Rate of H2 formation on amorphous grains

The rate of formation of molecular hydrogen from hydrogen atoms adsorbed on grains is analyzed, assuming that the grains are single crystals, polycrystalline or amorphous. On polycrystalline grains, and on graphite platelets, this rate could be orders of magnitude lower than on single crystal grains. The same is true for amorphous grains because there, at low temperatures, only atoms absorbed on neighboring sites can form molecules. Suitable formulae are derived and compared with the classical results for single crystal grains. Quantitative results are given for crystalline and amorphous ice, but with small changes these should also be valid for other solids. The rates for amorphous grains can approximate, within a factor of 10 or so, those for crystalline grains if the density of H atoms is high and the density of H₂ molecules is low and only when the temperature of the grains satisfies a relation which for ice and graphite leads to a value in the proximity of 15–17 K. This maximum rate occurs only a degree or so above the temperature at which the grains are totally covered by an H₂ layer and the reaction ceases. Furthermore, for a constant number density of grains, the rates on amorphous grains are second order while those on crystalline grains are first order. Both these circumstances predict amorphous grains to lead to H₂ clouds with irregular and sharply delineated features in contrast to more uniform clouds formed on crystalline grains.     

Interstellar catalysis

Although it is generally accepted that most, if not all, of the molecular hydrogen in interstellar space is formed through recombination reactions on grains, the exact mechanism by which this is accomplished is far from certain. In the past, great emphasis had been placed on the physical adsorption of H atoms on cold dielectric grains and their subsequent recombination and desorption as H₂ molecules. However, a careful re-examination of the problem leads us to believe that a rate coefficient ofk10^–17 cm³ s^–1—the value usually quoted in the literature—is a very strong overestimate. The same thing can be said for the recombination of H atoms on graphite grains. Since two-body gas phase reactions are not sufficient by themselves to account for the observed abundances of H₂, an alternate mechanism must exist. It is suggested that the chemisorption of hydrogen on transition metal grains may be just that formation mechanism. After separating the adsorption rate equations from those of desorption and using experimentally determined parameters, it is shown that transition metal grains can successfully catalyze as much H₂ as the theoretical maximum predicted for cold ice grains, even though metal grains are probably less than 10% as abundant (by mass) than dielectrics.     

Cosmic ray interactions with lunar materials: Nature and composition of species formed

The solar and galactic cosmic rays interact directly with lunar surface materials, and the dominant nature of interactions is essentially the complete absorption of corpuscles. These corpuscles damage the lattice structure, and induce a complex set of reactions in the materials producing various species. The cosmic ray damage of the lattice would not produce an amorphous layer, similar to that produced by the solar wind, because the solar wind erosion rate is faster than the cosmic ray-induced amorphous layer formation rate. The species formation rate considered in this paper are those produced by protons, the dominant component of cosmic rays. Protons produce H, H₂, OH, H₂O, and hydrogenated species of carbon, nitrogen, sulfur, etc. These species, while migrating in the material, encounter oncoming cosmic ray corpuscles, and undergo a complex set of reactions. Although a variety of species are produced by protons, the dominant contributor to the atmosphere is H₂. The H₂ flux (molecules cm^–2 sec^–1) is about 1.5 × 10⁵ as compared to the H flux of 8.4 × 10¹ and the H₂O flux of 4.6 × 10^–2. These fluxes are about 10^–3 smaller than the fluxes of the same species produced by the solar wind protons. Thus the contributions of the cosmic ray-induced species to the atmosphere is very small compared to the solar wind-induced species. Although simulated experiments showed high concentractions of OH and H₂O in the terrestrial materials of lunar type, these species concentrations in the lunar materials under the lunar environment is much smaller than those observed in the simulated experiments.     

