Icy Grains Research Articles

Interplay of gas and ice during cloud evolution

During the evolution of diffuse clouds to molecular clouds, gas-phase molecules freeze out on surfaces of small dust particles to form ices. On dust surfaces, water is the main constituent of the icy mantle in which a complex chemistry is taking place. We aim to study the formation pathways and the composition of the ices throughout the evolution of diffuse clouds. For this purpose, we use time-dependent rate equations to calculate the molecular abundances in both gas phase and on solid surfaces (onto dust grains). We fully consider the gas-dust interplay by including the details of freeze-out, chemical and thermal desorption, as well as the most important photo-processes on grain surfaces. The difference in binding energies of chemical species on bare and icy surfaces is also incorporated into our equations. Using the numerical code FLASH, we perform a hydrodynamical simulation of a gravitationally bound diffuse cloud and follow its contraction. We find that while the dust grains are still bare, water formation is enhanced by grain surface chemistry which is subsequently released into the gas phase, enriching the molecular medium. The CO molecules, on the other hand, tend to freeze out gradually on bare grains. This causes CO to be well mixed and strongly present within the first ice layer. Once one monolayer of water ice has formed, the binding energy of the grain surface changes significantly and an immediate and strong depletion of gas-phase water and CO molecules occur. While hydrogenation converts solid CO into formaldehyde (H$_2$CO) and methanol (CH$_3$OH), water ice becomes the main constituent of the icy grains. Inside molecular clumps formaldehyde is more abundant than water and methanol in the gas phase owing its presence in part to chemical desorption.

THE PRE-PERIHELION ACTIVITY OF DYNAMICALLY NEW COMET C/2013 A1 (SIDING SPRING) AND ITS CLOSE ENCOUNTER WITH MARS

We used the UltraViolet-Optical Telescope on board Swift to systematically follow the dynamically new comet C/2013 A1 (Siding Spring) on its approach to the Sun. The comet was observed from a heliocentric distance of 4.5 AU pre-perihelion to its perihelion at 1.4 AU. From our observations, we estimate that the water production rate during closest approach to Mars was 1.5 +/- 0.3 x 1E28 molecules/s, that peak gas delivery rates were between 4.5-8.8 kg/s, and that in total between 3.1-5.4 x 1E4 kg cometary gas was delivered to the planet. Seasonal and evolutionary effects on the nucleus govern the pre-perihelion activity of comet Siding Spring. The sudden increase of its water production between 2.46-2.06 AU suggests the onset of the sublimation of icy grains in the coma, likely driven by CO2. As the comet got closer to the Sun, the relative contribution of the nucleus' water production increased, while CO2 production rates decreased. The changes in the comet's activity can be explained by a depletion of CO2, but the comet's high mass loss rate suggests they may also reflect primordial heterogeneities in the nucleus.

Detection of glycolaldehyde toward the solar-type protostar NGC 1333 IRAS2A

Glycolaldehyde is a key molecule in the formation of biologically relevant molecules such as ribose. We report its detection with the Plateau de Bure interferometer toward the Class 0 young stellar object NGC 1333 IRAS2A, which is only the second solar-type protostar for which this prebiotic molecule is detected. Local thermodynamic equilibrium analyses of glycolaldehyde, ethylene glycol (the reduced alcohol of glycolaldehyde), and methyl formate (the most abundant isomer of glycolaldehyde) were carried out. The relative abundance of ethylene glycol to glycolaldehyde is found to be ~5 – higher than in the Class 0 source IRAS 16293-2422 (~1), but similar to the lower limits derived in comets (≥3–6). The different ethylene glycol-to-glycolaldehyde ratios in the two protostars might be related to different CH3OH:CO compositions of the icy grain mantles. In particular, a more efficient hydrogenation on the grains in NGC 1333 IRAS2A would favor the formation of both methanol and ethylene glycol. In conclusion, it is possible that like NGC 1333 IRAS2A, other low-mass protostars show high ethylene glycol-to-glycolaldehyde abundance ratios. The cometary ratios might consequently be inherited from earlier stages of star formation if the young Sun experienced conditions similar to NGC 1333 IRAS2A.

