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At least 343 records · Page 19
All sourcesNASA NTRSDOE OSTI
NASA NTRS1993

Visible-infrared properties of controlled laboratory soils

Almost all surfaces available for remote observation consist of particulate materials or soils. The distribution of mean particle sizes depend on the original material and physical and chemical processes that have acted on the surface over time. It is well known that the optical and infrared spectral properties of materials depends on the particle size. There has been little detailed study, however, of natural soils, namely particulate materials with a range of particle sizes. Current models for intimate mixing typically use an average particle size in calculations and are most successful when the particle size is constrained by known sieve fractions. Preliminary results of a study in which soils were prepared with a known composition and range of particle sizes are reported. This discussion presents the overall visible to infrared properties of these synthetic soils and evaluates the mid-infrared properties.

Pieters, C. M.↗
NASA NTRS1983

Strength of mineral absorption features in the transmitted component of near-infrared reflected light - First results from RELAB

Bidirectional reflectance measurements are the only type of reflectance data available to the remote observer. For compositional interpretations, data are desired not only for identification of possible mineral components but also for modal abundance. The latter requires detailed information about the strength of absorption features. Using a new laboratory facility, the RELAB, laboratory data in the near infrared are presented that document effects of particle size, mineral mixtures, and viewing geometry for selected materials with well-developed absorption bands. The commonly observed increase in reflectance with decrease in particle size is also observed for absorption bands as well as a related decrease in absorption strength. For small particles in parts of the spectrum of maximum reflectance, however, a minor decrease in reflectance with a decrese in particle size is sometimes observed. Small particles dominate the observed characteristics of particulate surfaces, which contain a range of particle sizes. The mean optical path length (transmission through particles) of reflected radiation measured for a variety of particle sizes has an apparent upper limit of about 2 mm for particles of less than 250 microns. The typical number of particles involved in the optical path is less than 50.

Pieters, C. M.↗
NASA NTRS2002

Solid Hydrogen Experiments for Atomic Propellants: Particle Formation Energy and Imaging Analyses

This paper presents particle formation energy balances and detailed analyses of the images from experiments that were conducted on the formation of solid hydrogen particles in liquid helium during the Phase II testing in 2001. Solid particles of hydrogen were frozen in liquid helium and observed with a video camera. The solid hydrogen particle sizes and the total mass of hydrogen particles were estimated. The particle formation efficiency is also estimated. Particle sizes from the Phase I testing in 1999 and the Phase II testing in 2001 were similar. Though the 2001 testing created similar particles sizes, many new particle formation phenomena were observed. These experiment image analyses are one of the first steps toward visually characterizing these particles and it allows designers to understand what issues must be addressed in atomic propellant feed system designs for future aerospace vehicles.

Palaszewski, Bryan↗
NASA NTRS1987

Uranian ring photometry - Results from Voyager 2

The rings of Uranus are characterized on the basis of an analysis of seven images obtained with a clear filter by Voyager 2 during its encounter with Uranus in January 1986. The data are presented graphically and compared with the predictions of theoretical models with different particle size distributions. Parameters determined include average particle Bond albedo 0.014 + or - 0.004, fractional area of the main rings in dust-size particles 0.002 or less, and average dust-particle size 1.0 + or - 0.3 microns for a Hansen-Hovenier (1974) distribution and a power-law distribution of spherical particles with spectral index 2.5 + or - 0.5.

Ockert, Maureen E.↗
NASA NTRS1980

Scattering by nonspherical particles of size comparable to wavelength - A new semi-empirical theory and its application to tropospheric aerosols

A semiempirical theory is developed which is based on simple physical principles and comparisons with laboratory measurements. The ultimate utility of this approach rests on its ability to successfully reproduce the observed single-scattering phase function for a wide variety of particle shapes, sizes and refractive indices. This approximate theory is developed for evaluating the interaction of randomly oriented, nonspherical particles with the total intensity component of electromagnetic radiation. Mie theory is used when the particle size parameter x (ratio of particle circumference to wavelength) is less than some upper bound x sub zero (about 5). For x greater than x sub zero, the interaction is divided into three components: diffraction, external reflection and transmission. The application of the theory is illustrated by considering the influence of the shape of tropospheric aerosols on their contribution to the earth's global albedo.

Pollack, J. B.↗
NASA NTRS1993

Martian global dust storms - Zonally symmetric numerical simulations including size-dependent particle transport

A zonally symmetric primitive-equation grid-point model of the Martian atmosphere is coupled with an aerosol transport/microphysical model in order to numerically investigate the size-dependent transport of dust particles in the Martian atmosphere. The coupled model accounts for diabatic heating due to a radiatively active evolving dust field, but neglects feedbacks between atmosphere-surface interactions and surface dust lifting. The differing suspension lifetimes of dust particles of various sizes (radius = 1-80 microns), in conjunction with spatially varying atmospheric dynamics, result in latitudinal differences in several measurements of the column integrated particle concentration. This work indicates the importance of considering the full range of particle sizes (and shapes) of the suspended dust during Martian global dust storms and their impact upon the spatial extent and wavelength-dependent radiative influence of such storms.

