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1.
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.  相似文献   

2.
The crystalline form of magnetic spherules, retrieved from the Pleistocene and Holocene mineral and organic surface sediments at the Northern foreland of Morasko meteorite-impact site, have been determined. While the magnetite (Fe3O4) composition of the spherules prevails, also hematite, maghemite and plagioclase crystalline inclusions have been observed. The magnetite form varies from amorphous, through fine powder, rough powder, holocrystalline to single crystals, as well as mixed forms, such as powder and holocrystalline, or amorphous and holocrystalline. The relieves on the surface of the spherules reflect the size and shape of the crystallites. The morphology of the spherules, their chemical composition and structure have been characterized and discussed in relation to the Morasko-Meteorite fall, and possible other cosmic, geological or anthropogenic origins.  相似文献   

3.
Abstract— This study examines the effects of shock metamorphism on fluid inclusions in crystalline basement target rocks from the Ries crater, Germany. The occurrence of two‐phase fluid inclusions decreases from shock stage 0 to shock stage 1, while single‐phase inclusions increase, likely as a result of re‐equilibration. In shock stages 2 and 3, both two‐phase and single‐phase inclusions decrease with increasing shock stage, indicating that fluid inclusion vesicles are destroyed due to plastic deformation and phase changes in the host minerals. However, quartz clasts entrained in shock stage 4 melts contain both single‐phase and two‐phase inclusions, demonstrating the rapid quenching of the melt and the heterogeneous nature of impact deformation. Inclusions in naturally shocked polycrystalline samples survive at higher shock pressures than those in single crystal shock experiments. However, fluid inclusions in both experimental and natural samples follow a similar trend in re‐equilibration at low to moderate shock pressures leading to destruction of inclusion vesicles in higher shock stages. This suggests that shock processing may lead to the destruction of fluid inclusions in many planetary materials and likely contributed to shock devolatilization of early planetesimals.  相似文献   

4.
A method for separating the spectral signatures of the Martian surface and atmosphere was developed and is applied to multiple emission angle data returned from the MGS TES instrument. This method includes correlated-k and index gas removal algorithms that may be applied to all nadir-pointing TES data. Initial results have provided new and refined measurements of the spectral shapes of atmospheric dust and the Martian surface. Surface temperatures and atmospheric dust opacities are also retrieved with improved accuracy over single observation temperature and opacity retrievals. Low-albedo surfaces display absorptions consistent in both shape and depth with previous studies. These surfaces may be closely modeled using a combination of previously derived basalt, andesitic, and high-albedo surface spectral shapes. Short wavelengths display no significant absorptions, indicating both the coarse particulate nature of the surface and the lack of significant amounts of carbonate. Moderate- and high-albedo surfaces have spectral shapes distinctive of fine particulate silicate materials. No single material can match the entire high-albedo surface spectrum, though there are clear indications of a material that closely matches intermediate to calcic plagioclase and an emission peak at ∼1620 cm−1 due to bound water. The lack of residual restrahlen silicate absorptions indicates that minerals such as olivine or pyroxene are not present in high-albedo surfaces at significant (but unknown) abundances. High-albedo surface results presented here are in agreement with and complementary to shorter wavelength observations. The Martian dust is composed of both primary and secondary minerals. Either chemical weathering has not completely altered its source material or the soil is a mixture of altered and unaltered sources. Further laboratory studies are needed to better establish detection limits and behavior of mineral mixtures of fine particulates in the thermal infrared portion of the spectrum.  相似文献   

