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At least 271 records · Page 15
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NASA NTRS1988

Space Station gas-grain simulation facility - Microgravity particle research

The proposed Space Station gas-grain simulation facility (GGSF) and the possibilities for research in the facility are discussed. The physics of particles in microgravity is reviewed. The proposed design of the GGSF is illustrated and examined. Examples of experiments which have been suggested for the GGSF are presented, including the formation of organic haze particles in Titan's atmosphere, organic compound synthesis on surfaces of growing particles, fractal particles, planetary ring particle dynamics, aggregation of fine geological particulates in planetary atmospheres, and dipolar grain coagulation and orientation.

Carle, Glenn C.↗
NASA NTRS1988

Formation of polar stratospheric clouds simulated in a two dimensional model of the atmosphere

A microphysics code has been implemented in a two dimensional model of the atmosphere to study formation of polar stratospheric clouds containing HCl or HNO3. The model range is from pole to pole in latitude and from the ground to about 20 km in altitude. Resolution in latitude is 10 deg and about 0.8 km in altitude. This is an Eulerian model with prescribed eddy diffusion coefficients and the circulation obtained from observations. The chemistry of the model follows the family approach for NO(x), Cl(x) and HO(x) while the ozone is fixed and changed seasonally. The aerosol code is based on an assigned population of condensation, coagulation and sedimentation. Aerosol growth is simulated in nine different size bins ranging between 0.01u and 2.56u. The model has been built to study aerosol layers formation in the upper troposphere and lower stratosphere and has been validated for sulfate aerosol resulting from a rather complex sulfur chemistry.

Visconti, Guido↗
NASA NTRS1988

A multidimensional model for aerosols - Description of computational analogs

The numerical algorithms used to simulate the advection, diffusion, sedimentation, coagulation, and condensational growth of atmospheric aerosols are described. The model can be used in one, two, or three spatial dimensions. The continuity equation in a generalized horizontal and vertical coordinate system is developed, which allows the model to be quickly adapted to a wide variety of dynamical models of global or regional scale. Algorithms are developed to treat the various physical processes, and the results of simulations are presented, which show the strengths and weaknesses of these algorithms. Although the emphasis is on the modeling of aerosols, the work is also applicable to the simulations of the transport of gases.

Toon, O. B.↗
NASA NTRS1988

Formation processes and time scales for meteorite parent bodies

The transition from small particles suspended in the solar nebula to the planetesimals (asteroids) that became the parent bodies of meteorites is examined. Planetesimals probably grew by coagulation of grain aggregates that collided due to different rates of settling and drag-induced orbital decay. Their growth was accompanied by radial transport of solids, possibly sufficient to deplete the primordial mass in the asteroid zone, but with relatively little mixing. The formation of asteroid-sized planetesimals was probably rapid, on a time scale less than 1 Myr.

Weidenschilling, S. J.↗
NASA NTRS1989

Self-limiting physical and chemical effects in volcanic eruption clouds

One-dimensional aerosol microphysical and photochemical models are used to study the chemistry of stratospheric volcanic clouds. The results indicate that the aerosol microphysical processes of condensation and coagulation produce larger particles as the SO2 injection rate is increased. Larger particles have a smaller optical depth per unit mass and settle out of the stratosphere at a faster rate than smaller ones, restricting the total number of particles in the stratosphere. The microphysical processes moderate the impact of volcanic clouds on the earth's radiation budget and climate, suggesting that volcanic effects may be self limiting. It is noted that the injection of HCl into the stratosphere, which could lead to large ozone changes, is limited by a cold trap effect in which HCl and water vapor condense on ash particles in the rising volcanic plume and fall out as ice.

Pinto, Joseph P.↗
NASA NTRS1990

Kinetics and mechanism of soot formation in hydrocarbon combustion

The focus of this work was on kinetic modeling. The specific objectives were: detailed modeling of soot formation in premixed flames, elucidation of the effects of fuel structure on the pathway to soot, and the development of a numerical technique for accurate modeling of soot particle coagulation and surface growth. Those tasks were successfully completed and are briefly summarized.