Murphy, James R.↗
NASA NTRS1994

Retrieval of sizes and total masses of particles in volcanic clouds using AVHRR bands 4 and 5

The advanced very high resolution radiometer (AVHRR) sensor on polar orbiting NOAA satellites can discriminate between volcanic clouds and meteorological clouds using two-band data in the thermal infrared. This paper is aimed at developing a retrieval of the particle sizes, optical depth, and total masses of particles from AVHRR two-band data of volcanic clouds. Radiative transfer calculations are used with a semi-transparent cloud model that is based on assumptions of spherical particle shape, a homogeneous underlying surface, and a simple thin cloud parallel to the surface. The model is applied to observed AVHRR data from a 13-hour old drifting cloud from the August 19, 1992, eruption of Crater Peak/Spurr Volcano, Alaska. The AVHRR data fit in the range of results calculated by the model, which supports its credibility. According to the model results, the average of effective particle radius in the test frame of this cloud is in the range of 2 to 2.5 micrometer, the optical depth at 12 micrometer is about 0.60 - 0.65. The total estimated mass of ash in the air amounts to 0.24 - 0.31 x 10(exp 6) tons, which is about 0.7-0.9% of the mass measured in the ashfall blanket. Sensitivity tests show that the mass estimate is more sensitive to the assumed ash size distribution than it is to the ash composition.

Wen, Shiming↗
NASA NTRS1991

The dynamics of submicron-sized dust particles lost from Phobos

The dynamics of submicron-sized dielectric particles lost from the Martian moon Phobos are studied in connection with the possible detection of dust by the Phobos 2 spacecraft. The motion of these small dust grains is influenced not only by gravity but also by solar radiation pressure and electromagnetic forces. The plasma environment of Mars is described by applying a hybrid gasdynamic-cometary model. Some of the submicron-sized grains ejected at speeds on the order of a few tens meters per second can stay in orbit around Mars for several months forming a nonuniform and time-dependent dust halo.

Horanyi, M.↗
NASA NTRS2004

Trajectory Studies of Large HNO3-Containing PSC Particles in the Arctic: Evidence for the Role of NAT

Large (5 to >20 micron diameter) nitric-acid-containing polar stratospheric cloud (PSC) particles were observed in the Arctic stratosphere during the winter of 1999-2000. We use a particle growth and sedimentation model to investigate the environment in which these particles grew and the likely phase of the largest particles. Particle trajectory calculations show that, while simulated nitric acid dihydrate (NAD) particle sizes are significantly smaller than the observed maximum particle sizes, nitric acid trihydrate (NAT) particle trajectories are consistent with the largest observed particle sizes.

McKinney, K. A.↗
NASA NTRS2002

Dynamics and Structure of Planetary Rings

We developed a novel technique to determine the macroscopic particle size distribution of Saturn's rings by exploiting diffraction effects during stellar occultations. This was a major undertaking, and resulted in two publications now in press. In the latter paper, we derived power-law particle size distributions for each of Saturn's main ring regions, using observations of the 3 July 1989 stellar occultation of 28 Sgr from Palomar, McDonald, and Lick Observatories. We used the Voyager PPS delta Sco optical depth profile to estimate and then remove the directly transmitted signal from the 28 Sgr observations, leaving high SNR scattered light profiles at wavelengths of 3.9, 2.1, and 0.9 micrometers. The angular distribution of this diffracted signal depends on the ring particle size distribution: the sharpness of the forward lobe is set by the largest particles, while the overall breadth and amplitude of the scattered signal reflects the abundance of smaller, cm-sized particles. We developed both a simple one-dimensional scattering model and a more realistic 2-D model. We assumed for simplicity a single power law particle size distribution for each major ring region, and determined the index q and lower and upper size cutoffs a(sub min) and a(sub max) that provide the best match to all three data sets in each region. Our results in the A and C Rings are fairly consistent with values of q and a(sub max) derived from Voyager radio occultation (RSS) measurements. We extended their results by determining lower limits to the particle size distributions and by probing the B Ring. This technique is applicable to imaging observations of the rings during the Cassini mission.

