U. Raut

Characterization of porosity in vapor-deposited amorphous solid water from methane adsorption

We have characterized the porosity of vapor-deposited amorphous solid water (ice) films deposited at 30-40 K using several complementary techniques such as quartz crystal microgravimetry, UV-visible interferometry, and infrared reflectance spectrometry in tandem with methane adsorption. The results, inferred from the gas adsorption isotherms, reveal the existence of microporosity in all vapor-deposited films condensed from both diffuse and collimated water vapor sources. Films deposited from a diffuse source show a step in the isotherms and much less adsorption at low pressures than films deposited from a collimated source with the difference increasing with film thickness. Ice films deposited from a collimated vapor source at 77 degrees incidence are mesoporous, in addition to having micropores. Remarkably, mesoporosity is retained upon warming to temperatures as high as 140 K where the ice crystallized. The binding energy distribution for methane adsorption in the micropores of ice films deposited from a collimated source peaks at approximately 0.083 eV for deposition at normal incidence and at approximately 0.077 eV for deposition at >45 degrees incidence. For microporous ice, the intensity of the infrared bands due to methane molecules on dangling OH bonds on pore surfaces increases linearly with methane uptake, up to saturation adsorption. This shows that the multilayer condensation of methane does not occur inside the micropores. Rather, filling of the core volume results from coating the pore walls with the first layer of methane, indicating pore widths below a few molecular diameters. For ice deposited at 77 degrees incidence, the increase in intensity of the dangling bond absorptions modified by methane adsorption departs from linearity at large uptakes.

Cosmic Ray Compaction of Porous Interstellar Ices

We studied the compaction of microporous vapor-deposited ice films under irradiation with different ions in the 80Y400keVenergyrange.Wefoundthatporositydecreasesexponentiallywithirradiationfluence,withameancompactionareaperionthatscaleslinearlywiththestoppingpowerof theprojectileSaboveathresholdSt ¼ 4eV8 � 1 .The experiments roughly follow a universal dependence of ion-induced compaction with restricted dose (eV molecule � 1 ). This behavior can be used to extrapolate our results to conditions of the interstellar medium. Relating our results to ionization rates of interstellar H2, we estimate that porous ice mantles on grains in dense molecular clouds are compacted by cosmic rays in � 10Y50 million years. Subject headingg cosmic rays — ISM: molecules — methods: laboratory — radiation mechanisms: nonthermal Online material: color figures

Compaction of microporous amorphous solid water by ion irradiation

We have studied the compaction of vapor-deposited amorphous solid water by energetic ions at 40 K. The porosity was characterized by ultraviolet-visible spectroscopy, infrared spectroscopy, and methane adsorption/desorption. These three techniques provide different and complementary views of the structural changes in ice resulting from irradiation. We find that the decrease in internal surface area of the pores, signaled by infrared absorption by dangling bonds, precedes the decrease in the pore volume during irradiation. Our results imply that impacts from cosmic rays can cause compaction in the icy mantles of the interstellar grains, which can explain the absence of dangling bond features in the infrared spectrum of molecular clouds.

Ozone Synthesis on the Icy Satellites

Condensed O2 and ozone on the surfaces of some icy satellites are thought to originate from the radiolytic decomposition of surface water ice by the impact of energetic magnetospheric ions, but decades of laboratory studies have produced no evidence for ozone from the radiolysis of pure water ice. Here we report for the first time the production of ozone in ice by 100 keV ions. Using a method that departs drastically from those used in all previous experiments, we have simulated more closely conditions on the icy satellites by performing ion irradiation while depositing water concurrently, which takes into account the effects of gravity and surface porosity. This codeposition causes the burial of a high concentration of radiolytic O2 from which ozone is formed. Our results demonstrate that the enhanced trapping of oxygen in surface ices depends on temperature and should vary locally, depending on the rates of irradiation and recondensation. The burial of radiolytic products by redeposition will likely occur in many varied astronomical environments, besides icy satellites.

Enceladus: A Source of Nitrogen and an Explanation for the Water Vapor Plume Observed by Cassini

Recently, the Cassini spacecraft observed an unexpected emission of plumes of water vapor, nitrogen, and icy particles from the southern polar region of Saturns icy moon Enceladus. While these findings support previous ideas of geological activity in this icy moon, there is no experimental evidence explaining how these plumes could be produced at the low (~130-160 K) surface temperatures. Here we show that similar behavior appears when heating water-ammonia ices that have been irradiated with protons that simulate Saturns energetic ion environment. In our experiments, the behavior results from the eruption of high-pressure bubbles of hydrogen and nitrogen molecules that accumulate in the ice due to the radiolytic decomposition of ammonia. The thermal release of nitrogen can explain the intriguing finding of N+ in the inner magnetosphere. Thus, our laboratory simulations indicate that radiation processing of the surface of Enceladus may explain much of the extraordinary phenomena that have been observed by Cassini.

