2O Ice Research Articles

The reactions of water vapour on the surfaces of stoichiometric and reduced uranium dioxide: A high resolution XPS study

The reaction of water with stoichiometric and O-defective UO 2 thin film surfaces is studied by high-resolution photoelectron spectroscopy using synchrotron X-rays radiation. The decomposition of D 2O molecules and the oxidative healing of defects on the reduced surfaces was observed and quantified. D 2O adsorption on the stoichiometric UO 2 surface at 300 K showed small amounts of OD species (ca. 532 eV) probably formed on trace amounts of surface defects, while at 95 K D 2O ice (533.5 eV) was the main surface species. On the contrary, a large signal of OD species was seen on the 300 K-Ar +-sputtered (reduced) surface, UO 2− x . This was concomitant with a rapid healing of surface defects as monitored by their U4f signal. Quantitative analysis of the OD signal with increasing temperature showed their disappearance by 550 K. The disappearance of these species while hydrogen molecules are still desorbing from the surface as monitored by TPD [S.D. Senanayake, H. Idriss, Surf. Sci. 563 (1–3) (2004) 135; S.D. Senanayake, R. Rousseau, D. Colegrave, H. Idriss, J. Nucl. Mater. 342 (2005) 179] is shedding light on the re-combinative desorption mechanism from dissociatively adsorbed water molecules on the surfaces of this defective metal oxide.

South Pole of Mars: Nature and composition of the icy terrains from Mars Express OMEGA observations

Monitoring the exchange of CO 2, H 2O and dust between the atmosphere, regolith, seasonal deposits and the permanent polar caps of Mars is required to study the climate of the planet and its evolution. The imaging spectrometer Observatoire pour la Minéralogie, l’Eau, les Glaces et l’Activité (OMEGA) of Mars Express contributes to such a task by regularly acquiring hyperspectral images of the polar regions in the 0.35– 5.10 μ m range. We analyze five OMEGA observations that were acquired over the high southern latitudes in 2004 at the end of summer ( L S = 335 – 348 ∘ ). We conduct statistical analyses of data as well as radiative transfer modelings of selected spectra in order to investigate the nature, distribution and properties of various icy terrains: the bright permanent polar cap (BPPC), the layered deposits (SPLD), and the seasonal accumulations of water frost (SFA). Annealed CO 2 ice, potentially polycrystalline, dominates the BPPC and contains trace amounts of dust and water probably trapped in the cap during southern winter. Vast areas of the BPPC are quite homogeneous with dust and water contents of ≈ 0.03 –0.06 and ≈ 0.02 –0.06 wt%, respectively, but we observe larger variations for specific regions. The BPPC lays on a water-rich ( ≈ 75 wt % ) basement that emerges at the edges of the cap. In the circumpolar region, we observe the superficial part of the SPLD that shows a mixture of perennial H 2O ice (up to ≈ 70 wt % ) and dust. The SPLD are generally buried under an optically thick layer of sand or of seasonal frost accumulations. The SFA are composed of water ice ( ≈ 65 –70 wt%) and dust and cover large areas at the outskirts of the BPPC or at distance from it. We also found intermediate situations ( ≈ 20 water wt%) between the SFA and the dust cover.

