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At least 1,045 records · Page 58
All sourcesNASA NTRSDOE OSTI
NASA NTRS1985

Does nucleation theory apply to the formation of refractory circumstellar grains?

It is pointed out that kinetic factors inherent in cosmic systems will prevent the attainment of an approximate equilibrium precondensation cluster distribution, and that therefore such systems will violate one of the most fundamental assumptions of nucleation theory. More importantly, it is shown that of the 11 refractory metal/metal oxide systems which have been studied experimentally none are consistent with the predictions of either classical nucleation theory or the Lothe-Pound modification. Since no valid method is known by which these deficiencies can be circumvented, it is concluded that nucleation theory cannot be made to work in expanding circumstellar shells. However, a stochastic, kinetic treatment which makes use of experimental and theoretical data for specific refractory systems does appear to be feasible.

Donn, B.↗
NASA NTRS1985

Minor-element signature of relic olivine grains in deep-sea particles - A match with forsterites from C2 meteorites

The results of high-precision electron microprobe analyses on relic forsterites in 13 different deep sea particles (DSP) are described, comparing their 'minor' element (Mn, Cr, Ca, Ti, Al) signatures with those of olivines found in several types of meteorites. The results show that the DSP olivines do not match those from terrestrial and lunar rocks and achondritic meteoroties, match poorly those of the carbonaceous Allende (CV3) and Orgeuil (CI) meteorites, but are well matched with olivines from C2 meteorites, including the unusual Belgica 7904 meteorite.

Steele, I. M.↗
NASA NTRS1985

Ion probe determinations of the rare earth concentrations of individual meteoritic phosphate grains

A new ion probe method for quantitative measurements of the concentrations of all the REE down to the ppm level in 5-20 micron spots is presented. The first application of the method is the determination of REE abundances in meteoritic phosphates. Results are shown to be in good agreement with previous INAA and ion probe determinations. The merrillites in the St. Severin amphoterite are richer in REE than the apatites (the enrichment factors, for various REE, range from 2.3 to 14.2) in contradiction with the results of Ebihara and Honda (1983). Provided good standards for other mineral phases are found or implanted marker ion techniques are used, the method should find a wide range of applications for the study of both terrestrial and extraterrestrial crystals at the microscopic level.

Crozaz, G.↗
NASA NTRS1985

Role of Convection in Grain Refining

A quantitative representation of the heat and fluid flow fields in electromagnetically levitated (positioned) metallic specimens is examined. Experimental studies as well as modelling of those studies on undercooling and solidification of levitated metal droplets are also considered. Thermal measurements will be made of the undercooling and solidification of levitated metal droplets. Thermal measurements will be made of the undercooling and of the details of the ultra-rapid recalescence. This levitation technique combined with quenching will be used to aid in the development of coherent model for the solidification process related to structural studies, including thermodynamics and kinetics for nucleation and metastable solute partition, kinetics for solid/liquid interface velocity with high undercooling, and heat flow consideration.

Szekely, J.↗
NASA NTRS1986

Experimental investigations relating to the properties and formation of cosmic grains

The interpretation of observations or theoretical analyses of interstellar processes requires a sound knowledge of relevant data. In many instances this can only be obtained by experiments carried out under appropriate conditions. The availability of such data applicable to the subject and the techniques for obtaining the data are surveyed.

Donn, B.↗
NASA NTRS1986

Interrelationships between interstellar and interplanetary grains

The relationship between solar system dust (SSD) and interstellar dust particles (ISMD) is being reconsidered because of the discovery of isotopic anomalies in meteorites. Meteoritic, circumstellar/meteoritic, interstellar/meteoritic, planetary, and cometary data are reviewed.

Clayton, D. D.↗
NASA NTRS1985

Ultraviolet albedo of cometary grains

Several dusty comets, including two at heliocentric distances greater than 3 AU (Bowell 1982 I and Cernis 1983l), have been observed with the International Ultraviolet Explorer. Ultraviolet albedos have been determined for the wavelength range 2600-3050 A by comparison of the IUE data with both recent solar flux measurements and with IUE observations of solar analog stars. All of the comets show a similar reddening within this spectral range as well as with respect to the visible and infrared. No spectral signatures of possible ices are evident in the data. The ratio of gas to dust production rates, evaluated for 15 comets observed by IUE, shows a well-defined dependence on heliocentric distance.

Feldman, P. D.↗
NASA NTRS1986

The organic component of interstellar grains

The 3.4 micron absorption feature observed in the spectrum of a number of Galactic Center (GC) sources indicates the presence of organic molecules in the interstellar medium. It is ascribed to the C-H stretch vibration of tetrahedrally bonded carbon. From the observed features due to the interstellar organic material, an estimate was made of its composition and abundance. The ratio of the number of C-H groups of tetrahedrally to those of trigonally bonded carbon was 1.5, the cosmic abundance of carbon was .00037, and the depth of the silicate absorption toward the GC was taken equal to 3.6.