5.
Abstract— We examined decomposition products of lepidocrocite, which were produced by heating the phase in air at temperatures up to 525 °C for 3 and 300 h, by x-ray diffraction (XRD), transmission electron microscopy (TEM), magnetic methods, and reflectance spectroscopy (visible and near-infrared (IR)). Single-crystal lepidocrocite particles dehydroxylated to polycrystalline particles of disordered maghemite that subsequently transformed to polycrystalline particles of hematite. Essentially pure maghemite was obtained at 265 and 223 °C for the 3 and 300 h heating experiments, respectively. Its saturation magnetization (Js) and mass specific susceptibility are ～50 Am2/kg and ～400 × 10?6 m3/kg, respectively. Because hematite is spectrally dominant, spectrally hematitic samples (i.e., a minimum near 860 nm and a maximum near 750 nm) also could be strongly magnetic (Js up to ～30 Am2/kg) from the masked maghemite component. Analyses by TEM showed that individual particles are polycrystalline with respect to both maghemite and hematite. The spectrally hematitic and magnetic Mh + Hm particles can satisfy the spectral and magnetic constraints for Martian surface materials over a wide range of values of Mh/(Mh + Hm) either as pure oxide powders or (within limits) as components of multiphase particles. These experiments are consistent with lepidocrocite as the precursor of Mh + Hm assemblages on Mars, but other phases (e.g., magnetite) that decompose to Mh and Hm are also possible precursors. Simulations done with a copy of the Mars Pathfinder magnet array showed that spectrally hematitic Mh + Hm powders having Js equal to 20.6 Am2/kg adhered to all five magnets.  相似文献   

6.
We discuss the composition and size distribution of the dust in the coma of Comet Hale-Bopp. We do this using a model fit for the infrared emission measured by the Infrared Space Observatory (ISO) and the measured degree of linear polarization of scattered light at various phase angles and wavelengths. The effects of particle shape on the modeled optical properties of the dust grains are taken into account. Both the short wavelength (7-44 μm) and the long wavelength (44-120 μm) infrared spectrum are fitted using the same dust parameters, as well as the degree of linear polarization at twelve different wavelengths in the optical to near-infrared domains. We constrain our fit by forcing the abundances of the major rock forming chemical elements to be equal to those observed in meteorites. The infrared spectrum at long wavelengths reveals that large grains are needed in order to fit the spectral slope. The size and shape distribution we employ allows us to estimate the sizes of the crystalline silicates. The ratios of the strength of various forsterite features show that the crystalline silicate grains in Hale-Bopp must be submicrometer-sized. On the basis of our analysis the presence of large crystalline silicate grains in the coma can be excluded. Because of this lack of large crystalline grains combined with the fact that we do need large amorphous grains to fit the emission spectrum at long wavelengths, we need only approximately 4% of crystalline silicates by mass (forsterite and enstatite) to reproduce the observed spectral features. After correcting for possible hidden crystalline material included in large amorphous grains, our best estimate of the total mass fraction of crystalline material is ∼7.5%, which is significantly lower than deduced in previous studies in which the typical derived crystallinity is ∼20-30%. The implications of this low abundance of crystalline material on the possible origin and evolution of the comet are discussed. We conclude that the crystallinity we observe in Hale-Bopp is consistent with the production of crystalline silicates in the inner Solar System by thermal annealing and subsequent radial mixing to the comet forming region (∼30 AU).  相似文献   

7.
The Thermal Emission Spectrometer (TES) on the Mars Global Surveyor spacecraft has detected deposits of coarse-grained, gray crystalline hematite in Sinus Meridiani, Aram Chaos, and Vallis Marineris. We argue that the key to the origin of gray hematite is that it requires crystallization at temperatures in excess of about 100 °C. We discuss thermal crystallization (1) as diagenesis at a depth of a few kilometers of sediments originally formed in low-temperature waters, or (2) as precipitation from hydrothermal solution. In Aram Chaos, a combination of TES data, Mars Orbiter Camera images, and Mars Orbiter Laser Altimeter (MOLA) topography suggests that high concentrations of hematite were formed in planar strata and have since been exposed by erosion of an overlying light-toned, caprock. Lesser concentrations of hematite are found adjacent to these strata at lower elevations, which we interpret as perhaps due to accumulation from physical weathering. The topography and the collapsed nature of the chaotic terrain favor a hydrothermally charged aquifer as the original setting where the hematite formed. Concentration of iron into such an ore-like body would be chemically favored by saline, Cl-rich hydrothermal fluids. An alternative sedimentary origin requires post-depositional burial to a depth of ∼3-5 km to induce thermally driven recrystallization of fine-grained iron oxides to coarse-grained hematite. This depth of burial and re-exposure is difficult to reconcile with commonly inferred martian geological processes. However, shallow burial accompanied by post-burial hydrothermal activity remains plausible. When the hematite regions originally formed, redox balance requires that much hydrogen must have been evolved to complement the extensive oxidation. Finally, we suggest that the coexistence of several factors required to form the gray hematite deposits would have produced a favorable environment for primitive life on early Mars, if it ever existed. These factors include liquid water, abundant electron donors in the form of H2, and abundant electron acceptors in the form of Fe3+.  相似文献   

