Water Ice Layer Research Articles

H ATOM IRRADIATION OF CARBON GRAINS UNDER SIMULATED DENSE INTERSTELLAR MEDIUM CONDITIONS: THE EVOLUTION OF ORGANICS FROM DIFFUSE INTERSTELLAR CLOUDS TO THE SOLAR SYSTEM

We present the results of experiments aimed at studying the interaction of hydrogen atoms at 80 K with carbon grains covered with a water ice layer at 12 K. The effects of H processing have been analyzed, using IR spectroscopy, as a function of the water ice layer. The results confirm that exposure of the samples to H atoms induces the activation of the band at 3.47 μm with no evidence for the formation of aromatic and aliphatic C-H bonds in the CH2 and CH3 functional groups. The formation cross section of the 3.47 μm band has been estimated from the increase of its integrated optical depth as a function of the H atom fluence. The cross section decreases with increasing thickness of the water ice layer, indicating an increase of adsorption of H atoms in the water ice layer. A penetration depth of 100 nm has been estimated for H atoms in the porous water ice covering carbon grains. Sample warm-up at room temperature causes the activation of the IR features due to the vibrations of the CH2 and CH3 aliphatic functional groups. The evolution of the 3.47 μm band carrier has been evaluated for dense and diffuse interstellar clouds, using the estimated formation cross section and assuming that the destruction cross section by energetic processing is the same as that derived for the 3.4 μm band. In both environments, the presence of the 3.47 μm band carrier is compatible with the evolutionary timescale limit imposed by fast cycling of materials between dense and diffuse regions of the interstellar medium. In diffuse regions the formation of the CH2 and CH3 aliphatic bands, inhibited in dense regions, takes place, masking the 3.47 μm band. The activation of the CH2 and CH3 aliphatic vibrational modes at the end of H processing after sample warm-up represents the first experimental evidence supporting an evolutionary connection between the interstellar carbon grain population, which is responsible for the 3.4 μm band (diffuse regions) and contributes to the absorption at 3.47 μm (dense regions), and the organics observed in interplanetary dust particles and cometary Stardust grains.

Phoenix and MRO coordinated atmospheric measurements

The Phoenix and Mars Reconnaissance Orbiter (MRO) missions collaborated in an unprecedented campaign to observe the northern polar region summer atmosphere throughout the Phoenix mission (25 May to 2 November 2008; Ls = 76°–150°) and slightly beyond (∼Ls = 158°). Five atmospherically related campaigns were defined a priori and were executed on 37 separate Martian days (sols). Phoenix and MRO observed the atmosphere nearly simultaneously. We describe the observation strategy and history, the participating experiments, and some initial results. We find that there is general agreement between measurements from different instruments and platforms and that complementary measurements provide a consistent picture of the atmosphere. Seasonal water abundance behavior matches with historical measurements. Winds aloft, as measured by cloud motions, showed the same seasonally consistent, diurnal rotation as the winds measured at the lander, during the first part of the mission (Ls = 76°–118°). A diurnal cycle recorded from Ls ∼ 108.3°–109.1°, in which a dust front was approaching the Phoenix Lander, is examined in detail. Cloud heights measured on subsequent orbits showed that in areas of active lifting, dust can be lofted quite high in the atmosphere, doubling in height over 2 h. The combination of experiments also revealed that there were discrete vertical layers of water ice and dust. Water vapor column abundances compared to near‐surface water vapor pressure indicate that water is not well mixed from the surface to a cloud condensation height and that the depth of the layer that exchanges diurnally with the surface is 0.5–1 km.

Models of high velocity impacts into dust-covered ice: Application to Martian northern lowlands

The detection of fresh impact craters with bright floors and ejecta (arising from fresh clean water ice) in the northern lowlands of Mars ( Byrne et al., 2009b, Science 325, 1674), together with observations of polygonal structures and evidence from the Phoenix probe, suggests that there are substantial water ice deposits just below the surface over large areas. Specifically in cases of the larger craters observed, the impacts themselves may have raised the temperature and the pressure of the water ice deposits locally to values which allow phase changes. In this paper, we use smoothed particle hydrodynamics to model hyper-velocity impacts. We estimate peak shock pressures in a solid water ice target covered by a layer of loose material, modeled by pre-damaged dunite. In addition, we account for the possibility of a thin layer of sub-surface water ice by using a three-layer model where the ice is surrounded by dunite. We find that the peak shock pressures reached in the simulated events are high enough to produce several 100–1000 kg of liquid water depending upon the impact parameters and the exact shock pressure needed for the phase change. A difficulty remains however in determining whether liquid is generated or whether a type of fluidized ice is produced (or indeed some combination of the two). We also note that the process can become rather complex as the number of layers increases because of reflections of the shock at sub-surface boundaries—a process which should lead to increased fluidization.

