air compressor is obtained. Compression calculations are close to experimental data. Studies have shown the real technical possibility of air compression when water freezes in a rigid sealed tank and its use for automatic drainage of water from solar panels at the onset of frost. 1. The proposed and experimentally investigated method of ensuring the movement of the ice surface relative to the walls of the tank when compression of the air in the tank, by thermal insulation of the airspace. Obtained thermodynamic analysis of the equation for the final pressure of the air compressed by ice. When using a tank with a constant horizontal section, the final pressure does not depend on the area of ​​the cross section and increases with increasing water level. The experiments on two installations showed that excess pressure reaches 0.3 - 0.5 bar. Freezing water tank is proposed to be used as a temperature sensor, and compressed air can drive a direct action valve. Calculations of water freezing time for compression of the air showed its significant reduction with the growth of the tank coefficient.

High pressure behavior of ethylene and water: From clathrate hydrate to polymerization in solid ice mixtures.

Among the ice mixtures that can be found in our universe, those involving ethylene are poorly studied even though ethylene reportedly exists in the presence of water in several astrochemical domains. Here, we report on the chemistry of ethylene and water mixtures in both pressure (0-15GPa) and temperature (300-370K) ranges relevant to celestial bodies conditions. The behavior of the binary mixture has been tracked, starting from the ethylene clathrate hydrate and following its evolution through two different crystalline phases up to 2.10GPa, where it decomposes into a solid mixture of water ice and crystalline ethylene. The pressure and temperature evolution of this mixture has been studied up to the complete transformation of ethylene into polyethylene and compared with that of the pure hydrocarbon, reporting here for the first time its spectroscopic features upon compression. The spectroscopic analysis of the recovered polymers from the ice mixtures provided hints about the reactivity of the monomer under the environmental stress exerted by the water network. The results of this study are expected to be significant in a variety of fields ranging from astrochemistry to material science and also to fundamental chemistry, particularly regarding the study and modelization of the behavior of complex mixtures.

Proton and Electron Irradiations of CH4:H2O Mixed Ices

The organic chemistry occurring in interstellar environments may lead to the production of complex molecules that are relevant to the emergence of life. Therefore, in order to understand the origins of life itself, it is necessary to probe the chemistry of carbon-bearing molecules under conditions that simulate interstellar space. Several of these regions, such as dense molecular cores, are exposed to ionizing radiation in the form of galactic cosmic rays, which may act as an important driver of molecular destruction and synthesis. In this paper, we report the results of a comparative and systematic study of the irradiation of CH4:H2O ice mixtures by 1 MeV protons and 2 keV electrons at 20 K. We demonstrate that our irradiations result in the formation of a number of new products, including both simple and complex daughter molecules such as C2H6, C3H8, C2H2, CH3OH, CO, CO2, and probably also H2CO. A comparison of the different irradiation regimes has also revealed that proton irradiation resulted in a greater abundance of radiolytic daughter molecules compared to electron irradiation, despite a lower radiation dose having been administered. These results are important in the context of the radiation astrochemistry occurring within the molecular cores of dense interstellar clouds, as well as on outer Solar System objects.

Аморфный лед средней плотности, полученный разложением водно-гелиевого геля

The article presents experimental studies of structural changes that occur during heating of nanosized powders of amorphous ice obtained by decay of a water-helium gel. Thermal annealing of the obtained samples was carried out by short exposures (about 15 minutes) at different temperatures in the range of 110-230K. The behavior of the amorphous phase during annealing was analyzed within the framework of its description by a mixture of amorphous ices of low and medium density (LDA and MDA, respectively). It was found that at the such description, the virgin sample was predominantly in the MDA state, while the proportion of the LDA phase was about 7 times less (MDA/LDA ≈ 7:1). It has been established that during annealing, a multistage process of structural transformations of the initial LDA + MDA sample takes place: from initial changes in the amorphous state at 110 K through crystallization of the cubic ice phase Ic with its intensive growth at a temperature of 130 K to the transformation of cubic ice into the hexagonal phase Ih in the temperature range T =135÷230K.

Thermal behavior of astrophysical amorphous molecular ices.