Vapor deposited ethanol–H 2O ice mixtures investigated by micro-Raman scattering

Solid deposits have been formed at 88 K and 10 −1 Torr from ethanol–water gas collected above aqueous solutions of ethanol (EtOH) (0.6, 2, 4.5, 9 and 17 mol%). The composition of different gas mixtures varying between 1:16 and 1:0.8 EtOH:H 2O are determined at 295 K using our experimental vapor–liquid equilibrium (VLE) data in combination with the Wilson model [28]. The Wilson constants derived at this temperature are Λ 12 = 0.37(4) and Λ 21 = 0.58(5). The concentration of EtOH in the ice mixture can be calculated using these data and a kinetic model of condensation. It is found to vary between 9 and 65 mol% EtOH. The ice mixtures are analyzed in situ in a modified cryostage by micro-Raman spectroscopy. The distinct vibrational signatures of pure EtOH, EtOH aqueous solutions and EtOH–ice mixtures are identified in the 400–3800 cm −1 spectral range. Internal vibrational motions of EtOH molecules are affected by temperature and concentration. The presence of amorphous EtOH–ice phases at 88 K is demonstrated by the characteristic vibrational signatures of the ν OH stretching modes. The crystallization of an EtOH hydrate is proposed during annealing at ∼140 K of a 65 mol% EtOH–ice mixture. According to our preliminary X-ray diffraction work, this phase has apparently a distinct structure from that of solid EtOH or from EtOH–clathtrate structures usually found in frozen aqueous solutions. For ice mixtures of lower EtOH content, a distinct hydrate phase crystallizes at ∼170 K. These results suggest that ice mixtures obtained by vapor deposition reflect the existence of EtOH clusters of a distinctive structural nature with respect to those encountered in frozen aqueous mixtures.

Deuteron ordering in ice containing impurities: A neutron diffraction study

We measured neutron powder diffraction of 0.13-M KOD-doped D 2O ice at temperatures in the range of 52–80 K to investigate the growth mechanism of ice XI, a deuteron-ordered phase of ice Ih. We observed that the diffraction profiles of the doped ice change with time in the temperature range of 67–74 K. To obtain the mass fraction f (the ratio of mass of ice XI to that of the doped ice), we carried out Rietveld analysis using a two-phase model. The results show that the growth rate of ice XI increases and approaches the transition temperature T c=76 K. The f value reaches 0.125 in about 3 days of measurement at a temperature just below T c. Further, we found that the growth rate is accelerated by annealing the ice at temperatures lower than 64 K for sufficient time in advance to produce ice XI. We conclude that the temperature history of an ice sample is important for understanding the growth mechanisms of ice XI.

Distributions of H 2O and CO 2 ices on Ariel, Umbriel, Titania, and Oberon from IRTF/SpeX observations

We present 0.8–2.4 μm spectral observations of uranian satellites, obtained at IRTF/SpeX on 17 nights during 2001–2005. The spectra reveal for the first time the presence of CO 2 ice on the surfaces of Umbriel and Titania, by means of 3 narrow absorption bands near 2 μm. Several additional, weaker CO 2 ice absorptions have also been detected. No CO 2 absorption is seen in Oberon spectra, and the strengths of the CO 2 ice bands decline with planetocentric distance from Ariel through Titania. We use the CO 2 absorptions to map the longitudinal distribution of CO 2 ice on Ariel, Umbriel, and Titania, showing that it is most abundant on their trailing hemispheres. We also examine H 2O ice absorptions in the spectra, finding deeper H 2O bands on the leading hemispheres of Ariel, Umbriel, and Titania, but the opposite pattern on Oberon. Potential mechanisms to produce the observed longitudinal and planetocentric distributions of the two ices are considered.

Ion irradiation of crystalline H 2O–ice: Effect on the 1.65-μm band

We have found that 0.8 MeV proton irradiation of crystalline H 2O–ice results in temperature dependent amorphization. The H 2O–ice's phase was determined using the near infrared spectrum from 1.0 μm (10,000 cm −1) to 2.5 μm (4000 cm −1). In crystalline H 2O–ice, the 1.65-μm (6061 cm −1) band is strong while it is nearly absent in the amorphous spectrum [Schmitt, B., Quirico, E., Trotta, F., Grundy, W.M., 1998. In: Schmitt, B., de Bergh, C., Festou, M. (Eds.), Solar System Ices. Kluwer Academic, Norwell, MA, 1998, pp. 199–240]. In this experiment, at low temperatures (9, 25, and 40 K), irradiation of crystalline H 2O–ice produced the amorphous H 2O–ice's spectrum. However, at 50 K, some crystalline absorptions persisted after irradiation and at 70 and 100 K the crystalline spectrum showed only slight changes after irradiation. Our results agree with previous H 2O–ice irradiation studies examining the crystalline peaks near 44 and 62 μm by Moore and Hudson [Moore, M.H., Hudson, R.L., 1992. Astrophys. J. 401, 353–360] and near 3.07 μm by Strazzulla et al. [Strazzulla, G., Baratta, G.A., Leto, G., Foti, G., 1992. Europhys. Lett. 18, 517–522] and by Leto and Baratta [Leto, G., Baratta, G.A., 2003. Astron. Astrophys. 397, 7–13]. We present a method of measuring band areas to quantify the phase and radiation dose of icy Solar System surfaces.

