Pure CO2 Ice Research Articles

JWST observations of 13CO2 ice

The structure and composition of simple ices can be severely modified during stellar evolution by protostellar heating. Key to understanding the involved processes are thermal and chemical tracers that can be used to diagnose the history and environment of the ice. The 15.2 µm bending mode of 12CO2 in particular has proven to be a valuable tracer of ice heating events but suffers from grain shape and size effects. A viable alternative tracer is the weaker 13CO2 isotopologue band at 4.39 µm, which has now become accessible at high S/N with the James Webb Space Telescope (JWST). In this study, we present JWST NIRSpec observations of 13CO2 ice in five deeply embedded Class 0 sources that span a wide range in masses and luminosities (0.2–104 L⊙) taken as part of the Investigating Protostellar Accretion Across the Mass Spectrum (IPA) program. The band profiles vary significantly depending on the source, with the most luminous sources showing a distinct narrow peak at 4.38 µm. We first applied a phenomenological approach with which we demonstrate that a minimum of three to four Gaussian profiles are needed to fit the absorption feature of 13CO2. We then combined these findings with laboratory data and show that a 15.2 µm 12CO2 bending-mode-inspired five-component decomposition can be applied to the isotopologue band, with each component representative of CO2 ice in a specific molecular environment. The final solution consists of cold mixtures of CO2 with CH3OH, H2O, and CO as well as segregated heated pure CO2 ice at 80 K. Our results are in agreement with previous studies of the 12CO2 ice band, further confirming that 13CO2 is a useful alternative tracer of protostellar heating and ice composition. We also propose an alternative solution consisting only of heated mixtures of CO2:CH3OH and CO2:H2O ices and warm pure CO2 ice at 80 K (i.e., no cold CO2 ices) for decomposing the ice profiles of HOPS 370 and IRAS 20126, the two most luminous sources in our sample that show strong evidence of ice heating resulting in ice segregation.

Influence of temperature on the chemical evolution and desorption of pure CO ices irradiated by cosmic-rays analogues

ABSTRACT Carbon monoxide (CO) plays a vital role in interstellar chemistry, existing abundantly in both gaseous and frozen environments. Understanding the radiation-driven chemistry of CO-rich ices is crucial for comprehending the formation and desorption of C-bearing molecules in the interstellar medium (ISM), particularly considering the potential impact of temperature on these processes. We report experimental data on irradiation processing of pure CO ice by cosmic ray analogues (95.2 MeV 136Xe23+ ions) at temperatures of 10, 15, and 20 K, in the IGLIAS set-up coupled to the IRRSUD beamline at GANIL (Caen, France). The evolution of the irradiated frozen samples was monitored by infrared spectroscopy. The computational PROCODA code allows us to quantify the chemical evolution of the samples, determining effective reaction rates coefficients (ERCs), molecular abundances at the chemical equilibrium (CE) phase, and desorption processes. The model integrated 18 chemical species – 8 observed (CO, CO2, C3, O3, C2O, C3O, C3O2, and C5O3) and 10 non-observed but predicted (C, O, C2, O2, CO3, C4O, C5O, C2O2, C2O3, C4O2) – linked via 156 reactions. Our findings reveal temperature-driven influences on molecular abundances at chemical equilibrium, desorption yields and rates, and ERC values. Certain reaction routes exhibit distinct thermochemical behaviours of gas- and ice-phase reactions which may be attributed to the presence of neighbouring molecules within the ice matrix. This study provides pivotal insights into the chemical evolution of CO-enriched ice under irradiation, impacting solid-state astrochemistry, clarifying molecular abundances, and advancing our understanding of ISM chemistry and temperature effects on ionized radiation-processed frozen ices.

