Mantle Growth Research Articles

Adsorption of CO on amorphous water-ice surfaces

We present the results of classical trajectory calculations of the adsorption of thermal CO on the surface of compact amorphous water ice, with a view to understanding the processes governing the growth and destruction of icy mantles on dust grains in the interstellar medium and interpreting solid CO infrared spectra. The calculations are performed at normal incidence, for Ei = 0.01 eV (116 K) and surface temperature Ts = 90 K. The calculations predict high adsorption probabilities (~1), with the adsorbed CO molecules having potential energies ranging from -0.15 to -0.04 eV with an average energy of -0.094 eV. In all the adsorbing trajectories, CO sits on top of the surface. No case of CO diffusion inside the ice or into a surface valley with restricted access was seen. Geometry minimizations suggest that the maximum potential energy of adsorbed CO (-0.155 eV) occurs when CO interacts with a “dangling OH” group, associated with the 2152 cm-1 band seen in laboratory solid-state CO spectra. We show that relatively few “dangling OH” groups are present on the amorphous ice surface, potentially explaining the absence of this feature in astronomical spectra. CO also interacts with “bonded OH” groups, which we associate with the 2139 cm-1 infrared feature of solid CO. Our results for CO adsorption on amorphous ice are compared with those previously obtained for CO adsorption to crystalline ice. The implications of the spectroscopic assignments are discussed in terms of the solid-CO infrared spectra observed in interstellar regions. Using the Frenkel model, the lifetime τ for which CO may remain adsorbed at the surface is calculated. At temperatures relevant to the interstellar medium, i.e. 10 K, it is longer than the age of the universe, but decreases dramatically with increasing Ts, such that at Ts = 90 K, τ = 300 ns. The pre-exponential factor used in the Frenkel model is found to be 0.95 ± 0.02 ps. These data are compared to recent experimental results. The astrophysical implications of these calculations are discussed, with particular reference to the CO binding sites identified on amorphous ice surfaces, their adsorption energies, probabilities and lifetimes.

Reproduction in the deepwater squid Moroteuthis ingens, what does it cost?

The cost of reproduction for the terminal spawning onychoteuthid squid, Moroteuthis ingens, was analysed using measures of condition and tissue biochemistry. Both males and females showed a dramatic drop in the weight of the gonad in stage 6 (spent) individuals. The mantle weight and nidamental gland weight of females also decreased during the maturation process. Males, however, had a marked increase in both the penis and spermatophoric complex weight in spent individuals, while female oviducal gland weight and nidamental gland length also increased in stage 6 individuals. Residual analysis indicated that testis growth was not developing at the expense of mantle growth, although there was a suggestion of cost to the fins. Females showed that the development of the ovary occurred at a cost to both the mantle and fins. Overall body condition also declined with maturity stage for both males and females, with stage 6 individuals of both sexes in poor condition. Very few females had eggs in the oviducts, suggesting that the oviducts are used as ducts instead of storage organs. Proximal analysis revealed a loss of constituents within the mantle during maturation, with an associated increase in water, indicating the remobilization of energy from the mantle to fuel reproduction. This study suggests that the digestive gland is not used as an energy store in this species.

Insulin-like system and growth regulation in the Pacific oyster Crassostrea gigas: hrIGF-1 effect on protein synthesis of mantle edge cells and expression of an homologous insulin receptor-related receptor

The involvement of molecules belonging to the insulin/IGF family in regulation of growth has been investigated in the Pacific oyster Crassostrea gigas. In vitro biological effects of human recombinant IGF-1 (hrIGF-1) on mantle edge cells, involved in oyster shell and soft body growth, were studied over an annual cycle. In mantle edge cells hrIGF-1 stimulates protein synthesis of 56 ± 5.1% over basal for 10 −10 M in September with in addition a clear dose-effect corresponding to the highest shell growth period, and 57.5 ± 3.45% over basal for 10 −11 M in March and 51 ± 5.4% over basal for 10 −10 M in April corresponding to the period of mantle growth. These insulin-like effects were associated with the expression of a recently identified C. gigas insulin receptor-related receptor (CIR) in mantle edge cells as demonstrated by RT-PCR. Moreover, in situ hybridisation (ISH) confirmed this expression at the level of the inner and outer epithelia involved in mantle growth and shell formation.