Near-infrared laboratory spectra of solid H 2O/CO 2 and CH 3OH/CO 2 ice mixtures

We present near-IR spectra of solid CO 2 in H 2O and CH 3OH, and find they are significantly different from that of pure solid CO 2. Peaks not present in either pure H 2O or pure CO 2 spectra become evident when the two are mixed. First, the putative theoretically forbidden CO 2 ( 2 ν 3 ) overtone near 2.134 μm (4685 cm −1), that is absent from our spectrum of pure solid CO 2, is prominent in the spectra of H 2O/CO 2=5 and 25 mixtures. Second, a 2.74-μm (3650 cm −1) dangling OH feature of H 2O (and a potentially related peak at 1.89 μm) appear in the spectra of CO 2–H 2O ice mixtures, but are probably not diagnostic of the presence of CO 2. Other CO 2 peaks display shifts in position and increased width because of intermolecular interactions with H 2O. Warming causes some peak positions and profiles in the spectrum of a H 2O/CO 2=5 mixture to take on the appearance of pure CO 2. Absolute strengths for absorptions of CO 2 in solid H 2O are estimated. Similar results are observed for CO 2 in solid CH 3OH. Since the CO 2 ( 2 ν 3 ) overtone near 2.134 μm (4685 cm −1) is not present in pure CO 2 but prominent in mixtures, it may be a good observational (spectral) indicator of whether solid CO 2 is a pure material or intimately mixed with other molecules. These observations may be applicable to Mars polar caps as well as outer Solar System bodies.

Cratering of icy targets by different impactors: Laboratory experiments and implications for cratering in the Solar System

Studies of impacts (impactor velocity about 5 km s −1) on icy targets were performed. The prime goal was to study the response of solid CO 2 targets to impacts and to find the differences between the results of impacts on CO 2 targets with those on H 2O ice targets. The crater dimensions in CO 2 ice were found to scale with impact energy, with little dependence on projectile density (which ranged from nylon to copper, i.e., 1150–8930 kg m −3). At equal temperatures, craters in CO 2 ice were the same diameter as those in water ice, but were shallower and smaller in volume. In addition, the shape of the radial profiles of the craters was found to depend strongly on the type of ice and to change with impact energy. The impact speed of the data is comparable to that for impacts on many types of icy bodies in the outer Solar System (e.g., the satellites of the giant planets, the cometary nuclei and the Kuiper Belt objects), but the size and thus energy of the impactors is lower. Scaling with impact energy is demonstrated for the impacts on CO 2 ice. The issue of impact disruption (rather than cratering) is discussed by analogy with that on water ice. Expressions for the critical energy density for the onset of disruption rather than cratering are established for water ice as a function of porosity and silicate content. Although the critical energy density for disruption of CO 2 ice is not established, it is argued that the critical energy to disrupt a CO 2 ice body will be greater than that for a (non-porous) water ice body of the similar mass.

The importance of pores in the electron stimulated production of D 2 and O 2 in low temperature ice