Incorporation of water into olivine during nebular condensation: Insights from density functional theory and thermodynamics,and implications for phyllosilicate formation and terrestrial water inventory

Using density functional theory, we have examined the hydration mechanism of olivine with the objective of understanding the reaction pathways toward the formation of crystalline serpentine and brucite. It is found that further supply of water beyond saturation of the adsorption sites on olivine surfaces leads to the formation of amorphous brucite and serpentine molecules, with the latter forming in the subsurface domain. The calculated activation energy for this process is ~25 kJ mol^?1, which permits formation of the amorphous materials well within the life span of the solar nebula. In addition, molecular dynamic simulations show that the adsorbed water in olivine is stable at least up to 900 K—a finding that is in accord with independent experimental studies. Thus, adsorption plus subsurface reaction of H₂O in olivine could have taken place at temperatures considerably higher than the stability limit of hydrous minerals in the nebular condition. Using the DFT derived enthalpy of adsorption data, and reasonable approximation for the entropy of adsorption, we have calculated the fractional coverage of the reactive surface sites of olivine grains of spherical geometry by adsorbed water, and the corresponding ocean equivalent water (OEW) that could have been accreted into the Earth. These results suggest that adsorption and the associated subsurface hydroxylation of olivine grains might have been responsible for a significant fraction of the Earth's water budget. The adsorption of water into olivine crystals in the solar nebula might also have led to the delivery of water to other planetary bodies.     

Io's 4-μm band and the role of adsorbed SO2

The role of adsorbed SO₂ on Io's surface particles in producing the observed spectral absorption band near 4 μm in Io's reflectance spectrum is explored. Calculations show that a modest 50% monolayer coating of adsorbed SO₂ molecules on submicron grains of sulfur of alkali sulfide, assumed to make up Io's uppermost optical surface (“radialith”), will result in a ν₁ + ν₃ absorption band near 4 μm with depth ～30% below the adjacent continuum, consistent with the observed strength of the Io band. The precise wavelength position of the ν₁ + ν₃ band of SO₂ in different phase states such as frost, ice, adsorbate, and gas are summarized from the experimental literature and compared with the available telescopic measurements of the Io band position. The results suggest that the 4-μm band in Io's full disk spectrum can best be explained by the presence on Io's surface of widespread SO₂ in the form of adsorbate rather than ice or frost.     

Formaldehyde polymers in comets

The possibility that crystalline formaldehyde polymers are present in cometary dust is discussed. In common with most other parent molecules proposed for comets, (H₂CO) _n is difficult to detect, even if it is present in relatively high concentrations. The optical properties of these polymers in the visual and infrared regions are similar to those of silicate grains, and crystalline formaldehyde polymers provide no emission at 6 cm wavelength. The lifetime of gaseous H₂CO in the solar radiation field is too short, and the expected transitions in the microwave region would be too weak to be detected. However, the available data concerning the physical properties of comets indicate that polymerized formaldehyde cannot be ruled out as a major constituent of cometary material.     

On the production of positive molecular ions in cometary comas

Positively charged molecular ions, such as H₂O⁺, which have been observed in cometary. comas, may be efficiently produced by the evaporation of positively charged clathrate grains of radii in the range 10^–6–10^–5 cm. Such grains may be expelled from nuclei of comets, along with gaseous molecules. Grain charging occurs via interaction with solar ultraviolet photons and/or solar wind protons. Observational data on the total quantities as well as the distributions of H₂O and H₂O⁺ in cometary comas are shown to be in accord with detailed model calculations.On leave from: Tata Institute of Fundamental Research, Bombay, India.     