French, Richard G.↗
NASA NTRS1996

Size Segregation and Number Density Enhancement of Particles in Accretion Disk Eddies

We investigate the conditions for trapping solid dust particles in eddies and discuss the behavior of particles in a non-laminar protoplanetary accretion disk. We considered particle sizes from small dust grains to larger objects, 10(exp -4) cm less than a(sub p) less than 10(exp 2) cm. Independent of the source of turbulence, one can expect eddies to exist in the gas flow of a accretion disk, in the form of randomly occurring turbulent features or as convective cells. Due to the centrifugal force, solid particles are driven out of an eddy. It will be shown that this process is inhibited by the gravitational force induced by the protostar. Because of the mass dependence of the friction time, a given eddy becomes a trap for particles of a characteristic size and causes a local change in the dust density. Thus, the size distribution of the grains is no longer spatially homogeneous on small scales. Our general estimates do not depend on special turbulence or convection models. We calculate the maximal inhomogeneity due to this process. The strongest effect was observed for mm-sized particles, which can be concentrated by a factor of 100 within only 100 years.

Klahr, H. H.↗
NASA NTRS1985

In-situ detection of micron-sized dust particles in near-Earth space

In situ detectors for micron sized dust particles based on the measurement of impact ionization have been flown on several space missions (Pioneer 8/9, HEOS-2 and Helios 1/2). Previous measurements of small dust particles in near-Earth space are reviewed. An instrument is proposed for the measurement of micron sized meteoroids and space debris such as solid rocket exhaust particles from on board an Earth orbiting satellite. The instrument will measure the mass, speed, flight direction and electrical charge of individually impacting debris and meteoritic particles. It is a multicoincidence detector of 1000 sq cm sensitive area and measures particle masses in the range from 10 to the -14th power g to 10 to the -8th power g at an impact speed of 10 km/s. The instrument is lightweight (5 kg), consumes little power (4 watts), and requires a data sampling rate of about 100 bits per second.

Gruen, E.↗
NASA NTRS1993

Thermal conductivity measurements of particulate materials under Martian conditions

The mean particle diameter of surficial units on Mars has been approximated by applying thermal inertia determinations from the Mariner 9 Infrared Radiometer and the Viking Infrared Thermal Mapper data together with thermal conductivity measurement. Several studies have used this approximation to characterize surficial units and infer their nature and possible origin. Such interpretations are possible because previous measurements of the thermal conductivity of particulate materials have shown that particle size significantly affects thermal conductivity under martian atmospheric pressures. The transfer of thermal energy due to collisions of gas molecules is the predominant mechanism of thermal conductivity in porous systems for gas pressures above about 0.01 torr. At martian atmospheric pressures the mean free path of the gas molecules becomes greater than the effective distance over which conduction takes place between the particles. Gas particles are then more likely to collide with the solid particles than they are with each other. The average heat transfer distance between particles, which is related to particle size, shape and packing, thus determines how fast heat will flow through a particulate material.The derived one-to-one correspondence of thermal inertia to mean particle diameter implies a certain homogeneity in the materials analyzed. Yet the samples used were often characterized by fairly wide ranges of particle sizes with little information about the possible distribution of sizes within those ranges. Interpretation of thermal inertia data is further limited by the lack of data on other effects on the interparticle spacing relative to particle size, such as particle shape, bimodal or polymodal mixtures of grain sizes and formation of salt cements between grains. To address these limitations and to provide a more comprehensive set of thermal conductivities vs. particle size a linear heat source apparatus, similar to that of Cremers, was assembled to provide a means of measuring the thermal conductivity of particulate samples. In order to concentrate on the dependence of the thermal conductivity on particle size, initial runs will use spherical glass beads that are precision sieved into relatively small size ranges and thoroughly washed.

Presley, M. A.↗
NASA NTRS1977

Physical properties of the particles composing the Martian dust storm of 1971-1972

Data obtained with the infrared interferometric spectrometer (IRIS) aboard Mariner 9 during the global Martian dust storm of 1971-1972 are analyzed to study the particle sizes in detail, estimate the dust particle size distribution, and determine the dust optical depth. Theoretical spectra of the emergent intensity from dust clouds with properties similar to those of the Martian cloud are generated, deficiencies of the approximation used are discussed, and the information content of the IR aerosol spectra is assessed. Analysis of IRIS spectra from a series of orbits spread through the early months of the Mariner 9 mission indicates that the dust could have been either igneous material with an SiO2 content similar to or higher than that of a typical terrestrial sediment or clay minerals similar to those on earth. In either case, the Martian dust was differentiated by geologic or weathering processes. The dust particle size distribution between 1 and 10 microns is found to have a slope similar to that of terrestrial dust size distributions far from source regions.