Low Energy Ion Scattering Investigations of n-Butanol−Ice System in the Temperature Range of 110−150 K

We have investigated the interaction of n-butanol (NBA) with thin layers of water ice prepared in ultra high vacuum in the temperature range of 110−150 K. From the mass spectra of the chemically sputtered species, created upon the collision of low energy (≥30 eV) Ar+ ions, we study the process of diffusive mixing of NBA with water ice, at the molecular level. The results show that NBA undergoes diffusive mixing with H2O. Even after depositing 1000 monolayers (MLs) of amorphous solid water (ASW) over NBA, both the species are observed on the surface. However, when NBA is deposited over ASW, no water is seen on the surface above 3−5 MLs of NBA. This could be interpreted as the absence of diffusive mixing in this system or surface segregation of NBA, in view of its lower surface energy just as in the case of liquid alcohols. An isomeric alcohol, namely, tert-butyl alcohol (TBA), also behaves similarly. Although the presence of NBA and TBA is detected, in the presence of ASW, they undergo selective ionization, giving specific peaks in the mass spectrum. D2O behaves in a manner similar to that of H2O. Preliminary experiments with other alcohols; namely, methanol, ethanol, and propanol were also done, and the results suggest that incomplete diffusion or surface segregation begins with propanol.

Origin of ice diapirism, true polar wander, subsurface ocean, and tiger stripes of Enceladus driven by compositional convection

We consider the scenario in which the presence of ammonia in the bulk composition of Enceladus plays a pivotal role in its thermochemical evolution. Because ammonia reduces the melting temperature of the ice shell by 100 K below that of pure water ice, small amounts of tidal dissipation can power an “ammonia feedback” mechanism that leads to secondary differentiation of Enceladus within the ice shell. This leads to compositionally distinct zones at the base of the ice shell arranged such that a layer of lower density (and compositionally buoyant) pure water ice underlies the undifferentiated ammonia-dihydrate ice layer above. We then consider a large scale instability arising from the pure water ice layer, and use a numerical model to explore the dynamics of compositional convection within the ice shell of Enceladus. The instability of the layer can easily account for a diapir that is hemispherical in scale. As it rises to the surface, it co-advects the warm internal temperatures towards the outer layers of the satellite. This advected heat facilitates the generation of a subsurface ocean within the ice shell of Enceladus. This scenario can simultaneously account for the origin of asymmetry in surface deformation observed on Enceladus as well as two global features inferred to exist: a large density anomaly within the interior and a subsurface ocean underneath the south polar region.

Recent rheologic processes on dark polar dunes of Mars: Driven by interfacial water?

In springtime on HiRISE images of the Southern polar terrain of Mars flow-like or rheologic features were observed. Their dark color is interpreted as partly defrosted surface where the temperature is too high for CO 2 but low enough for H 2O ice to be present there. These branching streaks grow in size and can move by an average velocity of up to about 1 m/day and could terminate in pond-like accumulation features. The phenomenon may be the result of interfacial water driven rheologic processes. Liquid interfacial water can in the presence of water ice exist well below the melting point of bulk water, by melting in course of interfacial attractive pressure by intermolecular forces (van der Waals forces e.g.), curvature of water film surfaces, and e.g. by macroscopic weight, acting upon ice. This melting phenomenon can be described in terms of “premelting of ice”. It is a challenging consequence, that liquid interfacial water unavoidably must in form of nanometric layers be present in water ice containing soil in the subsurface of Mars. It is the aim of this paper to study possible rheologic consequences in relation to observations, which seem to happen at sites of dark polar dunes on Mars at present. The model in this work assumes that interfacial water accumulates at the bottom of a translucent water-ice layer above a dark and insolated ground. This is warmed up towards the melting point of water. The evolving layer of liquid interfacial water between the covering ice sheet and the heated ground is assumed to drive downward directed flow-like features on slopes, and it can, at least partially, infiltrate (seep) into a porous ground. There, in at least temporarily cooler subsurface layers, the infiltrated liquid water refreezes and forms ice. The related stress built-up is shown to be sufficient to cause destructive erosive processes. The above-mentioned processes may cause change in the structure and thickness of the covering ice and/or may cause the movement of dune grains. All these processes may explain the observed springtime growing and downward extension of the slope streaks analyzed here.