Ice is a major component of astrophysical environments - from interstellar molecular clouds through protoplanetary disks to evolved solar systems. Ice and complex organic matter coexist in these environments as well, and it is thought primordial ice brought the molecules of life to Earth four billion years ago, which could have kickstarted the origin of life on Earth. To understand the journey of ice and organics from their origins to becoming a part of evolved planetary systems, it is important to complement high spatial and spectral resolution telescopes such as JWST with laboratory experimental studies that provide deeper insight into the processes that occur in these astrophysical environments. Our laboratory studies are aimed at providing this knowledge. In this article we present simultaneous mass spectrometric and infrared spectroscopic investigation on how molecular ice mixtures behave at different temperatures and how this information is critical to interpret observational data from protoplanetary disks as well as comets. We find that amorphous to crystalline water ice transformation is the most critical phenomenon that differentiates between outgassing of trapped volatiles such as CO2vs. outgassing of pure molecular ice domains of the same in a mixed molecular ice. Crystalline water ice is found to trap only a small fraction of other volatiles (<5%), indicating ice grain composition in astrophysical and planetary environments must be different depending on whether the ice is in amorphous phase or transformed into crystalline phase, even if the crystalline ice undergoes radiation-induced amorphization subsequently. Crystallization of water ice is a key differentiator for many ices in astronomical environments as well as in our Solar System.

Target temperature of local cryotherapy

BACKGROUND: local cryotherapy (LC) is a promising method of treating various diseases, as a result of which the effects of anesthesia and reduction of inflammation are achieved. The main factor of efficiency from a physical point of view is the achievement of the target temperature on the surface of the impact area. At the same time, during the procedure, the surface temperature of the tissue should not fall below the minimum permissible the limit temperature for safety.&#x0D; AIMS: identification of the range of target and limit temperatures in terms of safety parameters based on the results of literature analysis, carrying out experiments with the use of ice bag for cooling the biotissue phantom, determining the most appropriate impact modes.&#x0D; MATERIALS AND METHODS: a series of experiments was carried out on the developed stand to compare the two modes of the contact method of LC, depending on the working substance used (sodium chloride solution, a mixture of ice and water). The comparison was carried out according to the following parameters: the time to reach the target temperature on the surface, the temperature at the depth of the model medium. The temperature was measured by resistance thermometers (Pt100).&#x0D; RESULTS: when cooled with a sodium chloride solution, the surface temperature of the model medium dropped to 10 C in 6 minutes of exposure, and after 17 minutes reached its minimum value of 6.2 C. In the case of using a mixture of water and ice, the minimum temperature of 12.5 C was reached in 58 minutes. The minimum temperature at a depth of 8 mm in both cases was approximately the same and was about 17 C. It was reached by 19 minutes with a solution of sodium chloride and for 58 minutes with a mixture of water and ice.&#x0D; CONCLUSIONS: as a result of the experiments carried out, the most suitable contact cooling mode with the use of ice bag was determined - a mode using sodium chloride as a working substance with an initial temperature of minus 18.4 C with a possible impact time from 6 to 19 minutes.

Dynamic mode I fracture properties of saturated white sandstone under ambient sub-zero temperatures

Dynamic fracture properties of frozen rocks are essential for better designs of rock engineering infrastructures in cold regions. Thus, a dynamic-cryogenic testing system combined with a high-speed camera is adopted to conduct dynamic tests under ambient room and sub-zero temperatures (20 °C, −10 °C, −20 °C, and −30 °C). A porous white sandstone (WS) is used to fabricate notched semi-circle bend (NSCB) specimens for mode I dynamic fracture tests. The percentage of ice and unfrozen water in rock pores under different temperatures is quantified using the nuclear magnetic resonance (NMR) method. For comparison purpose, dry specimens are also tested under the same ambient temperatures. The results show that the dynamic fracture toughness (DFT) for two sets of specimens all show obvious rate dependency. Except for −30 °C, DFTs of dry specimens are marginally influenced by the sub-zero temperatures. On the contrary at a given loading rate, DFT of frozen saturated specimens first drops to the lowest value at −10 °C before further increasing with the dropping ambient temperature. Compared with dry specimens, saturated specimens show lower DFTs and crack propagation velocities that are quantified with the digital image correlation (DIC) method. The experimental results are explained invoking the rock weakening and enhancing mechanisms of the ice and unfrozen water mixture in rock pores. These results are useful to blasting designs in cold areas.