8.
GOLD (Grupo de Óptica de Láminas Delgadas) is devoted to the development of novel coatings with challenging performance in the far and the extreme ultraviolet (FUV-EUV, 50–200 nm). One of the main goals of this research is to provide the communities of astronomy, solar physics and atmospheric physics with coatings with high reflectance or transmittance at a target wavelength or band, and high rejection of the out-of-band at this complicated spectral range. Above the transparency cutoff of MgF2 (115 nm), transmittance filters based on Al/MgF2 multilayers have been developed peaked at wavelengths as short as 124 nm, with a peak transmittance of 27% and a FWHM of 12 nm for a non-aged coating. Below 115 nm, a research on reflectance filters has recently started with very promising results on filters peaked at the 83.4 nm OII spectral line. Fresh filters with 27% peak reflectance at normal incidence and a FWHM of 14 nm have been obtained. Furthermore, the peak reflectance wavelength of these filters can be tuned by rotation. A filter peaked at 83 nm at normal incidence will shift to ～73 nm at 30 deg from the normal and to ～58 nm at 45 deg. These novel reflective filters based on Al, Yb and SiO must still demonstrate stability over time.  相似文献   

9.
A random, hydrogen-free, assembly of microscopic sp2 carbon chips, forming a macroscopically homogeneous and isotropic solid, is proposed as a model carrier for the ultraviolet (UV) interstellar extinction band. The validity of this model is based on the calculation of the Bruggeman average dielectric function of a mixture of the known parallel and perpendicular dielectric functions of graphite. The π absorption feature of Rayleigh-sized spheres of this mixture falls near  4.6 μm−1  (2175 Å), but its width is  1.5 μm−1  , somewhat larger than the astronomically observed average,  1 μm−1  . This is confirmed by measurements of the reflectance of an industrial material, polycrystalline graphite. A better fit to the interstellar feature position and width is obtained with a hypothetical material, having the same dielectric functions as natural graphite, except for less extended wings of the π resonance. Physically, this could result from changes in the electronic band structure due to previous thermal histories. In this model, the Frölich feature central wavelength depends only on the π resonance frequency, while its width depends only on the damping constant of the same resonance. This explains the range of observed feature widths at constant feature wavelength.  相似文献   

10.
We present TUNIS, a double‐pass spectro‐imager built in THEMIS as a proof‐of‐concept for EST. Basic concepts and selected first results are shown. We introduce the concept of a Hadamard spectral mask as a proposition to move forward from the present implementation of a single‐wavelength per image pixel to a more general one of multiplexed spectral information that improves the temporal coherence of the spectral measurement (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

11.
PSR B1259−63 is a 48-ms radio pulsar in a highly eccentric 3.4-yr orbit with a Be star SS 2883. Unpulsed γ-ray, X-ray and radio emission components are observed from the binary system. It is likely that the collision of the pulsar wind with the anisotropic wind of the Be star plays a crucial role in the generation of the observed non-thermal emission. The 2007 periastron passage was observed in unprecedented details with Suzaku , Swift , XMM–Newton and Chandra missions. We present here the results of this campaign and compare them with previous observations. With these data we are able, for the first time, to study the details of the spectral evolution of the source over a 2-month period of the passage of the pulsar close to the Be star. New data confirm the pre-periastron spectral hardening, with the photon index reaching a value smaller than 1.5, observed during a local flux minimum. If the observed X-ray emission is due to the inverse Compton (IC) losses of the 10-MeV electrons, then such a hard spectrum can be a result of Coulomb losses, or can be related to the existence of the low-energy cut-off in the electron spectrum. Alternatively, if the X-ray emission is a synchrotron emission of very high-energy electrons, the observed hard spectrum can be explained if the high-energy electrons are cooled by IC emission in Klein–Nishina regime. Unfortunately, the lack of simultaneous data in the TeV energy band prevents us from making a definite conclusion on the nature of the observed spectral hardening and, therefore, on the origin of the X-ray emission.  相似文献   