Photochemical Processes in CO2/H2O Ice Mixtures with Trapped Pyrene, a Model Polycyclic Aromatic Hydrocarbon

Abstract Vacuum ultraviolet irradiation of a polycyclic aromatic hydrocarbon molecule, pyrene, trapped in carbon dioxide (CO2)-enriched astrophysical ice analogues has been studied and presented here. Pyrene is co-deposited with a gaseous mixture of H2O and CO2 (100% CO2; 50:50 CO2:H2O; and 100% H2O) and is subjected to Ly α photons from a hydrogen-flow microwave discharge lamp. To our surprise, we found ionization of pyrene in all three ices, with ionization yields following the H2O content in the ice, indicating that H2O ice stabilizes charged ions and electrons better than CO2 ice by a factor of 10. Caution needs to be exercised not to overinterpret our finding that pyrene is ionized in pure CO2 ice, because even trace amounts of water in the CO2 matrix could result in an increased ionization of pyrene. In addition to ionization of pyrene in CO2 ice, photochemical products of the CO2 ice itself, namely CO and CO3, were found to form efficiently, as detected using Fourier transform infrared spectroscopy, in agreement with earlier studies. UV–vis spectra showed formation of ozone (O3) with prolonged irradiation. Surprisingly, O3 yields followed CO2 concentration in the ice mixtures, with the strongest bands in photolyzed CO2 ice and the weakest in photolyzed H2O ice, indicating that CO2 ice is very protoreactive and produces more O atoms than H2O ice. Pyrene-containing photoproducts, incorporating CO2 or CO or O, such as Py-COOH, Py-OH, or Py-CHO, are not explicitly seen in the UV–vis absorption spectra, but we cannot rule out the possibility that their UV absorption may be swamped under the strong absorption of O3.

Nature, distribution and origin of CO2 on Enceladus

We present the first map of CO2 at the surface of Enceladus using data obtained by the Cassini Visible-Infrared Mapping Spectrometer (VIMS). In order to measure the weak and narrow CO2 absorption band depths around 4.26 µm, we improved: (1) the calibration of VIMS spectra; (2) the calculation of geographic coordinates; and (3) the projection techniques. We averaged multiple observations of a given area to obtain a signal to noise ratio high enough to map the CO2 abundances. CO2 is reliably detected mostly in the South Polar Region. This region includes active faults (Tiger Stripes), the highest observed surface temperatures, and locations of active plume eruptions. The occurrence here of CO2 is consistent with an endogenic origin controlled by tectonics. Both pure CO2 ice and complexed CO2 are detected from the positions of absorption bands between 4.27 and 4.24 µm. The highest concentrations of CO2 are between the main active faults of the South Polar Region, where the surface temperature is low. Pure CO2 ice deposits at the surface of Enceladus are best modeled by the formation of gas pockets below the icy crust and above the surface of the internal ocean. These pockets eventually release cold CO2 gas (∼70 to ∼119 K) at low-velocity (seeping) between the Tiger Stripes [Matson et al., 2018, Icarus, 302, 18–26]. CO2 clathrate hydrates may form in the ocean and may be subsequently released when a CO2 gas pocket blows out and erupts. Other mechanisms may contribute to reinforcing the anti-correlation of the CO2 distribution (of any type) with respect to the location of the Tiger Stripes, such as successive sublimation of CO2 and condensation on colder areas, and partial frost cover by H2O releases from plume eruptions.

Mesospheric CO2 ice clouds on Mars observed by Planetary Fourier Spectrometer onboard Mars Express

We have investigated mesospheric CO2 ice clouds on Mars through analysis of near-infrared spectra acquired by Planetary Fourier Spectrometer (PFS) onboard the Mars Express (MEx) from MY 27 to MY 32. With the highest spectral resolution achieved thus far in the relevant spectral range among remote-sensing experiments orbiting Mars, PFS enables precise identification of the scattering peak of CO2 ice at the bottom of the 4.3 μm CO2 band. A total of 111 occurrences of CO2 ice cloud features have been detected over the period investigated. Data from the OMEGA imaging spectrometer onboard MEx confirm all of PFS detections from times when OMEGA operated simultaneously with PFS. The spatial and seasonal distributions of the CO2 ice clouds detected by PFS are consistent with previous observations by other instruments. We find CO2 ice clouds between Ls = 0° and 140° in distinct longitudinal corridors around the equatorial region (± 20°N). Moreover, CO2 ice clouds were preferentially detected at the observational LT range between 15–16 h in MY 29. However, observational biases prevent from distinguishing local time dependency from inter-annual variation. PFS also enables us to investigate the shape of mesospheric CO2 ice cloud spectral features in detail. In all cases, peaks were found between 4.240 and 4.265 μm. Relatively small secondary peaks were occasionally observed around 4.28 μm (8 occurrences). These spectral features cannot be reproduced using our radiative transfer model, which may be because the available CO2 ice refractive indices are inappropriate for the mesospheric temperatures of Mars, or because of the assumption in our model that the CO2 ice crystals are spherical and composed by pure CO2 ice.