Perspectives of ammonite paleobiology from shell abnormalities in the genus Baculites

Many Baculites specimens from the Upper Cretaceous of the United States Western Interior show exceptional preservation of the original aragonitic shell and its fine-scale surface ornamentation. Growth lines are ubiquitous, with two orders of these structures represented on some shells, and reflect the incremental addition of new shell at the apertural margin. Growth line interruption in the form of repair of minor shell damage at the aperture, commonplace in contemporary Nautilus, is essentially absent in Baculites, suggesting that its members fed on small prey in the water column. As typical of Mesozoic ammonites in general, and in striking contrast to contemporary Nautilus, no in vivo epizoans have been recognized on specimens of Baculites. It is inferred that the shell of Baculites was covered in periostracum to eliminate epizoic colonization. By analogy with Nautilus, a distinctive micro-ornament oriented at right angles to growth lines and visible on parts of some specimens was probably associated with periostracal attachment. A small proportion of Baculites specimens show abnormalities in shell growth categorized as v-shaped indentations of growth lines, shell grooves, fine-scale folds on the surface of growth lines and feather structures. We view this entire set of structures as due to abnormalities in mantle growth induced at the leading edge, impressed into the periostracum during its fabrication, and then in turn into the shell surface. As many of the Baculites with shell abnormalities are smooth, the proposal by Checa linking homologous structures recognized on other ammonites to the formation of comarginal ribs is rejected. A case of sutural inversion, in which the form of minor divisions of the major saddles and lobes are transposed, is recognized in a specimen of Baculites codyensis Reeside. We consider sutural pattern in ammonites, an expression of septal fluting, as replicating the genetically specified standing form of an elastic adapical visceral mass. The inverted sutural pattern, and by implication the style of septal fluting, was transcribed exactly in the three successive septa preserved on the specimen. The abnormality appears to be a case of homeotic mutation in which the plan for one body region becomes translocated to another. The conservatism of major elements of sutural (=septal) patterns for Mesozoic ammonites in their evolutionary spectrum suggests that a homeobox of conserved DNA sequence, with the transcription factors encoded in homeotic genes, is likely to have been involved.

Stochastic Theory of Molecule Formation on Dust

The formation of interstellar molecules on the surfaces of dust grains is calculated in the framework of stochastic reaction kinetics. The master equation and the state transition probabilities are defined, and the growth of grain mantles following accretion and reaction of gas-phase species is computed. The results are compared to the observed composition and structure of interstellar ices. The differences between this approach to gas-grain kinetics and previous work is discussed, and possible extensions of the theory are outlined.

An elementary model for the dust cycle in galaxies

We give an elementary model for the evolution of dust in galaxies, based on abundance arguments. The model takes account of grain core production in both supernovae and giant stars, and includes mantle growth in the interstellar medium. Destruction of grain cores does not appear to be a dominant effect. We show that a self-consistent picture can be made in which the interstellar dust mass is an approximately constant fraction of the heavy element mass in the interstellar medium. This result is demonstrated to be essentially independent of outflow or inflow of interstellar material

Interstellar Extinction and Polarization in the Taurus Dark Clouds: The Optical Properties of Dust near the Diffuse/Dense Cloud Interface

Observations of interstellar linear polarization in the spectral range 0.35-2.2??m are presented for several stars reddened by dust in the Taurus region. Combined with a previously published study by Whittet et al., these results represent the most comprehensive data set available on the spectral dependence of interstellar polarization in this nearby dark cloud (a total of 27 sight lines). Extinction data for these and other reddened stars in Taurus are assembled for the same spectral range, combining published photometry and spectral classifications with photometry from the Two Micron All Sky Survey. The polarization and extinction curves are characterized in terms of the parameters ?max (the wavelength of maximum polarization) and RV (the ratio of total to selective extinction), respectively. The data are used to investigate in detail the question of whether the optical properties of the dust change systematically as a function of environment, considering stars observed through progressively more opaque (and thus progressively denser) regions of the cloud. At low visual extinctions (0 3, real changes in grain properties occur, characterized by observed RV values in the range 3.5-4.0. A simple model for the development of RV with AV suggests that RV may approach values of 4.5 or more in the densest regions of the cloud. The transition between normal extinction and dense cloud extinction occurs at AV ~ 3.2, a value coincident with the threshold extinction above which H2O-ice is detected on grains within the cloud. Changes in RV are thus either a direct consequence of mantle growth or occur under closely similar physical conditions. Dust in Taurus appears to be in a different evolutionary state compared with other nearby dark clouds, such as ? Oph, in which coagulation is the dominant physical process.