CO DIFFUSION INTO AMORPHOUS H2O ICES

The mobility of atoms, molecules and radicals in icy grain mantles regulate ice restructuring, desorption, and chemistry in astrophysical environments. Interstellar ices are dominated by H2O, and diffusion on external and internal (pore) surfaces of H2O-rich ices is therefore a key process to constrain. This study aims to quantify the diffusion kinetics and barrier of the abundant ice constituent CO into H2O dominated ices at low temperatures (15-23 K), by measuring the mixing rate of initially layered H2O(:CO2)/CO ices. The mixed fraction of CO as a function of time is determined by monitoring the shape of the infrared CO stretching band. Mixing is observed at all investigated temperatures on minute time scales, and can be ascribed to CO diffusion in H2O ice pores. The diffusion coefficient and final mixed fraction depend on ice temperature, porosity, thickness and composition. The experiments are analyzed by applying Fick's diffusion equation under the assumption that mixing is due to CO diffusion into an immobile H2O ice. The extracted energy barrier for CO diffusion into amorphous H2O ice is ~160 K. This is effectively a surface diffusion barrier. The derived barrier is low compared to current surface diffusion barriers in use in astrochemical models. Its adoption may significantly change the expected timescales for different ice processes in interstellar environments.

Ongoing hydrothermal activities within Enceladus.

Detection of sodium-salt-rich ice grains emitted from the plume of the Saturnian moon Enceladus suggests that the grains formed as frozen droplets from a liquid water reservoir that is, or has been, in contact with rock. Gravitational field measurements suggest a regional south polar subsurface ocean of about 10kilometres thickness located beneath an ice crust 30 to 40 kilometres thick. These findings imply rock-water interactions in regions surrounding the core of Enceladus. The resulting chemical 'footprints' are expected to be preserved in the liquid and subsequently transported upwards to the near-surface plume sources, where they eventually would be ejected and could be measured by a spacecraft. Here we report an analysis of silicon-rich, nanometre-sized dust particles (so-called stream particles) that stand out from the water-ice-dominated objects characteristic of Saturn. We interpret these grains as nanometre-sized SiO2 (silica) particles, initially embedded in icy grains emitted from Enceladus' subsurface waters and released by sputter erosion in Saturn's E ring. The composition and the limited size range (2 to 8 nanometres in radius) of stream particles indicate ongoing high-temperature (>90°C) hydrothermal reactions associated with global-scale geothermal activity that quickly transports hydrothermal products from the ocean floor at a depth of at least 40kilometres up to the plume of Enceladus.

Comet 67P/Churyumov-Gerasimenko sheds dust coat accumulated over the past four years.

Comets are composed of dust and frozen gases. The ices are mixed with the refractory material either as an icy conglomerate, or as an aggregate of pre-solar grains (grains that existed prior to the formation of the Solar System), mantled by an ice layer. The presence of water-ice grains in periodic comets is now well established. Modelling of infrared spectra obtained about ten kilometres from the nucleus of comet Hartley2 suggests that larger dust particles are being physically decoupled from fine-grained water-ice particles that may be aggregates, which supports the icy-conglomerate model. It is known that comets build up crusts of dust that are subsequently shed as they approach perihelion. Micrometre-sized interplanetary dust particles collected in the Earth's stratosphere and certain micrometeorites are assumed to be of cometary origin. Here we report that grains collected from the Jupiter-family comet 67P/Churyumov-Gerasimenko come from a dusty crust that quenches the material outflow activity at the comet surface. The larger grains (exceeding 50 micrometres across) are fluffy (with porosity over 50 per cent), and many shattered when collected on the target plate, suggesting that they are agglomerates of entities in the size range of interplanetary dust particles. Their surfaces are generally rich in sodium, which explains the high sodium abundance in cometary meteoroids. The particles collected to date therefore probably represent parent material of interplanetary dust particles. This argues against comet dust being composed of a silicate core mantled by organic refractory material and then by a mixture of water-dominated ices. At its previous recurrence (orbital period 6.5years), the comet's dust production doubled when it was between 2.7 and 2.5 astronomical units from the Sun, indicating that this was when the nucleus shed its mantle. Once the mantle is shed, unprocessed material starts to supply the developing coma, radically changing its dust component, which then also contains icy grains, as detected during encounters with other comets closer to the Sun.