A novel approach to measure photodesorption rates of interstellar ice analogues

Context. In recent years photodesorption rates have been determined in dedicated laboratory experiments for a number of different interstellar ice analogues. These rates are important in order to model non-thermal desorption processes that, for example, affect gas-phase abundances of species and determine the position of photo-induced snow lines in protoplanetary disks. However, different groups using similar experiments have found significant deviating photodesorption values.Aims. Here a new measurement concept is introduced that allows photodesorption rates to be determined following a different experimental approach. The potential of this method is demonstrated using the example of pure CO ice, the solid that gives the most striking discrepancies in the published results.Methods. The new experimental approach uses laser desorption post-ionisation time-of-flight mass spectrometry. It is based on the concept that the physical and geometrical properties of the plume obtained by laser induced desorption of the ice directly depend on the original ice thickness. This allows the ice loss to be determined as function of vacuum ultraviolet (VUV) fluence, which results in a photodesorption rate. The method has the additional advantage that it records all ice species, including photoproducts generated by the VUV irradiation. As a consequence, the method introduced here is also suited to determine the overall photodesorption rate of mixed ices.Results. The photodesorption rate for CO ice at 20 K has been determined as (1.4 ± 0.7) × 10-3 molecules per incident VUV photon. This result is compared to existing experimental and theoretical values and the astronomical relevance is discussed.

Analysis of R-band observations of an outburst of Comet 29P/Schwassmann-Wachmann 1 to place constraints on the nucleus’ rotation state

We present analysis of five nights of R-band observations of Comet 29P/Schwassmann-Wachmann 1 (SW1) taken on September 2008 which show the comet undergoing an outburst. Coma morphology shows a projected asymmetric shell of material expanding radially and four linear features on the northern side of the coma at position angles 37°, 78°, 300°, and 353°. Using the measured projected radial outflow velocity of 0.11 ±0.02 km/s for the shell material, we calculate an outburst time of UT 2008-09-21.03 ±0.95 days. By tracking the inner and outer extent of the northern linear features, we found that the features are fully contained within the expanding shell of material. This suggested both shell and linear features originated during the same event and activity originating from different regions on the nuclear surface are not necessary to generate both types of morphological structure observed. A 3-D Monte Carlo coma model was used to model the outburst. Morphological features present in the observations were modeled allowing constraints to be placed on the spin state of SW1’s nucleus. The evolution of morphological features allows constraints on the rotation period P assuming an outburst duration Δt and the spin period constraints are expressed in terms of their ratio P/Δt. Since the spin-pole orientation could not be constrained, four spin-pole orientations were chosen for modeling the coma. Spin-period constraints for each assumed pole orientation are discussed. Overall, modeling suggested either a spin period on the order of days, a spin-pole orientation nearly along the sub-Earth direction, or a combination of both. To place an independent constraint on the outburst duration, radial surface-brightness profiles of the observations were compared with profiles from synthetic models, giving an upper-limit of Δt ≤ 1.5 days. Longer outbursts resulted in a higher number of dust grains in close proximity to the nucleus during the observations and a profile slope too steep to model observations. Lastly, from photometry of the five nights of observation, a lower limit of (1.8 ± 0.07) × 109 kg was estimated for the total amount of dust emitted during the outburst. Assuming the outburst was triggered by either the sublimation of pure CO or CO2 ice and a dust to gas ratio of ∼4 (Rosetta results for Comet 67P, Rotundi et al. 2015), a lower limit for the outburst duration on the order of hours was obtained.