Stacking Increments: A New Model and Morphospace for the Analysis of Bivalve Shell Growth

A model employing stacking increments is introduced for the analysis of bivalve shell growth and form. The model is based on the components of shell growth that are potentially independent: the rate of mantle cell proliferation, the rate of precipitation of shell material, and the rate of translation of the pallial line, where the mantle is attached to the shell. This model is defined in terms of the following parameters: (1) the ratio of accretion of shell material at the shell margin to growth of the mantle by cell division, (2) the ratio of shell accretion at the pallial line to mantle growth, and (3) the ratio of the amount of pallial muscle translation, away from the umbo toward the shell margin, to mantle growth. In this model, the shape of a radial section through the shell is simulated by stacking of internal microgrowth increments. The mode of stacking of the increments is determined by the balance among the parameters defining growth. A theoretical morphospace defined on the basis of this model is largely consistent with the range of forms of naturally occurring bivalve shells. Analysis of the distribution of actual shell forms in relation to this morphospace suggests that the absolute rate of shell precipitation and the gradient in precipitation rate away from the shell margin along a radial cross-section are physiologically as well as geometrically constrained.

Garnet zoning and the identification of equilibrium mineral compositions in high‐pressure–temperature granulites from the Moldanubian Zone, Austria

A detailed investigation of the compositional variation in garnet has been undertaken in a garnet–pyroxene‐bearing granulite from the high‐grade Gföhl Unit, Moldanubian Zone, Lower Austria. Textural observations, together with the interpretation of the preserved garnet chemistry, enables the recognition of both prograde core and peak metamorphic garnet mantle growth stages, an extremely rare feature in high‐P–T granulite facies rocks. Initial thermobarometric calculations undertaken across whole garnet zoning profiles show how correct interpretation of a zoning profile is essential if the maximum peak metamorphic P–T conditions are to be recovered. The effect of retrograde decompression‐ and cooling‐driven reactions on inclusion and host garnet compositions has also been assessed. The results indicate that caution should be exercised when utilizing inclusion and adjacent garnet compositions for the thermobarometric evaluation of peak metamorphic equilibration conditions. Peak P–T conditions were determined by the TWEEQU thermobarometric method, utilizing the core compositions of matrix phases combined with the interpreted high‐P–T garnet mantle composition, to give 15.6 kbar and 1090 °C, consistent with previously determined results for Moldanubian granulites. Similar high‐P–T estimates are also provided by a re‐evaluation of previously published results for a granulite sample from the same lithological unit, using a modified interpretation of garnet and plagioclase compositional data. The new estimates presented confirm the previously disputed idea that the Gföhl Unit underwent a high‐pressure granulite facies stage and is therefore distinctly different from the underlying tectonostratigraphic units. It is emphasized that any interpretation of the peak metamorphic conditions in high‐grade rocks must be based on detailed petrographic observations combined with a thorough understanding of the co‐existing equilibrium mineral compositions.