Evolution of H2O, CO, and CO2 production in Comet C/2009 P1 Garradd during the 2011–2012 apparition

We present analysis of high spectral resolution NIR spectra of CO and H2O in Comet C/2009 P1 (Garradd) taken during its 2011–2012 apparition with the CSHELL instrument on NASA’s Infrared Telescope Facility (IRTF). We also present analysis of observations of atomic oxygen in Comet Garradd obtained with the ARCES echelle spectrometer mounted on the ARC 3.5-m telescope at Apache Point Observatory and the Tull Coude spectrograph on the Harlan J. Smith 2.7-m telescope at McDonald Observatory. The observations of atomic oxygen serve as a proxy for H2O and CO2. We confirm the high CO abundance in Comet Garradd and the asymmetry in the CO/H2O ratio with respect to perihelion reported by previous studies. From the oxygen observations, we infer that the CO2/H2O ratio decreased as the comet moved towards the Sun, which is expected based on current sublimation models. We also infer that the CO2/H2O ratio was higher pre-perihelion than post-perihelion. We observe evidence for the icy grain source of H2O reported by several studies pre-perihelion, and argue that this source is significantly less abundant post-perihelion. Since H2O, CO2, and CO are the primary ices in comets, they drive the activity. We use our measurements of these important volatiles in an attempt to explain the evolution of Garradd’s activity over the apparition.

Quantum chemical protocols for modeling reactions and spectra in astrophysical ice analogs: the challenging case of the C⁺ + H₂O reaction in icy grain mantles.

Icy grain mantles that accrete on refractory dust particles in the very cold interstellar medium or beyond the snow line in protoplanetary disks serve as minute incubators for heterogeneous chemistry. Ice mantle chemistry can differ significantly from the gas phase chemistry that occurs in these environments and is often richer. Modeling ices and their chemistry is a challenging task for quantum theoretical methods, but theory promises insight into these systems that is difficult to attain with experiments. Density functional theory (DFT) is predominately employed for modeling reactions in icy grain mantles due to its favorable scalability, but DFT has limitations that risk undercutting its reliability for this task. In this work, basic protocols are proposed for identifying the degree to which DFT methods are able to reproduce experimental or higher level theoretical results for the fundamental interactions upon which ice mantle chemistry depends, including both reactive interactions and non-reactive scaffolding interactions. The exemplar of this study is the reaction of C(+) with H2O, where substantial methodological differences are found in the prediction of gas phase relative energetics for stationary points (about 10 kcal mol(-1) for the C-O bond energy of the H2OC(+) intermediate), which in turn casts doubt about employing it to treat the C(+) + H2O reaction on an ice surface. However, careful explorations demonstrate that B3LYP with small correlation consistent basis sets performs in a sufficiently reliable manner to justify using it to identify plausible chemical pathways, where the dominant products were found to be neutral HOC and the CO(-) anion plus one and two H3O(+) cations, respectively. Predicted vibrational and electronic spectra are presented that would serve to verify or disconfirm the pathways; the latter were computed with time-dependent DFT. Conclusions are compared with those of a recent similar study by McBride and coworkers (J. Phys. Chem. A, 2014, 118, 6991).

The role of energetic processing on solid-phase chemistry in star forming regions

It is generally accepted that complex molecules observed in star forming regions are formed in the solid phase on icy grain mantles and are released to the gas-phase after desorption of icy mantles. Most of our knowledge on the physical and chemical properties of ices in star forming regions is based on the comparison between observations and laboratory experiments performed at low temperature (10–100 K). Here we present some recent laboratory experiments which show the formation of (complex) molecular species after ion bombardment of simple ices.

Report of an international workshop "Icy Grain Chemistry for Formation of Complex Organic Molecules: From Molecular Clouds to Protoplanetary Disks, Comets and Meteorites"

Report of an international workshop "Icy Grain Chemistry for Formation of Complex Organic Molecules: From Molecular Clouds to Protoplanetary Disks, Comets and Meteorites"