Toon, O. B.↗
NASA NTRSDate not supplied

Development of Challenge Aerosols for Testing Filters in Spacecraft Air Revitalization Systems

The common means for reducing particle concentrations in air in enclosed spaces, including in space habitats, are source prevention and particle removal by air filters. While air filtration and testing is a well-established discipline and industry, testing and classifying filters according to commonly used standards rely on a test aerosol that is often arbitrary and chosen for the convenience of the test method. In space habitats, the particle size distributions are expected to be quite different than the particle size distributions prescribed in test standards, due to the partial or low gravity environment affecting sedimentation of large particulates like hair or cloth fibers, or the introduction of planetary dust to the pressurized volume. This means that the efficacy of the filter will be quite different in the space habitat than specified according to a prevailing standard. This paper will present a means to specify and generate a “composite” test aerosol that is similar to the measured and reported particle sizes utilizing the International Space Station (ISS) as an example. Application of this test aerosol is expected to yield filter efficiencies and loading effects closer to what one can expect on the ISS and to be useful in determining filter lifetime and replacement cycles. The method can be tuned, within reason, to match other particle size distributions one may encounter, such as the intrusion of Lunar dust for a Lunar lander or habitat.

Filtration↗
NASA NTRS1991

Dust in the Mars atmosphere

The amount of dust suspended in the Martian atmosphere is highly variable with location and with time. The opacity of the sky is best known at the two Viking Lander sites, where visual, vertical-column optical depth never fell below a value of a few tenths during the 1.25 Mars years of observations and yet exceeded 2 to 3 during two great dust storms in 1977. Elsewhere on the planet, optical depths have been estimated from orbiter visible imaging of surface contrasts and from mapping of infrared emission from the surface and the overlying (dusty) atmosphere. In many cases these opacities (and thus dust amounts) may be uncertain by as much as a factor of two. Viking Lander observations of twilight indicate that the background dust haze is more or less uniformly mixed with altitude in the lower atmosphere. Observations from spacecraft indicate that there may be come seasonal variations to the height of these dust hazes, which sometimes extend above 30 km. (Ice haze layers may occur as high as 80 km.) The existing observations do not constrain the composition or the size distribution of the suspended dust particles very well. Remote sensing observations depend principally upon the product of the number of particles, the geometric cross-sections (and so particle size and shape), and the extinction efficiency of the particles (and so the particle composition), as integrated over the particle size distribution and along the line of sight. While the observed variation of dust opacity with wavelength constrains these quantities, it does not often permit the unique determination of the individual properties of the suspended dust. A size distribution having a cross-section weighted mean particle radius of 2.5 microns was deduced from a synthesis of the IR thermal emission spectra observed in the Southern Hemisphere by Mariner 9 during the 1971 global dust storm. Although the IR thermal emission is relatively insensitive to the sub-micron sized particles which tend to dominate visible opacity, this same size distribution was consistent with modeling of the sky brightness variation near the sun, as seen through the background haze above the Viking lander sites in the Northern Hemisphere.

Zurek, R. W.↗
NASA NTRS1986

Effect of backscatter-to-extinction ratio on lidar inversions

One way of deriving the atmospheric extinction coefficient from lidar measurements is to start from the single scattering lidar equation. Simulations of lidar experiments and extinction coefficient calculation were conducted and the resulting errors examined. In order to make the simulation realistic, the volume backscatter coefficients and extinction coefficients used in the simulation were based on measured particle distributions, in this case on the particle size distributions measured in Meppen, Germany in the fall of 1980. In that experiment particle size distributions were measured with a balloon-borne particle spectrometer during balloon descents from about 600 meters above ground to the surface. Each particle spectrum represented a layer of at most a few meter thickness, so an altitude resolved profile of particle size spectra is measured. A Mie code was used to compute linear extinction coefficient and volume backscatter coefficient profiles from the particle size spectra for each balloon sounding.

Measure, E. M.↗
NASA NTRS1997

Combustion of PTFE: The Effects of Gravity and Pigmentation on Ultrafine Particle Generation

Ultrafine particles generated during polymer thermodegradation are a major health hazard, owing to their unique pathway of processing in the lung. This hazard in manned spacecraft is poorly understood, because the particulate products of polymer thermodegradation are generated under low gravity conditions. Particulate generated from the degradation of PolyTetraFluoroEthylene (PTFE), insulation coating for 20 AWG copper wire (representative of spacecraft application) under intense ohmic heating were studied in terrestrial gravity and microgravity. Microgravity tests were done in a 1.2-second drop tower at the Colorado School of Mines (CSM). Thermophoretic sampling was used for particulate collection. Transmission Electron Microscopy (TEM) and Scanning Transmission Electron Microscopy (STEM) were used to examine the smoke particulates. Image software was used to calculate particle size distribution. In addition to gravity, the color of PTFE insulation has an overwhelming effect on size, shape and morphology of the particulate. Nanometer-sized primary particles were found in all cases, and aggregation and size distribution was dependent on both color and gravity; higher aggregation occurred in low gravity. Particulates from white, black, red and yellow colored PTFE insulations were studied. Elemental analysis of the particulates shows the presence of inorganic pigments.

McKinnon, J. Thomas↗