Pressure dependent trace gas trapping in amorphous water ice at 77 K: Implications for determining conditions of comet formation

The nature of cometary volatile materials is subject to debate. Theoretical models of cometary nuclei and laboratory studies suggest that these objects could be made of amorphous water ice in addition to other volatile molecules and refractory grains. This water ice structure has the ability to encapsulate the gases of surrounding environment, reflecting the physical and chemical conditions during their deposition. Therefore, the knowledge of the chemical composition of volatile molecules trapped in amorphous water ice provides a tool for probing the formation environment of cometary ice grains. Experimental studies of gas trapping efficiency in amorphous water ice have been previously conducted mostly under kinetic conditions, where dynamic pumping and temperature gradients prevented rigorous calibrations. In this work, we investigated the trapping efficiencies of Ar, CO, CH₄, Kr and N₂ by depositing water vapor as ice in the presence of trace gases in a volume submerged in liquid nitrogen at 77 K. The gas trapping efficiencies were determined simply by monitoring the pressure difference of the trace gases before and after the deposition of a known amount of water molecules as amorphous ice.Our results show that the trapped gas to water molecule ratio in amorphous ice is controlled primarily by the partial pressure of the gas during water ice deposition, and is independent of the ice deposition rate as well as the gas to water ratio in the vapor phase. The trapping efficiencies of gases decrease in the order of Kr > CH₄ > CO > Ar > N₂ in accordance with previous studies. Assuming that the water ice structure of comets is at least partially amorphous water ice at the time of their formation, these results suggest that the total pressure and composition of the surrounding environment of amorphous ice formation are significant controlling factors of trace gas concentrations in cometary ice. This further indicates that the evolution of the solar nebula and timing of cometary ice condensation can also be important parameters in linking the volatile contents of comets and their formation process.     

Trapping and release of gases by water ice and implications for icy bodies

The trapping and release of H₂, CO, CO₂, CH₄, Ar, Ne, and N₂ by amorphous water ice was studied experimentally under dynamic conditions, at low temperatures starting at 16°K, with gas pressure of 5 × 10^?8?10^?6 Torr. CO, CH₄, Ar, and N₂ were found to be released in three or four distinct temperature ranges, each resulting from a different trapping mechanism: (a) 30–55°K, where the gas frozen on the water ice evaporates; (b) 135–155°K, where gas is squeezed out of the water ice during the transformation of amorphous ice to cubic ice; (c) 165–190°K, where gas and water are released simultaneously, probably by the evaporation of a clathrate hydrate, and, occasionally (d) 160–175°K, where deeply buried gas is released during the transformation of cubic ice to hexagonal ice. If the third range is indeed due to clathrate formation, CO was found to form this compound. CO₂ did not form a clathrate under the experimental conditions. Excess hydrogen did not affect the occlusion of other gases. Hydrogen itself was trapped only at 16°K. Neon was not trapped at 25°K. With cubic ice, the only trapping mechanism is freezing of gas on the ice surface. No fractionation between the gas phase and the ice was observed with a mixture of CO and Ar. Massive ejection of ice grains was observed during the evaporation of the gas in three (a,c,d) out of the four ranges. The experimental results are used to explain several cometary phenomena, especially those occurring at large heliocentric distances, and are applied also to Titan's atmospheric composition and to the possible ejection of ice grains from Enceladus.     

Origin of water in the terrestrial planets

Abstract— I examine the origin of water in the terrestrial planets. Late‐stage delivery of water from asteroidal and cometary sources appears to be ruled out by isotopic and molecular ratio considerations, unless either comets and asteroids currently sampled spectroscopically and by meteorites are unlike those falling to Earth 4.5 Ga ago, or our measurements are not representative of those bodies. However, the terrestrial planets were bathed in a gas of H, He, and O. The dominant gas phase species were H₂, He, H₂ O, and CO. Thus, grains in the accretion disk must have been exposed to and adsorbed H2 and water. Here I conduct a preliminary analysis of the efficacy of nebular gas adsorption as a mechanism by which the terrestrial planets accreted “wet.” A simple model suggests that grains accreted to Earth could have adsorbed 1‐3 Earth oceans of water. The fraction of this water retained during accretion is unknown, but these results suggest that examining the role of adsorption of water vapor onto grains in the accretion disk bears further study.     

