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At least 235 records · Page 13
All sourcesNASA NTRSDOE OSTI
NASA NTRS1990

Recycled grains in lunar soils as an additional, necessary, regolith evolution parameter

Recycled lunar soil grains are defined as those soil grains that have been a part of either regolith breccias or agglutinates; thus, mineral grains, rock fragments, older agglutinates, and volcanic glass spherules, if dislodged from an agglutinate or a regolith breccia, would all qualify as recycled grains. This paper shows that it is possible to estimate the proportion of recycled material in lunar soils. Optical data from 12 soils in the Apollo 16 core 64001/2 were collected to estimate the proportion (W) of recycled crystalline grains in each of these soils. The W values show a correspondence with other independently derived parameters and the history of the core soils, indicating that W can be used as a valid soil-evolution parameter.

Basu, A.↗
NASA NTRS1989

The evolution of organic mantles on interstellar grains

By laboratory simulation of the chemical processes on dust grains it was investigated how solid organic materials can be produced in the interstellar medium. The ice mantles that accrete on grains in molecular clouds, consisting primarily of H2O, CO, H2CO, NH3, and O2, are irradiated by the internal UV field, resulting in the storage of radicals upon photodissociation of the original molecules. Transient heating events lead to the production of oxygen-rich organic species by recombination reactions. The experiments indicated that in this way the observed amount of organic material can be produced if a grain passes a few times through a molecular cloud during its life. After the destruction of the cloud the grains enter a more diffuse medium. Here they are subjected to the interstellar UV field as well as to collisions with atomic hydrogen. Experiments show that the intense photoprocessing results in the removal of small species like H2O and NH3 as well as in carbonization of the organic molecules. Contrary to this, the atomic H flux will maintain a certain hydrogen level in the mantle. These processes likely convert the original, oxygen-rich organics into an unsaturated hydrocarbon type material such as that observed towards IRS 7 and in Comet Halley grains.

Schutte, Willem A.↗
NASA NTRS1989

Infrared studies of dust grains in infrared reflection nebulae

IR reflection nebulae, regions of dust which are illuminated by nearby embedded sources, were observed in several regions of ongoing star formation. Near IR observation and theoretical modelling of the scattered light form IR reflection nebulae can provide information about the dust grain properties in star forming regions. IR reflection nebulae were modelled as plane parallel slabs assuming isotropically scattering grains. For the grain scattering properties, graphite and silicate grains were used with a power law grain size distribution. Among the free parameters of the model are the stellar luminosity and effective temperature, the optical depth of the nebula, and the extinction by foreground material. The typical results from this model are presented and discussed.

Pendleton, Yvonne J.↗
NASA NTRS1989

Circumstellar grain extinction properties of recently discovered post AGB stars

The circumstellar grains of two hot evolved post asymptotic giant branch (post AGB) stars, HD 89353 and HD 213985 were examined. From ultraviolet spectra, energy balance of the flux, and Kurucz models, the extinction around 2175 A was derived. With visual spectra, an attempt was made to detect 6614 A diffuse band absorption arising from the circumstellar grains so that we could examine the relationship of these features to the infrared features. For both stars, we did not detect any diffuse band absorption at 6614 A, implying the carrier of this diffuse band is not the carrier of the unidentified infrared features not of the 2175 A bump. The linear ultraviolet extinction of the carbon-rich star HD 89353 was determined to continue across the 2175 A region with no sign of the bump; for HD 213985 it was found to be the reverse: a strong, wide bump in the mid-ultraviolet. The 213985 bump was found to be positioned at 2340 A, longward of its usual position in the interstellar medium. Since HD 213985 was determined to have excess carbon, the bump probably arises from a carbonaceous grain. Thus, in view of the ultraviolet and infrared properties of the two post AGB stars, ubiquitous interstellar infrared emission features do not seem to be associated with the 2175 A bump. Instead, the infrared features seem related to the linear ultraviolet extinction component: hydrocarbon grains of radius less than 300 A are present with the linear HD 89353 extinction; amorphous anhydrous carbonaceous grains of radius less than 50 A might cause the shifted ultraviolet extinction bump of HD 213985.