12.
Spectra of asteroid 4 Vesta obtained in October 1990 with the International Ultraviolet Explorer are reanalyzed and reinterpreted. A large portion of the eastern hemisphere (based on the prime meridian definition of Thomas et al., 1997a) is darker at UV wavelengths than much of the western hemisphere. The UV lightcurve is in contrast with the visible lightcurve, which shows that the eastern hemisphere is brighter than the western. These IUE spectra of Vesta thus may be evidence for the “spectral reversal,” first seen on the Moon by Apollo 17, where the visibly brighter lunar highlands are darker than the maria at far-UV wavelengths. This effect was linked to space weathering when it was noted (Wagner et al., 1987) that the spectral reversal appears in the laboratory spectra of lunar soils but not powdered lunar rocks.We investigate Vesta’s UV lightcurve and spectral reversal, and its possible connection with space weathering. The addition to grain coatings of small amounts of submicroscopic iron (SMFe) through vapor deposition causes drastic spectral changes at UV-visible wavelengths (Hapke, 2001), while the longer wavelength spectrum remains largely unaffected. Other laboratory results (e.g., Hiroi and Pieters, 1998) indicate that the UV-visible wavelength range is affected by simulated weathering processes in a manner similar to what is seen on Vesta. It is likely that Vesta has experienced relatively minor amounts of space weathering, as indicated by the spectral reversal, along with the subtle visible-near infrared weathering effects (e.g., Binzel et al., 1997).  相似文献   

13.
Abstract— Synthetic MgSiO3 glasses were irradiated at room temperature by 300 keV electrons in a transmission electron microscope (TEM). One of the samples had been previously irradiated by 50 keV He+ ions. Electron irradiation induces the nucleation and growth of randomly oriented nanometer‐sized crystallites. The crystallites first consist of MgO and subsequently of forsterite (Mg2SiO4). Both are seen to form within an amorphous SiO2 matrix. The rate of crystallization of the samples has been monitored by conventional TEM imaging and electron diffraction. The sample that had been pre‐irradiated with He+ ions is found to transform faster than the as‐quenched glass. The crystallization of metastable MgSiO3 glasses is explained by ionizing radiation‐induced elemental diffusion that allows the reorganization of matter into a more favourable thermodynamic state. These results show that ionizing radiation interactions could account for crystal formation as observed in infrared spectroscopy in some young stellar environments.  相似文献   

14.
The distinct patterns, relatively low intensities and peak positions of overtone-combination bands of silicates and oxides suggest that the 5–8 μm spectral region can provide clues for the dust composition when near optically thick conditions exist for the 10-μm silicate feature. We present 1000–2500 cm−1 room-temperature laboratory spectra obtained from powders of silicate, aluminate and nitride minerals and silicate glasses. The spectra exhibit overtone absorption bands with mass absorption coefficients ∼100 times weaker than the fundamentals. These data are compared with the 5–8 μm spectra of deeply embedded young stellar objects observed with the Short Wavelength Spectrometer on the Infrared Space Observatory . Fits of the laboratory data to the observations, after subtraction of the 6.0-μm H2O ice feature and the 6.0-μm feature identified with organic refractory material, indicate that crystalline melilite (a silicate) or metamict hibonite (a radiation-damaged crystalline aluminate) may be responsible for much of the 6.9-μm absorption feature in the observations, with melilite providing the best match. A weaker 6.2-μm absorption in the young stellar object spectra is well matched by the spectra of hydrous crystalline amphibole silicates (actinolite and tremolite). Relative abundances of Si–O in room-temperature amphiboles to low-temperature H2O ice are in the range 0.46–3.9 and in melilite are in the range 2.5–8.6. No astronomical feature was matched by the overtones of amorphous silicates because these bands are too broad and peak at the wrong wavelength. Hence, this analysis is consistent with the 10-μm features of these objects being due to a mixture of crystalline and amorphous silicates, rather than only amorphous silicates.  相似文献   