Frenklach, Michael↗
NASA NTRS1990

Early stages of accumulation in the solar nebula

Coagulation of grains in the solar nebula should result in formation of low-density aggregates with fractal structure. The opacity of the nebula remains much higher than computed for the assumption that particles are compact. Timescales for particle growth, settling to the central plane of the nebula, and formation of planetesimals are lengthened significantly.

Weidenschilling, S. J.↗
NASA NTRS1989

The physics of planetesimal formation

Physical processes involved in the planetesimal formation are discussed with special attention given to the nature of aerodynamic interactions between solid bodies and gas in the solar nebula. It is emphasized that the model of planetesimal formation by gravitational instability of a dust layer yields predictions that are simple but almost certainly wrong. It is suggested instead that the formation of planetesimals began with the process of coagulation of grains into larger aggregates, and that gravitational forces became more important than gas drag only after objects as large as many meters in diameter had formed.

Weidenschilling, Stuart J.↗
NASA NTRS1989

The relationship between IR, optical, and UV extinction

An analysis is presented for the variability of absolute IR, optical, and UV extinction, A(sub lambda), derived through the ratio of total-to-selective extinction, R, for 31 lines of sight for which reliable UV extinction parameters were derived. These data sample a wide range of environments and are characterized by 2.5 is less than or equal to R is less than or equal to 6.0. It was found that there is a strong linear dependence between extinction expressed as A(sub lambda)/A(sub V) and 1/R for 1.25 micron is less than or equal to lambda is less than or equal to 0.12 micron. Differences in the general shape of extinction curves are largely due to variations in shape of optical/near-UV extinction corresponding to changes in R, with A(sub lambda)/A(sub V) decreasing for increasing R. From a least-squares fit of the observed R-dependence as a function of wavelength for 0.8/micron is less than or greater than 1/lambda is less than or equal to 8.3/micron, an analytic expression was generated from which IR, optical, and UV extinction curves of the form A(sub lambda)/A(sub V) can be reproduced with reasonable accuracy from a knowledge of R. It was also found that the absolute bump strength normalized to A(sub V) shows a general decrease with increasing R, suggesting that some fraction of bump grains may be selectively incorporated into coagulated grains. Finally, it was found that absolute extinction normalized by suitably chosen color indices results in a minimization of the R-dependence of portions of the UV curve, allowing A(sub lambda) to be estimated for these wavelengths independent of R.

Cardelli, Jason A.↗
NASA NTRS1989

Grains charges in interstellar clouds

The charge of cosmic grains could play an important role in many astrophysical phenomena. It probably has an influence on the coagulation of grains and more generally on grain-grain collisions, and on interaction between charged particles and grains which could lead to the formation of large grains or large molecules. The electrostatic charge of grains depends mainly on the nature of constitutive material of the grain and on the physical properties of its environment: it results from a delicate balance between the plasma particle collection and the photoelectron emission, both of them depending on each other. The charge of the grain is obtained in two steps: (1) using the numerical model the characteristics of the environment of the grain are computed; (2) the charge of a grain which is embedded in this environment is determined. The profile of the equilibrium charge of some typical grains through different types of interstellar clouds is obtained as a function of the depth of the cloud. It is shown that the grain charge can reach high values not only in hot diffuse clouds, but also in clouds with higher densities. The results are very sensitive to the mean UV interstellar radiation field. Three parameters appear to be essential but with different levels of sensitivity of the charge: the gas density, the temperature, and the total thickness of the cloud.