Buss, Richard H., Jr.↗
NASA NTRS1991

Astrophysical dust grains in stars, the interstellar medium, and the solar system

Studies of astrophysical dust grains in circumstellar shells, the interstellar medium, and the solar system may provide information about stellar evolution and about physical conditions in the primitive solar nebula. The following subject areas are covered: (1) the cycling of dust in stellar evolution and the formation of planetary systems; (2) astrophysical dust grains in circumstellar environments; (3) circumstellar grain formation and mass loss; (4) interstellar dust grains; (5) comet dust and the zodiacal cloud; (6) the survival of dust grains during stellar evolution; and (7) establishing connections between stardust and dust in the solar system.

Gehrz, Robert D.↗
NASA NTRS1991

Size and density distribution of very small dust grains in the Barnard 5 cloud

The effects of the temperature fluctuations in small graphite grains on the energy spectrum and the IR surface brightness of an isolated dust cloud heated externally by the interstellar radiation field were investigated using a series of models based on a radiation transport computer code. This code treats self-consistently the thermal coupling between the transient heating of very small dust grains and the equilibrium heating of conventional large grains. The model results were compared with the IRAS observations of the Barnard 5 (B5) cloud, showing that the 25-micron emission of the cloud must be produced by small grains with a 6-10 A radius, which also contribute about 50 percent to the observed 12-micron emission. The remaining 12 micron flux may be produced by the polycyclic aromatic hydrocarbons. The 60-and 100-micron radiation is dominated by emission from large grains heated under equilibrium conditions.

Lis, Dariusz C.↗
NASA NTRS1991

Infrared emission from isolated dust clouds in the presence of very small dust grains

Models of the effects of small grain-generated temperature fluctuations on the IR spectrum and surface brightness of externally heated interstellar dust clouds are presently constructed on the basis of a continuum radiation transport computer code which encompasses the transient heating of small dust grains. The models assume a constant fractional abundance of large and small grains throughout the given cloud. A comparison of model results with IRAS observations indicates that the observed 12-25 micron band emissions are associated with about 10-A radius grains, while the 60-100 micron emission is primarily due to large grains which are heated under the equilibrium conditions.

Lis, Dariusz C.↗
NASA NTRS1991

Very small grains and the infrared colors of galaxies

The contribution of very small grains with fluctuating temperature to the IR emission of disk galaxies is evaluated in light of IRAS data, and the abundance variation of these small grains among galaxies, relative to the 'classical', thermally stable grains, is estimated. It is suggested that the low dispersion in the 'cold' galaxies is representative of the true dispersion in the small-to-large grain abundance ratio among galaxies, while the larger dispersion among warmer galaxies results from increasing optical depth-heating dust effects due to AGNs, in conjunction with the destruction of small dust grains at high radiative energy densities.

Helou, G.↗
NASA NTRS1991

The origin of amorphous rims on lunar plagioclase grains: Solar wind damage or vapor condensates

A distinctive feature of micron sized plagioclase grains from mature lunar soils is a thin (20 to 100 nm) amorphous rim surrounding the grains. These rims were originally described from high voltage electron microscope observations of lunar plagioclase grains by Dran et al., who observed rims up to 100 nm thick on plagioclase grains from Apollo 11 and 12 soils. These rims are believed to be the product of solar wind damage. The amorphous rims were studied on micron sized plagioclase grains from a mature Apollo 16 soil using a JEOL 200FX transmission electron microscope equipped with an energy dispersive x ray spectrometer. It was found that the amorphous rims are compositionally distinct from the interior plagioclase and it is proposed that a major component of vapor condensates is present in the rims.

Keller, Lindsay P.↗
NASA NTRS1991

A Large-Grain Mapping Approach for Multiprocessor Systems Through Data Flow Model Ph.D. Thesis

A large-grain level mapping method is presented of numerical oriented applications onto multiprocessor systems. The method is based on the large-grain data flow representation of the input application and it assumes a general interconnection topology of the multiprocessor system. The large-grain data flow model was used because such representation best exhibits inherited parallelism in many important applications, e.g., CFD models based on partial differential equations can be presented in large-grain data flow format, very effectively. A generalized interconnection topology of the multiprocessor architecture is considered, including such architectural issues as interprocessor communication cost, with the aim to identify the 'best matching' between the application and the multiprocessor structure. The objective is to minimize the total execution time of the input algorithm running on the target system. The mapping strategy consists of the following: (1) large-grain data flow graph generation from the input application using compilation techniques; (2) data flow graph partitioning into basic computation blocks; and (3) physical mapping onto the target multiprocessor using a priority allocation scheme for the computation blocks.