Radiation Synthesis of Carbon Dioxide in Ice-coated Carbon: Implications for Interstellar Grains and Icy Moons

We report the synthesis of carbon dioxide on an amorphous carbon-13 substrate coated with amorphous water ice from irradiation with 100 keV protons at 20 K and 120 K. The quantitative studies show that the CO{sub 2} is dispersed in the ice; its column density increases with ion fluence to a maximum value (in 10{sup 15} molecules cm{sup -2}) of {approx}1 at 20 K and {approx}3 at 120 K. The initial yield is 0.05 (0.1) CO{sub 2} per incident H{sup +} at 20 (120) K. The CO{sub 2} destruction process, which limits the maximum column density, occurs with an effective cross section of {approx}2.5 (4.1) Multiplication-Sign 10{sup -17} cm{sup 2} at 20 (120) K. We discuss radiation-induced oxidation by reactions of radicals in water with the carbon surface and demonstrate that these reactions can be a significant source of condensed carbon dioxide in interstellar grains and in icy satellites in the outer solar system.

SOLID-STATE CO OXIDATION BY ATOMIC O: A ROUTE TO SOLID CO2 SYNTHESIS IN DENSE MOLECULAR CLOUDS

Investigations with infrared spectroscopy and microgravimetry show that CO2 forms in small quantities during codeposition of CO and cooled O and O2 into thin films at 20 K: ~3 × 1014 CO2 cm–2 within a film containing 2.7 × 1017 CO cm–2. The reason for the low CO2 yield is that O atoms react preferentially with O to form O2, and with O2 to form ozone, which was not considered in previous studies with atomic beams. Heating a CO + O + O2 film capped with a ~2.5 μm thick water ice layer to 80 K increased CO2 by 30%, showing additional reactions between O and CO diffusing in the ice layer. Although the CO2:CO = 10–3 measured in our experiments is at least two orders of magnitude smaller than reported interstellar ratios, sputtering and differential desorption during transient heating events, e.g., by impacts of cosmic rays or stellar winds, can increase the ratio to observed values.

PHOTOSYNTHESIS OF CARBON DIOXIDE FROM CARBON SURFACES COATED WITH OXYGEN: IMPLICATIONS FOR INTERSTELLAR MOLECULAR CLOUDS AND THE OUTER SOLAR SYSTEM

We investigate via infrared spectroscopy the synthesis of CO2 by ultraviolet irradiation (6.41 eV) of amorphous carbon covered with solid O2 at 21 K. Oxidation occurs at the O2-carbon interface promoted by photon excitation or dissociation of O2 molecules. The CO2 production is linear with photon fluence with a yield of 3.3 ± 0.3 × 10–5 CO2 photon–1; the yield does not decrease at high fluences (at least up to 2 × 1019 photons cm–2) since CO2 is not photodissociated at this photon energy. Replacing oxygen with water ice did not produce CO2 since H2O does not dissociate at this photon energy. The CO2 synthesis process discussed in this Letter does not require H2O or CO and may be important in cold astrophysical environments where O2 could be locally segregated on carbonaceous grains, such as in molecular clouds and icy objects in the outer solar system.

Radiation Effects in Water Ice in the Outer Solar System

Water ice in the outer solar system can either have condensed from the gas phase or have been brought in by colliding bodies, such as interplanetary ice grains to comets. Since icy bodies lack a protective atmosphere, their surface is subject to irradiation by photons, ions and electrons. This chapter discusses how energetic radiation affects the physical and chemical properties of a pure water ice surface, and how the outcome of radiation processes depends on the properties of the surface and on the environment (atmosphere, particle flux and energies).

Effect of microstructure on spontaneous polarization in amorphous solid water films

Amorphous solid water (ASW) films grown by vapor deposition below 110 K develop negative surface voltages Vs with respect to the substrate. This polarization is due to a partial alignment of the water molecules during condensation. Kelvin probe measurements show that the magnitude of the surface potential, |Vs|, increases linearly with film thickness at a rate that decreases with increasing deposition temperature. |Vs| decreases with increasing deposition temperature and increasing incidence angle of the vapor source. After film growth, |Vs| decreases irreversibly by 80% when the ice film is heated to ∼30 K above the deposition temperature. The measurements of |Vs| as a function of film porosity indicate that polarization in ASW is governed by incompletely coordinated water molecules, dangling with unbalanced dipoles at the internal surface of the pores and weakly aligned by the anisotropic film-vacuum interface. This idea is supported by the strikingly similar behavior of |Vs| and the infrared absorption due to the most pliable, two-coordinated surface molecules with annealing temperature.