The effects of porosity and morphology of amorphous and crystalline D 2O ices on the electron stimulated generation and trapping of D 2 and O 2 have been studied by post-irradiation thermal desorption. The trapped product yields increase from crystalline ice to amorphous ice to highly porous amorphous ice, similar to observed ESD yields. This is attributed to the increased number of defects, traps and pores in amorphous ice. Molecular deuterium is released in the temperature range from 55 to 105 K for each of the samples, with two notable bursts at 115 and 132 K for porous amorphous ice. Low temperature release is associated with diffusion of D 2 through micropores in the ice matrix. Highly porous amorphous ice shows a striking spike in the release of D 2 at 80 K which has not previously been observed and must be a result of trapping of D 2 within enclosed macropores. The trapped D 2 in macropores may result from transport of excitation energy to the pore interface or from transport of D 2 through micropores at lower temperatures during TPD, or both. Molecular oxygen is retained within ice until much higher temperatures (>140 K). The release at this temperature is attributed to sintering and diffusion along grain boundaries in crystalline ice. The majority of trapped O 2 evolves with the desorption of the ice matrix, suggesting that clathrate hydrates may be important trapping sites. Amorphous ice releases a surge of trapped O 2 at the 160 K amorphous to crystalline phase transition, which is consistent with codeposition experiments. Highly porous ice exhibits much higher total yields of oxygen and shows a second strong spike in trapped O 2 release at 175 K.

Formation and growth of ice XI: A powder neutron diffraction study

We measured neutron diffraction profiles of powdered KOD-doped D 2O ice to investigate the formation and growth of ice XI, a deuterium-ordered phase of ice, from ice Ih, a disordered phase. During the measurements, the ice was annealed at 57 K for about 17 h, and then warmed up and kept at 68 K for about 55 h, which are below the transition temperature T c = 76 K . The diffraction profiles for the doped ice at 57 K did not change with time and were close to those for ice Ih with the deuterium-disordered arrangement. However, when the temperature increased from 57 to 68 K, the diffraction profiles changed. We carried out the Rietveld analysis with the parameters for a two-phase model to obtain the structural evidence of the appearance and the growth of ice XI. The analysis provided evidence of ice XI as space group Cmc2 1 at 68 K. The temporal increase of the mass fraction of ice XI showed that nucleation of ice XI began just after warming up to 68 K. Then, ice XI grew for several days in the doped ice. This study provides first structural observations of the growth of ice XI.

PFS-MEX observation of ices in the residual south polar cap of Mars

The Mars Express spacecraft has a highly inclined orbit around Mars and so has been able to observe the south pole of Mars in illuminated conditions at the end of the southern summer ( L s = 330 ). Spectra from the planetary Fourier spectrometer (PFS) short wavelength (SW) channel were recorded over the permanent ice cap to study its composition in terms of CO 2 ice and H 2O ice. Models are fitted to the observed data, which include a spatial mixture of soil (not covered by ice) and CO 2 frost (with a specific grain size and a small amount of included dust and H 2O ice). Two different kinds of spectra were observed: those over the permanent polar cap with almost pure CO 2 ice, negligible water ice, no soil fraction required, and bright; and those over mixed terrain (at the edge of the cap or near troughs) containing a significant soil spatial fraction, more water ice and smaller CO 2 grain size. The amount of water ice given by fits to scaled albedo models is less than 10 ppm by weight. When using multi-stream reflectance models with the appropriate lighting geometry, the water amount must be 2–5 times greater than the albedo fit (less than 50 ppm). At the periphery of the residual polar cap, we found a region almost completely covered by water frost, modeled as a mixture of micron-sized and sub-mm sized grains. Our result using a granular mixture of micron-sized grains of water ice and dust with the CO 2 grains is different from the modeling of OMEGA polar cap observations using molecular mixtures.

CO 2 production by ion irradiation of H 2O ice on top of carbonaceous materials and its relevance to the Galilean satellites

In this work we report on new experiments of ion irradiation of water ice deposited on top of solid carbonaceous materials to study the production of CO 2 at the interface ice/refractory material and discuss the possibility that this mechanism accounts for the quantity of CO 2 ice detected on the surfaces of the Galilean satellites. The used experimental technique has been in situ infrared spectroscopy. We have irradiated thin films of H 2O frost on carbonaceous layers with 200 keV of He + and Ar +, and 30 keV of He + at 16 and 80 K. The used carbonaceous layers have been asphaltite, a natural bitumen, and solid organic residues obtained by irradiation of frozen benzene. In both cases the results show that CO 2 is produced very efficiently after irradiation obtaining a maximum quantity of the order of 6 × 10 15 molecules cm −2 . These results are, also quantitatively similar, to those recently obtained for water ice deposited on amorphous carbon films [Mennella, V., Palumbo, M.E., Baratta, G.A., 2004. Formation of CO and CO 2 molecules by ion irradiation of water ice covered hydrogenated carbon grains. Astrophys. J. 615, 1073–1080]. Thus we suggest that, whatever is the carbonaceous residue, CO 2 will be produced efficiently by the studied process. These results have interest in the context of the surfaces of the icy Galilean satellites in which CO 2 has been detected mainly trapped in the non-ice material, not in the pure water ice. We suggest that radiolysis of mixtures of water ice and refractory carbonaceous materials is the primary formation mechanism responsible for the CO 2 formation on the surfaces of the Galilean satellites.