OBSERVATIONS OF COMET ISON (C/2012 S1) FROM LOWELL OBSERVATORY

We observed dynamically new sungrazing comet ISON (C/2012 S1) extensively at Lowell Observatory throughout 2013 in order to characterize its behavior prior to perihelion. ISON had "typical" abundances for an Oort Cloud comet. Its dust production, as measured by Afrho, remained nearly constant during the apparition but its CN gas production increased by ~50x. The minimum active area necessary to support observed water production rates exceeded the likely surface area of the nucleus and suggests a population of icy grains in the coma. Together with the flattening of the dust radial profile over time, this is consistent with ejection of a large quantity of slow moving dust and icy grains in the coma at large heliocentric distance. The dust morphology was dominated by the tail, but a faint sunward dust fan was detected in March, April, May, and September. We imaged multiple gas species in September, October, and November. Excess CN signal was observed in the sunward hemisphere in September and early October. In November the excess CN signal was in the tailward hemisphere and two faint CN features appeared approximately orthogonal to the tail with position angles varying by about +/-20 degrees from night to night. Using numerical modeling, we best reproduced the orientation and shape of these features as well as the bulk brightness with a pole oriented approximately towards the Sun and a single source located within ~35 degrees of the equator. The production rates and coma morphology suggest a nucleus that was active over nearly its entire sunward facing hemisphere in September and October but which underwent a significant mass loss event, potentially including fragmentation, shortly before November 1. Significant mass loss likely continued at the same site over subsequent days/weeks and may have catastrophically weakened the nucleus prior to perihelion.

Formation and spectroscopy of dicyanotriacetylene (NC₈N) in solid Kr.

Thermally induced creation of dicyanotriacetylene (NC8N) was observed in solid krypton. Samples were obtained by cryogenic trapping of gaseous cyanoacetylene/Kr mixtures subjected to electric discharges. Strong a (3)Σ(+)(u) → X (1)Σ(+)(g) phosphorescence of NC8N is reported here for the first time; its vibronic structure permitted the measurement of several ground-state vibrational frequencies. Other chemical species, mostly smaller than the precursor molecule, have also been formed, among them the dicarbon molecule (C2), and these may serve as indispensable building blocks in the NC8N synthesis. Processes leading to the elongation of cyanoacetylenic chains are of potential importance for the chemistry of icy grains present in the interstellar gas clouds.

LABORATORY FORMATION OF FULLERENES FROM PAHS: TOP-DOWN INTERSTELLAR CHEMISTRY

Interstellar molecules are thought to build up in the shielded environment of molecular clouds or in the envelope of evolved stars. This follows many sequential reaction steps of atoms and simple molecules in the gas phase and/or on (icy) grain surfaces. However, these chemical routes are highly inefficient for larger species in the tenuous environment of space as many steps are involved and, indeed, models fail to explain the observed high abundances. This is definitely the case for the C$_{60}$ fullerene, recently identified as one of the most complex molecules in the interstellar medium. Observations have shown that, in some PDRs, its abundance increases close to strong UV-sources. In this letter we report laboratory findings in which C$_{60}$ formation can be explained by characterizing the photochemical evolution of large PAHs. Sequential H losses lead to fully dehydrogenated PAHs and subsequent losses of C$_{2}$ units convert graphene into cages. Our results present for the first time experimental evidence that PAHs in excess of 60 C-atoms efficiently photo-isomerize to Buckminsterfullerene, C$_{60}$. These laboratory studies also attest to the importance of top-down synthesis routes for chemical complexity in space.

Combined quantum chemical and modeling study of CO hydrogenation on water ice

Context. Successive hydrogenation reactions of CO on interstellar icy grain surfaces are considered one of the most e cient mechanisms in interstellar environments for the formation of H2CO and CH3OH, two of the simplest organic molecules detected in space. In the past years, several experimental and theoretical works have been focused on these reactions, providing relevant information both at the macroscopic and atomic scale. However, several questions still remain open, such as the exact role played by water in these processes, a crucial aspect because water is the dominant constituent of the ice mantles around dust grain cores. Aims. We here present a quantum chemical description of the successive H additions to CO both in the gas phase and on the surfaces of several water clusters. Methods. The hydrogenation steps were calculated by means of accurate quantum chemical methods and structural cluster models consisting of 3, 18, and 32 water molecules. Results. Our main result is that the interaction of CO and H2CO with the water cluster surfaces through H-bonds with the O atoms increases the C O polarization, thus weakening the C O bond. Consequently, the C atoms are more prone to receiving H atoms, which in turn lowers the energy barriers for the H additions compared to the gas-phase processes. The calculated energy barriers and transition frequencies associated with the reaction coordinate were adopted as input parameters in our numerical model of the surface chemistry (GRAINOBLE) to simulate the distribution of the H2CO and CH3OH ice abundances (with respect to water). Our GRAINOBLE results based on the energy barriers and transition frequencies for the reactions on the 32 water molecule cluster compare well with the observed abundances in low-mass protostars and dark cores.