Electronic Sputtering from H2O:C2H6 ice bombarded by energetic heavy ions

H2O:C2H6 ice mixture, in a proportion of approximately 1:10 and at 60 K in average, is irradiated by energetic multicharged ions (≈65MeV252Cf fission fragments). The sputtered positive and negative secondary ions are analyzed by time-of-flight plasma desorption mass spectrometry (PDMS-TOF). Several ionic species are identified, indicating strong surface fragmentation and clusterization. The bombardment induces the formation of the ion cluster series: (CH2)nX, where X = CH3+, C2H6+ or CH+, and the carbon cluster series (C)nX, where X = CH3+ or CH−. It is found that, in general, the yield distributions exhibit exponential decays, Y=Y0Fexp(−kFn)+Y0Sexp(−kSn), where kF=1.11 u−1 and kS=0.11 u−1. Such finding may be useful to understand the processes by which, in cosmic space, ionized molecular species of hydrocarbons are delivery to the gas phase.

LIDA: The Leiden Ice Database for Astrochemistry

Context. High-quality vibrational spectra of solid-phase molecules in ice mixtures and for temperatures of astrophysical relevance are needed to interpret infrared observations toward protostars and background stars. Such data are collected worldwide by several laboratory groups in support of existing and upcoming astronomical observations. Over the last 25 yr, the Laboratory for Astrophysics at Leiden Observatory has provided more than 1100 (high-resolution) spectra of diverse ice samples. Aims. In time with the recent launch of the James Webb Space Telescope, we have fully upgraded the Leiden Ice Database for Astrochemistry (LIDA) adding recently measured spectra. The goal of this paper is to describe what options exist regarding accessing and working with a large collection of infrared (IR) spectra, and the ultraviolet-visible (UV/vis) to the mid-infrared refractive index of H2O ice. This also includes astronomy-oriented online tools to support the interpretation of IR ice observations. Methods. This ice database is based on open-source Python software, such as Flask and Bokeh, used to generate the web pages and graph visualization, respectively. Structured Query Language (SQL) is used for searching ice analogs within the database and Jmol allows for three-dimensional molecule visualization. The database provides the vibrational modes of molecules known and expected to exist as ice in space. These modes are characterized using density functional theory with the orca software. The IR data in the database are recorded via transmission spectroscopy of ice films condensed on cryogenic substrates. The real UV/vis refractive indices of H2O ice are derived from interference fringes created from the simultaneous use of a monochromatic HeNe laser beam and a broadband Xe-arc lamp, whereas the real and imaginary mid-IR values are theoretically calculated. LIDA not only provides information on fundamental ice properties, but it also offers online tools. The first tool, SPECFY, is directly linked to the data in the database to create a synthetic spectrum of ices towards protostars. The second tool allows the uploading of external files and the calculation of mid-infrared refractive index values. Results. LIDA provides an open-access and user-friendly platform to search, download, and visualize experimental data of astrophysically relevant molecules in the solid phase. It also provides the means to support astronomical observations; in particular, those that will be obtained with the James Webb Space Telescope. As an example, we analysed the Infrared Space Observatory spectrum of the protostar AFGL 989 using the resources available in LIDA and derived the column densities of H2O, CO and CO2 ices.

Radiation-induced D-to-H Exchange in Ices Containing Ethane or Benzene: Reactions and Rate Constants

The rate constants for radiation-induced deuterium to hydrogen (D-to-H) exchange were recently measured for CD4 in H2O and C2D6 in H2O at 20 K. In the current study, rate constants for radiation-induced D-to-H exchange were obtained for C6D6 in H2O at 20, 50, and 100 K. The rate constants for D-to-H exchange for an organic molecule in water were found to be an order of magnitude greater for benzene than ethane. Additionally, D-to-H exchange for benzene was found to be far more favorable than the production of any other radiolysis product, a result that contrasts with our previous findings for methane and ethane. Finally, rate constants were obtained for C2D6 in H2O at 50 K. For all ice mixtures, increasing the temperature of the experiment yielded a larger rate constant. There is a notable difference in the magnitude of the response of the rate constant for D-to-H exchange as a function of temperature for C2D6 versus C6D6 containing ices. The results suggest that radiation will have an asymmetric effect on D-to-H exchange between different types of organic compounds and water ice matrices. These results suggest that the radiation history of an extraterrestrial sample will influence the D/H ratios of its organic compounds.

