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At least 433 records · Page 24
All sourcesNASA NTRSDOE OSTI
NASA NTRS1988

A possible explanation for the inconsistency between the Giotto grain mass distribution and ground-based observations

Giotto measured the in situ Halley dust grain mass distribution with 2 instruments, Particle Impact Analyzer and Dust Impact Detection System (DIDSY), as well as the total intercepted mass from the deceleration of the spacecraft (Giotto Radio-Science Experiment, GRE). Ground based observations made shortly before encounter have fluxes much higher than would be predicted from Giotto data. It is concluded that Giotto DIDSY and GRE data represent observations of dust originating from a narrow track along the nucleus. They are consistent with ground based data, if assumptions are made about the level of activity along this track. The actual size distribution that should be used for modeling of the whole coma should not include the large mass excess actually observed by Giotto. Extrapolation of the small grain data should be used, since for these grains the velocity dispersion is low and temporal changes at the nucleus would not affect the shape of the mass distribution.

Perry, C. H.↗
NASA NTRS1989

Gas-Grain Simulation Facility: Fundamental studies of particle formation and interactions. Volume 1: Executive summary and overview

An overview of the Gas-Grain Simulation Facility (GGSF) project and its current status is provided. The proceedings of the Gas-Grain Simulation Facility Experiments Workshop are recorded. The goal of the workshop was to define experiments for the GGSF--a small particle microgravity research facility. The workshop addressed the opportunity for performing, in Earth orbit, a wide variety of experiments that involve single small particles (grains) or clouds of particles. The first volume includes the executive summary, overview, scientific justification, history, and planned development of the Facility.

Fogleman, Guy↗
NASA NTRS1988

The formation of filamentary sublimate residues (FSR) from mineral grains

The significant interparticle forces observed between solar system dust grains upon desorption or sublimation of excess volatiles in simulated Martian or cometary environments are presently investigated, in order to more precisely define these mechanisms and to simulate the types of deposits thereby formed. Some classes of phyllosilicate mineral grains are noted to bond together to form a highly porous filamentary sublimate residue (FSR) exhibiting an exceptionally high tensile strength for its density; this may be important in its control of erosion and sublimation in Martian and cometary environments. It is concluded that FSR formation from clean mineral grains in water ice may be important in the formation of the Martian polar layered terrain.

Storrs, A. D.↗
NASA NTRS1989

Thermoelastic micromechanical stresses associated with a large alpha-silicon carbide grain in a polycrystalline beta-silicon carbide matrix

The thermoelastic micromechanical stresses associated with a single large hexagonal alpha-SiC grain within a fine-grain-size cubic (3C) beta-SiC matrix were calculated. The naturally occurring residual stresses which are created during cooling from the processing temperatures and the effects of superimposed applied external stresses are both considered. A significant effect of the shape or geometry of the alpha-SiC grain is revealed, with the largest residual stresses associated with the naturally occurring tabular or platelet structure. The stresses are compared with the published strength results for these materials, which suggests that the residual stresses assume a significant role in the strength reduction that is observed.

Li, Zhuang↗
NASA NTRS1989

Ultrafine-grained mineralogy and matrix chemistry of olivine-rich chondritic interplanetary dust particles

Olivine-rich chondritic interplanetary dust particles (IDPs) are an important subset of fluffy chondritic IDPs collected in the earth's stratosphere. Particles in this subset are characterized by a matrix of nonporous, ultrafine-grained granular units. Euhedral single crystals, crystals fragments, and platey single crystals occur dispersed in the matrix. Analytical electron microscopy of granular units reveals predominant magnesium-rich olivines and FeNi-sulfides embedded in amorphous carbonaceous matrix material. The variable ratio of ultrafine-grained minerals vs. carbonaceous matrix material in granular units support variable C/Si ratios, and some fraction of sulfur is associated with carbonaceous matrix material. The high Mg/(Mg+Fe) ratios in granular units is similar to this distribution in P/Comet Halley dust. The chondritic composition of fine-grained, polycrystalline IDPs gradually breaks down into nonchondritic, and ultimately, single mineral compositions as a function of decreased particle mass. The relationship between particle mass and composition in the matrix of olivine-rich chondritic IDPs is comparable with the relationship inferred for P/Comet Halley dust.

Rietmeijer, F. J. M.↗
NASA NTRS1989

Microgravity particle research on the Space Station - The gas-grain simulation facility

In the gravitational field on earth, the large settling rate of micron-sized particles and the effects of gravity-induced convection prohibit many interesting studies of phenomena such as coagulation, collisions, and mutual interactions of droplets, dust grains and other particles. Examples of exobiology experiments involving these phenomena are the simulation of organic aerosol formation in Titan's atmosphere, studies of the role of comets in prebiotic chemical evolution, and simulations of carbon grain interactions in various astrophysical environments. The Gas-Grain Simulation Facility (GGSF) is a proposed earth-orbital laboratory that will allow present ground-based experimental programs which study processes involving small particles and weak interactions to be extended to a new domain. Physics issues that scientists wishing to propose GGSF experiments must consider are reviewed in this paper. Specifically, coagulation, motion in gases and vacua, and wall deposition of particles in a microgravity environment are discussed.