Schutte, W. A.↗
NASA NTRS1987

Infrared properties of dust grains derived from IRAS observations

The analysis of several diffuse interstellar clouds observed by the Infrared Astronomy Satellite (IRAS) is presented. The 60/100 micron flux ratios appear to be nearly constant in clouds with up to 1 sup m visual extinction at the center. Observations of a highly regular cloud in Chamaeleon show that the 12/100 micron ratio peaks at an intermediate radial distance and declines towards the center of the cloud. These observations indicate that nonequilibrium emission accounts only for the 12 and 25 micron bands; strong emission observed at the 60 micron band is probably due to equilibrium thermal radiation. The correlation of the 12 micron emission with a red excess observed for a high latitude cloud, L1780, is shown to be consistent with the assumption that both features are due to fluorescence by the same molecular species.

Chlewicki, G.↗
NASA NTRS1986

Space Station gas-grain simulation facility - Application to exobiology

The technical issues involved in performing experiments on the behavior and properties of aerosols in a microgravity environment provided by the Space Station are reviewed. The displacement of a particle resulting from g-jitter for ballistic, Knudsen, and Stokes flow regimes is examined in detail, and the radiation, acoustic, electrostatic, and electromagnetic mechanisms for the control of this motion are described. The simulation of organic haze production on Titan has been selected as an example experiment for detailed study. The purpose of this experiment was to simulate the photolysis of methane and the subsequent formation of the organic haze particles in the Titan upper atmosphere.

Mckay, C. P.↗
NASA NTRS1988

A two component model for thermal emission from organic grains in Comet Halley

Observations of Comet Halley in the near infrared reveal a triple-peaked emission feature near 3.4 micrometer, characteristic of C-H stretching in hydrocarbons. A variety of plausible cometary materials exhibit these features, including the organic residue of irradiated candidate cometary ices (such as the residue of irradiated methane ice clathrate, and polycyclic aromatic hydrocarbons. Indeed, any molecule containing -CH3 and -CH2 alkanes will emit at 3.4 micrometer under suitable conditions. Therefore tentative identifications must rest on additional evidence, including a plausible account of the origins of the organic material, a plausible model for the infrared emission of this material, and a demonstration that this conjunction of material and model not only matches the 3 to 4 micrometer spectrum, but also does not yield additional emission features where none is observed. In the case of the residue of irradiated low occupancy methane ice clathrate, it is argued that the lab synthesis of the organic residue well simulates the radiation processing experienced by Comet Halley.

Chyba, Christopher↗
NASA NTRS1988

Light scattering of large rough particles application to cometary grains

While the electromagnetic field scattered by a spherical particle is classically obtained by the Helmholtz equation, the general case of an arbitrary particle may be investigated in the general framework of the interaction of a wave with a scattering potential. A wave function then satisfies the Schroedinger equation. The general solution of the Schroedinger equation is given. The main disadvantage of this approach are its restriction to large particles and its scalar nature preventing the calculation of the polarization. However, Perrin and Lamy have shown how to avoid the second limitation and retrieve a vectorial description. They proved that in the case of large spheres when the ad hoc assumptions are satisfied, the expression of the scattering amplitude may be approximated by an expansion series in partial waves, i.e., on a discrete basis. The analogy may be generalized, and the ratio of the two components for a rough particle obtained by taking the ratio of the reflectivities for the two directions of polarization. These reflectivities involve the simple and double reflections calculated following the method developed by Wolff for rough surfaces. The theory is further detailed.

Lamy, Philippe L.↗
NASA NTRS1988

Protostellar collapse, dust grains and solar system formation

Attempts to understand the origin of the solar system come from two perspectives. The first is that of the insider looking out; the system components beyond the earth are examined in detail, their properties categorized, and explanations are sought that make sense of the patterns and relations among them. The primitive meteorites are prime objects of study from this perspective. The other perspective is that of the outsider looking in, considering other stars that are now in their formative stages and trying to understand the physical processes that govern all such events, under the assumption that the solar system is not unique. A compelling theory of the origin of the solar system must, of course, merge the fruits of both perspectives.

Cassen, Patrick↗
NASA NTRS1988

Dynamically allocating sets of fine-grained processors to running computations

Researchers explore an approach to using general purpose parallel computers which involves mapping hardware resources onto computations instead of mapping computations onto hardware. Problems such as processor allocation, task scheduling and load balancing, which have traditionally proven to be challenging, change significantly under this approach and may become amenable to new attacks. Researchers describe the implementation of this approach used by the FFP Machine whose computation and communication resources are repeatedly partitioned into disjoint groups that match the needs of available tasks from moment to moment. Several consequences of this system are examined.

Middleton, David↗