CONSTRAINING THE DUST COMA PROPERTIES OF COMET C/SIDING SPRING (2013 A1) AT LARGE HELIOCENTRIC DISTANCES

The close encounter of Comet C/2013 A1 (Siding Spring) with Mars on October 19, 2014 presented an extremely rare opportunity to obtain the first flyby quality data of the nucleus and inner coma of a dynamically new comet. However, the comet's dust tail potentially posed an impact hazard to those spacecraft. To characterize the comet at large heliocentric distances, study its long-term evolution, and provide critical inputs to hazard modeling, we imaged C/Siding Spring with the Hubble Space Telescope when the comet was at 4.58, 3.77, and 3.28 AU from the Sun. The dust production rate, parameterized by the quantity Af$\rho$, was 2500, 2100, and 1700 cm (5000-km radius aperture) for the three epochs, respectively. The color of the dust coma is 5.0$\pm$0.3$\%$/100 nm for the first two epochs, and 9.0$\pm$0.3$\%$/100 nm for the last epoch, and reddens with increasing cometocentric distance out to ~3000 km from the nucleus. The spatial distribution and the temporal evolution of the dust color are most consistent with the existence of icy grains in the coma. Two jet-like dust features appear in the north-northwest and southeast directions projected in the sky plane. Within each epoch of 1-2 hour duration, no temporal variations were observed for either feature, but the PA of the southeastern feature varied between the three epochs by ~30$^\circ$. The dust feature morphology suggests two possible orientations for the rotational pole of the nucleus, (RA, Dec) = (295$^\circ\pm$5$^\circ$, +43$^\circ\pm$2$^\circ$) and (190$^\circ\pm$10$^\circ$, 50$^\circ\pm$5$^\circ$), or their diametrically opposite orientations.

Dynamical modelling of river deltas on Titan and Earth

The surface of Titan hosts a unique Earth-like environment with lakes and rivers, and active ‘hydrologic’ cycle of methane. We investigate sediment transport in Titanian rivers and deposition in Titanian lakes with particular attention to formation of river deltas. The obtained results are compared with analogous terrestrial processes. The numerical model based on Navier–Stokes equations for depth-integrated two dimensional turbulent flow and additional equations for bed-load and suspended-load sediment transport was used in our research. It is found that transport of icy grains in Titanian rivers is more effective than silicate grains of the same size in terrestrial rivers for the same assumed total discharge. This effect is explained theoretically using dimensionless form of equations or comparing forces acting on the grains. Our calculations confirm previous results (Burr et al., 2006. Icarus. 181, 235–242). We calculate also models with organic sediments of different densities, namely 1500 and 800kgm−3. We found substantial differences between materials of varying densities on Titan, but they are less pronounced than differences between Titan and Earth.

MASS TRANSPORT AROUND COMETS AND ITS IMPACT ON THE SEASONAL DIFFERENCES IN WATER PRODUCTION RATES

Comets are surrounded by a thin expanding atmosphere, and although the nucleus' gravity is small, some molecules and grains, possibly with the inclusion of ices, can get transported around the nucleus through scattering (atoms/molecules) and gravitational pull (grains). Based on the obliquity of the comet, it is also possible that volatile material and icy grains get trapped in regions, which are in shadow until the comet passes its equinox. When the Sun rises above the horizon and the surface starts to heat up, this condensed material starts to desorb and icy grains will sublimate off the surface, possibly increasing the comet's neutral gas production rate on the outbound path. In this paper we investigate the mass transport around the nucleus, and based on a simplified model, we derive the possible contribution to the asymmetry in the seasonal gas production rate that could arise from trapped material released from cold areas once they come into sunlight. We conclude that the total amount of volatiles retained by this effect can only contribute up to a few percent of the asymmetry observed in some comets.