Secondary ion emission from CO 2–H 2O ice irradiated by energetic heavy ions : Part II: Analysis–search for organic ions

Secondary ion mass spectrometry is used to investigate ion emission from a frozen-gas mixture of CO 2 and H 2O ( T = 80–90 K) bombarded by MeV nitrogen ions and by 252Cf fission fragments. The aim of the experiment is to detect organic molecules, produced in the highly excited material around the nuclear track, which appear as ions in the flux of sputtered particles. Part I of the present work [L.S. Farenzena, V.M. Collado, C.R Ponciano, E.F. da Silveira, K. Wien. Int. J. Mass Spectrom. 243 (2005) 85–93] described the method and presented the time-of-flight mass spectra; a list of the CO 2 specific and H 2O specific reaction products was provided. In Part II, the peaks of the TOF mass spectra are analyzed. Projectile-ice direct coulomb interaction leads to the production in the track of the H +, C +, O +, O 2 +, CO + and CO 2 + primarily ions, which react in the highly energized nuclear track plasma mainly with CO 2 and H 2O or H 2CO 3. The positive molecular hybrid ions formed are identified as organic species like COH +, COOH +, CH n = 1–3 +, H n = 1,2 COOH + and cluster ions. Similarly, the negative primarily ions O − and OH − formed by electron capture produce negative hybrid ions such as (CO 2) n OH −, the four ions (CO 4H m = 0–3 ) − and the corresponding cluster ions. By far, most of the molecular ions have been formed by one-step reactions; exceptions are eventually the CO 4H m − ions created by a reaction between CO 3 − and water molecules. An intense mass line corresponding to HCO 3 + has been observed, but not the one due to formaldehyde ion. Weak signals of ionic ketene, hydrogen peroxide and carbonic acid were seen.

Influence of helium-ion bombardment on the surface properties of pure and ammonia-adsorbed water thin films

The influence of the ion bombardment on the surface properties of water-ice films has been investigated. The films are irradiated with 1.5 keV He + ions and analyzed sequentially on the basis of time-of-flight secondary-ion mass spectrometry (TOF-SIMS). In order to minimize any temperature-induced effects, the measurements were made at 15 K. The damage of the films, as estimated from the H/D exchange between NH 3 and the D 2O ice and the intermixing of NH 3 with the H 2 18O ice, is recognized at the fluence above 2 × 10 14 ions/cm 2. The sputtering yield of the D 2O ice is determined as 0.9 ± 0.2 molecules per incoming He + ion. The temperature-programmed TOF-SIMS analysis of the water-ice films has been completed within the fluence of 5.8 × 10 12 ions/cm 2, so that no appreciable damage of the film should be induced during the measurement.

Application of the metastable impact electron spectroscopy (MIES), in combination with UPS and TPD, to the study of processes at ice surfaces

