An approach to numerical simulation of the gas distribution in the atmosphere of Enceladus

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/94942/1/jgra20509.pd

The composition and structure of the Enceladus plume

The Cassini Ultraviolet Imaging Spectrograph (UVIS) observed an occultation of the Sun by the water vapor plume at the south polar region of Saturn\u27s moon Enceladus. The Extreme Ultraviolet (EUV) spectrum is dominated by the spectral signature of H(2)O gas, with a nominal line-of-sight column density of 0.90 +/- 0.23 x 10(16) cm(-2) (upper limit of 1.0 x 10(16) cm(-2)). The upper limit for N(2) is 5 x 10(13) cm(-2), or \u3c 0.5% in the plume; the lack of N(2) has significant implications for models of the geochemistry in Enceladus\u27 interior. The inferred rate of water vapor injection into Saturn\u27s magnetosphere is similar to 200 kg/s. The calculated values of H(2)O flux from three occultations observed by UVIS have a standard deviation of 30 kg/s (15%), providing no evidence for substantial short-term variability. Collimated gas jets are detected in the plume with Mach numbers of 5-8, implying vertical gas velocities that exceed 1000 m/sec. Observations at higher altitudes with the Cassini Ion Neutral Mass Spectrometer indicate correlated structure in the plume. Our results support the subsurface liquid model, with gas escaping and being accelerated through nozzle-like channels to the surface, and are consistent with recent particle composition results from the Cassini Cosmic Dust Analyzer

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.Fil: Raut, U.. University of Virginia; Estados UnidosFil: Teolis, B. D.. University of Virginia; Estados UnidosFil: Loeffler, M. J.. University of Virginia; Estados UnidosFil: Vidal, Ricardo Alberto. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Santa Fe. Instituto de Desarrollo Tecnológico para la Industria Química. Universidad Nacional del Litoral. Instituto de Desarrollo Tecnológico para la Industria Química; ArgentinaFil: Famá, M.. University of Virginia; Estados UnidosFil: Baragiola, R. A.. University of Virginia; Estados Unido

On the origin and evolution of the material in 67P/Churyumov-Gerasimenko

International audiencePrimitive objects like comets hold important information on the material that formed our solar system. Several comets have been visited by spacecraft and many more have been observed through Earth- and space-based telescopes. Still our understanding remains limited. Molecular abundances in comets have been shown to be similar to interstellar ices and thus indicate that common processes and conditions were involved in their formation. The samples returned by the Stardust mission to comet Wild 2 showed that the bulk refractory material was processed by high temperatures in the vicinity of the early sun. The recent Rosetta mission acquired a wealth of new data on the composition of comet 67P/Churyumov-Gerasimenko (hereafter 67P/C-G) and complemented earlier observations of other comets. The isotopic, elemental, and molecular abundances of the volatile, semi-volatile, and refractory phases brought many new insights into the origin and processing of the incorporated material. The emerging picture after Rosetta is that at least part of the volatile material was formed before the solar system and that cometary nuclei agglomerated over a wide range of heliocentric distances, different from where they are found today. Deviations from bulk solar system abundances indicate that the material was not fully homogenized at the location of comet formation, despite the radial mixing implied by the Stardust results. Post-formation evolution of the material might play an important role, which further complicates the picture. This paper discusses these major findings of the Rosetta mission with respect to the origin of the material and puts them in the context of what we know from other comets and solar system objects

Distillation Kinetics of Solid Mixtures of Hydrogen Peroxide and Water and the Isolation of Pure Hydrogen Peroxide in Ultrahigh Vacuum

We present results of the growth of thin films of crystalline H2O2 and H2O2.2H2O (dihydrate) in ultrahigh vacuum by distilling an aqueous solution of hydrogen peroxide. We traced the process using infrared reflectance spectroscopy, mass loss on a quartz crystal microbalance, and in a few cases ultraviolet-visible reflectance. We find that the different crystalline phases-water, dihydrate, and hydrogen peroxide-have very different sublimation rates, making distillation efficient to isolate the less volatile component, crystalline H2O2

A Model Study of the Thermal Evolution of Astrophysical Ices

We address the question of the evolution of ices that have been exposed to radiation from stellar sources and cosmic rays. We studied in the laboratory the thermal evolution of a model ice sample: a mixture of water, hydrogen peroxide, dioxygen, and ozone produced by irradiating solid H2O2 with 50 keV H(+) at 17 K. The changes in composition and release of volatiles during warming to 200 K were monitored by infrared spectroscopy, mass spectrometry, and microbalance techniques. We find evidence for voids in the water component from the infrared bands due to dangling H bonds. The absorption from these bands increases during heating and can be observed at temperatures as high as approx. 155 K. More O2 is stored in the radiolyzed film than can be retained by codeposition of O2 and H2O. This O2 remains trapped until approx. 155 K, where it desorbs in an outburst as water ice crystallizes. Warming of the ice also drastically decreases the intrinsic absorbance of O2 by annealing defects in the ice. We also observe loss of O3 in two stages during heating, which correlates with desorption and possibly chemical reactions with radicals stored in the ice, triggered by the temperature increase

Angular dependence of the sputtering yield of water ice by 100 keV proton bombardment

We measured the total sputtering yield of amorphous water ice for 100 keV H+ as a function of the projectile incidence angle, and the angular distribution of the ejected H2O and O2 molecules, using a quartz-crystal microbalance and mass spectrometry, respectively, at temperatures of 20 K and 100 K. The total sputtering yield follows a cos−f θ dependence, with f ≅ 1.3, regardless of the irradiation temperature. This is explained by the action of fast binary δ-electrons that relocate the electronic energy deposited by the ion near the surface into the bulk of the material. We found that the O2 emission follows a cosine dependence, as expected from isotropic collision cascades or if transport of the oxygen to the surface is by thermal diffusion. In contrast, H2O emission is more outward peaked than cosine, which could be attributed to the blocking of large angle emission by the transient crater formed during sputtering of multiple water molecules by a given projectile.Fil: Vidal, Ricardo Alberto. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Santa Fe. Instituto de Desarrollo Tecnológico para la Industria Química. Universidad Nacional del Litoral. Instituto de Desarrollo Tecnológico para la Industria Química; Argentina. University of Virginia; Estados UnidosFil: Teolis, B. D.. University of Virginia; Estados UnidosFil: Baragiola, Raul Antonio. University of Virginia; Estados Unido

Mechanisms of O2 Sputtering from Water Ice by keV Ions

We have conducted experiments on the sputtering of water ice by 100 keV Ar(+) between 20 and 150 K. Our findings indicate that the temperature dependence of the total sputtering yield is heavily influenced by the thermal and irradiation history of the ice, showing a complex dependence on irradiation fluence that is correlated to the ejection of O2 molecules. The results suggest that O2 produced by the ions inside the ice diffuses to the surface where it is trapped and then ejected via sputtering or thermal desorption. A high concentration of O2 can trap in a subsurface layer during bombardment at 130 K, which we relate to the formation of hydrogen and its escape from that region. A simple model allows us to determine the depth profile of the absolute concentration of O2 trapped in the ice