Laboratory analogues to cosmic dust

A pulsed laser has been used to vaporize olivine, pyroxene, nickel-iron alloy, Al₂O₃, carbon, calcium carbonate, and silicon carbide, as well as mixtures of immiscible phases (Au–Al₂O₃ and Au-olivine) in oxidizing, reducing, and inert atmospheres. The collected condensates usually consist of strings of grains which have a median diameter of 20–30 nm, which is comparable to the calculated sizes of some interstellar and circumstellar dust grains. The silicate minerals vaporized in O₂ as well as calcium carbonate and carbon vaporized in Ar or H₂, are collected as glassy grains while the other materials produced crystalline grains. The systems of immiscible phases when vaporized produced condensates consisting of intermixed 2–50 nm grains of both components. The type of size distribution, crystal structures, and qualitiative elemental analyses of the condensates are given. Possible similarities between the mechanism of grain growth, structure, morphology, and chemistry of laboratory grains compared to interstellar and circumstellar grains, phases in meteorites and extraterrestrial dust collected in the stratosphere are examined. Applications of the experimental technique include the production of grain systems to serve as laboratory analogues for spectral studies of grain materials believed to exist in astronomical environments, and studies of the structure of grains condensed from complex gas mixtures.Paper presented at the Conference on Protostars and Planets, held at the Planetary Science Institute, University of Arizona, Tucson, Arizona, between January 3 and 7, 1978.     

Formaldehyde Polymers in Comets

The possibility that crystalline formaldehyde polymers are present in cometary dust is discussed. In common with most other parent molecules proposed for comets, (H₂CO)_n is difficult to detect, even if it is present in relatively high concentrations. The optical properties of these polymers in the visual and infrared regions are similar to those of silicate grains, and crystalline formaldehyde polymers provide no emission at 6 cm wavelength. The lifetime of gaseous H₂CO in the solar radiation field is too short, and the expected transitions in the microwave region would be too weak to be detected. However, the available data concerning the physical properties of comets indicate that polymerized formaldehyde cannot be ruled out as a major constituent of cometary material. This revised version was published online in July 2006 with corrections to the Cover Date.     

Is the D/H Ratio in the Comet Coma Equal to the D/H Ratio in the Comet Nucleus?

We present a simple, semianalytic model of the vaporization of H₂O and HDO ice from a comet nucleus. We use this model to show that the flux of HDO relative to H₂O can be much higher, at times, than would be expected from the D/H ratio in the nuclear ice itself. This effect varies with position in the comet's orbit. It is negligible sufficiently near the Sun but could lead to erroneous interpretations of the primordial D/H ratio in cometary ice if measurements are made in other parts of the cometary orbit.     

Crystalline comet dust: Laboratory experiments on a simple silicate system

Abstract— Spectra for certain comets show the presence of crystalline silicate dust grains believed to have been incorporated during comet formation. While grain crystallization is widely assumed to result from the thermal annealing of precursor amorphous grains, the physical processes behind the silicate amorphous‐to‐crystalline transition are poorly understood. This makes it difficult to place constraints on the evolutionary histories of both grains and comets, and consequently, on the nebular conditions in which they formed. It has, therefore, become necessary to study this process in the laboratory using simulated grain materials. In this paper, we discuss recent results from laboratory investigations into a basic amorphous MgSiO₃ silicate annealed in the region of 1000 K. Our object is not to model the behavior of dust grains per se, but to study the underlying process of crystallization and separate the physics of the material from the astrophysics of dust grains. In our experiments, we bring together spectroscopic measurements made in the infrared with the high resolution structural probing capabilities of synchrotron X‐ray powder diffraction. The combined use of these complementary techniques provides insights into the crystallization process that would not be easily obtained if each was used in isolation. In particular, we focus on the extent to which the identification of certain spectral features attributed to crystalline phases extends to the physical structure of the grain material itself. Specifically, we have identified several key features in the way amorphous MgSiO₃ behaves when annealed. Rather than crystallize directly to enstatite (MgSiO₃) structures, in crystallographic terms, amorphous MgSiO₃ can enter a mixed phase of crystalline forsterite (Mg₂SiO₄) and SiO₂‐rich amorphous silicate where structural evolution appears to stall. Spectroscopically, the evolution of the 10 μm band does not appear to correlate directly with structural evolution, and therefore, may be a poor indicator of the degree of crystallinity. Indeed, certain features in this band may not be indicators of crystal type. However, the 20 μm band is found to be a good indicator of crystal structure. We suggest that forsterite forms from the ordering of pre‐existing regions rich in SiO₄ and that this phase separation is aided by a dehydrogenation processes that results in the evolutionary stall. The implications of this work regarding future observations of comets are discussed.     