We report studies of the interaction (80–200 K) of atoms and molecules with ice films, deposited on tungsten, by combining photoelectron spectroscopy, UPS with HeI and II, metastable impact electron spectroscopy (MIES), and temperature programmed desorption (TPD). The focus is on the following issues: (1) Interaction of Na Atoms with films of solid NH 3 and CH 3OH: the delocalization of the 3sNa electrons and their role in the deprotonation of ice molecules (seen for CH 3OH, but not for NH 3) is studied. DOS (density of states) information from density functional theory (DFT) is compared with the MIES spectra. It is concluded that the 3s electron, delocalized from its Na core and trapped between the core and surrounding molecules, triggers the CH 3OH deprotonation; for NH 3, Na dimers appear to play an important role in the delocalization process. (2) Interaction of salt (NaCl) and acid (HCOOH) molecules with H 2O ice: neither NaCl nor HCOOH penetrate into the ice film during adsorption below 90 K. However, ionic dissociation of NaCl occurs as a consequence of the NaCl–ice interaction. At 105 K the solvation of ionic species becomes significant. The desorption of H 2O from the mixed film takes place between 145 and 170 K; those species bound to Na + and Cl − are removed last. No dissociation of HCOOH is observed as a consequence of the interaction with ice. Partial solvation of formic acid species takes place above about 120 K whereby these species become embedded into the rim of the water film. However, no deeper penetration of formic acid molecules into the water film can be detected before H 2O desorption becomes significant. The TPD spectra of HCOOH display structure, around 160 K, close to or coinciding with the sublimation temperature of water, and around 180 K, caused by species chemisorbed on the tungsten substrate supporting the ice film. The interaction of both NaCl and HCOOH with ice is discussed on the basis of qualitative free energy profiles.

Radiative habitable zones in martian polar environments

The biologically damaging solar ultraviolet (UV) radiation (quantified by the DNA-weighted dose) reaches the martian surface in extremely high levels. Searching for potentially habitable UV-protected environments on Mars, we considered the polar ice caps that consist of a seasonally varying CO 2 ice cover and a permanent H 2O ice layer. It was found that, though the CO 2 ice is insufficient by itself to screen the UV radiation, at ∼1 m depth within the perennial H 2O ice the DNA-weighted dose is reduced to terrestrial levels. This depth depends strongly on the optical properties of the H 2O ice layers (for instance snow-like layers). The Earth-like DNA-weighted dose and Photosynthetically Active Radiation (PAR) requirements were used to define the upper and lower limits of the northern and southern polar Radiative Habitable Zone (RHZ) for which a temporal and spatial mapping was performed. Based on these studies we conclude that photosynthetic life might be possible within the ice layers of the polar regions. The thickness varies along each martian polar spring and summer between ∼1.5 and 2.4 m for H 2O ice-like layers, and a few centimeters for snow-like covers. These martian Earth-like radiative habitable environments may be primary targets for future martian astrobiological missions. Special attention should be paid to planetary protection, since the polar RHZ may also be subject to terrestrial contamination by probes.

A spectroscopic study of the surfaces of Saturn's large satellites: H 2O ice, tholins, and minor constituents

We present spectra of Saturn's icy satellites Mimas, Enceladus, Tethys, Dione, Rhea, and Hyperion, 1.0–2.5 μm, with data extending to shorter (Mimas and Enceladus) and longer (Rhea and Dione) wavelengths for certain objects. The spectral resolution ( R = λ / Δ λ ) of the data shown here is in the range 800–1000, depending on the specific instrument and configuration used; this is higher than the resolution ( R = 225 at 3 μm) afforded by the Visual-Infrared Mapping Spectrometer on the Cassini spacecraft. All of the spectra are dominated by water ice absorption bands and no other features are clearly identified. Spectra of all of these satellites show the characteristic signature of hexagonal H 2O ice at 1.65 μm. We model the leading hemisphere of Rhea in the wavelength range 0.3–3.6 μm with the Hapke and the Shkuratov radiative transfer codes and discuss the relative merits of the two approaches to fitting the spectrum. In calculations with both codes, the only components used are H 2O ice, which is the dominant constituent, and a small amount of tholin (Ice Tholin II). Tholin in small quantities (few percent, depending on the mixing mechanism) appears to be an essential component to give the basic red color of the satellite in the region 0.3–1.0 μm. The quantity and mode of mixing of tholin that can produce the intense coloration of Rhea and other icy satellites has bearing on its likely presence in many other icy bodies of the outer Solar System, both of high and low geometric albedos. Using the modeling codes, we also establish detection limits for the ices of CO 2 (a few weight percent, depending on particle size and mixing), CH 4 (same), and NH 4OH (0.5 weight percent) in our globally averaged spectra of Rhea's leading hemisphere. New laboratory spectral data for NH 4OH are presented for the purpose of detection on icy bodies. These limits for CO 2, CH 4, and NH 4OH on Rhea are also applicable to the other icy satellites for which spectra are presented here. The reflectance spectrum of Hyperion shows evidence for a broad, unidentified absorption band centered at 1.75 μm.