Kim, Hwa-Soo↗
NASA NTRS1991

Grain size control of rhenium strip

Ensuring the desired grain size in the pure Re strip employed by the SP-100 space nuclear reactor design entails the establishment of an initial grain size in the as-received strip and the avoidance of excessive grain growth during subsequent fabrication. Pure Re tapered tensile specimens have been fabricated and tested in order to quantify the effects of grain-boundary migration. Grain size could be rendered fine and uniform by means of a rolling procedure that uses rather large reductions between short intermediate anneals. The critical strain regime varies inversely with annealing temperature.

Schuster, Gary B.↗
NASA NTRS1993

On the negative polarization of light scattered by subwavelength regolithic grains

Recent laboratory polarimetric measurements of light scattering by layers of alumina grains of different size are used to examine the relevance of the Wolff & Dollfus (1990) theory of negative polarization, which has been developed to describe light scattering by regolithic grains of arbitrary size. It is demonstrated that, while reproducing satisfactorily the main polarization characteristics of light scattering by grains much greater than the wavelength, this theory does not describe the negative polarization of light measured by Geake & Geake (1990) for subwavelength-sized grains. The smallest grains exhibit a polarization feature that may be called the 'polarization opposition effect'. This feature requires an alternative explanation and may be due to the so-called weak localization of photons in discrete random media.

Kolokolova, L. O.↗
NASA NTRS1993

Oxygen isotopic composition of individual olivine grains from the Allende meteorite

The oxygen isotopic composition of a variety of individual olivine grains (including refractory forsterite grains, cores of isolated olivine grains, FeO-rich rims, and individual matrix olivine grains) from the Allende CV3 meteorite was investigated by ion microprobe mass spectrometry, in order to obtain information on the formation mechanism of these samples. It was found that the most primitive (i.e., refractory) olivine in Allende is far less enriched in O-16 than are spinel and pyroxene in Ca,Al-rich inclusions, suggesting that Allende olivine must have formed in an environment that is less enriched in O-16 compared to the gas from which Ca,Al-inclusions are formed. FeO-rich (26-30 wt pct) rims of isolated olivine grains are significantly higher in delta-O-17 and delta-O-18 than forsteritic cores, suggesting that these rims formed by condensation from an oxidized gas with higher delta-O-17 than delta-O-18. Matrix olivine was found to be highest in FeO and to have the lowest enrichment in O-16.

Weinbruch, S.↗
NASA NTRS1993

Interstellar grains in primitive meteorites - Diamond, silicon carbide, and graphite

Primitive meteorites contain a few parts per million (ppm) of pristine interstellar grains that provide information on nuclear and chemical processes in stars. Their interstellar origin is proven by highly anomalous isotopic ratios, varying more than 1000-fold for elements such as C and N. Most grains isolated thus far are stable only under highly reducing conditions (C/O greater than 1), and apparently are 'stardust' formed in stellar atmospheres. Microdiamonds, of median size about 10 A, are most abundant (about 400-1800 ppm) but least understood. They contain anomalous noble gases including Xe-HL, which shows the signature of the r- and p-processes. Silicon carbide, of grain size 0.2-10 microns and abundance about 6 ppm, shows the signature of the s-process and apparently comes mainly from red giant carbon (AGB) stars of 1-3 solar masses. Some grains appear to be not less than 10 exp 9 a older than the solar system. Graphite spherules of grain size 0.8-7 microns and abundance less than 2 ppm contain highly anomalous C and noble gases, as well as large amounts of fossil Mg-26 from the decay of extinct Al-26. They seem to come from at least three sources, probably AGB stars, novae, and Wolf-Rayet stars.

Anders, Edward↗
NASA NTRS1994

What does the fine-scale petrography of IDPs reveal about grain formation and evolution in the early solar system?