Fogleman, G.↗
NASA NTRS1990

Laboratory simulation of interstellar grain chemistry and the production of complex organic molecules

During the past 15 years considerable progress in observational techniques has been achieved in the middle infrared (5000 to 500 cm(-1), 2 to 20 microns m), the spectral region most diagnostic of molecular vibrations. Spectra of many different astronomical infrared sources, some deeply embedded in dark molecular clouds, are now available. These spectra provide a powerful probe, not only for the identification of interstellar molecules in both the gas solid phases, but also of the physical and chemical conditions which prevail in these two very different domains. By comparing these astronomical spectra with the spectra of laboratory ices one can determine the composition and abundance of the icy materials frozen on the cold (10K) dust grains present in the interior of molecular clouds. These grains and their ice mantles may well be the building blocks from which comets are made. As an illustration of the processes which can take place as an ice is irradiated and subsequently warmed, researchers present the infrared spectra of the mixture H2O:CH3OH:CO:NH3:C6H14 (100:50:10:10:10). Apart from the last species, the ratio of these compounds is representative of the simplest ices found in interstellar clouds. The last component was incorporated into this particular experiment as a tracer of the behavior of a non-aromatic hydrocarbon. The change in the composition that results from ultraviolet photolysis of this ice mixture using a UV lamp to simulate the interstellar radiation field is shown. Photolysis produces CO, CO2, CH4, HCO, H2CO, as well as a family of moderately volatile hydrocarbons. Less volatile carbonaceous materials are also produced. The evolution of the infrared spectrum of the ice as the sample is warmed up to room temperature is illustrated. Researchers believe that the changes are similar to those which occur as ice is ejected from a comet and warmed up by solar radiation. The warm-up sequence shows that the nitrile or iso-nitrile bearing compound produced during photolysis evaporates between 200 and 250K, suggesting that it is carried by a small molecular species. These molecules could be similar to the source material in Comet Halley that is ejected in grains into the coma, freed by sublimation, and photolyzed by solar radiation to produce the observed jets.

Allamandola, L. J.↗
NASA NTRS1988

Experimental studies of circumstellar, interstellar and interplanetary refractory grain analogs

Experimental techniques for studying the processes which occur during the life cycle of a refractory grain are reviewed. Consideration is given to the construction of a differentially-pumped beam-mass spectrometer which is expected to determine the relative stability of small clusters in the Fe-Mg-Si-O and Ca-Ti-Al-O systems as a function of temperature and pressure. Also, a smoke generator has been constructed to produce gram quantities of amorphous grains for use in thermal annealing and hydrous alteration studies of smokes containing Fe-Mg-Al-Ti-Ca-Si-O and Cl. Topics of the studies include the properties of grains produced in a flow condensation apparatus, the sorption efficiency of Si2O3 smokes for AR gas, the processing of ices containing refractory precursors such as SiH4 or Fe(CO)5 plus water, and the residue from 1 MeV bombardment and warming of an amorphous Fe-SiO smoke.

Nuth, J. A.↗
NASA NTRS1989

Grain ultraviolet extinction properties of recently discovered post asymptotic giant branch stars

The circumstellar grains of two hot evolved postasymptotic giant branch stars, HD 89353 and HD 213985, are examined, deriving the extinction around 2175 A. An unsuccessful attempt to detect 6614 A diffuse band absorption arising from the grains implies that the carrier of this diffuse band is not the carrier of the unidentified IR features reported by other investigators nor of the 2175 A bump. It is found that the linear UV extinction of the carbon-rich star HD 89353 continues across the 2175 A region with no sign of the bump; for HD 213985 the reverse is found: a strong, wide bump in the mid-UV. A wavelength of 2300 A is derived for the HD 213985 bump, which lies longward of its unusual position in the interstellar medium. Since HD 89353 is metal-poor yet rich in hydrogen and carbon, the linear extinction arises from hydrocarbon grains of radius less than 300 A.

Buss, Richard H., Jr.↗
NASA NTRS1990

Cosmogenic neon from individual grains of CM meteorites - Extremely long pre-compaction exposure histories or an enhanced early particle flux

This paper presents the results on cosmogenic Ne extracted from individual meteoritic grains by a laser extraction system which used, at different times, two CW lasers: an Ar-ion laser and an Nd:YAG laser, with 20 and 70 W of deliverable power, respectively. Chemical etching was used to select grains exposed to solar flare VH particles. Results show that most of the grains with solar flare VH tracks (but not those which did not exhibit such tracks) contain spallation-produed Ne in significant excess of that due to the nominal cosmic-ray exposure, providing evidence for extensive energetic particle exposure during the precompaction era.

