The signature of condensed molecular oxygen has been reported in recent optical-reflectance measurements of the jovian moon Ganymede, and a tenuous oxygen atmosphere has been observed on Europa. The surfaces of these moons contain large amounts of water ice, and it is thought that O2 is formed by the sputtering of ice by energetic particles from the jovian magnetosphere. Understanding how O2 might be formed from low-temperature ice is crucial for theoretical and experimental simulations of the surfaces and atmospheres of icy bodies in the Solar System. Here we report laboratory measurements of the threshold energy, cross-section and temperature dependence of O2 production by electronic excitation of ice in vacuum, following electron-beam irradiation. Molecular oxygen is formed by direct excitation and dissociation of a stable precursor molecule, rather than (as has been previously thought) by diffusion and chemical recombination of precursor fragments. The large cross-section for O2 production suggests that electronic excitation plays an important part in the formation of O2 on Ganymede and Europa.

[1] Evidence for an ocean beneath the icy crust of Europa includes reflectance spectra of disrupted surface regions indicating hydrated materials such as salts. We simulated exposure of salty brine on the cold surface of Europa by flash-freezing sulfate and carbonate solutions. This produces materials that have near-infrared reflectance spectra distinct from those for crystalline minerals and more similar to those for Europa's non-ice regions. These new spectroscopic data, along with geophysical evidence, geochemical models, and meteorite studies, strongly suggest that the non-ice materials in the disrupted regions on Europa's surface contain large amounts of disordered and heavily hydrated MgSO 4 and perhaps Na 2 SO 4 that are endogenic in origin.

Abstract. We report studies on the thermal and radiolytic stability of the hydrated salt minerals epsomite (MgSO4o7H20), mirabilite (Na2SO4 ß 10H20 ), and natron (Na2CO3 ß 10H20 ) under the low-temperature and ultrahigh vacuum conditions characteristic of the surface of the Galilean satellite Europa. We prepared samples, ran temperature-programmed dehydration (TPD) profiles and irradiated the samples with electrons. The TPD profiles are fit using Arrhenius-type first-order desorption kinetics. This analysis yields activation energies of 0.90+0.10, 0.70+0.07, and 0.45_+0.05 eV for removal of the hydration water for epsomite, natron, and mirabilite, respectively. A simple extrapolation indicates that at Europa surface temperatures (_<130 K), epsomite should remain hydrated over geologic timescales (-1011-10 TM years), whereas natron and mirabilite may dehydrate appreciably in approximately 108 and 103 years, respectively. A small amount of SO 2 was detected during and after 100 eV electron-beam irradiation of dehydrated epsomite and mirabilite samples, whereas products such as 0 2 remained below detection limits. The upper limit for the 100 eV electron-induced damage cross section of mirabilite and epsomite is -10 -19 cm 2. The overall radiolytic stability of these minerals is partially due to (1) the multiply charged nature of the sulfate anion, (2) the low probability of reversing the attractive Madelung (mostly the attractive electrostatic) potential via Auger decay, and (3) solid-state caging effects. Our laboratory results on the thermal and radiolytic stabilities of these salt minerals indicate that hydrated magnesium sulfate and perhaps other salts could exist for geologic timescales on the surface of Europa. [1998, 1999a] proposed from the NIMS spectroscopic evidence that these absorptions were due to hydrated (and not just hydroxylated) minerals, with the most likely candidate matehals being

The electron-stimulated desorption ͑ESD͒ of D Ϫ ions from condensed D 2 O films is investigated. Three low-energy peaks are observed which are identified as arising from excitation of 2 B 1 , 2 A 1 , and 2 B 2 dissociative electron attachment ͑DEA͒ resonances. A fourth, higher energy feature is also seen in the D Ϫ yield which is likely due to the formation of a transient anion state that dissociates and/or decays into a dissociative excited state. The energies and ion yields of the resonances vary with the temperature and morphology of the D 2 O film. Below 60 K, the work function of the ice films changes with temperature and the DEA resonances shift in energy. The D Ϫ ESD yield generally increases with temperature, but it deviates from this trend at temperatures corresponding to structural phase transitions in ice. The (2 B 1) D Ϫ temperature dependence is remarkably similar to that observed for the ESD of low-energy D ϩ ions from D 2 O ice, even though the two originate from different electronic excitations. These results are attributed to thermally induced changes in the hydrogen bonding network, which changes the lifetimes of the predissociative states that lead to ESD and which also allows for the reorientation of surface molecules.

We present a study of the electron-stimulated desorption of deuterium cations ͑D ϩ) from thin ͑1-40 ML͒ D 2 O ice films vapor deposited on a Pt͑111͒ substrate. Measurements of the total yield and velocity distributions as a function of temperature from 90 to 200 K show that the D ϩ yield changes with film thickness, surface temperature, and ice phase. We observe two energy thresholds for cation emission, near 25 and 40 eV, which are weakly dependent upon the ice temperature and phase. The cation time-of-flight ͑TOF͒ distribution is at least bimodal, indicating multiple desorption channels. A decomposition of the TOF distributions into ''fast'' and ''slow'' channels shows structure as a function of excitation energy, film thickness, and temperature. The D ϩ yield generally increases with temperature, rising near 120 K on amorphous ice, and near 135 K on crystalline ice. The amorphous-crystalline phase transition at ϳ160 K causes a drop in total desorption yield. The temperature dependence of D Ϫ desorption via the 2 B 1 dissociative electron attachment resonance is very similar to the slow D ϩ yield, and likely involves similar restructuring and lifetime effects. The data collectively suggest that a thermally activated reduction of surface hydrogen bonding increases the lifetime of the excited states responsible for ion desorption, and that these lifetime effects are strongest for excited states involving a 1 bands ͓S0163-1829͑97͒05931-6͔

VERA, the Virtual Environment for Reactor Applications, is the system of physics capabilities being developed and deployed by the Consortium for Advanced Simulation of Light Water Reactors (CASL). CASL was established for the modeling and simulation of commercial nuclear reactors. VERA consists of integrating and interfacing software together with a suite of physics components adapted and/or refactored to simulate relevant physical phenomena in a coupled manner. VERA also includes the software development environment and computational infrastructure needed for these components to be effectively used. We describe the architecture of VERA from both software and numerical perspectives, along with the goals and constraints that drove major design decisions, and their implications. We explain why VERA is an environment rather than a framework or toolkit, why these distinctions are relevant (particularly for coupled physics applications), and provide an overview of results that demonstrate the use of VERA tools for a variety of challenging applications within the nuclear industry.

The clean GaSb(100) surface exhibits a variety of reconstructions, including a c(2 X 10), a c(2 X 6), and a (1 X 3)/c(2X 6), in order of a decreasing surface Sb/Ga ratio. Core-level photoemission spectroscopy and reAection high-energy electron diffraction were employed to study each of these reconstructions in detail. Our results show that the c(2X6) and (1X3)/c(2X6) surfaces have significantly different stoichiometries and core-level line shapes, and are, in fact, different reconstructions, rather than variations of the same c(2X 6) surface due to disorder. Analysis of the photoemission data taken with a wide range of incident photon energy suggests that the c (2 X 10) surface is very Sb rich, with more than 2 ML of Sb, while the c(2X6) has 1 -3 ML of Sb, and the (1X3)/c(2X6) has about 1 --ML of Sb with -, 'ML of Ga atoms intermixed on the surface. Quantum-mechanical diffraction and interference effects are found to significantly affect the measurement of surface-to-bulk photoemission intensity ratios. Surface structure models are presented for each reconstruction which are consistent with the observed data.

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