Bel, N.↗
NASA NTRS1991

Historical evidence for a connection between volcanic eruptions and climate change

The times of historical volcanic aerosol clouds were compared with changes in atmospheric temperatures on regional, hemispheric, and global scales. These involve either a direct comparison of individual significant eruption years with temperature records, or a comparison of eruption years with composited temperature records for several years before and after chosen sets of eruptions. Some studies have challenged the connection between individual eruptions and climate change. Mass and Portman (1989) recently suggested that the volcanic signal was present, but smaller than previously thought. In a study designed to test the idea that eruptions could cause small changes in climate, Hansen and other (1978) chose one of the best monitored eruptions at the time, the 1963 eruption of Agung volcano on the island of Bali. Using a simple radiation-balance model, in which an aerosol cloud in the tropics was simulated, this basic pattern of temperature change in the tropics and subtropics was reproduced. There may be natural limits to the atmospheric effects of any volcanic eruption. Self-limiting physical and chemical effects in eruption clouds were proposed. Model results suggest that aerosol microphysical processes of condensation and coagulation produce larger aerosols as the SO2 injection rate is increased. The key to discovering the greatest effects of volcanoes on short-term climate may be to concentrate on regional temperatures where the effects of volcanic aerosol clouds can be amplified by perturbed atmospheric circulation patterns, especially changes in mid-latitudes where meridional circulation patterns may develop. Such climatic perturbations can be detected in proxy evidence such as decreases in tree-ring widths and frost damage rings in climatically sensitive parts of the world, changes in treelines, weather anomalies such as unusually cold summers, severity of sea-ice in polar and subpolar regions, and poor grain yields and crop failures.

Rampino, Michael R.↗
NASA NTRS1991

Medical evaluations on the KC-135 1990 flight report summary

The medical investigations completed on the KC-135 during FY 1990 in support of the development of the Health Maintenance Facility and Medical Operations are discussed. The experiments are comprised of engineering evaluations of medical hardware and medical procedures. The investigating teams are made up of both medical and engineering personnel responsible for the development of medical hardware and medical operations. The hardware evaluated includes dental equipment, a coagulation analyzer, selected pharmaceutical aerosol devices, a prototype air/fluid separator, a prototype packaging and stowage system for medical supplies, a microliter metering system, and a workstation for minor surgical procedures. The results of these engineering evaluations will be used in the design of fleet hardware as well as to identify hardware specific training requirements.

Lloyd, Charles W.↗
NASA NTRS1990

Formation of the earth

The origin of the earth is discussed in the context of the formation of the sun and the planets, and a standard model for such a formation assuming gravitational instability in a dense interstellar molecular cloud is outlined, along with the most significant variant of the model in which the loss of the nebular gas occurred after the formation of the earth. The formation of the sun and solar nebulae is addressed, and the coagulation of grains and the formation of small planetesimals are covered, along with the gravitational accumulation of planetesimals into planetary embryos and final stages of accumulation - embryos of planets. It is pointed out that the final stage of accumulation consists of the collision of these embryos; because of their large size, particularly after their further growth, these collisions represent giant impacts. It is concluded that the earth was initially an extremely hot and melted planet, surrounded by a fragile atmosphere and subject to violent impacts by bodies of the size of Ceres and even the moon.

Wetherill, George W.↗
NASA NTRS1991

UV spectroscopy of Titan's atmosphere, planetary organic chemistry and prebiological synthesis. II - Interpretation of new IUE observations in the 220-335 nm range

The theoretical model developed by McKay et al. (1989) to characterize the size distribution, thermal structure, and chemical composition of the stratospheric haze of Titan is applied to new 220-335-nm albedo measurements obtained with the long-wavelength prime camera of the IUE during August 1987. Data and model predictions are presented in extensive graphs and discussed in detail. It is shown that a simple model with particles of one size at a given altitude does not accurately reproduce the observed features in all spectral regions, but that good general agreement is obtained using a model with a uniformly mixed layer at 150-600 km and a bimodal distribution of small 'polymer' haze particles (radius less than 20 nm) and larger haze particles (radius 100-500 nm). The number densities implied by this model require, however, a mechanism such as electrostatic charging or reaction kinetics to inhibit coagulation of the smaller particles.