Ammonia, carbon dioxide, and the non-detection of the 2152 cm−1 CO band

Context. COa title should be crisp and newspaper-style, and direct questions are to be avoided in scientific writing. Please rephrase your title and take this into account. Your running title may be a good place to start is one of the most abundant ice components on interstellar dust grains. When it is mixed with amorphous solid water (ASW) or located on its surface, an absorption band of CO at 2152 cm−1 is always present in laboratory measurements. This spectral feature is attributed to the interaction of CO with dangling-OH bonds (dOH) in ASW. However, this band is absent in observational spectra of interstellar ices. This raises the question whether CO forms a relatively pure layer on top of ASW or is in close contact with ASW, but not via dangling bonds. Aims. We aim to determine whether the incorporation of NH3 or CO2 into ASW blocks the dOH and therefore reduces the 2152 cm−1 band. Methods. We performed laboratory experiments to simulate the layered structure of the ice mantle, that is, we grew CO ice on top of 1) pure ASW, 2) NH3:H2O=10:100 mixed ice, and 3) CO2:H2O=20:100 mixed ice. Infrared spectra were measured to quantify the strength of the 2152 cm−1 band. In addition, a second set of experiments were performed to determine how the incorporation of NH3 into ASW affects the dOH band. Results. We found that annealing the ice reduces the 2152 cm−1 band and that NH3 blocks the dOH on ASW surface and therefore reduces the 2152 cm−1 band more effectively than CO2. We suggest that this difference between NH3 and CO2 can be ascribed to the polarity of the guest molecule (NH3 is a polar species, whereas CO2 is apolar). The polarity implies that the formation of an H-bond between the N atom of ammonia and the dOH is a barrier-less reaction. We also determined the pore surface area of the ice mixtures as a function of the annealing temperature, and found that the nondetection of 2152 cm−1 band does not necessarily exclude the possibility of a porous ice mantle.

Formation of c-C6H5CN Ice Using the SPACE TIGER Experimental Setup

Benzonitrile (c-C6H5CN) has been recently detected in cold and dense regions of the interstellar medium, where it has been used as a signpost of a rich aromatic organic chemistry that might lead to the production of polycyclic aromatic hydrocarbons. One possible origin of this benzonitrile is interstellar ice chemistry involving benzene (c-C6H6) and nitrile molecules (organic molecules containing the −C≡N group). We have addressed the plausibility of this c-C6H5CN formation pathway through laboratory experiments using our new setup SPACE TIGER. The SPACE TIGER experimental setup is designed to explore the physics and chemistry of interstellar ice mantles using laser-based ice processing and product detection methods. We have found that c-C6H5CN is formed upon irradiation of c-C6H6:CH3CN binary ice mixtures with 2 keV electrons and Lyα photons at low temperatures (4−10 K). Formation of c-C6H5CN was also observed when c-C6H6 and CH3CN were embedded in a CO ice matrix, but it was efficiently quenched in a H2O ice matrix. The results presented in this work imply that interstellar ice chemistry involving benzene and nitrile molecules could contribute to the formation of the observed benzonitrile only if these species are present on top of the ice mantles or embedded in the CO-rich ice layer, instead of being mixed into the H2O-rich ice layer.

Microstructure‐based effective stress for unsaturated frozen soils

AbstractCurrent knowledge on unsaturated soils suggests that the effective stress should be related to soil microstructures, by which the immobile water attached to the soil solid in micropores and the free water mainly governed by capillary effects in relatively large pores can be distinguished. Paying attention to this knowledge, the micro‐ and macroscopic degrees of saturation to separate micro‐ and macropores occupied by the equivalent water (ice and liquid water) are used and the microstructurally based effective stress for unsaturated frozen soils is proposed using the first law of thermodynamics and the mixture theory. The formulated effective stress considers morphological and mechanical differences of ice in two porosity scales, and can reduce to familiar forms under various assumptions for both frozen–unfrozen and saturated–unsaturated states. Regarding the mixture of microscopic ice and soil particles as the solid skeleton, the elastic bulk modulus of the solid matrix for frozen soils in the regime of compressible solids has also been derived. In addition, other emerging energy‐conjugate pairs have been identified and the second law of thermodynamics is employed to obtain the dissipation inequality in the regime of inelastic deformation for unsaturated frozen soils. The theoretical work presented in this paper could provide an important insight into mechanical constitutive study for unsaturated (saturated) frozen soils in cold regions engineering.

Bibliometric Analysis of Accidental Oil Spills in Ice-Infested Waters.

