M. Famá

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

Radiolysis of water ice in the outer solar system: Sputtering and trapping of radiation products

We performed quantitative laboratory radiolysis experiments on cubic water ice between 40 and 120 K, with 200 keV protons. We measured sputtering of atoms and molecules and the trapping of radiolytic molecular species. The experiments were done at fluences corresponding to exposure of the surface of the Jovian icy satellites to their radiation environment up to thousands of years. During irradiation, O 2 molecules are ejected from the ice at a rate that grows roughly exponentially with temperature; this behavior is the main reason for the temperature dependence of the total sputtering yield. O 2 trapped in the ice is thermally released from the ice upon warming; the desorbed flux starts at the irradiation temperature and increases strongly above 120 K. Several peaks in the desorption spectrum, which depend on irradiation temperature, point to a complex distribution of trapping sites in the ice matrix. The yield of O 2 produced by the 200 keV protons and trapped in the ice is more than 2 orders of magnitude smaller than used in recent models of Ganymede. We also found small amounts of trapped H 2 O 2 that desorb readily above 160 K.

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.

Atomic collisions in solids: Astronomical applications

Airless bodies in space are subject to irradiation with energetic atomic particles, which generate atmospheres by sputtering and alter the surface composition. Astronomical observations with telescopes and space probes continuously provide new data that require new laboratory experiments for their interpretation. Many of these experiments also serve to expand the current frontier of atomic collisions in solids by discovering previously unknown phenomena. Some of the experimental techniques used in these experiments could find applications in other areas of atomic collisions in solids. We present results from our current experimental research program on sputtering and surface modification of ices and minerals and point out opportunities for research in this area.

Low density solid ozone

We report a very low density ( approximately 0.5 g/cm(3)) structure of solid ozone. It is produced by irradiation of solid oxygen with 100 keV protons at 20 K followed by heating to sublime unconverted oxygen. Upon heating to 47 K the porous ozone compacts to a density of approximately 1.6 g/cm(3) and crystallizes. We use a detailed analysis of the main infrared absorption band of the porous ozone to interpret previous research, where solid oxygen was irradiated by UV light and keV electrons.

Ion beam induced chemistry: the case of ozone synthesis and its influence on the sputtering of solid oxygen

Abstract To understand chemical effects caused by ion beams in solids we study the simplest case, the formation of ozone in solid oxygen. We irradiate thin O 2 films with 100 keV protons and study the fluence dependence of the sputtering of solid O 2 with a microbalance and a mass spectrometer. The formation of ozone in the film after irradiation is identified by optical reflectance spectroscopy and thermal desorption. We discuss the processes involved and the implications of the fluence dependence of the ozone concentration upon the sputtering yield of oxygen.

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).

Sputtering from a Porous Material by Penetrating Ions

Porous materials are ubiquitous in the universe and weathering of porous surfaces plays an important role in the evolution of planetary and interstellar materials. Sputtering of porous solids in particular can influence atmosphere formation, surface reflectivity, and the production of the ambient gas around materials in space. Several previous studies and models have shown a large reduction in the sputtering of a porous solid compared to the sputtering of the non-porous solid. Using molecular dynamics simulations we study the sputtering of a nanoporous solid with 55% of the solid density. We calculate the electronic sputtering induced by a fast, penetrating ion, using a thermal spike representation of the deposited energy. We find that sputtering for this porous solid is, surprisingly, the same as that for a full-density solid, even though the sticking coefficient is high.