South polar residual cap of Mars: Features, stratigraphy, and changes

The south residual polar cap of Mars, rich in CO 2 ice, is compositionally distinct from the north residual cap which is dominantly H 2O ice. The south cap is also morphologically distinct, displaying a bewildering variety of depressions formed in thin layered deposits, which have been observed to change by scarp retreat over an interval of one Mars year (Malin et al., 2001, Science 294, 2146–2148). The climatically sensitive locale of the residual caps suggests that their behavior may help in the interpretation of recent fluctuations or repeatability of the Mars climate. We have used Mars Global Surveyor Mars Orbiter Camera (MOC) images obtained in three southern summers to map the variety of features in the south residual cap and to evaluate changes over two Mars years (Mars y). The images show that there are two distinct layered units which were deposited at different times separated by a period of degradation. The older unit, ∼ 10 m thick, has layers approximately 2 m thick. The younger unit has variable numbers of layers, each ∼ 1 m thick. The older unit is eroding by scarp retreat averaging 3.6 m/Mars y, a rate greater than the retreat of 2.2 m/Mars y observed for the younger unit. The rates of scarp retreat and sizes of the different types of depressions indicate that the history of the residual cap has been short periods of deposition interspersed with longer erosional periods. Erosion of the older unit probably occupied ∼ 100 – 150 Mars y. One layer may have been deposited after the Mariner 9 observations in 1972. Residual cap layers appear to differ from normal annual winter deposits by having a higher albedo and perhaps by having higher porosities. These properties might be produced by differences in the depositional meteorology that affect the fraction of high porosity snow included in the winter deposition.

Secondary ion emission from CO 2–H 2O ice irradiated by energetic heavy ions : Part I. Measurement of the mass spectra

Secondary ion mass spectrometry is used to investigate ion emission from a frozen-gas mixture ( T = 80–90 K) of CO 2 and H 2O bombarded by MeV nitrogen ions and by 252Cf fission fragments (FF). The aim of the experiments is to produce organic molecules in the highly excited material around the nuclear track and to detect them in the flux of sputtered particles. Such sputter processes are known to occur at the icy surfaces of planetary or interstellar objects. Time-of-flight (TOF) mass spectrometry is employed to identify the desorbed ions. Mass spectra of positive and negative ions were taken for several molecular H 2O/CO 2 ratios. In special, positive ions induced by MeV nitrogen beam were analyzed for 9 and 18% H 2O concentrations of the CO 2–H 2O ice and negative ions for ∼5% H 2O. The ion peaks are separated to generate exclusive the spectra of CO 2 specific ions, H 2O specific ions and hybrid molecular ions, the latter ones corresponding to ions that contain mostly H and C atoms. In the mass range from 10 to 320 u, the latter exhibits 35 positive and 58 negative ions. The total yield of the positive ions is 0.35 and 0.57 ions/impact, respectively, and of negative ions 0.066 ions/impact. Unexpected effects of secondary ion sputtering yields on H 2O/CO 2 ratio are attributed to the influence of water molecules concentration on the ionization process.

The legacy of SWAS: Water and molecular oxygen in the interstellar medium

The Submillimeter Wave Astronomy Satellite has recently completed 5.5 years of successful operation. Among the legacies of the mission has been a greater understanding of the abundance and spatial distribution of H 2O and O 2 within molecular clouds. The model summarized here provides an explanation for the relatively low gaseous H 2O abundance and the high H 2O–ice abundance in cold dense clouds, the higher H 2O abundance toward diffuse clouds, the low O 2 abundance toward all clouds observed.