15.
A.C. Marra  M.D. Lane  A. Blanco 《Icarus》2011,211(1):839-848
Hematite is an iron oxide that is very important for the study of climatic evolution of Mars. It can occur in three forms: nanophase (dark purple), fine-grained (red) and coarse-grained (gray).In a previous work, we studied the influence of particle size and shape on the infrared spectra (in the wavelength range 6.25-50 μm) of submicron red hematite particles and found that bulk optical constants did not fit the spectra of very fine particles with several classes of models.In the present paper, we derive bulk optical constants of a sample of the same parent material of hematite already used in a previous work in order to determine the particulate optical constants. As a first result we find that, also in this case, bulk and particulate optical constants are different from each other. Furthermore, we show that these bulk optical constants, although derived starting from the same parent material of hematite and used with a model adopting the laboratory measured grain size distribution of the sample, cannot be used to reproduce the spectra of submicron particles. Our results can help the scientific community to appropriately model the contribution of hematite submicron grains to the martian dust for a better understanding of the geologic evolution of the planet.  相似文献   

16.
Yan Tang  Yujie Huang 《Icarus》2006,180(1):88-92
The detection of gray crystalline hematite deposits on Mars by Thermal Emission Spectrometer (TES) has been used to argue for the presence of liquid water on Mars in the distant past. By methanol-thermal treatment of anhydrous FeCl3 at low temperatures (70-160 °C), crystalline gray hematite with layered structure was synthesized, based on this result an alternative explanation for the origin of martian hematite deposits is suggested. Methane could be abundant in the early martian atmosphere; process such as photochemical oxidation of methane could result in the formation of ocean or pool of organic compounds such as methanol, which provides an environment for the formation of large-scale hematite deposits on Mars.  相似文献   

17.
Abstract– The grains returned by NASA’s Stardust mission from comet 81P/Wild 2 represent a valuable sample set that is significantly advancing our understanding of small solar system bodies. However, the grains were captured via impact at ～6.1 km s?1 and have experienced pressures and temperatures that caused alteration. To ensure correct interpretations of comet 81P/Wild 2 mineralogy, and therefore preaccretional or parent body processes, an understanding of the effects of capture is required. Using a two‐stage light‐gas gun, we recreated Stardust encounter conditions and generated a series of impact analogs for a range of minerals of cometary relevance into flight spare Al foils. Through analyses of both preimpact projectiles and postimpact analogs by transmission electron microscopy, we explore the impact processes occurring during capture and distinguish between those materials inherent to the impactor and those that are the product of capture. We review existing and present additional data on olivine, diopside, pyrrhotite, and pentlandite. We find that surviving crystalline material is observed in most single grain impactor residues. However, none is found in that of a relatively monodisperse aggregate. A variety of impact‐generated components are observed in all samples. Al incorporation into melt‐derived phases allows differentiation between melt and shock‐induced phases. In single grain impactor residues, impact‐generated phases largely retain original (nonvolatile) major element ratios. We conclude that both surviving and impact‐generated phases in residues of single grain impactors provide valuable information regarding the mineralogy of the impacting grain whilst further studies are required to fully understand aggregate impacts and the role of subgrain interactions during impact.  相似文献   