The possible formation of a hydrogen coma around comets at large heliocentric distances

An observational test--the detection of a hydrogen coma around comets at large heliocentric distances--is proposed for determining whether comets were formed by the agglomeration of unaltered, ice-coated, interstellar grains. Laboratory experiments showed that amorphous water ice traps H2, D2, and Ne below 20 K and does not release them completely until the ice is heated to 150 K. Gas/ice ratios as high as 0.63 are obtainable. Thus, if the ice-coated interstellar grains were not heated above approximately 110 K, prior to their agglomeration into cometary nuclei, the inward propagating heat waves should release from the comets a continuous flux of molecular hydrogen. This flux would exceed that of water molecules at approximately 3 AU preperihelion and approximately 4 AU postperihelion.     

Interstellar catalysis

Since gas-phase reactions alone cannot account for the observed abundances of H₂ in the typical interstellar cloud, one or more surface reactions are probably involved. Of the three possible candidates, only the catalytic production of H₂ on transition metal grains is supported by laboratory evidence. Using the rate equations developed in a previous paper for this process, the steady-state equilibrium abundances of H, H₂,e ^–, H⁺, H^–, H₂ ⁺, and H₃ ⁺ are calculated for large (r>10 pcs;M10² M ), tenuous (n=10²–10⁴ cm^–3) hydrogen dust clouds under a wide variety of conditions. In addition to the four rate equations involved in the catalytic reactions, 18 gas-phase and one additional surface reaction—the physical adsorption of H-atoms on cold, dielectric surfaces and their subsequent recombination and desorption as H₂ molecules—are included in the calculations. It is found that metal grains can produce as much interstellar H₂ as the best physical adsorption mechanism under optimum conditions if the extinction in the visible is less than 5^m.0. The three critical parameters for efficient catalysis (activation energy of desorption, grain temperature, and the number density of available sites) are examined, and it is shown that catalytic reactions are efficient producers of H₂ under all but the most unfavorable conditions.     

Trace constituents of Europa's atmosphere

Europa is bombarded by intense radiation that erodes the surface, launching molecules into a thin “atmosphere” representative of surface composition. In addition to atoms and molecules created in the mostly water ice surface such as H₂O, O₂, H₂, the atmosphere is known to have species representative of trace surface materials. These trace species are carried off with the 10-10⁴ H₂O molecules ejected by each energetic heavy ion, a process we have simulated using molecular dynamics. Using the results of those simulations, we found that a neutral mass spectrometer orbiting ∼100 km above the surface could detect species with surface concentrations above ∼0.03%. We have also modeled the atmospheric spatial structure of the volatile species CO₂ and SO₂ under a variety of assumptions. Detections of these species with moderate time and space resolution would allow us to constrain surface composition, chemistry and to study space weathering processes.     

The influence of grain mantles on the formation of hydrogen molecules on grain surfaces

The physical adsorption energy,E, of hydrogen molecules on various substrates at temperatures between 5 and 30 K and at the lowest practicable gas densities has been measured. Values ofE/k are for condensed CO 340 K, CO₂ 800 K, H₂O 850 K and for ‘dirty’ graphite 980 K and ‘dirty’ copper 800 K. From these measurements temperature ranges in which H atoms might combine on the surface to form H₂ molecules are estimated. Duley has discussed the formation and composition of condensed gas mantles on interstellar grains. The effects of such mantles in promoting and poisoning hydrogen molecule formation are discussed.     