Infrared Observations of Interstellar Ices

In the recent years revolutionary results concerning the nature of icy dust particles have been obtained with the help of the Infrared Space Observatory (ISO) and ground based observations. To date interstellar ice features of H2O, CO, CO2, CH3OH, CH4, H2CO, OCS and HCOOH as well as other minor species are observed. Interstellar grains act as important catalysts in the interstellar medium. Processes such as UV irradiation, cosmic ray processing and temperature variations determine the grain mantle growth and chemical evolution. ISO has revealed that ice segregation is an important and ubiquitous process in the vicinity of massive protostars and reflects the extensive thermal processing of grains in such environments.In this paper a recent view on the inventory of interstellar ices is presented. Constraints on the reservoirs of oxygen in dense clouds are discussed, taking into account recent measurements of oxygen-bearing species. Large abundances of CO2 and CH3OH in dense molecular clouds provide challenging perspectives to investigate the differences of ice chemistry in the vicinity of high and low-mass protostars. Accurate abundances of ice species and knowledge on the ice distribution in the protostellar regions are an important tool to define the environmental conditions in molecular clouds. A global understanding of interstellar ice chemistry also allows monitoring the incorporation and evolution of volatiles in planetesimals and comets and to reveal processes predominant in the early Solar System.

Thermal evolution of comet nuclei

The properties of cometary surfaces which influence the thermal behavior of the nucleus such as the albedo, the emissivity and the surface temperature are critically reviewed. It is shown that these properties are less firmly established than generally believed. A brief presentation is given of the state of the art in comet modelling. It is concluded that models which are supposed to give an adequate desription of cometary activity must take into account short term variations in local temperature due to the nucleus rotation and due to the orientation of the rotation axis. The activity can also be strongly influenced by the nucleus topography. If comets are porous it is further necessary to include all type of transport mechanisms of heat conduction, advection, local deposition and extraction of latent heat and all mechanisms of redistribution of volatile matter (advection, diffusion, sublimation, recondensation). For porous nuclei, no direct relationship between the coma composition and the abundance ratio in the nucleus can be established. An adequate description of the formation and growth of dust mantles can only be obtained from multdimensional models running over several revolutions with time steps much smaller than the rotation period.

Hyperplastic growth of mucous cells in the mantle of the mussel Modiolus kurilensis from a heavily polluted area of Amursky Bay, Sea of Japan.

This histology of an external mantle growth in 1 of 500 Modiolus kurilensis from a heavily polluted area of Amursky Bay, Sea of Japan, was described. The growth consisted of subepithelial basophilic mucous cells containing glucosaminoglycans, eosinophilic large-granular cells with proteins and neutral polysaccharides, and mixed (acid + neutral mucopolysaccharides) cell types. Some subepithelial eosinophilic and basophilic gland cells were dividing and seemed to be the source of tumor growth. The mitotic index of growth cells reached 0.5% on some growth sections; however, many mitoses were pycnotic. The emergence of the tumor on the mussel mantle is probably related to a compensatory or regenerative hyperplasia of subepithelial mucous cells.

Water Ice in the Disk Around the Protostar AFGL 2136 IRS 1

The protostar AFGL 2136 IRS 1 illuminates the Juggler nebula and drives a massive molecular outflow. The line of sight toward AFGL 2136 also displays a rich spectrum of solid state absorption features. To investigate the distribution of ice-coated grains in the environment of IRS 1, we obtained narrowband images at six near-infrared wavelengths surrounding and including the 3.08 μm water ice feature. These images (specifically, a color map constructed from 2.14 and 3.60 μm images) provide vivid evidence for the presence of a disk around IRS 1 and appear to reveal, for the first time, the detailed distribution of water ice in the local environment of a protostar. Models of the 2-4 μm spectral energy distributions (SEDs) toward IRS 1 and the Juggler nebula suggest that the abundance of icy grains is larger along lines of sight through the disk than along lines of sight away from the equatorial plane of IRS 1. Thus, while we find evidence that icy grains exist throughout the cloud core containing AFGL 2136, it appears that the physical conditions in the circumstellar disk around IRS 1 are more conducive to ice mantle survival and/or growth than are physical conditions in its rarefied polar regions.

Flow mixing, object-matrix coherence, mantle growth and the development of porphyroclast tails

The kinematic ‘stability fields’ of common porphyroclast types have been defined using a computer program that simulates low Reynolds number Newtonian shearing flow in the vicinity of a rigid spherical object. The principal variables governing the geometry of porphyroclast tails at a given finite shear strain are: (1) porphyroclast-matrix coherence, and (2) mantle recrystallization history, both of which affect the extent to which mantle material becomes mixed with the matrix as deformation progresses.