18.
The outcome of collisions between small icy bodies, such as Kuiper belt objects, is poorly understood and yet a critical component of the evolution of the trans-neptunian region. The expected physical properties of outer Solar System materials (high porosity, mixed ice-rock composition, and low material strength) pose significant computational challenges. We present results from catastrophic small body collisions using a new hybrid hydrocode to N-body code computational technique. This method allows detailed modeling of shock propagation and material modification as well as gravitational reaccumulation. Here, we consider a wide range of material strengths to span the possible range of Kuiper belt objects. We find that the shear strength of the target is important in determining the collision outcome for 2 to 50-km radius bodies, which are traditionally thought to be in a pure gravity regime. The catastrophic disruption and dispersal criteria, , can vary by up to a factor of three between strong crystalline and weak aggregate materials. The material within the largest reaccumulated remnants experiences a wide range of shock pressures. The dispersal and reaccumulation process results in the material on the surfaces of the largest remnants having experienced a wider range of shock pressures compared to material in the interior. Hence, depending on the initial structure and composition, the surface materials on large, reaccumulated bodies in the outer Solar System may exhibit complex spectral and albedo variations. Finally, we present revised catastrophic disruption criteria for a range of impact velocities and material strengths for outer Solar System bodies.  相似文献   

19.
One of the intrinsic properties of particulate sulfur allotropes is a change in UV-visible reflectivity with temperature change of the material. The surface of Io experiences temperature changes during eclipse which are sufficient to cause a detectable change in the spectral reflectivity of sulfur; thus, if the surface of Io is composed primarily of sulfur allotropes, a change in reflectivity at certain wavelengths should be observable shortly after eclipse reappearance. We observed four eclipse reappearances during July and August of 1983 and saw no posteclipse brightening effects in filter bands selected for sensitivity to color changes in sulfur. Our model of the brightness change for S8 (“yellow” sulfur) implies that this material covers less than 50% of Io's surface. Negative posteclipse brightening observations were also obtained with a filter chosen for the high contrast between SO2 frost and the average albedo of the surface of Io at that wavelength. We conclude that no significant condensation of optically thick SO2 occurred on the surface of Io during these eclipses.  相似文献   

20.
This paper presents and discusses selected reflectance spectra of 40 Main Belt asteroids. The spectra have been obtained by the author in the Crimean Laboratory of the Sternberg Astronomical Institute (2003–2009). The aim is to search for new spectral features that characterize the composition of the asteroids’ material. The results are compared with earlier findings to reveal substantial irregularities in the distribution of the chemical-mineralogical compositions of the surface material of a number of minor planets (10 Hygiea, 13 Egeria, 14 Irene, 21 Lutetia, 45 Eugenia, 51 Nemausa, 55 Pandora, 64 Angelina, 69 Hesperia, 80 Sappho, 83 Beatrix, 92 Undina, 129 Antigone, 135 Hertha, and 785 Zwetana), which are manifest at different rotation phases. The vast majority of the analyzed high-temperature asteroids demonstrate subtle spectral features of an atypical hydrated and/or carbonaceous chondrite material (in the form of impurities or separate units), which are likely associated with the peculiarities of the formation of these bodies and the subsequent dynamic and impact processes, which lead, inter alia, to the delivery of atypical materials. Studies of 4 Vesta aboard NASA’s Dawn spacecraft have found that asteroids of similar types can form their own phyllosilicate generations provided that their surface material contains buried icy or hydrated fragments of impacting bodies. The first evidence has been obtained of a spectral phase effect (SPE) at small phase angles (≤4°) for 10 Hygiea, 21 Lutetia, and, possibly, 4 Vesta. The SPE manifests itself in an increasing spectral coefficient of brightness in the visible range with decreasing wavelength. This effect is present in the reflectance spectrum of CM2 carbonaceous material at a phase angle of 10° and absent at larger angles (Cloutis et al., 2011a). The shape of Hygeia’s reflectance spectra at low phase angles appears to be controlled by the SPE during the most part of its rotation period, which may indicate a predominantly carbonaceous chondrite composition on a part of the asteroid’s surface. For Vesta, the SPE may manifest itself in the flat or slightly concave shape of the asteorid’s reflectance spectra at some of the rotation phases, which is likely caused by the increased number of dark spots on corresponding parts of its surface.  相似文献   

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