EXTREMELY ORGANIC-RICH COMA OF COMET C/2010 G2 (HILL) DURING ITS OUTBURST IN 2012

We performed high-dispersion near-infrared spectroscopic observations of comet C/2010 G2 (Hill) at 2.5 AU from the Sun using NIRSPEC (R ≈ 25,000) at the Keck II Telescope on UT 2012 January 9 and 10, about a week after an outburst had occurred. Over the two nights of our observations, prominent emission lines of CH{sub 4} and C{sub 2}H{sub 6}, along with weaker emission lines of H{sub 2}O, HCN, CH{sub 3}OH, and CO were detected. The gas production rate of CO was comparable to that of H{sub 2}O during the outburst. The mixing ratios of CO, HCN, CH{sub 4}, C{sub 2}H{sub 6}, and CH{sub 3}OH with respect to H{sub 2}O were higher than those for normal comets by a factor of five or more. The enrichment of CO and CH{sub 4} in comet Hill suggests that the sublimation of these hypervolatiles sustained the outburst of the comet. Some fraction of water in the inner coma might exist as icy grains that were likely ejected from nucleus by the sublimation of hypervolatiles. Mixing ratios of volatiles in comet Hill are indicative of the interstellar heritage without significant alteration in the solar nebula.

ENANTIOMERIC EXCESSES INDUCED IN AMINO ACIDS BY ULTRAVIOLET CIRCULARLY POLARIZED LIGHT IRRADIATION OF EXTRATERRESTRIAL ICE ANALOGS: A POSSIBLE SOURCE OF ASYMMETRY FOR PREBIOTIC CHEMISTRY

The discovery of meteoritic amino acids with enantiomeric excesses of the L-form (ee L) has suggested that extraterrestrial organic materials may have contributed to prebiotic chemistry and directed the initial occurrence of the ee L that further led to homochirality of amino acids on Earth. A proposed mechanism for the origin of ee L in meteorites involves an asymmetric photochemistry of extraterrestrial ices by UV circularly polarized light (CPL). We have performed the asymmetric synthesis of amino acids on achiral extraterrestrial ice analogs by VUV CPL, investigating the chiral asymmetry transfer at two different evolutionary stages at which the analogs were irradiated (regular ices and/or organic residues) and at two different photon energies (6.6 and 10.2 eV). We identify 16 distinct amino acids and precisely measure the L-enantiomeric excesses using the enantioselective GC × GC-TOFMS technique in five of them: α-alanine, 2,3-diaminopropionic acid, 2-aminobutyric acid, valine, and norvaline, with values ranging from ee L = –0.20% ± 0.14% to ee L = –2.54% ± 0.28%. The sign of the induced ee L depends on the helicity and the energy of CPL, but not on the evolutionary stage of the samples, and is the same for all five considered amino acids. Our results support an astrophysical scenario in which the solar system was formed in a high-mass star-forming region where icy grains were irradiated during the protoplanetary phase by an external source of CPL of a given helicity and a dominant energy, inducing a stereo-specific photochemistry.

INVESTIGATION OF HNCO ISOMER FORMATION IN ICE MANTLES BY UV AND THERMAL PROCESSING: AN EXPERIMENTAL APPROACH

Current gas phase models do not account for the abundances of HNCO isomers detected in various environments, suggesting a formation in icy grain mantles. We attempted to study a formation channel of HNCO and its possible isomers by vacuum-UV photoprocessing of interstellar ice analogues containing H$_2$O, NH$_3$, CO, HCN, CH$_3$OH, CH$_4$, and N$_2$ followed by warm-up, under astrophysically relevant conditions. Only the H$_2$O:NH$_3$:CO and H$_2$O:HCN ice mixtures led to the production of HNCO species. The possible isomerization of HNCO to its higher energy tautomers following irradiation or due to ice warm-up has been scrutinized. The photochemistry and thermal chemistry of H$_2$O:NH$_3$:CO and H$_2$O:HCN ices was simulated using the Interstellar Astrochemistry Chamber (ISAC), a state-of-the-art ultra-high-vacuum setup. The ice was monitored in situ by Fourier transform mid-infrared spectroscopy in transmittance. A quadrupole mass spectrometer (QMS) detected the desorption of the molecules in the gas phase. UV-photoprocessing of H$_2$O:NH$_3$:CO/H$_2$O:HCN ices lead to the formation of OCN$^-$ as main product in the solid state and a minor amount of HNCO. The second isomer HOCN has been tentatively identified. Despite its low efficiency, the formation of HNCO and the HOCN isomers by UV-photoprocessing of realistic simulated ice mantles, might explain the observed abundances of these species in PDRs, hot cores, and dark clouds.