Grain destruction in shocks in the interstellar medium

view Abstract Citations (431) References (35) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Grain Destruction in Shocks in the Interstellar Medium Jones, A. P. ; Tielens, A. G. G. M. ; Hollenbach, D. J. ; McKee, C. F. Abstract Destruction of interstellar dust occurs predominantly in supernova shock waves in the warm neutral/ionized medium (density approximately = 0.25/cu cm, temperature approximately = 104 K). Recent theoretical developments and laboratory data for sputtering processes and grain-grain collisional vaporization allows us to better evaluate the grain destruction rate in interstellar shocks in the warm medium. We find that, independent of composition, grain destruction in supernova blast waves is dominated by nonthermal sputtering for shock velocities greater than 50 km/s and less than or equal to 150 km/s and thermal sputtering at higher shock velocities. We use a detailed scheme for the vaporization of grains colliding at high velocities (vs greater than or equal to 20 km/s) and show that the grain-grain collision destruction process is only dominant for shock velocities of less than or equal to 50-80 km/s and is less important than previously assumed. Nevertheless, the grain-grain destruction rates are of order 30%-90% of the sputtering rates at vs greater than 100 km/s and less than 200 km/s and are important in vaporizing the cores of grains. Detailed results for grain destruction as a function of grain size and composition are presented. We also present results for silicon carbide, iron, ice, and porous test particles. For carbonaceous grains we find that the fractional destruction is less than or equal to 0.29, and for silicate it is less than or equal to 0.45, for vs less than or equal to 200 km/s. We have calculated grain lifetimes, using the three-phase model of the interstellar medium, and find lifetimes of 4 x 108 yr for carbonaceous grains and 2.2 x 108 yr for silicate grains. Given that the typical stardust injection timescale of 2.5 x 109 yr, we conclude that efficient mechanisms for grain growth in the interstellar medium must exist in order that a significant fraction of the refractory elements be incorporated in dust, as observed. Therefore, although our improved model has less vaporization of dust due to grain-grain collisions, sputtering still destroys dust efficiently and grain mantle growth in the interstellar medium is required, a conclusion reached in previous models of grain destruction in the interstellar medium. Carbonaceous mantles on silicate grains can protect the silicate cores from sputtering destruction in interstellar shock waves, provided that the protective mantles can efficiently reform in the interstellar medium. Also, if the grains are porous the postshock grain velocities are lower than for solid particles, and grain destruction is reduced. Porosity and mantling may increase the grain lifetime by factors of approximately 3 and approximately 3-4, respectively. The fraction of interstellar silicon in silicate stardust is therefore less than or equal to 0.25, but more silicon might be depleted in the form of a grain mantle. Publication: The Astrophysical Journal Pub Date: October 1994 DOI: 10.1086/174689 Bibcode: 1994ApJ...433..797J Keywords: Astronomical Models; Cosmic Dust; Particle Collisions; Shock Waves; Sputtering; Supernovae; Vaporizing; Carbonaceous Materials; Ice; Iron; Numerical Analysis; Porous Materials; Silicates; Silicon Carbides; Astrophysics; ISM: DUST; EXTINCTION; SHOCK WAVES; STARS: SUPERNOVAE: GENERAL full text sources ADS |

The ice threshold in molecular clouds: a diagnostic of the infrared radiation field

The threshold extinction at which ice is deposited on dust in molecular clouds is observed to show wide variation from cloud to cloud. The desorption of H2O molecules following absorption at 3 μm is proposed to account for the ice deposition threshold extinction. However, this mechanism is effective only up to H2O monolayer coverage and does not restrict the subsequent ice mantle growth, once started. It is shown that the threshold extinction is a measure of the local infrared radiation field near 3 μm in each cloud.