CO2 INFRARED PHONON MODES IN INTERSTELLAR ICE MIXTURES

ABSTRACT CO2 ice is an important reservoir of carbon and oxygen in star- and planet-forming regions. Together with water and CO, CO2 sets the physical and chemical characteristics of interstellar icy grain mantles, including desorption and diffusion energies for other ice constituents. A detailed understanding of CO2 ice spectroscopy is a prerequisite to characterize CO2 interactions with other volatiles both in interstellar ices and in laboratory experiments of interstellar ice analogs. We report laboratory spectra of the CO2 longitudinal optical (LO) phonon mode in pure CO2 ice and in CO2 ice mixtures with H2O, CO, and O2 components. We show that the LO phonon mode position is sensitive to the mixing ratio of various ice components of astronomical interest. In the era of the James Webb Space Telescope, this characteristic could be used to constrain interstellar ice compositions and morphologies. More immediately, LO phonon mode spectroscopy provides a sensitive probe of ice mixing in the laboratory and should thus enable diffusion measurements with higher precision than has been previously possible.

ICE GRAIN COLLISIONS IN COMPARISON: CO2, H2O, AND THEIR MIXTURES

ABSTRACT Collisions of ice particles play an important role in the formation of planetesimals and comets. In recent work, we showed that CO2 ice behaves like silicates in collisions. The resulting assumption was that it should therefore stick less efficiently than H2O ice. Within this paper, a quantification of the latter is presented. We used the same experimental setup to study collisions of pure CO2 ice, pure water ice, and 50% mixtures by mass between CO2 and water at 80 K, 1 mbar, and an average particle size of ∼90 μm. The results show a strong increase of the threshold velocity between sticking and bouncing with increasing water content. This supports the idea that water ice is favorable for early growth phases of planets in a zone within the H2O and the CO2 iceline.

N2 AND CO DESORPTION ENERGIES FROM WATER ICE

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

UV photoprocessing of CO2ice: a complete quantification of photochemistry and photon-induced desorption processes

Context. Ice mantles that formed on top of dust grains are photoprocessed by the secondary ultraviolet (UV) field in cold and dense molecular clouds. UV photons induce photochemistry and desorption of ice molecules. Experimental simulations dedicated to ice analogs under astrophysically relevant conditions are needed to understand these processes.Aims. We present UV-irradiation experiments of a pure CO2 ice analog. Calibration of the quadrupole mass spectrometer allowed us to quantify the photodesorption of molecules to the gas phase. This information was added to the data provided by the Fourier transform infrared spectrometer on the solid phase to obtain a complete quantitative study of the UV photoprocessing of an ice analog.Methods. Experimental simulations were performed in an ultra-high vacuum chamber. Ice samples were deposited onto an infrared transparent window at 8K and were subsequently irradiated with a microwave-discharged hydrogen flow lamp. After irradiation, ice samples were warmed up until complete sublimation was attained.Results. Photolysis of CO2 molecules initiates a network of photon-induced chemical reactions leading to the formation of CO, CO3 , O2 , and O3 . During irradiation, photon-induced desorption of CO and, to a lesser extent, O2 and CO2 took place through a process called indirect desorption induced by electronic transitions, with maximum photodesorption yields (Y pd ) of ~1.2 × 10-2 molecules incident photon-1 , ~9.3 × 10-4 molecules incident photon-1 , and ~1.1 × 10-4 molecules incident photon-1 , respectively.Conclusions. Calibration of mass spectrometers allows a direct quantification of photodesorption yields instead of the indirect values that were obtained from infrared spectra in most previous works. Supplementary information provided by infrared spectroscopy leads to a complete quantification, and therefore a better understanding, of the processes taking place in UV-irradiated ice mantles.

Highly resolved infrared spectra of pure CO2ice (15–75 K)

Context. The ν 2 bending mode of pure CO2 ice around 15.2 μ m exhibits a fine double-peak structure that offers a sensitive probe to study the physical and chemical properties of solid CO2 in space. Current laboratory spectra do not fully resolve the CO2 ice features.Aims. To improve the fitting of the observed CO2 features, high-resolution solid-state infrared spectra of pure CO2 ice are recorded in the laboratory for a series of astronomically relevant temperatures and at an unprecedented level of detail. Methods. The infrared spectra of pure CO2 ice were recorded in the 4000 to 400 cm-1 (2.5–25 μ m) region at a resolution of 0.1 cm-1 using Fourier transform infrared spectroscopy. Results. Accurate band positions and band widths (FWHM) of pure CO2 ice are presented for temperatures of 15, 30, 45, 60, and 75 K. The focus of this spectroscopic work is on the CO2 (ν 2 ) bending mode, but more accurate data are also reported for the 12 CO2 and 13 CO2 (ν 3 ) stretching mode, and CO2 (ν 1 +ν 3 ) and (2ν 2 +ν 3 ) combination bands.