Courtin, Regis↗
NASA NTRS1991

Tridymite and maghemite formation in an Fe-SiO smoke

Grains produced in porous Fe-SiO smoke are analyzed by means of electron microscopy to determine their crystallographical, morphological, and chemical properties. The nucleation of metal oxide condensates takes place as the vapor evolves in a condensation-flow apparatus, initially producing simple crystalline phases. Fe nucleation follows, and the subsequent heterogeneous nucleation of quenched mixed high-Fe silica grains exhibits associated Fe-depletion, eventually producing pure silica grains. The coalescence exhibited by the pure silica grains is compatible with the higher surface energy found in the pure silica phase. During coagulation the grains melt and anneal to a chemically pure mixture of alpha- and beta-tridymite, and the precipitation of amorphous silica material is observed. The settling of grains in evolving solar nebula is theorized to be related to the stickiness of this material, and maghemite is suggested to contribute to the Fe/Si fractionation in solar nebula.

Rietmeijer, F. J. M.↗
NASA NTRS1991

Condensational growth and trace species scavenging in stratospheric sulfuric acid/water aerosol droplets

Stratospheric aerosols play a significant role in the environment. The composition of aerosols is believed to be a liquid solution of sulfuric acid and water with numerous trace species. Of these trace species, ozone in particular was recognized as being very important in its role of shielding the environment from harmful ultraviolet radiation. Also among the trace species are HCl and ClONO2, the so called chlorine reservoir species and various oxides of nitrogen. The quantity of stratospheric aerosol and its particle size distribution determines, to a large degree, the chemistry present in the stratosphere. Aerosols experience 3 types of growth: nucleation, condensation, and coagulation. The application of condensation investigations to the specific problem of stratospheric aerosols is discussed.

Tompson, Robert V., Jr.↗
NASA NTRS1992

Numerical experiments with flows of elongated granules

Theory and numerical results are given for a program simulating two dimensional granular flow (1) between two infinite, counter-moving, parallel, roughened walls, and (2) for an infinitely wide slider. Each granule is simulated by a central repulsive force field ratcheted with force restitution factor to introduce dissipation. Transmission of angular momentum between particles occurs via Coulomb friction. The effect of granular hardness is explored. Gaps from 7 to 28 particle diameters are investigated, with solid fractions ranging from 0.2 to 0.9. Among features observed are: slip flow at boundaries, coagulation at high densities, and gross fluctuation in surface stress. A videotape has been prepared to demonstrate the foregoing effects.

Elrod, Harold G.↗
NASA NTRS1992

Plausible surface models for Titan

Current understanding of the nature of Titan's surface and some new ideas for explaining the curious radar returns from Saturn's largest satellite are reviewed. Pre-Voyager models of the surface, based largely on cosmochemistry and the discovery of atmospheric methane, allowed for a range of possibilities, including pure methane oceans. The Voyager 1 flyby ruled out this last possibility, replacing it with compelling observational arguments in favor of a mixed light hydrocarbon and nitrogen ocean. Ground based radar observations indicated a surprisingly reflective surface which is inconsistent with a hydrocarbon ocean and more reminiscent of the Galilean Satellites. Nonetheless, passive radiometric measurements of the surface do not support the notion that Titan's surface is like that of the Galilean satellites. One of the arguments against hydrocarbon oceans reflecting radar energy is that most solid, complex hydrocarbon and nitriles will be denser than the liquid and sink. Nonetheless, many of the aerosol species will coagulate in highly nonspherical patterns, and some species probably polymerize in long chains. Such chains will have very low sedimendation velocities in the ocean and may remain near the surface through ocean mixing process. The prospect of an oceanic 'soup' of polar polymers acting as volume reflectors at radio wevelengths suggests that the interpretation of radar observations needs evaluation.

Lunine, Jonathan I.↗