Textural evolution of the rapakivi granites, south Greenland ?Sr, O and H isotopic investigations

The development of rapakivi texture in feldspars from the Ketilidian granitoids of south Greenland has been investigated using Sr, O and H isotopes. A low temperature signature is found in the Sr and O data which seemingly contradicts some textural features that point to a magmatic origin of the plagioclase mantles around the K-feldspar ovoids. An origin for these mantles involving exsolution from an original alkali feldspar solid solution is proposed, which involves growth of mantles over a range of conditions determined by the mobility of the exsolving sodic feldspar. This mobility may be enhanced at high temperatures in the presence of melts or increased fluid pressures and at lower temperatures by the processes responsible for the transformation of K-feldspar to microcline. Rapakivi granites with both white and dark green feldspar occur in south Greenland but show no major isotopic differences, although the dark alkali feldspars contain significantly more fluid. Equivalent fluids in the white alkali feldspars may have escaped during plagioclase exsolution.

The formation and annealing of circumstellar dust, as gauged by IRAS low-resolution spectra and the microwave maser chronology

By comparing IRAS mean colors and LRS spectral differences in a large sample of Mira variables with the chronological sequence for development of microwave maser emission among such stars, it appears possible to describe a dust grain formation and annealing sequence. The early time spectra are dominated by aluminum oxides, which are then overrun by the emergence of silicates. Rather than the growth of silicate mantles on aluminate cores, this sequence is interpreted as a simple result of the higher electron affinity of aluminum for oxygen, resulting in an initial abundance of AlO bonds in the underoxidized grains. Once all the Al becomes fully oxidized, the relative abundances of Al and Si dictate that the AlO signature will be swamped by the growth of the SiO stretching fundamental. The correlations among the proposed dust chronology, the increasingly thick circumstellar envelopes, and the light curve asymmetries of Mira variables are instructive for understanding the evolutionary changes occurring in such stars.

Mantle desorption from amorphous grains

Amorphous carbons have been shown to consist of graphitic islands which are thought to be poorly connected to each other thermally. The heat produced by absorption of a single photon in an amorphous grain should therefore be localized in the absorbing and neighbouring islands, resulting in a temperature excursion for that region of the grain which is larger than that expected for a homogeneous particle. The temperature excursions resulting from the absorption of ultraviolet and infrared photons by amorphous carbon grains are estimated, and the effectiveness of this process in limiting the growth of molecular mantles on the grains is discussed. Observational evidence relating to the desorption of CO from grains in the dark-cloud environment is interpreted on the basis of this model.

Grains in Diffuse Clouds: Carbon-Coated Silicate Cores

A new model of interstellar grains is proposed in which grains in diffuse clouds consist of small (radius ≲ 10nm) and large asymmetric (0.05μm – 0.25μm) silicate cores thinly coated with mantles of amorphous carbon (thickness ≲ 5nm). This model can account successfully for many of the observed properties of interstellar dust and gives a simple interpretation of the interstellar extinction curve. The extinction bump at 220nm is attributed to absorption by finely divided silicates, as indicated by laboratory data. The large silicates provide a “grey” background extinction through the visual and UV, but the bulk of the extinction in this region is attributed, on the basis of laboratory data, to the amorphous carbon coatings. The average interstellar carbon depletion required in this model is about 50%.Wide variations in the observed interstellar extinction along different lines of sight are well known. These variations have a natural explanation in this model in terms of two parameters: the thickness of the carbon coatings, and the proportion of graphitic to diamond-like structure within the amorphous carbon. The underlying silicate cores are generally unchanged in these variations. The average interstellar extinction curve requires roughly equal proportions of graphitic and diamond-like forms of amorphous carbon. A higher graphitic fraction produces more visual extinction and a higher diamond-like fraction produces more far UV extinction. Varying both the proportions, and the total carbon content encompasses a wide range of extinction curve shapes.Amorphous carbon deposited at low temperature is generally diamond-like. Temperature excursions in the material reduce the hydrogen content and enhance the graphitic nature of the material. In the interstellar medium, this process has a time scale ≳ 106 yr. Carbon coatings are therefore expected to be deposited in the interstellar medium in diamond-like form and to be slowly converted to graphitelike. The growth of carbon mantles will be reversed by intermittent shocks Thus, interstellar space should contain grains with a range of thicknesses of carbon coatings, and compositions between graphitic and diamond-like.The chemical and physical properties of amorphous carbon have been the subject of intensive laboratory study. These properties enable an understanding of a variety of observations of dust (especially in the IR) and lead to a number of predictions which are described in this paper.