ANOMALOUS CO2ICE TOWARD HOPS-68: A TRACER OF PROTOSTELLAR FEEDBACK

We report the detection of a unique CO2 ice band toward the deeply embedded, low-mass protostar HOPS-68. Our spectrum, obtained with the Infrared Spectrograph onboard the Spitzer Space Telescope, reveals a 15.2 micron CO2 ice bending mode profile that cannot modeled with the same ice structure typically found toward other protostars. We develop a modified CO2 ice profile decomposition, including the addition of new high-quality laboratory spectra of pure, crystalline CO2 ice. Using this model, we find that 87-92% of the CO2 is sequestered as spherical, CO2-rich mantles, while typical interstellar ices show evidence of irregularly-shaped, hydrogen-rich mantles. We propose that (1) the nearly complete absence of unprocessed ices along the line-of-sight is due to the flattened envelope structure of HOPS-68, which lacks cold absorbing material in its outer envelope, and possesses an extreme concentration of material within its inner (10 AU) envelope region and (2) an energetic event led to the evaporation of inner envelope ices, followed by cooling and re-condensation, explaining the sequestration of spherical, CO2 ice mantles in a hydrogen-poor mixture. The mechanism responsible for the sublimation could be either a transient accretion event or shocks in the interaction region between the protostellar outflow and envelope. The proposed scenario is consistent with the rarity of the observed CO2 ice profile, the formation of nearly pure CO2 ice, and the production of spherical ice mantles. HOPS-68 may therefore provide a unique window into the protostellar feedback process, as outflows and heating shape the physical and chemical structure of protostellar envelopes and molecular clouds.

An examination of Mars’ north seasonal polar cap using MGS: Composition and infrared radiation balance

A detailed analysis of data from one revolution of the Mars Global Surveyor (MGS) is presented. Approximately 80% of this revolution observes the mid-winter northern seasonal polar cap, which covers the surface to <60°N, and which is predominantly within polar night. The surface composition and temperature are determined through analysis of 6–50μm infrared spectra from the Thermal Emission Spectrometer (TES). The infrared radiative balance, which is the entire heat balance in the polar night except for small subsurface and atmospheric advection terms, is calculated for the surface and atmospheric column. The primary constituent, CO2 ice, also dominates the infrared spectral properties by variations in its grain size and by admixtures of dust and water ice, which cause large variations in the 20–50μm emissivity. This is modified by incomplete areal coverage, and clouds or hazes. This quantitative analysis reveals CO2 grain radii ranging from ∼100μm in isolated areas, to 1–5mm in more widespread regions. The water ice content varies from none to about one part per thousand by mass, with a clear increase towards the periphery of the polar cap. The dust content is typically a few parts per thousand by mass, but is as much as an order of magnitude less abundant in “cold spot” regions, where the low emissivity of pure CO2 ice is revealed. This is the first quantitative analysis of thermal spectra of the seasonal polar cap and the first to estimate water ice content. Our models show that the cold spots represent cleaner, dust-free ice rather than finer grained ice than the background. Our guess is that the dust in cold spots is hidden in the center of the CO2 frost particles rather than not present. The fringes of the cap have more dust and water ice, and become patchy, with warmer water snow filling the gaps on the night side, and warmer bare soil on the day side. A low optical depth (<1 in the visible) water ice atmospheric haze is apparent on the night side, and appears with smaller optical depth on the day side. The infrared radiative balance at the surface is typically 20–25Wm−2 in the central polar cap, with ∼25% dips in the regions of dust-free CO2. The atmospheric radiative terms are typically 1–3Wm−2.