Experiment LEND of the NASA Lunar Reconnaissance Orbiter for High-Resolution Mapping of Neutron Emission of the Moon

The scientific objectives of neutron mapping of the Moon are presented as 3 investigation tasks of NASA's Lunar Reconnaissance Orbiter mission. Two tasks focus on mapping hydrogen content over the entire Moon and on testing the presence of water-ice deposits at the bottom of permanently shadowed craters at the lunar poles. The third task corresponds to the determination of neutron contribution to the total radiation dose at an altitude of 50 km above the Moon. We show that the Lunar Exploration Neutron Detector (LEND) will be capable of carrying out all 3 investigations. The design concept of LEND is presented together with results of numerical simulations of the instrument's sensitivity for hydrogen detection. The sensitivity of LEND is shown to be characterized by a hydrogen detection limit of about 100 ppm for a polar reference area with a radius of 5 km. If the presence of ice deposits in polar "cold traps" is confirmed, a unique record of many millions of years of lunar history would be obtained, by which the history of lunar impacts could be discerned from the layers of water ice and dust. Future applications of a LEND-type instrument for Mars orbital observations are also discussed.

Activation of the 3.47 μm Band by H Atom Irradiation of Carbon Grains Covered with a Water Ice Layer at 12 K

We present the first results of laboratory research on the processing of nano-sized carbon particles under simulated dense-medium conditions. The experiments aim to study the variations of the degree of hydrogenation of carbon particles covered with a water ice layer in response to H atom exposure. The effects of H atom processing have been studied by means of IR spectroscopy. The results show that the formation of C-H bonds in the aliphatic CH2 and CH3 functional groups does not take place. The activation of a band at 3.47 μm is observed. It is assigned to the C-H stretching vibration of tertiary sp3 carbon atoms. The band profile is compatible with that present in the spectrum of sources embedded in dense clouds. We propose that the carbon grain population, which is responsible for the interstellar aliphatic 3.4 μm band in diffuse regions, absorbs at 3.47 μm in dense clouds.

A conceptual model for explanation of Albedo changes in Martian craters

We propose a conceptual model to interpret AM/PM high albedo events (HAEs) in crater interiors at the Martian seasonal polar caps. This model consists of two components: (1) a relatively permanent high-albedo water–ice body exposed in a crater interior and (2) a variable crater albedo in response to aerosol optical depth, dust contamination, and H 2O/CO 2 frost deposits or sublimes in four phases, based on temperature and solar longitude changes. Two craters (Korolev crater of fully exposed water–ice layer and ‘Louth’ crater of partially exposed water–ice layer) are used to demonstrate the model. This model explains the HAEs and their seasonal changes and suggests that many crater-like features formed in the last episodic advance of the polar ice cap in the last high obliquity period should have water–ice exposed or covered. For the AM-only HAEs craters, there seems no need of a water–ice layer to be fully exposed, but a subsurface water–ice layer (or ice-rich regolith) is a necessary condition.

Desorption of Hot Molecules from Photon Irradiated Interstellar Ices

We present experimental measurements of photodesorption from ices of astrophysical relevance. Layers of benzene and water ice were irradiated with a laser tuned to an electronic transition in the benzene molecule. The translational energy of desorbed molecules was measured by time-of-flight (ToF) mass spectrometry. Three distinct photodesorption processes were identified: a direct adsorbate-mediated desorption producing benzene molecules with a translational temperature of around 1200 K, an indirect adsorbate-mediated desorption resulting in water molecules with a translational temperature of around 450 K, and a substrate-mediated desorption of both benzene and water producing molecules with translational temperatures of around 530 and 450 K, respectively. The translational temperature of each population of desorbed molecules is well above the temperature of the ice matrix. The implications for gas-phase chemistry in the interstellar medium are discussed.