Hohenberg, Charles M.↗
NASA NTRS1989

Alignment mechanisms of paramagnetic grains revisited

Taking into account the tight coupling of grain axis with angular momentum due to effective dissipation of rotation energy, the alignment of spheroidal grains was investigated by paramagnetic relaxation. Alignment degree will be significantly improved in diffuse clouds. The inclusions of superparamagnetic (SPM) substances may play a key role in grain alignment in dark clouds as well as in diffuse clouds.

Seki, Munezo↗
NASA NTRS1989

Grains in galactic haloes

Researchers considered the effect of extensive forces on dust grains subjected to the light and matter distribution of a spiral galaxy (Greenberg et al. (1987), Ferrini et al. (1987), Barsella et al (1988). Researchers showed that the combined force on a small particle located above the plane of a galactic disk may be either attractive or repulsive depending on a variety of parameters. They found, for example, that graphite grains from 20 nm to 250 nm radius are expelled from a typical galaxy, while silicates and other forms of dielectrics, after initial expulsion, may settle in potential minimum within the halo. They discuss only the statistical behavior of the forces for 17 galaxies whose luminosity and matter distribution in the disk, bulge and halo components are reasonably well known. The preliminary results of the study of the motion of a dust grain for NGC 3198 are given.

Ferrara, Andrea↗
NASA NTRS1989

Experimental evidence for amorphous carbon grains in comets

Amorphous carbon grains similar to those produced in the laboratory, but with a higher hydrogen content, appear to be good candidates to simulate both the IR continuum emission and the 3.4 micron band measured for P/Halley. The comparison of the cometary features with those detected in the laboratory for carbon grains characterized by various sp(exp 2)/sp(exp 3) ratios seems to indicate that a prevalent diamond-like (sp(exp 3)) structure should be present in cometary particles. These kinds of solid particles seem also suitable to explain the daily and monthly variations of the 3.4 micron band intensity, relative to the continuum, and, at the same time,- to fulfill the abundance constraints. The same grains appear to be able to reproduce the absorption bands detected in the IR galactic source IRS 7. This result may be considered as a first experimental evidence of a relation existing between interstellar dust and cometary materials.

Colangeli, Luigi↗
NASA NTRS1991

Grain boundary sliding behaviour of copper and alpha brass at intermediate temperatures

The role of grain boundary sliding in copper and Cu-30 pct Zn in the temperature range 0.50-0.72 Tm, where Tm is the absolute melting point of the material, is examined. First, sliding data obtained on these materials are presented. The results indicate that the stress exponent for sliding is similar to that for lattice deformation, while the activation energy for sliding varies between 0.5 and 1.6 of the activation energy for creep. Several models proposed for grain boundary sliding are discussed, and it is shown that they do not account for the observed results on copper and alpha brass. A phenomenological model is proposed, where it is assumed that grain boundary sliding results from the glide of dislocations on secondary slip planes.

Raj, S. V.↗
NASA NTRS1991

Cometary and interstellar dust grains - Analysis by ion microprobe mass spectrometry and other techniques

A survey of microanalytical measurements on interplanetary dust particles (IDPs) and interstellar dust grains from primitive meteorites is presented. Ion-microprobe mass spectrometry with its capability to determine isotopic compositions of many elements on a micron spatial scale has played a special role. Examples are measurements of H, N, and O isotopes and refractory trace elements in IDPs; C, N, Mg, and Si isotopes in interstellar SiC grains; and C and N isotopes and H, N, Al, and Si concentrations in interstellar graphite grains.

Zinner, Ernst↗
NASA NTRS1991

Explosive desorption of icy grain mantles in dense clouds

The cycling of the condensible material in dense clouds between the gas phase and the icy grain mantles is investigated. In the model studied, desorption of the ice occurs due to grain mantle explosions when photochemically stored energy is released after transient heating by a cosmic ray particle. It is shown that, depending on the grain size distribution in dense clouds, explosive desorption can maintain up to about eight percent of the carbon in the form of CO in the gas phase at typical cloud densities.

Schutte, W. A.↗
NASA NTRS1992

Generic grain size distribution for liquid phase sintering

In order to develop an empirical model to represent a generic grain size distribution for liquid phase sintering, experimental data and theoretical models are analyzed using the plot of standard normal vs normalized size. From the experimental 3D grain size plot, the most representative line is deduced. The line best fits the Lifshitz-Slyozov encounter modified model except for the small size ranges of the lower 15 percent of the distribution. It is shown that grain coalescence might be a cause for this departure from theory.

German, R. M.↗
NASA NTRS1992

Effects of grain size and shape in modeling reflectance spectra of mineral mixtures

An 'isograin model' of reflectance from particulate material is presented. The model treats reflectance as a series of grain interactions, incorporating optical constants of each material and two parameters that model the activity of scattering and absorption in the vertical and the horizontal directions, respectively. Mixing ratios of components in mixture are treated as variables and are calculated using least-squares fit. Grain size and shape have a significant effect on the accuracy of predicting mixing ratios but can be modeled by introducing the concept of effective grain size. The model calculates spectra in good agreement with measured spectra and models reflectance within 1 percent and mixing ratios within 5 wt pct.

Hiroi, T.↗