CO2ICE TOWARD LOW-LUMINOSITY EMBEDDED PROTOSTARS: EVIDENCE FOR EPISODIC MASS ACCRETION VIA CHEMICAL HISTORY

We present Spitzer IRS spectroscopy of CO2 ice bending mode spectra at 15.2 micrometer toward 19 young stellar objects with luminosity lower than 1 Lsun (3 with luminosity lower than 0.1 Lsun). Ice on dust grain surfaces can encode the history of heating because pure CO2 ice forms only at elevated temperature, T > 20 K, and thus around protostars of higher luminosity. Current internal luminosities of YSOs with L < 1 Lsun do not provide the conditions needed to produce pure CO2 ice at radii where typical envelopes begin. The presence of detectable amounts of pure CO2 ice would signify a higher past luminosity. Many of the spectra require a contribution from a pure, crystalline CO2 component, traced by the presence of a characteristic band splitting in the 15.2 micrometer bending mode. About half of the sources (9 out of 19) in the low luminosity sample have evidence for pure CO2 ice, and six of these have significant double-peaked features, which are very strong evidence of pure CO2 ice. The presence of the pure CO2 ice component indicates that the dust temperature, and hence luminosity of the central star/accretion disk system, must have been higher in the past. An episodic accretion scenario, in which mixed CO-CO2 ice is converted to pure CO2 ice during each high luminosity phase, explains the presence of pure CO2 ice, the total amount of CO2 ice, and the observed residual C18O gas.

Compositional analysis of Hyperion with the Cassini Visual and Infrared Mapping Spectrometer

Compositional mapping of the surface of Hyperion using Cassini Visual and Infrared Mapping Spectrometer (VIMS) observations reveals a heterogeneous surface dominated by water ice accompanied by additional materials. Carbon dioxide, as evidenced by a prominent absorption band centered at 4.26μm, is distributed over most of the surface, including icy regions. This does not represent exposures of pure CO2 ice, but concentrations of CO2 molecules adsorbed on other materials or complexed in H2O, perhaps as a clathrate (Cruikshank, D.P., Meyer, A.W., Brown, R.H., Clark, R.N., Jaumann, R., Stephan, K., Hibbitts, C.A., Sandford, S.A., Mastrapa, R., Filacchione, G., Dalle Ore, C.M., Nicholson, P.D., Buratti, B.J., McCord, T.B., Nelson, R.M., Dalton, J.B., Baines, K.H., Matson, D.L., The VIMS Team [2010]. Icarus 206, 561–572). Localized deposits of low-albedo material in subcircular depressions exhibit spectral absorptions indicative of C–H in aromatic (3.29μm) and aliphatic (3.35–3.50μm) hydrocarbons. An absorption band at 2.42μm that is also seen on other saturnian satellites, tentatively identified as H2 (Clark, R.N. et al. [2011]. In: Proc. AAS-DPS Meeting, 43, 1563; Clark et al., in preparation, 2012) adsorbed on dark material grains, is also prominent. Our best spectral models included H2O and CO2 ice, with small amounts of nanophase Fe and Fe2O3. Weaker and more spatially scattered absorption features are also found at 4.48, 4.60, and 4.89μm, although no clear molecular identifications have yet been made. While strongest in the low-albedo deposits, the CO2, hydrocarbon and putative H2 bands vary in strength throughout the icy regions, as do the 4.48-, 4.60- and 4.89-μm bands, suggesting that this background ice is laced with a complex mixture of non-ice compounds.

Dust release and tensile strength of the non-volatile layer of cometary nuclei

We developed a thermophysical model for cometary nuclei, which is based upon the assumption that comets form by the gravitational instability of an ensemble of dust and ice aggregates. Under this condition, the tensile strength of the ice-free outer layers of a cometary nucleus can be calculated, using the dust–aggregate collision and adhesion model of Weidling et al. (Weidling, R. et al. [2012]. Icarus, 218, 688-700). Based on available laboratory data on the gas permeability and thermal conductivity of ice-free dust layers, we derived the temperature and pressure at the dust–ice interface for pure water and pure carbon dioxide ice. Comparison of the vapor pressure below the dust crust with its tensile strength allows the prediction of dust release from cometary surfaces. Our model predicts dust activity for pure CO2 ice and for heliocentric distances of ≲3AU, whereas pure H2O ice cannot explain the dust emission.