Interaction of Carboxylic Acids and Water Ice Probed by Argon Ion Induced Chemical Sputtering

In this paper, we investigated the interaction of simple carboxylic acids, formic acid (FA), acetic acid (AA), and propionic acid (PA) with thin layers of water ice in the temperature range of 110-190 K in ultrahigh vacuum. The focus, however, is on the AA-ice system. Molecularly thin layers of these systems were prepared on a pre-cooled polycrystalline copper substrate. The interactions and phase changes in the system were monitored with chemical sputtering using low-energy (≤30 eV) Ar + , which probes the topmost surface layers. At 110 K, the deposited AA exists as dimers in its amorphous solid form. At the same temperature, in the presence of water ice, this dimeric form gets converted to chainlike oligomers. Chemical sputtering spectra show distinct features for these two surface species. The data suggest that ion formation reflects the surface structure, implying a unique mechanism for its formation. Detailed studies have been made with amorphous solid water (ASW) and crystalline water (CW) to get a complete understanding of the system. Experiments carried out with AA-D 2 O ice confirmed the proton-transfer mechanism during chemical sputtering. Other studies were conducted with AA-CH 3 OH and AA-CCl 4 systems. Detailed investigations performed to understand the effect of thickness of AA and ice overlayers suggest that the extent of water molecules required to effect the structural transformation in the acid is dependent on the amount of the latter. Dimeric-to-oligomeric transformation does not occur for the PA-ice system. Detection of a structural transition at the very top of ice in molecularly thin films adds additional capabilities to the low-energy ion scattering technique.

On the origin of perennial water ice at the south pole of Mars: A precession‐controlled mechanism?

The poles of Mars are known to have recorded recent (<107 years) climatic changes. While the south polar region appears to have preserved its million‐year‐old environment from major resurfacing events, except for the small portion containing the CO2 residual cap, the discovery of residual water ice units in areas adjacent to the cap provides compelling evidence for recent glaciological activity. The mapping and characterization of these H2O‐rich terrains by Observatoire pour la Minéralogie, l'Eau, les Glaces et l'Activité (OMEGA) on board Mars Express, which have supplemented earlier findings by Mars Odyssey and Mars Global Surveyor, have raised a number of questions related to their origin. We propose that these water ice deposits are the relics of Mars' orbit precession cycle and that they were laid down when perihelion was synchronized with northern summer, i.e., more than 10,000 years ago. We favor precession over other possible explanations because (1) as shown by our General Circulation Model (GCM) and previous studies, current climate is not conducive to the accumulation of water at the south pole due to an unfavorable volatile transport and insolation configuration, (2) the residual CO2 ice cap, which is known to cold trap water molecules on its surface and which probably controls the current extent of the water ice units, is geologically younger, (3) our GCM shows that 21,500 years ago, when perihelion occurred during northern spring, water ice at the north pole was no longer stable and accumulated instead near the south pole with rates as high as 1 mm yr−1. This could have led to the formation of a meters‐thick circumpolar water ice mantle. As perihelion slowly shifted back to the current value, southern summer insolation intensified and the water ice layer became unstable. The layer recessed poleward until the residual CO2 ice cover eventually formed on top of it and protected water ice from further sublimation. In this polar accumulation process, water ice clouds play a critical role since they regulate the exchange of water between hemispheres. The so‐called “Clancy effect,” which sequesters water in the spring/summer hemisphere coinciding with aphelion due to cloud sedimentation, is demonstrated to be comparable in magnitude to the circulation bias forced by the north‐to‐south topographic dichotomy. However, we predict that the response of Mars' water cycle to the precession cycle should be asymmetric between hemispheres not only because of the topographic bias in circulation but also because of an asymmetry in the dust cycle. We predict that under a “reversed perihelion” climate, dust activity during northern summer is less pronounced than during southern summer in the opposite perihelion configuration (i.e., today's regime). When averaged over a precession cycle, this reduced potential for dust lifting will force a significantly colder summer in the north and, by virtue of the Clancy effect, will curtail the ability of the northern hemisphere to transfer volatiles to the south. This process may have helped create the observed morphological differences in the layered deposits between the poles and could help explain the large disparity in their resurfacing ages.