The 'pyroxene' interplanetary dust particles (IDP's) may be the best samples for investigation of primordial grain-forming reactions because they appear to have experienced negligible post-accretional alteration. They are likely to continue to yield information about gas-to-solid condensation and other grain-forming reactions that may have occurred either in the solar nebular or presolar interstellar environments. An immediate challenge lies in understanding the nanometer-scale petrography of the ultrafine-grained aggregates in 'pyroxene' IDP's. Whether these aggregates contain components from diverse grain-forming environments may ultimately be answered by systematic petrographic studies using electron microscopes capable of high spatial resolution microanalysis. It may be more difficult to decipher evidence of grain formation and evolution in 'olivine' and 'layer silicate' IDP's because they appear to have experienced post-accretional alteration. Most of the studied 'olivine' IDPs have been subjected to heating and equilibration, perhaps during atmospheric entry, while the 'layer silicate' IDP's have experienced aqueous alteration.

Bradley, John↗
NASA NTRS1994

Porphyrins in the interstellar medium (in grains)

Spectral sensitivity of the chromophores to their immediate chemical environment establishes some of the chemical constituents of the grains in which they reside. These are: (1) Paraffins, such as, octane, nonane, decane, and others...(needed for Shpolskii matrices and producing quasilines); and (2) Pyridine. The presence of pyridine is required not only to produce the spectral DIB matching, but also to produce the 36 cm(sup -1) crystal field splitting of the S(sub 1) electronic state. The presence of pyridine in the grains can be confirmed spectroscopically. Pyridine produces a transmission window at 2175 A, matching exactly the well known UV hump. On grain reflection, some of the incoming UV radiation is absorbed into the grain's outer layers. Spikes in the lab and in the astronomical data are due to vibronic transitions in pyridine. The lab spectroscopy reported here clearly establishes the presence of MgTBP, H2TPB, and pyridine in the interstellar grains. The high fluorescence efficiency of MgTBP (being optically pumped in the visible) apparently accounts for all the observed UIR emissions.

Johnson, Fred M.↗
NASA NTRS1994

The contribution of vapor deposition to amorphous rims on lunar soil grains

Recent analysis analytical electron microscope study of lunar soils showed that the approximately 60-nm-wide amorphous rims surrounding many lunar soils grains exhibit distinct compositional differences from their hosts. On average, the amorphous rim compositions reflect the local bulk soil composition with the exceptions of Si and S, which are enriched relative to the bulk soil. These chemical trends led us to propose that the amorphous rims were in fact deposits of impact-generated vapors produced during regolith gardening, a hypothesis that runs contrary to the generally accepted view that the rims are produced through amorphization of the outer parts of mineral grains by interaction with the solar wind. Analytical data are reported for amorphous rims on individual minerals in lunar soils in order to show that the magnitude of the chemical differences between rim and host are so great that they require a major addition of foreign elements to the grain surfaces. The average composition of amorphous rims is listed as a function of host mineralogy as determined in microtone thin sections using energy-dispersive X-ray spectrometry in the transmission electron microscope. As the host mineral becomes chemically more complex, the chemical differences are not as clear. The average rim compositions are remarkably similar and are independent of the host grain mineralogy. Whether there are 'sputtering' or radiation effects superimposed on the vapor-deposited material can be debated. We do not explicitly exclude the effects of radiation damage as a contributing factor to the formation of amorphous rims; we are merely emphasizing the major role played by condensed vapors in the formation of amorphous rims on lunar soil grains.

Keller, L. P.↗
NASA NTRS1995

The Grain Structure of Castings: Some Aspects of Modelling

The efficacy of the modelling of the solidification of castings is typically tested against observed cooling curves and the final grain structures and sizes. Without thermo solutal convection, equiaxed grain formation is promoted by introduction of heterogeneous substrates into the melt, as grain refiners. With efficient thermo solutal convection, dendrite fragments from the mushy zone can act as an intrinsic source of equiaxed grains and resort to grain refining additions is unnecessary. The mechanisms of dendrite fragmentation and transport of these fragments are briefly considered.

Hellawell, A.↗