Oil spills are environmental pollution events that occur due to natural disasters or human activities, resulting in a liquid petroleum hydrocarbon release in the environment, especially into the marine ecosystem. Once oil spills happen, they cause detrimental consequences to the environment, living organisms, and humans. Although there are increasing oil and gas activities in the Arctic region, which is abundant with undiscovered oil and gas resources, the harsh environmental conditions of the region, such as the ice coverage, cold temperatures, long periods of darkness, and its remoteness, pose significant challenges to managing the risk of accidental oil spills in ice-infested waters. In this paper, a bibliometric analysis has been applied to study the global work on oil spill research in ice-infested waters. The paper aims to present an overview of the available oil spill response methods in ice-infested waters, identify the current trends of the research on oil spills in ice-infested waters, and determine the challenges with the future research directions based on the bibliometric analysis. The analysis includes a total number of 77 articles that have been published in this research field which were available in the Scopus database, involving 193 authors from 17 countries dating from 1960 to September 2022. During the bibliometric analysis, the top five most productive authors and countries as well as the most cited publications on oil spills in ice-infested waters have been identified; the authors' cooperation network and the cooperation network between the countries in oil spills research in ice-infested waters have been created; a co-citation analysis and a terms analysis have been performed to identify the popular terms and topics. For future directions, it is recommended for researchers (1) to study real oil spills as much as possible to obtain a good overview through replication under different situations; (2) to develop a new technique for the careful examination and management of the potential risks; (3) to study oil separation from the recovered oil-ice mixture.

Dielectric constant of ice in natural rivers

Ground-penetrating radar (GPR) is a non-contact measurement method widely used to determine ice thickness, which is an important hydrological parameter of rivers in winter. The accuracy of GPR measurements of ice thicknesses is highly dependent on the dielectric constant value of ice. Research on the dielectric constant of ice has mainly focused on pure ice; however, pure ice is not commonly found in natural rivers. In fact, hydrodynamic and meteorological factors, as well as boundary conditions, affect ice in natural rivers, which can be treated as a mixture of pure ice, air bubbles, sediment, and pure water. Therefore, the dielectric constant of ice in natural rivers often differs from the theoretical value of pure ice, a fact that has often been neglected in prototype measurements. During eight years of prototype monitoring in the Heilong River and Yellow River (China), it was found that variations in the dielectric constants of ice in the two rivers produced errors in the GPR ice thickness measurements. In this study, we measure the dielectric constant of ice at 158 measurement points across more than 20 river sections, and we present a recommended range of values for the dielectric constant. Regarding media, such as air bubbles, sediment, and water in natural river ice, we analyze sand (both dry and wet) in ice-bound sediment and the effects of air bubbles, sediment, and water upon the ice's dielectric constant. Using temperature and voltage measuring rods installed in the ice, we demonstrate that the water content in the ice differs significantly according to the freeze-up and breakup periods. The present study recommends guidelines including suitable timing, ice-condition, and dielectric-constant reference values for GPR-based ice thickness measurements to improve the measurement accuracy. Thus, this work can serve as a reference for future applications in similar rivers.

Characterizing Seasonal and Residual Ices at the South Pole of Mars Using a Universal Set of CRISM Spectral Endmembers

AbstractCurrent maps of compositional variation across south polar ice exposures on Mars do not resolve the meter‐scales at which erosional processes are most active, ultimately limiting our understanding of how the deposits form and evolve and how they can be used to interpret long‐term climate records. In this study, we use k‐means clustering and random forest classification to identify and map a set of universal spectral endmembers across 167 high‐resolution observations acquired during southern summer by the Compact Reconnaissance Imaging Spectrometer for Mars. The 21 endmembers show distinct combinations and strengths of key infrared absorption features reflecting diverse mixtures of CO2 ice, H2O ice, and dust. The resulting compositional framework can be used to characterize the nature of both seasonal CO2 frost and the residual ices it overlies across a variety of terrains. Following the large dust event of Mars Year 28, the residual CO2‐ice deposits (RCD) were covered by an unusually thick or long‐lived deposit of seasonal frost. Within the RCD, low‐albedo material around erosional features display H2O ice absorptions consistent with exposures around the outer margins of the RCD. These peripheral water‐ice deposits show unexpected variation in CO2 ice and dust content. Most notably, regions within several km of the edge of the RCD display spectral contributions from CO2 ice even after seasonal frost has been removed. These results can inform investigations focused on the dynamics of seasonal CO2 deposition, the development of erosional morphologies, and the creation of climate records in south polar stratigraphy.