Phase equilibrium in the system CO2‐H2O: Application to Mars

Thermodynamic analysis of the system CO2‐H2O shows that although CO2clathrate and liquid CO2are stable over most of the pressure and temperature range expected for the Martian regolith, hydraulic equilibrium of the regolith pore gas with the overlying atmosphere, a condition most likely in equatorial regions, will destabilize both phases. Condensation on Mars comes in four predictable assemblages: pure H2O ice, CO2clathrate (hydrate), a eutectic mixture of solid CO2and either clathrate or H2O ice (if the clathrate fails to nucleate), and under limited conditions, pure solid CO2. Seasonal frosts are likely to consist of thin layers of water ice and/or clathrate, overlain by the eutectic assemblage. Growing ice caps are also likely to be layered, with sets of ice (lower) and eutectic (upper) layers interspersed with pure solid CO2layers. The saturated water content of a 1‐bar CO2atmosphere with surface temperatures >273 K would have reached 20 precipitable millimeters. Basal melting, triggered by trapped heat flow from the interior, limits the thickness and composition of the polar ice caps and is to be expected during periods of low obliquity, when there is extensive cooling and condensation at the poles. Introduction of liquid CO2into the Martian regolith by basal melting at the poles may be a major mechanism for loss of the ancient greenhouse atmosphere and its long‐term sequestration. Explosive release of CO2fluid coexisting with groundwater may trigger outflow channel floods and reset atmospheric isotope ratios.

Fundamental studies of the cluster ion bombardment of water ice

The fundamental sputtering properties of water ice are of interest for molecular depth profiling of biological samples in their native environment. We report on a method of studying amorphous water ice films of precise thicknesses in which pure water vapor is condensed onto a pre-cooled, silver-coated quartz crystal microbalance (QCM). This scheme allows for the determination of water ice sputter yields for any primary projectile as well as providing a means for studying escape depths of atoms and molecules beneath the deposited water ice layer. Specifically, we find a removal of approximately 2500 water molecule equivalents/20 keV C 60 + projectile with an underlying silver ion escape depth of 7.0 Å.

ABUNDANCE OF A NOVEL DINOFLAGELLATE PHYLOTYPE IN THE ROSS SEA, ANTARCTICA1

The biodiversity of protistan assemblages present in microhabitats of the Ross Sea, Antarctica, was examined using molecular biological methods to obtain a greater understanding of the genetic diversity present. Sequencing of 18S clone libraries indicated genetically diverse collections of organisms in the water column, ice, and meltwater layer (slush), but a single small subunit ribosomal DNA (srDNA) sequence type dominated clone libraries (>30%) from seawater and slush samples taken within the ice pack of this ecosystem. The BLAST searches indicated that this dominant clone was derived from a dinoflagellate, and that it shared sequence similarity (97.6%–98.3%) with both Karenia and Karlodinium species. Phylogenetic analyses based on small subunit ribosomal gene sequences supported its placement as a sister group to these taxa, and suggested that it represented a novel genus. The dinoflagellate was successfully recovered in culture, and morphological analyses have shown that it contains chloroplasts, is gymnodinoid, appears not to have thecal plates, and has an apical groove and sulcal structure that confirm its placement as a relative of the Karenia/Karlodinium group. The abundance of this phylotype in natural samples was confirmed by quantitative PCR analyses of water and slush communities, and suggests that this dinoflagellate can be a major constituent of the protistan assemblages of some Antarctic microhabitats of the Ross Sea.

Comparison between polar regions of Mars from HEND/Odyssey data

In this paper, we have analyzed neutron spectroscopy data gathered by the High Energy Neutron Detector (HEND) instrument onboard Mars Odyssey for comparison of polar regions. It is known that observation of the neutron albedo of Mars provides important information about the distribution of water–ice in subsurface layers and about peculiarities of the CO 2 seasonal cycle. It was found that there are large water-rich permafrost areas with contents of up to ∼50% water by mass fraction at both the north and south Mars polar regions. The water–ice layers at high northern latitudes are placed close to the surface, but in the south they are covered by a dry and relatively thick (10–20 cm) layer of soil. Analysis of temporal variations of neutron flux between summer and winter seasons allowed the estimation of the masses of the CO 2 deposits which seasonally condense at the polar regions. The total mass of the southern seasonal deposition was estimated as 6.3 × 10 15 kg , which is larger than the total mass of the seasonal deposition at the north by 40–50%. These results are in good agreement with predictions from the NASA Ames Research Center General Circulation Model (GCM). But, the dynamics of the condensation and sublimation processes are not quite as consistent with these models: the peak accumulation of the condensed mass of CO 2 occurred 10–15 degrees of L s later than is predicted by the GCM.