The loss and depth of CO2 ice in comet nuclei

An analytical model has been developed to simulate the chemical differentiation of a homogeneous, initially unmantled cometary nucleus composed of water ice, putative unclathrated CO₂ ice, and silicate dust in specified proportions. Selective sublimation of any free CO₂ ice present in a new comet should produce a surface layer of water ice and dust overlying the undifferentiated core. This surface layer modifies the temperature of buried CO₂ ice and restricts the outflow of gaseous CO₂. On each orbit, water sublimation closer to perihelion temporarily reduces the thickness of the water ice and dust layer and liberates dust. Most of the dust is blown off the nucleus, but a small amount of residual dust remains on the surface (cf. H. L. F. Houpis, W. H. Ip, and D. A. Mendis, 1986, Astrophys. J., in press). Our model includes the effects of nucleus rotation, arbitrary orientation of the rotation axis, latitude, heat conduction into the interior of the nucleus, restriction of CO₂ gas outflow by the water ice and dust layer, and the use of thermal conductivities for both amorphous and crystalline water ice as appropriate, featuresthat were not included in the Houpis et al. model. The model also accounts for the erosion of the water ice surface, which Houpis et al. appear to have accounted for and which is an important effect. Specifically, we investigate the effects of varying the permeability of the surface water ice layer, the mass fraction of CO₂, the orbit and the latitude, using the orbital parameters of Comets Halley and Tempel 2. It is found that CO₂ gas production should exceed H₂O gas production beyond ∼3 AU, and at 1 AU CO₂ gas production should be between 20 to 25% of H₂O gas production. The depth of CO₂ ice and the variation in the depth of CO₂ ice throughout an orbit are affected significantly by the perihelion of the orbit. The effects due to water ice permeability are significant but much less than expected on the basis of flow area. Latitude and CO₂ concentration produce relatively small effects. Under all conditions considered here, CO₂ ice should always be found within ∼1 m from the surface of comet nuclei if it is present as a free species to begin with. This result is probably generally valid for unmantled portions of most comets and qualitatively simulates the behavior of an abundant, highly volatile component in an H₂O/silicate matrix. Comparison of these and similar results with observations could yield information regarding the permeability and chemical composition of cometary material and suggest sampling strategies to minimize fractionation effects. The method is applicable to other nonwater ices.     

The molecular composition of Comet C/2007 W1 (Boattini): Evidence of a peculiar outgassing and a rich chemistry

We measured the chemical composition of Comet C/2007 W1 (Boattini) using the long-slit echelle grating spectrograph at Keck-2 (NIRSPEC) on 2008 July 9 and 10. We sampled 11 volatile species (H₂O, OH^∗, C₂H₆, CH₃OH, H₂CO, CH₄, HCN, C₂H₂, NH₃, NH₂, and CO), and retrieved three important cosmogonic indicators: the ortho-para ratios of H₂O and CH₄, and an upper-limit for the D/H ratio in water. The abundance ratios of almost all trace volatiles (relative to water) are among the highest ever observed in a comet. The comet also revealed a complex outgassing pattern, with some volatiles (the polar species H₂O and CH₃OH) presenting very asymmetric spatial profiles (extended in the anti-sunward hemisphere), while others (e.g., C₂H₆ and HCN) showed particularly symmetric profiles. We present emission profiles measured along the Sun-comet line for all observed volatiles, and discuss different production scenarios needed to explain them. We interpret the emission profiles in terms of release from two distinct moieties of ice, the first being clumps of mixed ice and dust released from the nucleus into the sunward hemisphere. The second moiety considered is very small grains of nearly pure polar ice (water and methanol, without dark material or apolar volatiles). Such grains would sublimate only very slowly, and could be swept into the anti-sunward hemisphere by radiation pressure and solar-actuated non-gravitational jet forces, thus providing an extended source in the anti-sunward hemisphere.