Thermal Reactions between H2S and O3: Implications for Europa Surface Chemistry

Here we present a laboratory study demonstrating a low-temperature thermal oxidation reaction within H2O + H2S + O3 solid ice mixtures that produces observable sulfur anion products at temperatures as low as 90 K. This reaction primarily produces SO2, sulfur anions (including HSO, HSO, and SO), and O2 at lower temperatures (90–140 K) and hydrated states of sulfuric acid (H2SO4: nH2O, where n = 0, 1, 4) at higher temperatures (150–250 K). We estimate that the overall activation energy to initiate these reactions is 20 ± 3 kJ mol−1, which is significantly lower than the activation energy required to oxidize SO2 to the sulfate ion. Given the detection of sulfur species on the surfaces of the Galilean satellites and the prevalence of radiolytically produced oxidants, we expect that these thermal reactions will play an important role in explaining the results obtained from future observations and missions that can measure the spatial distribution of these species.

Influence of grain growth on CO2 ice spectroscopic profiles

Context. Interstellar dust grain growth in dense clouds and protoplanetary disks, even when moderate, affects the observed interstellar ice profiles as soon as a significant fraction of dust grains are in the size range close to the wave vector at the considered wavelength. The continuum baseline correction made prior to analysing ice profiles influences the subsequent analysis and hence the estimated ice composition, which are typically obtained by band fitting using thin film ice mixture spectra. Aims. We explore the effect of grain growth on the spectroscopic profiles of ice mantle constituents, focusing particularly on carbon dioxide, with the aim of understanding how it can affect interstellar ice mantle spectral analysis and interpretation. Methods. Using the discrete dipole approximation for scattering and absorption of light, the mass absorption coefficients of several distributions of grains – composed of ellipsoidal silicate cores with water and carbon dioxide ice mantles – are calculated. A few models also include amorphous carbon in the core and pure carbon monoxide in the ice mantle. We explore the evolution of the size distribution starting in the dense core phase in order to simulate the first steps of grain growth up to three microns in size. The resulting mass absorption coefficients are injected into RADMC-3D radiative transfer models of spherical dense core and protoplanetary disk templates to retrieve the observable spectral energy distributions. Calculations are performed using the full scattering capabilities of the radiative transfer code. We then focus on the particularly relevant calculated profile of the carbon dioxide ice band at 4.27 µm. Results. The carbon dioxide anti-symmetric stretching mode profile is a meaningful indicator of grain growth. The observed profiles towards dense cores obtained with the Infrared Space Observatory and Akari satellites already show profiles possibly indicative of moderate grain growth. Conclusions. The observation of true protoplanetary disks at high inclination with the James Webb Space Telescope should present distorted profiles that will allow constraints to be placed on the extent of dust growth. The more evolved the dust size distribution, the more the extraction of the ice mantle composition will require both understanding and taking grain growth into account.

Stability of urea in astrophysical ices. A laboratory study of VUV irradiation and high-energy electron bombardment

ABSTRACT The recent detection of urea in the interstellar medium raises questions about its stability in different astronomical environments. In this work, we have studied the stability of urea ices and urea/water ice mixtures under vacuum-ultraviolet (VUV; 6.3–10.9 eV) irradiation and high-energy (5 keV) electron bombardment at 30, 100, and 200 K. The evolution of the ices was monitored with infrared spectroscopy. CO2, HNCO, and OCN− were identified as reaction products in the 30 K samples. At the higher temperatures CO2 and HNCO were hardly found in the processed ices. The measurements provided destruction cross-sections and allowed the derivation of radiation yields, G100, and half-life doses for urea. G100 values were found to be low (≈3.6–0.3 molecules/100 eV) both for VUV photons and high-energy electrons with electrons being slightly more efficient for the destruction of the molecule. These low G100 values are likely due to favourable mechanisms of energy dissipation or urea recombination. The stability of urea under irradiation increases with temperature which suggests that higher mobility improves the repair mechanisms. Estimates based on these laboratory data indicate that urea should be stable (≈108–109 yr) against irradiation in cold dense clouds and hot cores. It would not survive long (≈103–104 yr) on the bare surface of a Kuiper belt object, but would be well protected (≈109 yr) against radiation below a 30 $\mu$m ice layer. The high resistance of the molecule to radiation damage makes it a good candidate for prebiotic chemistry.