Seas under ice: Stability of liquid-water oceans within icy worlds

The present-day existence of internal oceans under the outer ice shell of several icy satellites of the Solar System has been recently proposed. The presence of antifreeze substances decreasing ice’s melting point (and tidal heating in Europa’s case) has been generally believed to allow the stability of such oceans; limited cooling of the water (ice plus liquid) layer, due to stability against convection or to stagnant lid convection in the icy shell, have been also considered. Here we propose that even pure liquidwater oceans could survive today within several icy worlds, and we consider some factors affecting thermal modeling in these bodies. So, the existence of such oceans would be a natural consequence of the physical properties of water ice, independently from the addition of antifreeze substances or any other special conditions. The inclusion of these substances would contribute to expand the conditions for water to stay liquid and to increase ocean’s volume.

Production of Oxidants by Ion Irradiation of Water/Carbon Dioxide Frozen Mixtures

We present new experimental results on the formation of oxidants, such as hydrogen peroxide, ozone, and carbonic acid, under ion irradiation of icy mixtures of water/carbon dioxide at different ratios and temperatures (16 and 80 K). Pure water ice layers and mixtures with carbon dioxide were irradiated by 200 keV He+ ions. We found that the CO(2)/H(2)O ratio progressively decreased to a value of about 0.1, the H(2)O(2) production increased with increasing CO(2) abundance at both 16 and 80 K, and the CO and H(2)CO(3) production increased with increasing CO(2) abundance at 16 K. At 80 K, the synthesis of CO was less efficient because of the high volatility of the molecule that partially sublimed from the target. The production of carbonic acid was connected with the production of CO(3). O(3) was detected only after ion irradiation of CO(2)-rich mixtures. The experimental results are discussed with regard to the relevance they may have in the production of an energy source for a europan or a martian biosphere.

Phasing diffraction data from a stream of hydrated proteins

We consider the problem of phase determination for continuous diffraction patterns obtained from a beam of identical, aligned large molecules (such as proteins), each coated with a layer of water or vitreous ice. Many laser-aligned molecules are assumed to lie within a wide continuous x-ray beam at any instant. An iterative phasing method is developed to extract the common target structure in three dimensions from diffraction patterns of these doped ice balls. Several measurements of the diffraction intensity in reciprocal space are needed. We found iteratively two boundaries (supports) (between protein and ice and the outer iceball support) by using the charge-flipping and multiple hybrid input-output algorithms, working with multiple sets of measured data. The approach is applied to simulated data from hydrated lysozyme proteins generated by the serial crystallography method of laser-aligned protein-beam diffraction proposed by Spence and Doak [Phys. Rev. Lett. 92, 198102 (2004)]. We consider also the effect of empty ice balls on the patterns. The algorithm can also be used to align images with different randomly chosen origins, so that the same embedded subunits overlap.

Ion irradiation of asphaltite as an analogue of solid hydrocarbons in the interstellar medium

We present the results of experiments of low temperature ion irradiation of pure asphaltite samples and those covered with water ice layers. Asphaltite (a natural bitumen) was used as an analogue of carbonaceous interstellar grains on which water- rich icy mantles can be formed in the dense and cold phases of the interstellar medium. We simulated irradiation by cosmic ions in the laboratory to study, by in situ infrared spectroscopy, the formation of molecular species in the asphaltite samples and at the interface between the refractory solid (asphaltite) and the water ice mantle. New molecular species were formed at the expense of water molecules and atoms (mainly carbon and hydrogen) from the asphaltite substrate. The vibration modes we detected at low temperature (16 K) include: C-H stretch (carbynoids), C-H asymmetric stretch (acetylene), C-H aromatic stretch, C=O stretch in CO2 ,C ≡O stretch in CO, and C≡C stretch (carbynoids). Some of these species remain stable after warming of the irradiated samples to room temperature.