CO ICE PHOTODESORPTION: A WAVELENGTH-DEPENDENT STUDY

UV-induced photodesorption of ice is a non-thermal evaporation process that can explain the presence of cold molecular gas in a range of interstellar regions. Information on the average UV photodesorption yield of astrophysically important ices exists for broadband UV lamp experiments. UV fields around low-mass pre-main sequence stars, around shocks and in many other astrophysical environments are however often dominated by discrete atomic and molecular emission lines. It is therefore crucial to consider the wavelength dependence of photodesorption yields and mechanisms. In this work, for the first time, the wavelength-dependent photodesorption of pure CO ice is explored between 90 and 170 nm. The experiments are performed under ultra high vacuum conditions using tunable synchrotron radiation. Ice photodesorption is simultaneously probed by infrared absorption spectroscopy in reflection mode of the ice and by quadrupole mass spectrometry of the gas phase. The experimental results for CO reveal a strong wavelength dependence directly linked to the vibronic transition strengths of CO ice, implying that photodesorption is induced by electronic transition (DIET). The observed dependence on the ice absorption spectra implies relatively low photodesorption yields at 121.6 nm (Ly-alpha), where CO barely absorbs, compared to the high yields found at wavelengths coinciding with transitions into the first electronic state of CO (singulet Pi at 150 nm); the CO photodesorption rates depend strongly on the UV profiles encountered in different star formation environments.

CO clathrate hydrate: Near to mid-IR spectroscopic signatures

Carbon monoxide is the second most abundant molecule after H 2 in the molecular universe, and as such an abundant constituent of interstellar and Solar System ices. To trace the possibility of this molecule to be found in a clathrate hydrate inclusion compound, its pure phase FTIR spectrum is investigated. We confirm the formation of a type I clathrate structure whereas simple guest size estimates would favour a type II clathrate hydrate, revealing interactions of this molecule with its water network during clathrate formation. The observed cage vibrational downshift with respect to pure CO ice is within 5 cm −1. The temperature dependent wavenumber separation between the two enclathrated CO vibrational transitions in the two distinct type I clathrate cages is less than a wavenumber below 140 K, implying that the spectral simplification for detailed spectroscopic analysis of the individual profiles is a difficult task. The dynamics of the CO molecules in its cage change considerably from 5 K to 140 K. At temperatures above 30 K, the molecule is extremely mobile in the cages, as revealed by the infrared profile, significantly different from CO entrapped in water ice and different from observed profiles in astrophysical objects.

Seasonal H2O and CO2 ice cycles at the Mars Phoenix landing site: 1. Prelanding CRISM and HiRISE observations

The condensation, evolution, and sublimation of seasonal water and carbon dioxide ices were characterized at the Mars Phoenix landing site from Martian northern midsummer to midspring (Ls ∼ 142° – Ls ∼ 60°) for the year prior to the Phoenix landing on 25 May 2008. Ice relative abundances and grain sizes were estimated using data from the Compact Reconnaissance Imaging Spectrometer for Mars and High Resolution Imaging Science Experiment aboard Mars Reconnaissance Orbiter and a nonlinear mixing model. Water ice first appeared at the Phoenix landing site during the afternoon in late summer (Ls ∼ 167°) as an optically thin layer on top of soil. CO2 ice appeared after the fall equinox. By late winter (Ls ∼ 344°), the site was covered by relatively pure CO2 ice (∼30 cm thick), with a small amount of ∼100 μm diameter water ice and soil. As spring progressed, CO2 ice grain sizes gradually decreased, a change interpreted to result from granulation during sublimation losses. The combined effect of CO2 sublimation and decreasing H2O ice grain sizes allowed H2O ice to dominate spectra during the spring and significantly brightened the surface. CO2 ice disappeared by early spring (Ls ∼ 34°) and H2O ice by midspring (Ls ∼ 59°). Spring defrosting was not uniform and occurred more rapidly over the centers of polygons and geomorphic units with relatively higher thermal inertia values.